Monday, March 30, 2009

Evaporator Types and Applications

Evaporators are often classified as follows:
  1. heating medium separated from evaporating liquid by tubular heating surfaces,
  2. heating medium confined by coils, jackets, double walls, flat plates, etc.,
  3. heating medium brought into direct contact with evaporating liquid, and
  4. heating with solar radiation.


Evaporators with tubular heating surfaces dominate the field. Circulation of the liquid past the surface may be induced by boiling (natural circulation) or by mechanical methods (forced circulation). In forced circulation, boiling may or may not occur on the heating surface

Solar evaporators require tremendous land areas and a relatively cheap raw material, since pond leakage may be appreciable. Solar evaporation generally is feasible only for the evaporation of natural brines, and then only when the water vapor is evaporated into the atmosphere and is not recovered.

Evaporators may be operated batchwise or continuously. Most evaporator systems are designed for continuous operation. Batch operation is sometimes employed when small amounts must be evaporated. Batch operation generally requires more energy than continuous operation.

Batch evaporators, strictly speaking, are operated such that filling, evaporating, and emptying are consecutive steps. This method of evaporation requires that the body be large enough to hold the entire charge of the feed and the heating element be placed low enough not to be uncovered when the volume is reduced to that of the product. Batch operation may be used for small systems, for products that require large residence times, or for products that are difficult to handle.

A more frequent method of operation is semibatch in which feed is continuously added to maintain a constant liquid level until the entire charge reaches the final concentration. Continuous-batch evaporators usually have a continuous feed, and over at least part of the cycle, a continuous discharge. One method of operation is to circulate from a storage tank to the evaporator and back until the entire tank is at a specified concentration and then finish the evaporation in batches.

Continuous evaporators have continuous feed and discharge. Concentrations of both feed and discharge remain constant during operation.

Evaporators may be operated either as once-through units or the liquid may be recirculated through the heating element. In once-through operation all the evaporation is accomplished in a single pass. The ratio of evaporation to feed is limited in single-pass operation; single-pass evaporators are well adapted to multiple-effect operation permitting the total concentration of the liquid to be achieved over several effects. Agitated-film evaporators are also frequently operated once through. Once-through evaporators are also frequently required when handling heat-sensitive materials.

Recirculated systems require that a pool of liquid be held within the equipment. Feed mixes with the pooled liquid and the mixture circulates across the heating element. Only part of the liquid is vaporized in each pass across the heating element; unevaporated liquid is returned to the pool. All the liquor in the pool is therefore at the maximum concentration. Recirculated systems are therefore not well suited for evaporating heat sensitive materials. Recirculated evaporators, however, can operate over a wide range of concentration and are well adapted to single-effect evaporation.

There is no single type of evaporator which is satisfactory for all conditions. It is for this reason that there are many varied types and designs. Several factors determine the application of a particular type for a specific evaporation result. The following sections will describe the various types of evaporators in use today and will discuss applications for which each design is best adapted.

Sunday, March 29, 2009

Mechanisms Flow-Induced Vibration

Induced vibration of any system involves the coupling of some exciting forces with an elastic structure. In the case of flow-induced vibration, the exciting forces result from the flow of the shellside fluid and the elastic structure in the bundle of tubes. The exciting forces fluctuate at characteristic frequencies which increase continuously with increasing flow rate. The tubes vibrate only at unique responding frequencies called their natural frequencies. Coupling occurs when the exciting frequencies match the responding frequencies and tube vibration results.

The natural frequency of tubes depends primarily on their geometry and material of construction. The intensity of vibration is evidenced by the amount of periodic movement; the extent of this peak-to-peak movement about the at-rest centerline is termed the amplitude of vibration. Energy must be available to excite the tubes into vibration. The energy of vibration is dissipated by internal and external damping. The exciting force could be the result of:
  1. fluid dynamic mechanisms as a result of flow parallel to or across the tubes
  2. pulsations of a compressor or pump
  3. mechanical vibrations transmitted through a structure.
Unless amplified by resonant phenomena, the flow forces normally enountered in equipment are not sufficient to cause damage. Resonance, which can increase the tube deflection by orders of magnitude, occurs when the frequency of a cyclic exciting force coincides with the natural frequency of the tube

In order to predict the occurrence of flow-induced vibration, the phenomena that produces the exciting forces and the dynamic response by the tubes must be understood. The determination of tube natural frequencies is relatively straight-forward. However, determination of the exciting forces created by the shellside fluid flow is extremely more difficult. The shellside flow in a heat exchanger follows a complex flow path. It is subjected to changes of direction, acceleration, and deceleration. At times, the flow is either perpendicular to the tubes (crossflow), axially along the tubes (parallel flow), or at any angle in between. Flow phenomena in crossflow include vortex shedding, turbulent buffeting, and fluid-elastic whirling, The flow phenomena found in parallel flow includesaxial-flow eddy formation.

Saturday, March 28, 2009

Flow-Induced Vibration in Evaporations

The concern for flow-induced tube vibration has become a serious consideration in the design of shell-and-tube equipment. These problems can lead to tubes and tubejoints that leak, increased shellside pressure drop, and intolerably loud noises. The result is that equipment must be removed from service for repair and modification.

Flow-induced vibrations can damage tubes in evaporators. All tubes vibrate under all flow conditions! However, we are concerned with vibrations which cause significant tube damage. As larger evaporators, greater flowrates and higher shellside velocities become more prevalent, damaging tube vibrations are more likely to occur. No evaporator design is complete without considering the possibility of damage as a result of flow-induced vibrations.

Damage is more likely to occur with gases or vapors on the shellside than with liquids. Flow-induced vibrations also occur with liquids on the shellside, but the damage is often limited to localized areas of relatively high velocity. In severe cases, tubes can leak within a few days or even in a few hours after the equipment has been placed in service. More often, damage will appear a year or so after startup. Additional tube damage will develop after the initial damage has been repaired, but the number and frequency of further damages will decrease with time.

In a number of cases, heat exchanger tube failures attributed to flowinduced vibration have resulted in consequential damage to other equipment within a plant. Failures of this nature have proven to be the most destructive, most costly, and have required the longest plant shutdowns for rectification.

Currently available methods for predicting flow-induced vibration damage are inadequate for predicting failures. At best, they identify the equipment that are susceptible to damage. The primary reason for this lack of precision is that flow-induced vibrations are extremely complicated. Much has been learned, but the probability of its occurrence is still not known. However, the cost penalty for equipment designed to completely avoid damaging vibration is modest and is almost always easily justified.

Some of the problem areas concerned with prediction of vibration include:
  1. the complex pattern of flow through a tube bundle
  2. the complicated fluid mechanics of a bank of vibrating tubes
  3. the role of damping
  4. the rates of wear and fatigue.
Nevertheless, it is possible to develop design criteria, especially when tempered with experience, to ensure that equipment will be safe from vibration damage.

Flow-induced vibrations problems in tubular equipment are commonly thought of as consisting entirely of mechanical failure of the tubes. However, the vibration can increase the shellside pressure drop, sometimes as much as double. Further, an acoustically vibrating unit can produce an intolerably high noise level. With an increasing emphasis on noise control, acoustic vibration must be an important consideration in design of tubular equipment.

Friday, March 27, 2009

Physical Properties in Evaporators

To calculate heat-transfer rates, physical-property data for the fluids being treated must be available. Physical property data should be as accurate as possible, especially as more accurate heat-transfer correlations become available. However, most physical properties of mixtures must be calculated or estimated; consequently there is little need to attempt to determine true film temperatures. Physical-property data at the average bulk fluid temperature are generally sufficient.

The following physical properties are usually required in order to obtain satisfactory calculated heat transfer rates:
  1. viscosity
  2. thermal conductivity
  3. density
  4. specific heat
  5. latent heat
  6. surface tension
When condensing or vaporizing over a temperature range, a curve representing heat load as a function of temperature should also be available. In addition, any concentration effects should be known.

Thursday, March 26, 2009

Types of Heat Transfer Operations

There are two types of heat transfer operation: sensible heat and change of phase. Sensible heat operations involve heating or cooling of a fluid in which the heat transfer results only in a temperature change of the fluid. Changeof-phase heat transfer results in a liquid being changed into a vapor or a vapor being changed into a liquid. Boiling or vaporization is the convection process involving a change in phase from liquid to vapor. Condensation is the convection process involving a change in phase from vapor to liquid. Many applications involve both sensible heat and change-of-phase heat transfer.

Sensible Heat Transfer Inside Tubes

Sensible heat transfer in most applications involves forced convection inside tubes or ducts or forced convection over exterior surfaces.

The heating and cooling of fluids flowing inside conduits are among the most important heat-transfer processes in engineering. The flow of fluids inside conduits may be broken down into three flow regimes. These flow regimes are measured by a ratio called the Reynolds number which is an indication of the turbulence of the flow inside the conduit. The three regimes are:

Laminar Flow Reynolds numbers less than 2,100
Transition Flow Reynolds numbers between 2,100 and 10,000
Turbulent Flow Reynolds numbers greater than 10,000

Wednesday, March 25, 2009

Modes of Heat Transfer in Evaporators

The literature of heat transfer generally recognizes three distinct modes of heat transfer: conduction, radiation, and convection. Strictly speaking, only conduction and radiation should be classified as heat-transfer processes, because only these two mechanisms depend for their operation on the mere existence of a temperature difference. The last of thre three, convection, does not strictly comply with the definition of heat transfer because it depends for its operation on mechanical mass transport also. But since convection also accomplishes transmission of energy from regions of high temperature to regions of lower temperature, the term “heat transfer by convection” has become generally accepted

In most situations heat flows not by one, but by several of these mechanisms simultaneously.

Conduction is the transfer of heat from one part of a body to another part of the same body, or from one body to another in physical contact with it, without appreciable displacement of the particles of the body. Conduction can occur in solids, liquids, or gases.

Radiation is the transfer of heat from one body to another, not in contact with it, by means of electromagnetic wave motion through space, even when a vacuum exists between them.

Convection is the transfer of heat from one point to another within a fluid, gas or liquid, by the mixing of one portion of the fluid with another. In natural convection, the motion of the fluid is entirely the result of differences in density resulting from temperature differences; in forced convection, the motion is produced by mechanical means. When the forced velocity is relatively low, it should be realized that “freeconvection” factors, such as density and temperature difference, may have an important influence.

In the solution of heat-transfer problems, it is necessary not only to recognize the modes of heat transfer which play a role, but also to determine whether a process is Steady or Unsteady. When the rate of heat flow in a system does not vary with time-when it is constant-the temperature at any point does not change and steady-state conditions prevail. Under steady-state conditions, the rate of heat input at any point of the system must be exactly equal to the rate of heat output, and no change in internal energy can take place. The majority of engineering heat-transfer problems are concerned with steady-state systems.

The heat flow in a system is transient, or unsteady, when the temperatures at various points in the system change with time. Since a change in temperature indicates a change in internal energy, we conclude that energy storage is associated with unsteady heat flow. Unsteady-heat-flow problems are more complex than are those of steady state and can often only be solved by approximate methods.

Tuesday, March 24, 2009

Improvements in Evaporators

Many improvements have been made in evaporator technology in the last half-century. The improvements have taken many forms but have served to effect the following:
  1. Greater evaporation capacity through better understanding of the heat transfer mechanisms.
  2. Better economy through more efficient use of evaporator types
  3. Longer cycles between cleaning because of better understanding of salting, scaling, and fouling.
  4. Cheaper unit costs by modern fabrication techniques and larger unit size.
  5. Lower maintenance costs and improved product quality by use of better materials of construction as a result of better understanding of corrosion.
  6. More logical application of evaporator types to specific services.
  7. Better understanding and application of control techniques and improved instrumentation has resulted in improved product quality and reduced energy consumption.
  8. Greater efficiency resulting from enhanced heat transfer surfaces and better energy economy.
  9. Compressor technology and availability has permitted the application of mechanical vapor compression.

Monday, March 23, 2009

Liquid Characteristics (Part 2)

Scaling

Scaling is the growth or deposition on heating surfaces of a material which is either insoluble or has a solubility that decreases with an increase in temperature. It may also result from a chemical reaction in the evaporator. Both scaling and salting liquids are usually best handled in an evaporator that does not rely upon boiling for operation.

Fouling

Fouling is the formation of deposits other than salt or scale. They may be due to corrosion, solid matter entering with the feed, or deposits formed on the heating medium side.

Corrosion

Corrosion may influence the selection of evaporator type since expensive materials of construction indicate evaporators affording high rates of heat transfer. Corrosion and erosion are frequently more severe in evaporators than in other types of equipment because of the high liquid and vapor velocities, the frequent presence of suspended solids, and the concentrations required.

Product Quality

Product quality may require low holdup and low temperatures. Low-holduptime requirements may eliminate application of some evaporator types. Product quality may also dictate special materials of construction.

Other Fluid Properties

Other fluid properties must also be considered. These include: heat of solution, toxicity, explosion hazards, radioactivity, and ease of cleaning. Salting, scaling, and fouling result in steadily diminishing heat transfer rates, until the evaporator must be shut down and cleaned. Some deposits may be difficult and expensive to remove.

Sunday, March 22, 2009

Liquid Characteristics (Part 1)

Concentration

The properties of the feed to an evaporator may exhibit no unusual problems. However, as the liquor is concentrated, the solution properties may drastically change. The density and viscosity may increase with solid content until the heat transfer performance is reduced or the solution becomes saturated. Continued boiling of a saturated solution may cause crystals to form which often must be removed to prevent plugging or fouling of the heat transfer surface. The boiling point of a solution also rises considerably as it is concentrated.

Foaming

Some materials may foam during vaporization. Stable foams may cause excessive entrainment. Foaming may be caused by dissolved gases in the liquor, by an air leak below the liquid level, and by the presence of surface-active agents or finely divided particles in the liquor. Many antifoaming agents can be used effectively. Foams may be suppressed by operating at low liquid levels, by mechanical methods, or by hydraulic methods.

Temperature Sensitivity

Many chemicals are degraded when heated to moderate temperatures for relatively short times. When evaporating such materials special techniques are needed to control the time/temperature characteristics of the evaporator system.

Salting

Salting refers to the growth on evaporator surfaces of a material having a solubility that increases with an increase of temperature. It can be reduced or eliminated by keeping the evaporating liquid in close or frequent contact with a large surface area of crystallized solid.

What an Evaporator Does

As stated above, the object of evaporation may be to concentrate a solution containing the desired product or to recover the solvent. Sometimes both may be accomplished. Evaporator design consists of three principal elements: heat transfer, vapor-liquid separation, and efficient utilization of energy.

In most cases the solvent is water, heat is supplied by condensing steam, and the heat is transferred by indirect heat transfer across metallic surfaces. For evaporators to be efficient, the equipment selected and used must be able to accomplish several things:
  1. Transfer large amounts of heat to the solution with a minimum amount of metallic surface area. This requirement, more than all other factors, determines the type, size, and cost of the evaporator system.
  2. Achieve the specified separation of liquid and vapor and do it with the simplest devices available. Separation may be important for several reasons: value of the product otherwise lost; pollution; fouling of the equipment downstream with which the vapor is contacted; corrosion of this same downstream equipment. Inadequate separation may also result in pumping problems or inefficient operation due to unwanted recirculation.
  3. Make efficient use of the available energy. This may take several forms. Evaporator performance often is rated on the basis of steam economy-pounds of solvent evaporated per pound of steam used. Heat is required to raise the feed temperature from its initial value to that of the boiling liquid, to provide the energy required to separate liquid solvent from the feed, and to vaporize the solvent. The greatest increase in energy economy is achieved by reusing the vaporized solvent as a heating medium. This can be accomplished in several ways to be discussed later. Energy efficiency may be increased by exchanging heat between the entering feed and the leaving residue or condensate.
  4. Meet the conditions imposed by the liquid being evaporated or by the solution being concentrated. Factors that must be considered include product quality, salting and scaling, corrosion, foaming, product degradation, holdup, and the need for special types of construction.
Today many types of evaporators are in use in a great variety of applications. There is no set rule regarding the selection of evaporator types. In many fields several types are used satisfactorily for identical services. The ultimate selection and design may often result from tradition or past experience. The wide variation in solution characteristics expand evaporator operation and design from simple heat transfer to a separate art.

Saturday, March 21, 2009

Evaporator Elements

Three principal elements are of concern in evaporator design: heat transfer, vapor-liquid separation, and efficient energy consumption. The units in which heat transfer takes place are called heating units or calandrias. The vapor-liquid separators are called bodies, vapor heads, or flash chambers.

The term body is also employed to label the basic building module of an evaporator, comprising one heating element and one flash chamber. An effect is one or more bodies boiling at the same pressure. A multiple-effect evaporator is an evaporator system in which the vapor from one effect is used as the heating medium for a subsequent effect boiling at a lower pressure. Effects can be staged when concentrations of the liquids in the effects permits; staging is two or more sections operating at different concentrations in a single effect. The term evaporator denotes the entire system of effects, not necessarily one body or one effect.

Friday, March 20, 2009

Evaporation

Evaporation is the removal of solvent as vapor from a solution or slurry. For the overwhelming majority of evaporation systems the solvent is water. The objective is usually to concentrate a solution; hence, the vapor is not the desired product and may or may not be recovered depending on its value. Therefore, evaporation usually is achieved by vaporizing a portion of the solvent producing a concentrated solution, thick liquor, or slurry.

Evaporation often encroaches upon the operations known as distillation, drying, and crystallization. In evaporation, no attempt is made to separate components of the vapor. This distinguishes evaporation from distillation. Evaporation is distinguished from drying in that the residue is always a liquid. The desired product may be a solid, but the heat must be transferred in the evaporator to a solution or a suspension of the solid in a liquid. The liquid may be highly viscous or a slurry. Evaporation differs from crystallization in that evaporation is concerned with concentrating a solution rather than producing or building crystals.

Thursday, March 19, 2009

Fouling Factors

When a heat exchanger is placed in service, the heat transfer surfaces are, presumably, clean. With time, in some services in the power and process industries, the apparatus may undergo a decline in its ability to transfer heat. This is due to the accumulation ofheat insulating substances on either or both ofthe heat transfer surfaces. The Tubular Exchanger Manufacturers’ Association (TEMA) undertook the establishment ofstandards defining design practices not covered by the ASME Code for Unfired Pressure Vessels. Because the ASME code is concerned primarily with safe pressure containment and the means for inspecting for it during construction, the contribution ofTEMA to sound mechanical construction has been substantial.

In addition, TEMA published a table of fouling factors to assist the designer in preventing the fouling ofa single item in a process, including several items ofheat transfer equipment. Resistances were tabulated which were to be added to the film resistances (1/Sihi and 1/hoSo) ofspecific process streams so that the operating period ofeach would be similar and assure some desired period ofcontinuous operation. The tables off ouling factors were intended as a crude guide toward the equalizations of cumulative fouling in all fouling streams in the assembly.

The fouling factors published by TEMA became entrenched in industrial heat exchanger design. Fouling factors, by the TEMA definition, are time dependent. They are not present when the apparatus is placed on stream; yet at some definite time in the future, when the apparatus has lost some of its heat transfer capabilities, the fouling factor is deemed to have arrived. TEMA does not delineate the in-between fouling process, and the fouling factor has shed little light on the nature of fouling. Significant is the fact that an item of equipment that failed to comply with the TEMA notion of a desired period ofcontinuous operation became a fouling problem. Within the scope of the definition of a fouling factor, the only means for ameliorating fouling was to employ larger fouling factors for repetitive services.

The entire concept ofthe fouling factor is somewhat indefinite. It is an unsteadystate effect that is added indiscriminately to steady-state heat transfer resistances. The difference between a clean and a fouled exchanger is that an intolerable portion of the available temperature difference between fluids must be used to overcome fouling. Thus, ifthe outside surface So ofa pipe or tube is the reference and rdo is the fouling or dirt factor.

Wednesday, March 18, 2009

Fouling Mechanisms

Somerscales and Knudsen (1981) have identified six categories off fouling:
  1. Particulate fouling. The accumulation ofsolid particles is suspended in the process stream on the heat transfer surfaces. Typical examples include dust deposition, particles carried in condenser cooling water, and unburned fuel, or fly ash. If the solid deposition is due to gravity, the process is referred to as sedimentation fouling.
  2. Precipitation fouling. Dissolved substances carried in the process stream are precipitated on the heat transfer surfaces. Examples include carbonates, sulfates, and carbonates. Scaling occurs when precipitation occurs on heated rather than cooled surfaces.
  3. Chemical reaction fouling. In certain cases, deposits on the heat transfer surfaces which are not, in themselves, reactants are formed by chemical reactions. In this type off ouling, cracking and coking ofhydrocarbons and polymerization are typical examples.
  4. Corrosion fouling. In this type of fouling, the heat transfer surface reacts, at certain pH levels, to produce products that adhere to the heat transfer surfaces, and in turn, this may promote the attachment of additional fouling materials. Sulfur in fuel oil and sulfur products in the flue gas, such as sulfur dioxide, can lead to sulfuric acid. This has caused, for example, significant damage to heat exchange surfaces in air heaters in the power industry.
  5. Biological fouling. Materials such as algae, bacteria, molds, seaweed, and barnacles carried in the process stream cause biological fouling of the heat transfer surfaces. A prime example of biological fouling is in marine power plant condensers.
  6. Freezing fouling. In this type off ouling, a liquid, or some ofits higher-eltingpoint components will deposit on a subcooled heat transfer surface.

Tuesday, March 17, 2009

Regenerator : Introduction

The regenerator represents a class ofheat exchangers in which heat is alternately stored and removed from a surface. This heat transfer surface is usually referred to as the matrix ofthe regenerator. For continuous operation, the matrix must be moved into and out ofthe fixed hot and cold fluid streams. In this case, the regenerator is called a rotary regenerator. If, on the other hand, the hot and cold fluid streams are switched into and out ofthe matrix, the regenerator is referred to as a fixed matrix regenerator. In both cases the regenerator suffers from leakage and fluid entrainment problems, which must be considered during the design process.

An example ofa rotary regenerator is shown in Fig. below. This is the Lungstrom air preheater used in power plants to warm the incoming combustion air using the exhaust or flue gases from the steam generator.


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A Lungstrom air preheater.

Classification of Compact Heat Exchangers

Compact heat exchangers may be classified by the kinds of compact elements that they employ. The compact elements usually fall into five classes:
  1. Circular and flattened circular tubes. These are the simplest form of compact heat exchanger surface. The designation ST indicates flow inside straight tubes (example: ST-1), FT indicates flow inside straight flattened tubes (example: FT-1) and FTD indicates flow inside straight flattened dimpled tubes. Dimpling interrupts the boundary layer, which tends to increase the heat transfer coefficient without increasing the flow velocity.
  2. Tubular surfaces. These are arrays oftubes ofsmall diameter, from 0.9535 cm down to 0.635 cm, used in service where the ruggedness and cleanability ofthe conventional shell-and-tube exchanger are not required. Usually, tubesheets are comparatively thin, and soldering or brazing a tube to a tubesheet provides an adequate seal against interleakage and differential thermal expansion.
  3. Surfaces with flow normal to banks of smooth tubes. Unlike the radial low fin tubes, smooth round tubes are expanded into fins that can accept a number oftube rows, as shown in Fig. 11.16a. Holes may be stamped in the fin with a drawn hub or foot to improve contact resistance or as a spacer between successive fins, as shown, or brazed directly to the fin with or without a hub. Other types reduce the flow resistance outside the tubes by using flattened tubes and brazing, as indicated in Fig b and c below. Flat tubing is made from strips similar to the manufacture of welded circular tubing but is much thinner and is joined by soldering or brazing rather than welding. The designation considers staggered (S) and in-line (I) arrangements oftubes and identifies transverse and longitudinal pitch ratios. The suffix (s) indicates data correlation from steady-state tests. All other data were correlated from a transient technique. Examples include the surface S1.50-1.25(s), which is a staggered arrangement with data obtained via steady-state tests with transverse pitch-to-diameter ratio of1.50 and longitudinal pitch-to-diameter ratio of1.25. The surface I1.25-1.25 has an inline arrangement with data obtained from transient tests with both transverse and longitudinal pitch-to-diameter ratios of1.25.
  4. Plate fin surfaces. These are shown in Figs. d through i below.
  5. Finned-tube surfaces. Circular tubes with spiral radial fins are designated by the letters CF followed by one or two numerals. The first numeral designates the number offins per inch, and the second (ifone is used) refers to the nominal tube size. With circular tubes with continuous fins, no letter prefix is employed and the two numerals have the same meaning as those used for circular tubes with spiral radial ins. For finned flat tubes, no letter prefix is used; the first numeral indicates the fins per inch and the second numeral indicates the largest tube dimension. When CF does not appear in the designation ofthe circular tube with spiral radial fins, the surface may be presumed to have continuous fins.
  6. Matrix surfaces. These are surfaces that are used in rotating, regenerative equipment such as combustion flue gas–air preheaters for conventional fossil furnaces. In this application, metal is deployed for its ability to absorb heat with minimal fluid friction while exposed to hot flue gas and to give up this heat to incoming cold combustion air when it is rotated into the incoming cold airstream. No designation is employed.


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Some compact heat exchanger elements

Monday, March 16, 2009

Compact Heat Exchanger

One variation ofthe fundamental compact exchanger element, the core, is shown in Fig below. The core consists ofa pair ofparallel plates with connecting metal members that are bonded to the plates. The arrangement of plates and bonded members provides both a fluid-flow channel and prime and extended surface. It is observed that ifa plane were drawn midway between the two plates, each halfofthe connecting metal members could be considered as longitudinal fins.


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Exploded view ofa compact heat exchanger core: 1, plates; 2, side bars; 3, corrugated fins stamped from a continuous strip of metal. By spraying braze powder on the plates, the entire assembly ofplates, fins, and bars can be thermally bonded in a single furnace operation.


Two or more identical cores can be connected by separation or splitter plates, and this arrangement is called a stack or sandwich. Heat can enter a stack through either or both end plates. However, the heat is removed from the successive separating plates and fins by a fluid flowing in parallel through the entire network with a single average convection heat transfer coefficient. For this reason, the stack may be treated as a finned passage rather than a fluid–fluid heat exchanger, and, ofcourse, due consideration must be given to the fact that as more and more fins are placed in a core, the equivalent or hydraulic diameter ofthe core is lowered while the pressure loss is increased significantly.

Next, consider a pair ofcores arranged as components ofa two-fluid exchanger in crossflow as shown in Fig. 11.15. Fluids enter alternate cores from separate headers at right angles to each other and leave through separate headers at opposite ends of the exchanger. The separation plate spacing need not be the same for both fluids, nor need the cores for both fluids contain the same numbers or kinds of fins. These are dictated by the allowable pressure drops for both fluids and the resulting heat transfer coefficients. When one coefficient is quite large compared with the other, it is entirely permissible to have no extended surface in the alternate cores through which the fluid with the higher coefficient travels. An exchanger built up with plates and fins as in Fig. below is a plate fin heat exchanger.


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Two-fluid compact heat exchanger with headers removed.


The discussion ofplate fin exchangers has concentrated thus far on geometries involving two or more fluids that enter the body ofthe compact heat exchanger by means ofheaders. In many instances, one ofthe fluids may be merely air, which is used as a cooling medium on a once-through basis. Typical examples include the air-fin cooler and the radiators associated with various types ofinternal combustion engines. Similarly, there are examples in which the compact heat exchanger is a coil that is inserted into a duct, as in air-conditioning applications.

Shell and Tube Heat Exchangers : Construction

Shell-and-tube heat exchangers are fabricated with round tubes mounted in cylindrical shells with their axes coaxial with the shell axis. The differences between the many variations ofthis basic type ofheat exchanger lie mainly in their construction features and the provisions made for handling differential thermal expansion between tubes and shell.

Awidely accepted standard is published by the Tubular Exchanger Manufacturers’ Association (TEMA). This standard is intended to supplement the ASME as well as other boiler and pressure vessel codes. The TEMA (1998) standard was prepared by a committee comprising representatives of27 U.S. manufacturing companies, and their combined expertise and experience provide exchangers ofhigh integrity at reasonable cost. TEMA provides a standard designation system that is summarized in and six examples ofthe shell-and-tube heat exchanger arrangements are shown in Fig. below.


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TEMA standard designation system for shell-and-tube heat exchangers. (From Saunders, 1988, with permission.)


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(a) Single-tube-pass baffled single-pass-shell shell-and-tube heat exchanger designed to give essentially counterflow conditions. The toroidal expansion joint in the center ofthe shell accommodates differential thermal expansion between the tubes and the shell. (b) U-tube single-pass-shell shell-and-tube heat exchanger. (c) Two-pass baffled single passshell shell-and-tube heat exchanger. (d) Heat exchanger similar to that of( c) except for the floating head used to accommodate differential thermal expansion between the tubes and the shell. (e) Heat exchanger that is similar to the heat exchanger in (d) but with a different type offloating head. (f) Single-tube-pass baffled single-pass-shell shell-and-tube heat exchanger with a packed joint floating head and double header sheets to assure that no fluid leaks from one fluid circuit into the other. (Courtesy ofthe Patterson-Kelley Co. and reproduced from Fraas, 1989, with permission.)

Sunday, March 15, 2009

Heat Exchanger : Introduction

A heat exchanger can be defined as any device that transfers heat from one fluid toanother or from or to a fluid and the environment. Whereas in direct contact heat exchangers, there is no intervening surface between fluids, in indirect contact heat exchangers, the customary definition pertains to a device that is employed in the transfer of heat between two fluids or between a surface and a fluid. Heat exchangers may be classified (Shah, 1981, or Mayinger, 1988) according to (1) transfer processes,(2) number offluids, (3) construction, (4) heat transfer mechanisms, (5) surface compactness, (6) flow arrangement, (7) number offluid passes, and (8) type ofsurf ace.

Recuperators are direct-transfer heat exchangers in which heat transfer occurs between two fluid streams at different temperature levels in a space that is separated by a thin solid wall (a parting sheet or tube wall). Heat is transferred by convection from the hot (hotter) fluid to the wall surface and by convection from the wall surface to the cold (cooler) fluid. The recuperator is a surface heat exchanger.

Regenerators are heat exchangers in which a hot fluid and a cold fluid flow alternately through the same surface at prescribed time intervals. The surface of the regenerator receives heat by convection from the hot fluid and then releases it by convection to the cold fluid. The process is transient; that is, the temperature of the surface (and of the fluids themselves) varies with time during the heating and cooling of the common surface. The regenerator is a also surface heat exchanger.

Saturday, March 14, 2009

Factors Influencing Corrosion (Part 2)

Temperature The rate of corrosion tends to increase with rising temperature. Temperature also has a secondary effect through its influence on the solubility of air (oxygen), which is the most common oxidizing substance influencing corrosion. In addition, temperature has specific effects when a temperature change causes phase changes which introduce a corrosive second phase. Examples include condensation systems and systems involving organics saturated with water.

Velocity Most metals and alloys are protected from corrosion, not by nobility [a metal’s inherent resistance to enter into an electrochemical reaction with that environment, e.g., the (intrinsic) inertness of gold to (almost) everything but aqua regia], but by the formation of a protective film on the surface. In the examples of film-forming protective cases, the film has similar, but more limiting, specific assignment of that exemplary-type resistance to the exposed environment (not nearly so broad-based as noted in the case of gold). Velocity-accelerated corrosion is the accelerated or increased rate of deterioration or attack on a metal surface because of relative movement between a corrosive fluid and the metal surface, i.e., the instability (velocity sensitivity) of that protective film.

An increase in the velocity of relative movement between a corrosive solution and a metallic surface frequently tends to accelerate corrosion. This effect is due to the higher rate at which the corrosive chemicals, including oxidizing substances (air), are brought to the corroding surface and to the higher rate at which corrosion products, which might otherwise accumulate and stifle corrosion, are carried away. The higher the velocity, the thinner will be the films which corroding substances must penetrate and through which soluble corrosion products must diffuse.

Whenever corrosion resistance results from the formation of layers of insoluble corrosion products on the metallic surface, the effect of high velocity may be to prevent their normal formation, to remove them after they have been formed, and/or to preclude their reformation. All metals that are protected by a film are sensitive to what is referred to as its critical velocity; i.e., the velocity at which those conditions occur is referred to as the critical velocity of that chemistry/temperature/velocity environmental corrosion mechanism. When the critical velocity of that specific system is exceeded, that effect allows corrosion to proceed unhindered. This occurs frequently in small-diameter tubes or pipes through which corrosive liquids may be circulated at high velocities(e.g., condenser and evaporator tubes), in the vicinity of bends in pipelines, and on propellers, agitators, and centrifugal pumps. Similar effects are associated with cavitation and mechanical erosion.

Films Once corrosion has started, its further progress very often is controlled by the nature of films, such as passive films, that may form or accumulate on the metallic surface. The classical example is the thin oxide film that forms on stainless steels.

Insoluble corrosion products may be completely impervious to the corroding liquid and, therefore, completely protective; or they may be quite permeable and allow local or general corrosion to proceed unhindered. Films that are nonuniform or discontinuous may tend to localize corrosion in particular areas or to induce accelerated corrosion at certain points by initiating electrolytic effects of the concentration-cell type. Films may tend to retain or absorb moisture and thus, by delaying the time of drying, increase the extent of corrosion resulting from
exposure to the atmosphere or to corrosive vapors.

It is agreed generally that the characteristics of the rust films that form on steels determine their resistance to atmospheric corrosion. The rust films that form on low-alloy steels are more protective than those that form on unalloyed steel.

In addition to films that originate at least in part in the corroding metal, there are others that originate in the corrosive solution. These include various salts, such as carbonates and sulfates, which may be precipitated from heated solutions, and insoluble compounds, such as “beer stone,” which form on metal surfaces in contact with certain specific products. In addition, there are films of oil and grease that may protect a material from direct contact with corrosive substances. Such oil films may be applied intentionally or may occur naturally, as in the case of metals submerged in sewage or equipment used for the processing of oily substances.

Other Effects Stream concentration can have important effects on corrosion rates. Unfortunately, corrosion rates are seldom linear with concentration over wide ranges. In equipment such as distillation columns, reactors, and evaporators, concentration can change continuously, making prediction of corrosion rates rather difficult. Concentration is important during plant shutdown; presence of moisture
that collects during cooling can turn innocuous chemicals into
dangerous corrosives.

As to the effect of time, there is no universal law that governs the reaction for all metals. Some corrosion rates remain constant with time over wide ranges, others slow down with time, and some alloys have increased corrosion rates with respect to time. Situations in which the corrosion rate follows a combination of these paths can
develop. Therefore, extrapolation of corrosion data and corrosion rates should be done with utmost caution.

Impurities in a corrodent can be good or bad from a corrosion standpoint. An impurity in a stream may act as an inhibitor and actually retard corrosion. However, if this impurity is removed by some process change or improvement, a marked rise in corrosion rates can result. Other impurities, of course, can have very deleterious effects on materials. The chloride ion is a good example; small amounts of chlorides in a process stream can break down the passive oxide film on stainless steels. The effects of impurities are varied and complex. One must be aware of what they are, how much is present, and where they come from before attempting to recommend a particular material of construction.

Friday, March 13, 2009

Factors Influencing Corrosion (Part 1)

Solution pH The corrosion rate of most metals is affected by pH. The relationship tends to follow one of three general patterns:

  1. Acid-soluble metals such as iron have a relationship as shown in Fig. a below. In the middle pH range (≈4 to 10), the corrosion rate is controlled by the rate of transport of oxidizer (usually dissolved O2) to the metal surface. Iron is weakly amphoteric. At very high temperatures such as those encountered in boilers, the corrosion rate increases with increasing basicity, as shown by the dashed line.

  2. Amphoteric metals such as aluminum and zinc have a relationship as shown in Fig. b. These metals dissolve rapidly in either acidic or basic solutions.

  3. Noble metals such as gold and platinum are not appreciably affected by pH, as shown in Fig. c.


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Oxidizing Agents In some corrosion processes, such as the solution of zinc in hydrochloric acid, hydrogen may evolve as a gas. In others, such as the relatively slow solution of copper in sodium chloride, the removal of hydrogen, which must occur so that corrosion may proceed, is effected by a reaction between hydrogen and some oxidizing chemical such as oxygen to form water. Because of the high rates of corrosion which usually accompany hydrogen evolution, metals are rarely used in solutions from which they evolve hydrogen at an appreciable rate. As a result, most of the corrosion observed in practice occurs under conditions in which the oxidation of hydrogen to form water is a necessary part of the corrosion process. For this reason, oxidizing agents are often powerful accelerators of corrosion, and in many cases the oxidizing power of a solution is its most important single property insofar as corrosion is concerned.

Oxidizing agents that accelerate the corrosion of some materials may also retard corrosion of others through the formation on their surface of oxides or layers of adsorbed oxygen which make them more resistant to chemical attack. This property of chromium is responsible for the principal corrosion-resisting characteristics of the stainless steels.

It follows, then, that oxidizing substances, such as dissolved air, may accelerate the corrosion of one class of materials and retard the corrosion of another class. In the latter case, the behavior of the material usually represents a balance between the power of oxidizing compounds to preserve a protective film and their tendency to accelerate corrosion when the agencies responsible for protective-film breakdown are able to destroy the films.

Thursday, March 12, 2009

Fluid Corrosion : General

Corrosion is the destructive attack upon a metal by its environment or with sufficient damage to its properties, such that it can no longer meet the design criteria specified. Not all metals and their alloys react in a consistent manner when in contact with corrosive fluids. One of the common intermediate reactions of a metal surface is achieved with oxygen, and those reactions are variable and complex. Oxygen can sometimes function as an electron acceptor and cause cathodic depolarization by removing the “protective” film of hydrogen from the cathodic area. In other cases, oxygen can form protective oxide films. The long-term stability of these films also varies: some are soluble in the environment, others form more stable and inert passive films. Electrochemically, a metal surface is in the active state (the anode), i.e., in which the metal tends to corrode, or is being corroded. When a metal is passive, it is in the cathodic state, i.e., the state of a metal when its behavior is much more noble (resists corrosion) than its position in the emf series would predict. Passivity is the phenomenon of an (electrochemically) unstable metal in a given electrolyte remaining observably unchanged for an extended period of time.

Metallic Materials Pure metals and their alloys tend to enter into chemical union with the elements of a corrosive medium to form stable compounds similar to those found in nature. When metal loss occurs in this way, the compound formed is referred to as the corrosion product and the metal surface is spoken of as being corroded.

Corrosion is a complex phenomenon that may take any one or more of several forms. It is usually confined to the metal surface, and this is called general corrosion. But it sometimes occurs along defective and/or weak grain boundaries or other lines of weakness because of a difference in resistance to attack or local electrolytic action.

In most aqueous systems, the corrosion reaction is divided into an anodic portion and a cathodic portion, occurring simultaneously at discrete points on metallic surfaces. Flow of electricity from the anodic to the cathodic areas may be generated by local cells set up either on a single metallic surface (because of local point-to-point differences on the surface) or between dissimilar metals.

Nonmetallics As stated, corrosion of metals applies specifically to chemical or electrochemical attack. The deterioration of plastics and other nonmetallic materials, which are susceptible to swelling, crazing, cracking, softening, and so on, is essentially physiochemical rather than electrochemical in nature. Nonmetallic materials can either be rapidly deteriorated when exposed to a particular environment or, at the other extreme, be practically unaffected. Under some conditions, a nonmetallic may show evidence of gradual deterioration. However, it is seldom possible to evaluate its chemical resistance by measurements of weight loss alone, as is most generally done for metals.

Wednesday, March 11, 2009

Introduction of Fluid Corrosion

In the selection of materials of construction for a particular fluid system, it is important first to take into consideration the characteristics of the system, giving special attention to all factors that may influence corrosion. Since these factors would be peculiar to a particular system, it is impractical to attempt to offer a set of hard and fast rules that would cover all situations.

The materials from which the system is to be fabricated are the second important consideration; therefore, knowledge of the characteristics and general behavior of materials when exposed to certain environments is essential.

In the absence of factual corrosion information for a particular set of fluid conditions, a reasonably good selection would be possible from data based on the resistance of materials to a very similar environment. These data, however, should be used with considerable reservations. Good practice calls for applying such data for preliminary screening. Materials selected thereby would require further study in the fluid system under consideration.

Tuesday, March 10, 2009

Compressors

A compressor is a device which pressurizes a working fluid. One of the basic purposes of using a compressor is to compress the fluid and to deliver it at a pressure higher than its original pressure. Compression is required for a variety of purposes, some of which are listed below:
  1. To provide air for combustion
  2. To transport process fluid through pipelines
  3. To provide compressed air for driving pneumatic tools
  4. To circulate process fluid within a process
Different types of compressors are shown in Fig. below. Positive displacement compressors are used for intermittent flow in which successive volumes of fluid are confined in a closed space to increase their pressures. Rotary compressors provide continuous flow. In rotary compressors, rapidly rotating parts (impellers) accelerate fluid to a high speed; this velocity is then converted to additional pressure by gradual deceleration in the diffuser or volute which surrounds the impeller. Positive-displacement compressors can be further classified as either reciprocating or rotary type. The reciprocating compressor has a piston having a reciprocating motion within a cylinder. The rotary positive-displacement compressors have rotating elements whose positive action results in compression and displacement. The rotary positive-displacement compressors can be further subdivided into sliding vane, liquid piston, straight lobe, and helical lobe
compressors. The continuous flow compressors can be classified as either dynamic compressors or ejectors. Ejectors entrain the in-flowing fluid by using a high-velocity gas or steam jet and then convert the velocity of the mixture to pressure in a diffuser. The dynamic compressors have rotating elements, which accelerate the inflowing fluid, and convert the velocity head to pressure head, partially in the rotating elements and partially in the stationary diffusers or blade. The dynamic compressors can be further subdivided into centrifugal, axial-flow, and mixed-flow compressors. The main flow of gas in the centrifugal compressor is radial. The flow of gas in an axial compressor is axial, and the mixed-flow compressor combines some characteristics of both centrifugal and axial compressors.

It is not always obvious what type of compressor is needed for an application. Of the many types of compressors used in the process industries, some of the more significant are the centrifugal, axial, rotary, and reciprocating compressors.

For very high flows and low pressure ratios, an axial-flow compressor would be best. Axial-flow compressors usually have a higher efficiency but a smaller operating region than does a centrifugal machine. Centrifugal compressors operate most efficiently at medium flow rates and high pressure ratios. Rotary and reciprocating compressors (positive-displacement machines) are best used for low flow rates and high pressure ratios. The positivedisplacement compressors and, in particular, reciprocating compressors were the most widely used in the process and pipeline industries up to and through the 1960s.

In turbomachinery the centrifugal flow and the axial-flow compressors are the ones used for compressing gases. Positive-displacement compressors such as reciprocating, gear-type, or lobe-type are widely used in the industry for many other applications such as slurry pumping

The industrial pressure ratio is low because the operating range needs to be large. The operating range is defined as the range between the surge point and the choke point. The surge point is the point at which the flow is reversed in the compressor. The choke point is the point at which the flow has reached Mach = 1.0, the point where no more flow can get through the unit, a “stone wall.” When surge occurs, the flow is reversed, and so are all the forces acting on the compressor, especially the thrust forces. Surge can lead to total destruction of the compressor. Thus surge is a region that must be avoided. Choke conditions cause a large drop in efficiency, but do not lead to destruction of the unit. Note that with the increase in pressure ratio and the number of stages, the operating range is narrowed in axial-flow and centrifugal compressors.

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Pump Diagnostics

Pump problems vary over a large range depending on the type of pumps and the usage of the pumps. They can be classified in the following manner by the pump type and the service:
  1. Positive-displacement pumps—reciprocating pumps problems can be classified into the following categories:

    • Compressor valve problems: plate valves, feather valves, concentric disk valves, relief valves
    • Piston and rod assembly: piston rings, cylinder chatter, cylinder cooling, piston-rod packing
    • Lubrication system

  2. Positive displacement pumps—gear-type and roots-type problems can be classified into the following categories:

    • Rotor dynamic problems: vibration problems, gear problems or roots rotor problems, bearing and seal problems
    • Lubrication systems

  3. Continuous flow pumps such as centrifugal pumps problems can be classified into the following categories:

    • Cavitation
    • Capacity flow
    • Motor overload
    • Impeller
    • Bearings and seals
    • Lubrication systems
Table below about classifies different types of centrifugal pump-related problems, their possible causes, and corrective actions that can be taken to solve some of the more common issues. These problems in the table are classified into three major categories for these type of pumps:
  1. Cavitation
  2. Capacity flow
  3. Motor overload
The use of vibration monitoring to diagnose pump and compressor problems is discussed at the end of the subsection on compressor problems.
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Monday, March 9, 2009

Introduction of Centrifugal Pumps

The centrifugal pump is the type most widely used in the chemical industry for transferring liquids of all types—raw materials, materials in manufacture, and finished products—as well as for general services of water supply, boiler feed, condenser circulation, condensate return, etc. These pumps are available through a vast range of sizes, in capacities from 0.5 m3/h to 2 × 104m3/h (2 gal/min to 105 gal/min), and for discharge heads (pressures) from a few meters to approximately 48 MPa (7000 lbf/in2). The size and type best suited to a particular application can be determined only by an engineering study of the problem.

The primary advantages of a centrifugal pump are simplicity, low first cost, uniform (nonpulsating) flow, small floor space, low maintenance expense, quiet operation, and adaptability for use with a motor or a turbine drive.

A centrifugal pump, in its simplest form, consists of an impeller rotating within a casing. The impeller consists of a number of blades, either open or shrouded, mounted on a shaft that projects outside the casing. Its axis of rotation may be either horizontal or vertical, to suit the work to be done. Closed-type, or shrouded, impellers are generally the most efficient. Open- or semiopen-type impellers are used for viscous liquids or for liquids containing solid materials and on many small pumps for general service. Impellers may be of the single-suction or the double-suction type—single if the liquid enters from one side, double if it enters from both sides.

Casings There are three general types of casings, but each consists of a chamber in which the impeller rotates, provided with inlet and exit for the liquid being pumped. The simplest form is the circular casing, consisting of an annular chamber around the impeller; no attempt is made to overcome the losses that will arise from eddies and shock when the liquid leaving the impeller at relatively high velocities enters this chamber. Such casings are seldom used.

Volute casings take the form of a spiral increasing uniformly in cross-sectional area as the outlet is approached. The volute efficiently converts the velocity energy imparted to the liquid by the impeller into pressure energy.

A third type of casing is used in diffuser-type or turbine pumps. In this type, guide vanes or diffusers are interposed between the impeller discharge and the casing chamber. Losses are kept to a minimum in a well-designed pump of this type, and improved efficiency is obtained over a wider range of capacities. This construction is often used in multistage high-head pumps.

Action of a Centrifugal Pump Power from an outside source is applied to shaft A, rotating the impeller B within the stationary casing C. The blades of the impeller in revolving produce a reduction in pressure at the entrance or eye of the impeller. This causes liquid to flow into the impeller from the suction pipe D. This liquid is forced outward along the blades at increasing tangential velocity. The velocity head it has acquired when it leaves the blade tips is changed to pressure head as the liquid passes into the volute chamber and thence out the discharge E.

Pump Specifications

Pump specifications depend upon numerous factors but mostly on application. Typically, the following factors should be considered while preparing a specification.
  1. Application, scope, and type
  2. Service conditions
  3. Operating conditions
  4. Construction application-specific details and special considerations
    • Casing and connections
    • Impeller details
    • Shaft
    • Stuffing box details—lubrications, sealing, etc.
    • Bearing frame and bearings
    • Baseplate and couplings
    • Materials
    • Special operating conditions and miscellaneous items

Table below is based on the API and ASME codes and illustrates a typical specification for centrifugal pumps.

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Sunday, March 8, 2009

Terminology of Pumps and Compressors (Part 2)

Static Suction Head The static suction head hss is the vertical distance measured from the free surface of the liquid source to the pump centerline plus the absolute pressure at the liquid surface.

Total Discharge Head The total discharge head hd is the reading hgd of a gauge at the discharge flange of a pump (corrected to the pump centerline*), plus the barometer reading and the velocity head hvd at the point of gauge attachment:
Again, if the discharge gauge pressure is below atmospheric, the vacuum-gauge reading is used for hgd in Eq. (10-45) with a negative sign.

Before installation it is possible to estimate the total discharge head from the static discharge head hsd and the discharge friction head hfd as follows:
Static Discharge Head The static discharge head hsd is the vertical distance measured from the free surface of the liquid in the receiver to the pump centerline,* plus the absolute pressure at the liquid surface. Total static head hts is the difference between discharge and suction static heads.

Velocity Since most liquids are practically incompressible, the relation between the quantity flowing past a given point in a given time and the velocity of flow is expressed as follows:

Q = Av

Velocity Head This is the vertical distance by which a body must fall to acquire the velocity v.

Viscosity In flowing liquids the existence of internal friction or the internal resistance to relative motion of the fluid particles must be considered. This resistance is called viscosity. The viscosity of liquids usually decreases with rising temperature. Viscous liquids tend to increase the power required by a pump, to reduce pump efficiency, head, and capacity, and to increase friction in pipe lines.

Friction Head This is the pressure required to overcome the resistance to flow in pipe and fittings.

Work Performed in Pumping To cause liquid to flow, work must be expended. A pump may raise the liquid to a higher elevation, force it into a vessel at higher pressure, provide the head to overcome pipe friction, or perform any combination of these. Regardless of the service required of a pump, all energy imparted to the liquid in performing this service must be accounted for; consistent units for all quantities must be employed in arriving at the work or power performed.

Classification of pumps. (Courtesty of Hydraulic Institute.)

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Terminology of Pumps and Compressors (Part 1)

Displacement Discharge of a fluid from a vessel by partially or completely displacing its internal volume with a second fluid or by mechanical means is the principle upon which a great many fluidtransport devices operate. Included in this group are reciprocatingpiston and diaphragm machines, rotary-vane and gear types, fluid piston compressors, acid eggs, and air lifts.

The large variety of displacement-type fluid-transport devices makes it difficult to list characteristics common to each. However, for most types it is correct to state that (1) they are adaptable to high-pressure operation, (2) the flow rate through the pump is variable (auxiliary damping systems may be employed to reduce the magnitude of pressure pulsation and flow variation), (3) mechanical considerations limit maximum throughputs, and (4) the devices are capable of efficient performance at extremely low-volume throughput rates.

Centrifugal Force Centrifugal force is applied by means of the centrifugal pump or compressor. Though the physical appearance of the many types of centrifugal pumps and compressors varies greatly, the basic function of each is the same, i.e., to produce kinetic energy by the action of centrifugal force and then to convert this energy into pressure by efficiently reducing the velocity of the flowing fluid.

In general, centrifugal fluid-transport devices have these characteristics: (1) discharge is relatively free of pulsation; (2) mechanical design lends itself to high throughputs, capacity limitations are rarely a problem; (3) the devices are capable of efficient performance over a wide range of pressures and capacities even at constant-speed operation; (4) discharge pressure is a function of fluid density; and (5) these are relatively small high-speed devices and less costly.

A device which combines the use of centrifugal force with mechanical impulse to produce an increase in pressure is the axial-flow compressor or pump. In this device the fluid travels roughly parallel to the shaft through a series of alternately rotating and stationary radial blades having airfoil cross sections. The fluid is accelerated in the axial direction by mechanical impulses from the rotating blades; concurrently, a positive-pressure gradient in the radial direction is established in each stage by centrifugal force. The net pressure rise per stage results from both effects.

Electromagnetic Force When the fluid is an electrical conductor, as is the case with molten metals, it is possible to impress an electromagnetic field around the fluid conduit in such a way that a driving force that will cause flow is created. Such pumps have been developed for the handling of heat-transfer liquids, especially for nuclear reactors.

Transfer of Momentum Deceleration of one fluid (motivating fluid) in order to transfer its momentum to a second fluid (pumped fluid) is a principle commonly used in the handling of corrosive materials, in pumping from inaccessible depths, or for evacuation. Jets and eductors are in this category.

Absence of moving parts and simplicity of construction have frequently justified the use of jets and eductors. However, they are relatively inefficient devices. When air or steam is the motivating fluid, operating costs may be several times the cost of alternative types of fluid-transport equipment. In addition, environmental considerations in today’s chemical plants often inhibit their use.

Mechanical Impulse The principle of mechanical impulse when applied to fluids is usually combined with one of the other means of imparting motion. As mentioned earlier, this is the case in axial-flow compressors and pumps. The turbine or regenerative-type pump is another device which functions partially by mechanical impulse.

Measurement of Performance The amount of useful work that any fluid-transport device performs is the product of (1) the mass rate of fluid flow through it and (2) the total pressure differential measured immediately before and after the device, usually expressed in the height of column of fluid equivalent under adiabatic conditions. The first of these quantities is normally referred to as capacity, and the second is known as head.

Capacity This quantity is expressed in the following units. In SI units capacity is expressed in cubic meters per hour (m3/h) for both liquids and gases. In U.S. customary units it is expressed in U.S. gallons per minute (gal/min) for liquids and in cubic feet per minute (ft3/min) for gases. Since all these are volume units, the density or specific gravity must be used for conversion to mass rate of flow. When gases are being handled, capacity must be related to a pressure and a temperature, usually the conditions prevailing at the machine inlet. It is important to note that all heads and other terms in the following equations are expressed in height of column of liquid.

Total Dynamic Head The total dynamic head H of a pump is the total discharge head hd minus the total suction head hs.

Total Suction Head This is the reading hgs of a gauge at the suction flange of a pump (corrected to the pump centerline∗), plus the barometer reading and the velocity head hvs at the point of gauge attachment:
If the gauge pressure at the suction flange is less than atmospheric, requiring use of a vacuum gauge, this reading is used for hgs in Eq. with a negative sign.

Before installation it is possible to estimate the total suction head as follows:


where hss = static suction head and hfs = suction friction head.

Other Coarse Crushers

Friable materials, such as coal, may be broken up without the application of large forces, and therefore less robust plant may be used. A common form of coal breaker consists of a large hollow cylinder with perforated walls. The axis is at a small angle to the horizontal and the feed is introduced at the top. The cylinder is rotated and the coal is lifted by means of arms attached to the inner surface and then falls against the cylindrical surface. The coal breaks by impact and passes through the perforations as soon as the size has been sufficiently reduced. This type of equipment is less expensive and has a higher throughput than the jaw or gyratory crusher. Another coarse rotary breaker, the rotary coal breaker, is similar in action to the hammer mill described later, and is shown in Figure below. The crushing action depends upon the transference of kinetic energy from hammers to the material and these pulverisers are essentially high speed machines with a speed of rotation of about 10 Hz (600 rpm) giving hammer tip velocities of about 40 m/s.

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Rotary coal breaker

Saturday, March 7, 2009

General Design Procedure of Gas Absorption Systems

The design engineer usually is required to determine (1) the best solvent; (2) the best gas velocity through the absorber, or, equivalently, the vessel diameter; (3) the height of the vessel and its internal members, which is the height and type of packing or the number of contacting trays; (4) the optimum solvent circulation rate through the absorber and stripper; (5) temperatures of streams entering and leaving the absorber and stripper, and the quantity of heat to be removed to account for the heat of solution and other thermal effects; (6) pressures at which the absorber and stripper will operate; and (7) mechanical design of the absorber and stripper vessels (predominantly columns or towers), including flow distributors and packing supports.

The problem presented to the designer of a gas absorption system usually specifies the following quantities: (1) gas flow rate; (2) gas composition of the component or components to be absorbed; (3) operating pressure and allowable pressure drop across the absorber; (4) minimum recovery of one or more of the solutes; and, possibly, (5) the solvent to be employed. Items 3, 4, and 5 may be subject to economic considerations and therefore are left to the designer.

Recovery of the solvent, occasionally by chemical means but more often by distillation, is almost always required and is considered an integral part of the absorption system process design. A more complete solvent-stripping operation normally will result in a less costly absorber because of a lower concentration of residual solute in the regenerated (lean) solvent, but this may increase the overall cost of the entire absorption system.

Design Applications of Double Pipe Heat Exchangers

One benefit of the hairpin exchanger is its ability to handle high tubeside pressures at a lower cost than other removable-bundle exchangers. This is due in part to the lack of pass partitions at the tubesheets which complicate the gasketing design process. Present mechanical design technology has allowed the building of dependable, removable bundle, hairpin multitubes at tubeside pressures of 825 bar (12,000 psi).

The best known use of the hairpin is its operation in true countercurrent flow which yields the most efficient design for processes that have a close temperature approach or temperature cross. However, maintaining countercurrent flow in a tubular heat exchanger usually implies one tube pass for each shell pass. As recently as 30 years ago, the lack of inexpensive, multiple-tube pass capability often diluted the
advantages gained from countercurrent flow.

The early attempts to solve this problem led to investigations into the area of heat transfer augmentation. This familiarity with augmentation techniques inevitably led to improvements in the efficiency and capacity of the small heat exchangers. The result has been the application of the hairpin heat exchanger to the solution of unique process problems, such as dependable, once-through, convective boilers offering high-exit qualities, especially in cases of process-temperature crosses

Principles of Construction Double Pipe Heat Exchangers

Hairpin heat exchangers (often also referred to as “double pipes”) are characterized by a construction form which imparts a U-shaped appearance to the heat exchanger. In its classical sense, the term double pipe refers to a heat exchanger consisting of a pipe within a pipe, usually of a straight-leg construction with no bends. However, due to the need for removable bundle construction and the ability to handle differential thermal expansion while avoiding the use of expansion joints (often the weak point of the exchanger), the current U-shaped configuration has become the standard in the industry. A further departure from the classical definition comes when more than one pipe or tube is used to make a tube bundle, complete with tubesheets and tube supports similar to the TEMA style exchanger.

Hairpin heat exchangers consist of two shell assemblies housing a common set of tubes and interconnected by a return-bend cover referred to as the bonnet. The shell is supported by means of bracket assemblies designed to cradle both shells simultaneously. These brackets are configured to permit the modular assembly of many hairpin sections into an exchanger bank for inexpensive future-expansion capability and for providing the very long thermal lengths demanded by special process applications.

The bracket construction permits support of the exchanger without fixing the supports to the shell. This provides for thermal movement of the shells within the brackets and prevents the transfer of thermal stresses into the process piping. In special cases the brackets may be welded to the shell. However, this is usually avoided due to the resulting loss of flexibility in field installation and equipment reuse at other sites and an increase in piping stresses.

The hairpin heat exchanger, unlike the removable bundle TEMA styles, is designed for bundle insertion and removal from the return end rather than the tubesheet end. This is accomplished by means of removable split rings which slide into grooves machined around the outside of each tubesheet and lock the tubesheets to the external closure flanges. This provides a distinct advantage in maintenance since bundle removal takes place at the exchanger end furthest from the plant process piping without disturbing any gasketed joints of this piping.

Approach to Heat Exchanger Design

The proper use of basic heat-transfer knowledge in the design of practical heat-transfer equipment is an art. Designers must be constantly aware of the differences between the idealized conditions for and under which the basic knowledge was obtained and the real conditions of the mechanical expression of their design and its environment. The result must satisfy process and operational requirements (such as availability, flexibility, and maintainability) and do so economically. An important part of any design process is to consider and offset the consequences of error in the basic knowledge, in its subsequent incorporation into a design method, in the translation of design into equipment, or in the operation of the equipment and the process. Heat-exchanger design is not a highly accurate art under the best of conditions.

The design of a process heat exchanger usually proceeds through the following steps:

  1. Process conditions (stream compositions, flow rates, temperatures, pressures) must be specified.

  2. Required physical properties over the temperature and pressure ranges of interest must be obtained.

  3. The type of heat exchanger to be employed is chosen.

  4. A preliminary estimate of the size of the exchanger is made, using a heat-transfer coefficient appropriate to the fluids, the process, and the equipment.

  5. A first design is chosen, complete in all details necessary to carry out the design calculations.

  6. The design chosen in step 5 is evaluated, or rated, as to its ability to meet the process specifications with respect to both heat transfer and pressure drop.

  7. On the basis of the result of step 6, a new configuration is chosen if necessary and step 6 is repeated. If the first design was inadequate to meet the required heat load, it is usually necessary to increase the size of the exchanger while still remaining within specified or feasible limits of pressure drop, tube length, shell diameter, etc. This will sometimes mean going to multiple-exchanger configurations. If the first design more than meets heat-load requirements or does not use all the allowable pressure drop, a less expensive exchanger can usually be designed to fulfill process requirements.

  8. The final design should meet process requirements (within reasonable expectations of error) at lowest cost. The lowest cost should include operation and maintenance costs and credit for ability to meet long-term process changes, as well as installed (capital) cost. Exchangers should not be selected entirely on a lowest-first-cost basis, which frequently results in future penalties.

The Gyratory Crusher

The gyratory crusher shown in Figure below employs a crushing head, in the form of a truncated cone, mounted on a shaft, the upper end of which is held in a flexible bearing, whilst the lower end is driven eccentrically so as to describe a circle. The crushing action takes place round the whole of the cone and, since the maximum movement is at the bottom, the characteristics of the machine are similar to those of the Stag crusher. As the crusher is continuous in action, the fluctuations in the stresses are smaller than in jaw crushers and the power consumption is lower. This unit has a large capacity per unit area of grinding surface, particularly if it is used to produce a small size reduction. It does not, however, take such a large size of feed as a jaw crusher, although it gives a rather finer and more uniform product. Because the capital cost is high, the crusher is suitable only where large quantities of material are to be handled.

The jaw crushers and the gyratory crusher all employ a predominantly compressive force.

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Gyratory crusher

Friday, March 6, 2009

The Dodge Jaw Crusher

In the Dodge crusher, shown in Figure below, the moving jaw is pivoted at the bottom. The minimum movement is thus at the bottom and a more uniform product is obtained, although the crusher is less widely used because of its tendency to choke. The large opening at the top enables it to take very large feed and to effect a large size reduction. This crusher is usually made in smaller sizes than the Stag crusher, because of the high fluctuating stresses that are produced in the members of the machine.


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Dodge crusher