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.

Water Treatment in General

Water must have eye appeal and taste appeal before we will drink it with much relish. Instinctively we draw back from the idea of drinking dirty, smelly water. Actually far more important to our well-being is whether or not a water is safe to drink. If it holds &sease bacteria, regardless of its clarity and sparkle, we should avoid it. Let's consider these two highly important aspects of water: potability and palatability.

Regardless of any other factors, water piped into the home must be potable. To be potable, it should be completely free of disease organisms. Water is the breeding ground for an almost unbelievably large variety of organisms. Water does not produce these organisms. It merely is an ideal medium in whch they can grow. These organisms gain entry into water through a variety of sources. They enter from natural causes, surface drainage and sewage. Many of the organisms in water are harmless. In fact, they are extremely beneficial to man. Others have a wide nuisance value and still others are the source of disease. In general, we are primarily concerned here with organisms which are potential disease-producers. These are of five types: bacteria, protozoa, worms, viruses, fungi. The presence of certain organisms of these various types can lead to such infectious diseases as typhoid fever, dysentery, cholera, jaundice, hepatitis, undulant fever and tularaemia.

There are other diseases as well, which spread through d r i i g unsafe water. Tremendous strides have been made in the control of these diseases within recent years. Much of the credit must go to sanitary engineers for their careful, consistent control of public water supplies. Biologically, there are two major classifications for our purposes. We can classify water organisms either as members of the plant or animal kingdoms.The following ways are the natural ways, in which water is purified: Bacteria and algae consume organic waste; Micro-organisms devour bacteria and algae; Oxidation renders organic matter harmless; Ultra-violet rays of

sun have germicidal effects. 

Under the broad heading of plant forms, we can classify the following:

Energy Intensive Treatment Technologies

OZONE

Ozone is used extensively in Europe to purify water. Ozone, a molecule composed of 3 atoms of oxygen rather than two, is formed by exposing air or oxygen to a high voltage electric arc. Ozone is much more effective as a disinfectant than chlorine, but no residual levels of disinfectant exist after ozone turns back into O2. (One source quotes a half life of only 120 minutes in distilled water at 20 "C). Ozone is expected to see increased use in the U.S. as a way to avoid the production and formation of trihalomethanes, and while ozone does break down organic molecules, sometimes this can be a disadvantage as ozone treatment can produce higher levels of smaller molecules that provide an energy source for microorganisms. If no residual disinfectant is present (as would happen if ozone were used as the only treatment method), these microorganisms will cause the water quality to deteriorate in storage. Ozone also changes the surface charges of dissolved organics and colloidially suspended particles. This causes microflocculation of the dissolved organics and coagulation of the colloidal particles.

Introducing Chemical Treatment

CHLORINE

Chlorine is familiar to most people as it is used to treat virtually all municipal water systems in the United States. Chlorine has a number of problems when used for field treatment of water. When chlorine reacts with organic material, it attaches itself to nitrogen containing compounds (ammonium ions and amino acids), leaving less free chlorine to continue disinfection. Carcinogenic trihalomethanes are also produced, though this is only a problem with long-term exposure. Trihalomethanes can also be filtered out with a charcoal filter, though it is more efficient to use the same filter to remove organics before the water is chlorinated. Unless free chlorine is measured, disinfection can not be guaranteed with moderate doses of chlorine. One solution is superchlorination, the addition of far more chlorine than is needed. This must again be filtered through activated charcoal to remove the large amounts of chlorine, or hydrogen peroxide can be added to drive the chlorine off. Either way there is no residual chlorine left to prevent recontamination. This isn't a problem, if the water is to be used at once.

Chlorine is sensitive to both the pH and temperature of the treated water. Temperature slows the reaction for any chemical treatment, but chlorine treatment is particularly susceptible to variations in the pH as at lower pHs, hypochlorous acid is formed, while at higher pHs, it will tend to dissociate into hydrogen and chlorite ions, which are less effective as a disinfectant. As a result, chlorine effectiveness drops off when the pH is greater than 8.Ordinary household bleach (such as Clorox) in the U.S. contains 5.25 % sodium hypochlorite (NaOCL) and can be used to purify water if it contains no other active ingredients, scents, or colorings. Some small treatment plants in Africa produce their own sodium hypochlorite on site from the electrolysis of brine. Power demands range from 1.7 to 4 kwh per lb. of NaOCL. 2 to 3.5 lbs. of salt are needed for each pound of NaOCL. These units are fairly simple and are made in both the U.S. and the U.K. Another system, designed for China, where the suitable raw materials were mined or manufactured locally, used a reaction between salt, manganese dioxide, and sulfuric acid to produce chlorine gas. The gas was then allowed to react with slaked lime to produce a bleaching powder that could then be used to treat water. A heat source is required to speed the reaction up. Bleaching Powder (or Chlorinated Lime) is sometimes used at the industrial scale. Bleaching powder is 33-37% chlorine when produced, but losses its chlorine rapidly, particularly when exposed to air, light or moisture.

Introducing The Physical Treatment Methods

The following technologies are among the most commonly used physical methods of purifying water:

Heat Treatment - Boiling is one way to purify water of all pathogens. Most experts feel that if the water reaches a rolling boil it is safe. A few still hold out for maintaining the boiling for some length of time, commonly 5 or 10 minutes, plus an extra minute for every l000 feet of elevation. One reason for the long period of boiling is to inactivate bacterial spores (which can survive boiling), but these spore are unlikely to be waterborne pathogens. Water can also be treated at below boiling temperatures, if contact time is increased. Commercial units are available for residential use, which treat 500 gals of water per day at an estimated cost of $1/1000 gallons for the energy. The process is similar to milk pasteurization, and holds the water at 161" F for 15 seconds. Heat exchangers recover most of the energy used to warm the water. Solar pasteurizers have also been built that can heat three gallons of water to 65 " C and hold the temperature for an hour. A higher temperature could be reached, if the device was rotated east to west during the day to follow the sunlight. Regardless of the method, heat treatment does not leave any form of residual to keep the water free of pathogens in storage.

Reverse Osmosis - Reverse osmosis forces water, under pressure, through a membrane that is impermeable to most contaminants. The membrane is somewhat better at rejecting salts than it is at rejecting non-ionized weak acids and bases and smaller organic molecules (molecular weight below 200). In the latter category are undissociated weak organic acids, amines, phenols, chlorinated hydrocarbons, some pesticides and low molecular weight alcohols. Larger organic molecules and all pathogens are rejected. Of course, it is possible to have a imperfection in the membrane that could allow molecules or whole pathogens to pass through. Using reverse osmosis to desalinate seawater requires considerable pressure (1000 psi) to operate. Reverse osmosis filters are available that will use normal municipal or private water pressure to remove contaminates from water. The water produced by reverse osmosis, like distilled water, will be close to pure H,O. Therefore mineral intake may need to be increased to compensate for the normal mineral content of water in much of the world.

The Clean Water Act

Drinking water standards are not the only regulations we need to comply with in the U.S. Today's Clean Water Act has its origins from the late 1940s. The original 1948 statute (Chapter 758; PL 845), the Water Pollution Control Act, authorized the Surgeon General of the Public Health Service, in cooperation with other federal, state, and local entities, to prepare comprehensive programs for eliminating or reducing the pollution of interstate waters and tributaries and improving the sanitary condition of surface and underground waters. Since 1948, the original statute has been amended extensively to authorize additional water quality programs, standards and procedures to govern allowable discharges, and funding for construction grants or general programs. Amendments in other years provided for continued authority to conduct program activities or administrative changes to related activities. This legislation was originally enacted as the Federal Water Pollution Control Act of 1972, and was amended in 1977 and renamed the Clean Water Act. It was

reauthorized in 1991.

The Clean Water Act strives to restore and maintain the chemical, physical, and biological integrity of the nation's water. The act sets up a system of water quality standards, discharge limitations, and permits. If a project may result in the placement of material into waters, a Corps of Engineers' Dredge and Fill Permit (Section 404) may be required. The permit also pertains to activities in wetlands and riparian areas. Certain Federal projects may be exempt from the requirements of Section 404, if the conditions set forth in section 404(r) are met. Before either a National Pollutant Discharge Elimination System (NPDES) (Section 402) or Section 404 permit can be issued, the applicant must obtain a Section 401 certification. This declaration states that any discharge complies with all applicable effluent limitations and water quality standards.

The Drinking Water Standards (Part 2)

In spite of the multitudinous regulations and standards that an existing public water system must comply with, the principles of conventional water treatment process have not changed significantly over half a century. Whether a filter contains sand, anthracite, or both, slow or rapid rate, constant or declining rate, filtration is still filtration, sedimentation is still sedimentation, and disinfection is still disinfection. What has changed, however, are many tools that we now have in our engineering arsenal. For example, , a supervisory control and data acquisition (SCADA) system can provide operators and managers with accurate process controI variables and operation and maintenance records. In addition to being able to look at the various options on the computer screen, engineers can conduct pilot plant studies of the multiple variables inherent in water treatment plant design. Likewise, operators and managers can utilize an ongoing pilot plant facility to optimize chemical feed and develop important information needed for future expansion and upgrading.

Technology and ultimately equipment selection depends on the standards set by the regulations. Drinking water standards are regulations that EPA sets to control the level of contaminants in the nation's drinking water. These standards are part of the Safe Drinking Water Act's "multiple barrier" approach to drinking water protection, which includes assessing and protecting drinking water sources; protecting wells and collection systems; making sure water is treated by qualified operators; ensuring the integrity of distribution systems; and making information available to the public on the quality of their drinking water. With the involvement of EPA, states, tribes, drinking water utilities, communities and citizens, these multiple barriers ensure that tap water in the U.S. and territories is safe to drink. In most cases, EPA delegates responsibility for implementing drinking water standards to states and tribes. There are two categories of drinking water standards:

The Drinking Water Standards (Part 1)

When the objective of water treatment is to provide drinking water, then we need to select technologies that are not only the best available, but those that will meet local and national quality standards. The primary goals of a water treatment plant for over a century have remained practically the same: namely to produce water that is biologically and chemically safe, is appealing to the consumer, and is noncorrosive and nonscaling. Today, plant design has become very complex from discovery of seemingly innumerable chemical substances, the multiplying of regulations, and trying to satisfy more discriminating palates. In addition to the basics, designers must now keep in mind all manner of legal mandates, as well as public concerns and environmental considerations, to provide an initial prospective of water works engineering planning, design, and operation.

The growth of community water supply systems in the United States started in the early 1800s. By 1860, over 400, and by the turn of the century over 3000 major water systems had been built to serve major cities and towns. Many older plants were equipped with slow sand filters. In the mid 1890s, the Louisville Water Company introduced the technologies of coagulation with rapid sand filtration. 

The first application of chlorine in potable water was introduced in the 1830s for taste and odor control, at that time diseases were thought to be spread by odors. It was not until the 1890s and the advent of the germ theory of disease that the importance of disinfection in potable water was understood. Chlorination was first introduced on a practical scale in 1908 and then became a common practice.

What We Mean by Water Purification

When we refer to water purification, it makes little sense to discuss the subject without first identifying the contaminants that we wish to remove from water. Also, the source of the water is of importance. Our discussion at this point focuses on drinking water. Groundwater sources are of a particular concern, because there are many communities throughout the U.S. that rely on this form. The following are some of the major contaminants that are of concern in water purification applications, as applied to drinking water sources, derived from groundwater.

Heavy Metals - Heavy metals represent problems in terms of groundwater pollution. The best way to identify their presence is by a lab test of the water or by contacting county health departments. There are concerns of chronic exposure to low levels of heavy metals in drinking water.

An Overview of Water And Waste-Water Treatment

We may organize water treatment technologies into three general areas: Physical Methods, Chemical Methods, and Energy Intensive Methods. Physical methods of wastewater treatment represent a body of technologies that we refer largely to as solid-liquid separations techniques, of which filtration plays a dominant role. Filtration technology can be broken into two general categories - conventional and non-conventional. This technology is an integral component of drinking water and wastewater treatment applications. It is, however, but one unit process within a modern water treatment plant scheme, whereby there are a multitude of equipment and technology options to select from depending upon the ultimate goals of treatment. To understand the role of filtration, it is important to make distinctions not only with the other technologies employed in the cleaning and purification of industrial and municipal waters, but also with the objectives of different unit processes.

Chemical methods of treatment rely upon the chemical interactions of the contaminants we wish to remove from water, and the application of chemicals that either aid in the separation of contaminants from water, or assist in the destruction or neutralization of harmful effects associated with contaminants. Chemical treatment methods are applied both as stand-alone technologies, and as an integral part of the treatment process with physical methods. 

Among the energy intensive technologies, thermal methods have a dual role in water treatment applications. They can be applied as a means of sterilization, thus providing high quality drinking water, and/or these technologies can be applied to the processing of the solid wastes or sludge, generated from water treatment applications. In the latter cases, thermal methods can be applied in essentially the same manner as they are applied to conditioning water, namely to sterilize sludge contaminated with organic contaminants, and/or these technologies can be applied to volume reduction. Volume reduction is a key step in water treatment operations, because ultimately there is a tradeoff between polluted water and hazardous solid waste.

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Lucky Luke Comic

Lucky Luke is a Franco-Belgian comics series created by Belgian cartoonist Maurice De Bevere, better known as Morris, and for one period written by René Goscinny. Set in the American Old West, it stars the titular character, Lucky Luke, the cowboy known to "shoot faster than his shadow". Along with The Adventures of Tintin and Asterix, Lucky Luke is one of the most popular and best-selling comic- ook series in continental Europe.[1] About half of the series' adventures have been translated into English. Lucky Luke comics have been translated into 23 languages, including many European languages, some African and Asian languages.

Both a tribute to the mythic Old West and an affectionate parody, the comics were created by the Belgian artist Morris who drew Lucky Luke from 1946 until his death in 2001. The first Lucky Luke adventure named Arizona 1880 appeared in the Almanach issue of the comics magazine Le Journal de Spirou on December 7, 1946.[2] After several years of solitary work on the strip, Morris began a collaboration with René Goscinny who became the series' writer for a period that is considered the golden age of the series. This started with the story Des rails sur la Prairie published on August 25, 1955 in Spirou.[3] Ending a long run of serial publications in Spirou, the series shifted to Goscinny's magazine Pilote in 1967 with the story La Diligence, subsequently leaving publisher Dupuis for Dargaud.