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.