Water contact cooler

In a cooling tower s operation, sensible heat also plays a role. When warm water contacts cooler air, the air cools the water and its temperature rises as it gains the sensible heat of the water. Roughly 25%,of the sensible heat transfer takes place within the tower, with the balance of the cooling phenomenon achieved from the evaporative effect of the latent heat of vaporization. In simple terms, a cooling tower is a device that transfers quantities of heat from one mass to another. As we will see in later chapters, a cooling tower is simply an air-mass heat exchanger. 

For fuel cell systems operating on reformates, high levels of aimnonia and CO2 may be present in the fuel stream. Ammonia is known to be detrimental to PEM fuel cell catalysts. It is desirable to reduce the concentration of aimnonia in the fuel stream to below 2 ppm. However, ion filters, if used alone, will be quickly saturated due to the high concentration of ammonia in the water. A water "contact cooler" has been used to transfer the ammonia into a reservoir of water, which needs to be dumped or cleaned. To achieve water neutral operation, Bonville et al with International Fuel Cells Corporation (Japan) [63] patented a method to remove the ammonia in the contaminated stack water via a steam stripping process, which removes ammonia around 400 ppm (and CO2 as well) from contaminated water to about 30 ppm. An ammonia-laden steam is discharged to the environment. A demineralization bed is then used to further reduce ammonia concentration in the water to an acceptable low level. 

In direct contact heal exchange, there is no wall to separate hot and cold streams, and high rales of heal transfer are achieved. Applications include reactor off-gas quenching, vacuum condensers, desuperheating, and humidification. Water-cooling lowers are a particular example of a direct contact heal e.xchanger. In direct contact cooler-condensers, the condensed liquid is frequently used as the coolant. 

A direct-contact gas cooler system operates as follows Approximately 35,000 lb/hr of bone-dry air is passed over hot trays. The air is heated from 150°F to 325°F as it passes over the trays. It exits from the unit with a due point of 105°F. The hot air is sent to a direct-contact cooler, where its temperature is reduced back to 150°F. During the cooling stage, the air is dehumidified with water that is heated frpm 75°F to 105°F. The unit is rated at 3.5 inches of water pressure drop (a) Determine the number of diffusion units needed for this operation and (b) Establish the required dimensions for the direct-contact cooling tower (Hint Use standard low-pressure-drop data from the literature. Some of the older literature give pressure drop data for simple fill. See Sherwood, T. K. and C. E. Reed [6]. 

Wet-dry towers. Figure 9.17(g), employ heat transfer surface as well as direct contact between air and water. Air coolers are used widely for the removal of sensible heat from cooling water on a comparatively small scale when cooling tower capacity is limited. Since dry towers cost about twice as much as wet ones, combinations of wet and dry sometimes are applied, particularly when the water temperatures are high (near 160°F) so that the evaporation losses are prohibitive and the plumes are environmentally undesirable. The warm water flows first through tubes... 

The ammoniac-free gas flows to the final coolers, u, where it is further cooled by direct water contact. The water plus condensed naphthalene flows from the cooler through tar, which absorbs the naphthalene. The water is cooled and recirculated into the final cooler through, v. 

Air is filtered (2) and compressed to 5.7 bar in a compressor (3), leaving the compressor s last stage with about 100 °C. Subsequently it is cooled first with cooling water (o) and then with chilled water (p) to temperatures between 5 and 20 °C in the direct contact cooler (4). This cooling reduces the moisture content of the saturated air, thus reducing the expenditure for the ensuing HjO removal in the zeolite adsorbers (6/6 ). The chilled water is supplied by the evaporative... 

These generators vaporize a liquid (oil/mineral oil or glycol and water), which then condenses into a fine aerosol on contact with cooler air. The amount of smoke produced should be controllable by the liquid feed rate and the temperature of the heating chamber, but in practice the output is not ea.sy to control. They will, however, produce a large amount of smoke over a long periled, dhe generators are relatively expensive (several hundred ECUs), are bulky, are not generally portable, and require an electrical connection. 

Figure 7.4 Microcomputer programming of a hatch cooling crystallizer. A, crystallization vessel, B, control heater, C, control cooler. surrounding the draft-tube), D, contact thermometer, E, discharge plug and conical baffle), F, recorder, G, relay, H, temperature programmer, I, cooling water pump, J, cooling water reservoir, K, water inflow L, water outflow after Jones and Mullin, 1974)...

The transfer of heat within a fluid as the result of mixing of the warmer and cooler portions of the fluid is convection. For example, air in contact with the hot plates of a radiator in a room rises and cold air is drawn off the floor of the room. The room is heated by convection. It is the mixing of the warmer and cooler portions of the fluid that conducts the heat from the radiator on one side of a room to the other side. Another example is a bucket of water placed over a flame. The water at the bottom of the bucket becomes heated and less dense than before due to thermal expansion. It rises through the colder upper portion of the bucket transferring its heat by mixing as it rises. 

A hot-water heating system forces water into pipes, or arrangements of pipes called registers that warm from contact with warm water. Air in the room warms from contact with the pipes. Usually, the pipes are on the floor of a room so that warmer, less dense air around the pipes rises somewhat like a helium-filled balloon rises in air. The warmer air cools as it mixes with cooler air near the ceiling and falls as its density increases. This process is called convection and the moving air is referred to as convection current. The process of convection described here is pipe-to-air and usually does a better job of heating evenly than in an air-to-air convection system—the circulation of air by fans as in a forced-air heating system. 

Convective heat transfer occurs when a fluid (gas or liquid) is in contact with a body at a different temperature. As a simple example, consider that you are swimming in water at 21°C (70°F), you observe that your body feels cooler than it would if you were in still air at 21°C (70°F). Also, you have observed that you feel cooler in your automobile when the air-conditioner vent is blowing directly at you than when the air stream is directed away from you. Both ot these observations are directly related to convective heat transfer, and we might hypothesize that the rate of energy loss from our body due to this mode of heat transfer is dependent on not only the temperature difference but also the typie of surrounding fluid and the velocity of the fluid. We can thus define the unit heat transfer for convection, q/A, as follows ... 

Condensation occurs when air temperatures cool. The cooling occurs in one of two ways. Either the air vapor cools as it rises and expands or as it comes into contact with a cool object such as a cold landmass or an ice-covered area. Air rises for several reasons. It can be forced up as it encounters a cooler, denser body of air, or when it meets mountains or other raised land masses. It can rise as it meets a very warm surface, like a desert, and become more buoyant than the surrounding air. Air also can be forced to rise by storms—during tornadoes particles of air circling to the center of a cyclone collide and are forced up. When the water vapor collides with a cold object, it can become fog, dew, or frost as it condenses. The vapor cools as it rises into the atmosphere and condenses to form clouds and, sometimes, rain. 

Figure 6-32 illustrates ejector systems with large condensable loads which can be at least partially handled in the precondenser. Controls are used to maintain constant suction pressure at varying loads (air bleed), or to reduce the required cooling water at low process loads or low water temperatures [2]. The cooler W ater must not be throttled below the minimum (usually 30%-50% of maximum) for proper contact in the condenser. It may be controlled by tailwater temperature, or by the absolute pressure. 

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