Evaporatif soğutma

Abstract

Bu çalışmada, havanın iklimlendirilmesi işlemlerin de kullanılan evaporatif soğutma olayı çeşitleriyle birlikte incelenerek, direktevaporatif soğutma için havanın evaporatif soğutucudan çıkış özellikleri elde edilmiştir. Hava içinde bulunan nem miktarı arttıkça, evaporatif soğutma işleminin etkenliği azalmaktadır. Başka bir deyişle, herhangi bir bölgede uygulanacak evaporatif soğutma işleminin başarısı, bölgenin iklim şartlarına bağlıdır. Buna göre, evaporatif soğutmanın hangi şehirlerimizde verimli olarak kullanılabileceği bulunmuştur. Bilinen önemli direktevaporatif soğutucu tiplerine değinilmiştir. Bu tiplere alternatif olabilecek, gözenekli plakalardan oluşan yeni bir direkt evaporatif soğutucunun analizi yapılmıştır. Söz konusu soğutucununta sarımında anahtar rolü oynayacak sonuçlar elde edilmiştir.

Air cooling by the evaporation of water is termed evaporative cooling. When water evaporates into the air being cooled, it creates direct evaporative cooling, the oldest and most common form. In the direct system, air is cooled by direct contact with the water, either by an extended wetted-surface material (as in wetted-pad type air coolers) or with a series of sprays {as in spray type evaporative coolers ). However, when the evaporation occurs separately and the air is cooled without humidity gain, the process is indirect evaporative cooling. In the indirect system, air is cooled in a heat exchanger, which uses a secondary stream of air and water that has been evaporatively cooled, such as by cooling tower and cooling coil. Compared with refrigerated air conditioning, evaporative cooling systems are quite inexpensive, and their use is widespread in residences and commercial and industrial establishments in many regions of the world. The initial (capital) cost of an evaporative cooling sys tem may be less than one-fourth that of a refrigerated system for the same structure; and the input energy to operate the evaporative cooler would typically be less than one-fifth of the refrigerated system's energy requirement. With energy costs escalating alarmingly, evaporative cooling is becoming an attractive option where ambient air conditions permit its use. The evaporative cooling process involves what is known as an adiabatic exchange of heat. The term adia- batic means that a process occurs at constant heat. As applied to evaporative cooling, this means that an air- water-vapour mixture is cooled (i.e., its dry bulb temperature is lowered) without any gain or loss of heat through the cabinet or casing of the cooling mechanism. The water in the cooler, if recirculation is used, assumes the wet bulb temperature of the air and cooling proceeds with the enthalpy or total heat content of the air remaining essentially constant. -VIII- On the standard psychrometric charts, lines of constant enthalpy are essentially the same as lines of constant wet bulb. If an evaporative cooler is to cool air without any heat transfer to or from, the outside of the unit, it follows inescapably that some form of heat transfer or exchange involves the evaporation of water, and the heat required to evaporate the water is taken from the sensible heat of the air into which the water evaporates. In other words, the heat and mass transfer process between the air and water lowers the air dry- bulb temperature and increases the humidity ratio and at constant wet bulb temperature. The maximum possible dry-bulb temperature reduction of the air in an evaporative cooler is the difference between the entering air dry bulb and wet bulb tempe ratures. This is usually referred to as the wet bulb depression. The efficiency of an evaporative cooler depends on the extent to which complete saturation is approached. Efficiency of an evaporative cooler is defined as the drop in dry- bulb temperature produced in the air passing through the cooler divided by the wet bulb depression of the entering air. Mathematically, for an evaporative cooler, % Eff = £Son " £p°ff * 10° ÜBon ~ w"on The first requirement for successful evaporative cooling is a low wet bulb temperature. The lowest pos sible dry bulb temperature of the air off an evaporative cooler (at 100 percent efficiency) is the wet-bulb tem perature of the ambiant air. The wet bulb temperature prevailing in the area is then, one of the important limitations on evaporative cooler performance. The second requirement is a high dry bulb temperature. The dry bulb temperature must be so uncomfortably high that any amount and kind of cooling will be considered a wel come relief. It should be emphasized that evaporative cooling, although it affords relief in hot, dry climates, does not perform all the functions of true air conditioning. Evaporative systems cool, circulate, ventilate and -IX- filter the air. They do not dehumidify but, on the contrary, humidify the air. Evaporative cooling then provides a measure of comfort in hot, relatively dry climates, but it isn't complete air conditioning. Evaporatively cooled air is nearly saturated as it comes off the cooler. Even through a satisfactory dry- bulb temperature results, the relative humidity off the cooler will be very high and therefore evaporative cooling cannot remove latent heat from a space. As cooled air mixes with room air, it warms up and absorbs sensible heat from the room air. The end result of the mixing and heat exchange is the room condition. If the room condition is to be within the comfort zone, the space lood must be almost entirely sensible. Because of large internal latent loads, theaters, restaurants and other spaces where people congregate in large numbers cannot be successfully air-conditioned by evaporative systems. A determination as to the feasibility of evapora tive cooling can be made by using the comfort chart. Effective temperature lines on the comfort chart represent a variety of different dry-bulb and wet-bulb temperatures at which people feel equally comfortable. The summer comfort chart zone is an area on the comfort chart within which any point represents a dry-bulb and, wet bulb combination which results in comfort for a definite percent of a test population. The upper limit of the summer comfort zone is 26 ° Effective Temperature. The average temperature of the spaces cooled by evaporative coolers should not exceed this limit for comfort cooling. In order to obtain comfort cooling by an evaporative cooling application, required outdoor climate conditions are found as follows. - A minumum design wet bulb depression of 12 C. - A maximum design wet bulb temperature of 25 C. In order to obtain relief cooling by an evaporative cooling application, required outdoor climate conditions are found as follows. - A minumum design wet bulb depression of 9 C - A maximum design wet bulb temperature of 25 C -X- Evaporative coolers are of three general types: the spray type air cooler, the rotary air cooler and the wetted-pad air cooler. Spray type air coolers may be nothing more than large sheet-metal housings fitted with spray nozzles. A blower pulls air through the cooler, where evaporative cooling of the air takes place in the presence of the fine water spray. Eliminator plates are provided to prevent the entrainment of water droplets in the air stream. This kind of air coolers are dependable, efficient, and economical to operate. They are best suited to large commercial and industrial evaporative cooling applications. Like all evaporative coolers, they operate on 100 percent outside air. Rotary air cooler is a packaged-type unit designed and built at the factory in a range of sizes for small to medium cooling jobs. The cooler and blower can be obtained separately. If the blower unit is not needed by virtue of the fact that an existing heating system blower is judged adequate, just the cooling unit can be installed. Wetted-pad type coolers contain evaporative pads and a water-circulating pump to lift the sump water up to a distributing system from which it runs down through the pads and back into the sump. A fan within the cooler pulls the air through evaporative pads and delivers it to the space to be cooled. This kind of air coolers are made in sizes from 1 to 10 m /s. Efficiency of wetted-pad type coolers is ordinarily not as high as that for the other two types. Combination systems can involve both direct and indirect principles, along with heat exchangers and cooling coils. In such systems, temperatures below the initial wet-bulb temperature may be produced. While such systems can be more complex, the higher cost of energy may justify their use in certain geographical areas. The use of combination systems for commercial applications can extend the range of atmospheric conditions under which comfort requirements can be met. For the same design conditions, combination systems will provide lower cool air temperatures and thereby reduce the required air flow rate. -XI- In the last stage of this study, a new kind of direct evaporative cooler which constitutes of porous plates is investigated. The porous plates are vertically placed in a casing and either upper or lower parts of the plates are in a reservoir. A blower pulls air through the plates which are wetted by the water. Water flow through porous plates is provided by capillary pumping forces. In order to obtain completely wetted surfaces, pressure drop of the water flowing upwards in the plates shouldn't exceed capillary pumping forces. Darcy-Brinkman-Ergun Model was used to model the flow inside the porous region. According to directions of the water and air flows, four different cases are investigated. Important results of engineering interest were obtained and are reported in this thesis.