Direct Contact Heat Transfer Research Paper

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DIRECT CONTACT HEAT TRANSFER DEVICES

Heat exchangers are used to transfer heat from one fluid to another. In any heat exchangers a wall divides the flowing fluid and forms a thermal resistance between them. However, direct contact heat transfer devices are exception for this situation. (1)
In direct type heat exchangers hot and cold fluid contact each other directly. In many cases mass transfer occurs in the mean time.(2) Direct contact heat transfer devices are used to process the atmospheric air to obtain desired state or to cool water for recurcilutaion though a heat removal process. (3) The most common types of direct contact heat transfer devices are air washers, cooling towers and spray dehumidifiers. There are also other direct contact
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Cooling Tower
Another type of direct contact heat transfer device is cooling tower. Cooling towers are used for heat dissipation from the water which carries heat from another process, which can be a power cycle, furnace etc. In a typical cooling tower, warm water is poured at the top of the tower and cool (and preferably dry) air is blown from the below. Within the cooling tower arrays of plates are set so the surface area between air and water is increased thus the heat transfer increases. When the water reaches bottom it cools and it is then pumped to system where moist and warm air leaves the tower. However, since there is water loss by evaporation water is supplied to system also. Although the process seems very easy the calculations regarding to cooling tower is rather complicated. Analytical solution to cooling towers are very hard to do so iterative solution is used.(4)
There are two methods to do this. First method is graphical method. İn this method Psychrometric chart is directly used. The second method is dimensionless DW diagram (a.k.a Stevens diagram). The two methods yield in similar results. The process regarding the cooling tower is step wise and is shown on psychrometric chart in appendix
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Spray dehumidifier in some sense is very close to both air washer and cooling tower. It is similar to air washer because air is blown in an AC unit while the water is sprayed on air. And it is similar to cooling tower because the process on psychrometric chart is similar to cooling tower rather than air washer. The process on psychrometric chart again is iterative and step wise and is shown in appendix section.
The purpose of the spray dehumidifier is as its name implies to dehumidify relatively moist air. Inlet conditions of spray dehumidifier is different than air washer. In air washer relatively dry air is blown into system where in spray dehumidifier it is humid air. And the water sprayed on air is close to air temperature in air washer where it is realyy cold in spray dehumidifier. To be specific, in order dehumidification to take place condensation must occur and in order condentation take place the sprayed water must be cooler than the dew point temperature of the humid

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