Cooling tower vs Desert cooler

In one line in a cooling tower, the water cools adiabatically while in a Desert cooler, the air cools adiabatically. In a cooling tower water transfers heat [enthalpy] to air and in the process water cools. In a Desert cooler, the wet air is at a higher energy level than dry air in the hot space. Wet air transfers its enthalpy to hot air in the hot space by evaporating its moisture, thereby cooling the air.

How does Desert Cooler work?

Although?air?coolers?and?air?conditioners?are?often?confused,?they?are?two?entirely?different?appliances. Because air coolers don’t use gas or complicated parts, they are less expensive.

Desert coolers draw in warm ambient air passed?over?the?unit's?damp?honeycomb?filter. The?unit's?cold-water tank keeps the honeycomb filter damp. A fan draws hot and dry [ low humidity] air from the atmosphere and pushes the dry air through the damp filters into the space to be cooled. The moist air carries moisture inside the hot space to be cooled where the moisture evaporates by the principle of evaporative cooling and cools the surroundings.

Air conditioners, on the other hand, utilize refrigerant gas to cool down the warm air in a room and then blow the now-cooled air around the space with internal fans. They must vent the hot air outside.?

Desert cooler vs Cooling tower

?Water and air are two different energy levels. Water with lots of potential energy is at a higher level. When they meet, they want to share their energies to reach an equilibrium. There is only one option, water must release its energy and go near the air's energy. Energy can flow from only high energy to low energy.

?In a cooling tower that provides mass transfer surface and contact time, water particles move into the air from the interface and these particles carry heat so water cools. The internal energy of cooling comes from the water itself.

?In an air cooler, there is nearly no mass transfer of water particles simply because it is not designed for mass transfer cooling. Inside the Desert cooler, the air picks up moisture to the extent its RH, contact time, and contact surface area allow. The wet air with moisture enters the hot space to be cooled. The moisture in the hot space evaporates adiabatically by using the energy stored in the moisture it carries. This cools the air.

In short, the key point is in a cooling tower the water provides energy for cooling in the process water cools. In the case of air coolers, the wet air provides the enthalpy of cooling to the hot air, and in the process, the air cools.

?Thermodynamics

?Desert cooler [air cooler]

?A Desert cooler is a device that cools the air. Desert coolers work well in hot summer in places where the humidity in the air is low. It consists of a water reservoir with a pump. The pump draws water from the reservoir and sprays water over the excelsior pads located on three sides of the cooler box to keep the pads wet.

Excelsior pads are constructed generally from softwood shavings. A centrifugal fan draws air from the atmosphere through wet pads and throws air into the room/building as applied.

Unlike cooling towers, there is a very limited air-water mass transfer interface in the desert cooler. The wet pads are about ? inch thick and can provide very limited surface area for air to come to mass transfer equilibrium with water when air is drawn through these wet pads. An air cooler is typically about a five-foot x five-foot box with practically little mass transfer area unlike fills in a cooling tower. There is very limited mass transfer in the air cooler box. It is predominantly sensible heat transfer in the air cooler box when air draws moisture from the spray of water to the extent its relative humidity and contact time allow.

While the hot incoming air draws energy from the circulating water there is very little cooling of air in the air cooler box due to big specific heat difference between air and water. Air’s specific heat is about 1/4th of water.

As the moist air is pushed out by the air cooler fan into the hot space the moisture carried by wet air evaporates [latent heat] in the hot space, the energy is supplied for PV work and heat of vaporization by the wet air. Evaporation of every gram of water removes 2.26 kj heat. Consequently, the air cools.

?Remember the wet air has higher enthalpy than the dry air in the hot space. The energy from the wet air flows to hot dry air at lower energy and by evaporating water from the wet air that cools the surroundings in the hot space. This is the fundamental of a Desert cooler.

In summary, in a cooling tower, the water at higher enthalpy transfers it to the air at a lower energy level. As a result, water cools. Air absorbs this enthalpy without doing any PV work at constant pressure and near-constant temperature. Air does no PV work in a cooling tower

In an air cooler or Desert cooler, the wet air transfers its enthalpy to dry-hot air at a lower enthalpy by doing PV work as the wet air expands and the moisture evaporates. Air does PV work in the Desert cooler. This is how air cools in the Desert cooler.