Heat exchanger

Methods of air cooling

Overview of air cooling methods in practice and theory

1. Cooling with winged pipes.

In air conditioning engineering, people commonly use surface heat exchangers to cool the air.

About structure: The most common is the aluminum fin copper tube heat exchanger. Air moves outside the heat exchanger. The interior can be either cold water (refrigerant) or the evaporating refrigerant itself.

Air treatment by refrigeration method

The air moving through the coil is cooled on the one hand, on the other hand, a part of the water vapor can condense on the heat exchanger surface and flow down the condensate trough. So on the I-d graph, the process of changing the state of the air will follow the A1 process or the cooling and drying process. When the surface temperature is higher than ts, the process takes place along the A2 curve, humidified isothermal cooling. Most of the air conditioners in life use surface type chillers.

2. Cooling with treated spray water

Air can be cooled through a mixed heat exchanger in which treated chilled water is sprayed in direct contact with the air for cooling. This device is also known as a spray chamber device.

When the air passes through the spray chamber, the temperature decreases and the moisture content can increase, remain constant or decrease depending on the temperature of the spray water. When the water temperature is small, the water vapor in the air will condense on the surface of the water droplets and reduce the moisture content. Thus, it is possible to adjust the moisture content of the air through adjusting the spray water temperatur

In spray chamber equipment, water is sprayed into tiny droplets by means of nozzles. Because the water droplets are very small, the contact area is extremely large, but in the spray chamber, the contact time between the air and the water is very small, so the heat exchange efficiency is more or less limited.

To increase the contact area, one can form a water film on solid surfaces. The effect of this method is similar to that of injection.

Spray chamber equipment is widely used in the textile industry and many other industries, requiring humidity control according to strict programs.

3. Natural water cooling (Air cooler)

Chilled water cooling is another high cost for water cooling. In cases where it is required that the air temperature to be cooled is not too low, natural, uncooled water can be used and evaporated into the air to reduce its temperature, this method is called evaporative cooling system (Air cooler) specialized for factories. The degree of cooling the air depends on its humidity and the temperature of the water.

Currently, there are many types of water fans on the market, all of which have the same working principle, which is to let water evaporate into the air when moving through the fan. On the figure 4.3 is a type of water fan. Water is pumped to the top by a small pump and allowed to flow through a layer of capillary porous material. Air moves through the capillary layer is wetted, water will evaporate adiabatic into the air, causing the air temperature to decrease along the adiabatic line A4.

In industry, for example, in textile factories, the use of spray chamber equipment with cooled water is very expensive. Therefore, on days with little sunshine and when the load is not too large, people do not use cold water, but use normal water to treat the air. The processing in this case also takes place along the A4 line.

The water-treated air temperature is usually lowered to as low as possible by the wet thermometer temperature.

4. Compressor cooling – air expansion

To cool the air on airplanes, air compression and expansion methods are used to achieve low-temperature air. Thanks to the availability of turbine compressors that can be used to compress air, there is no need to equip additional compressors, so it is often used.

The schematic diagram of the system is shown in Figure 4.4. In this device, compression and intermediate cooling are carried out twice before being put into the expansion machine to lower the temperature.

The working process of the system is as follows: The externally charged air is taken by the turbine compressor, one side is taken to the combustion chamber to fuel the aircraft engine, the other part is taken to the cooling device. 1, Here the compressed air is machined by the outdoor air. After that, the compressed air is brought to the centrifugal compressor to compress the level 2 to a higher pressure, the turbulence continues to be brought to the level 2 cooling device. The compressed air after level 2 cooling is brought to the turbine, perform adiabatic expansion, so that the pressure and temperature decrease (about 10oC). Cold air is introduced into the cabin. The turbine is coaxially connected to the level 2 compressor to take advantage of the mechanical energy generated by the expanding compressed air.

Such a compressor-expansion aircraft cooling system is normally used only when the aircraft is stationary. When the aircraft is operating at high altitude, it is possible to extract outside air to regulate the temperature in the cabin, because at high altitude, the outside air is quite cold.