One Air Washer - Five Types of Air Conditioning

What is an Air Washer?

An Air Washer is a simple air conditioning device rather I would say a simple, relatively economical and environmentally friendly air conditioning device. Air conditioning is wrongly understood in few cases as air cooling but conditioning means anything like cooling, heating, humidifying, dehumidifying, filtering etcetera. 

An air washer is almost similar to that of a simple desert air cooler used for residential purpose or similar to that of a little complicated cooling tower too. A simple air washer will have a blower, nozzles, small water pump, drift eliminators, filters and a small water collection pond. Air comes in direct contact with a spray of water and there will be an exchange of heat and mass (water vapour) between water and air. And here the interesting part comes...

This kind of simple air washer can actually condition your air output in 5 different ways by just tuning the temperature of the water spraying inside. In the air washer, the mean temperature of water droplets in contact with air actually decides the heat and mass transfer direction. As per the 2nd law of thermodynamics, the heat transfer between air and water droplets will be in the direction of decreasing

temperature gradient. Similarly, the mass transfer will be in the direction of decreasing vapour pressure gradient and hence the capability of conditioning the air in such different ways. 

Let's see them one by one now...

• Cooling and Dehumidification: ( tw < tDPT ) Since the exit enthalpy of air is less than its inlet value, from energy balance it can be shown that there is a transfer of total energy from the air to water. Hence to continue the process, water has to be externally cooled. Here both latent and sensible heat transfers are from air to water. This is shown by Process O-A in below figure. ( tw: water temperature ; tDPT = dew point temperature)

• Adiabatic saturation: ( tw = tWBT ) Here the sensible heat transfer from air to water is exactly equal to latent heat transfer from water to air. Hence, no external cooling or heating of water is required. That is this is a case of pure water recirculation. This is shown by Process O-B in the below figure. This the process that takes place in a perfectly insulated evaporative cooler. ( tWBT = wet bulb temperature)

• Cooling and humidification:( tDPT < tw < tWBT ) Here the sensible heat transfer is from air to water and latent heat transfer is from water to air, but the total heat transfer is from air to water, hence, water has to be cooled externally. This is shown by Process O-C in below figure. 

• Cooling and humidification:( tWBT < tw < tDBT ) Here the sensible heat transfer is from air to water and latent heat transfer is from water to air, but the total heat transfer is from water to air, hence, water has to be heated externally. This is shown by Process O-D in below figure. This is the process that takes place in a cooling tower. The airstream extracts heat from the hot water coming from the condenser and the cooled water is sent back to the condenser.( tDBT = dry bulb temperature)

• Heating and humidification: ( tw > tDBT ). Here both sensible and latent heat transfers are from water to air, hence, water has to be heated externally. This is shown by Process in the below figure. 

Thus, it is evident that an air washer works as a year-round air conditioning system. Unfortunately, these are the equipment seen maintained very poorly and not utilizing the benefits out of it at the extreme.