A very interesting topic that will interest anyone who is interested in eco-friendly solutions. What is adiabatic cooling? Simply put, it is a method of cooling that involves evaporating water into the air, in which the enthalpy of the air remains constant, but the temperature of the air is lowered and the moisture content increases. This is the oldest method of cooling and, interestingly, everyone has experienced this type of cooling in their life, they just didn't know it. For example, rainfall is an example of direct adiabatic cooling. After a summer storm, the temperature drops but unfortunately the humidity in the air increases. Sweating is another example, as it helps the body regulate body temperature. In addition, water is much cheaper than the electricity needed to produce cooling. For example, to cool 1000m3/h of air from an initial temperature of +30oC to a final temperature of +23oC, we need 3.6kW of cooling power generated by a chiller. Assuming that the chiller works with a coefficient of performance (COP) of 2.5 and the price of electricity is 0.6 PLN/kWh, then 1 kWh of cooling costs 0.24 PLN, and the total cost is 3.6*0.24=0.864 PLN. The same task achieved using a spray chamber to reach the same final temperature of +23oC requires only 3.27 dm3 of water. If 1000 dm3 of water costs about 5 PLN, then the cost of lowering the temperature will be only 0.01635 PLN. As we can see, adiabatic cooling is 52.8 times cheaper than cooling with a chiller!
Adiabatic cooling can be:
Direct - without using a heat exchanger, which occurs when we spray water directly into the air being blown in, which then evaporates in the air.
Theoretically, the lowest temperature that can be achieved is the wet bulb temperature, which for an initial temperature of +30°C is around +20.9°C. However, as with any natural process, there is a certain efficiency (around 80% in this case), so we can expect to achieve around +22.8°C.
So when can we use direct adiabatic cooling? Whenever there are high sensible heat gains, large air masses, and a high temperature gradient, meaning that the temperature of the air being discharged is higher than the temperature of the air being drawn in. In such cases, the second method of adiabatic cooling described below becomes pointless.
Indirect adiabatic cooling involves using an intermediate heat exchanger, such as a counterflow heat exchanger. In this method, water is sprayed directly into the air being discharged from the room, which reduces its temperature and increases its moisture content (the enthalpy, of course, remains constant, as befits an adiabatic process). The air then passes through the heat exchanger, where it gives up its coolness to the air being drawn in. For example, if we are discharging air from a room at a temperature of t=+25°C (taking into account the heat added by the fan) and a relative humidity of RH=55%, it is possible to achieve a temperature behind the spray chamber of t=+19.9°C with an efficiency of 80%.
With a counter-flow heat exchanger with a temperature efficiency of 80% at our disposal, we are able to lower the temperature of the drawn air from +30oC to +21.92oC with a humidity of 72.6%.
Can such air realistically be used to air-condition a room and still be within the comfort zone according to DIN EN 13779? It all depends on the humidity gains in the room, but with a "normal" room, it should be possible. Let's assume that the air is to air-condition a room with a total heat gain of 3 kW and a moisture gain of 0.5 kg/h. Let's look at the graph below - we are interested in point "P".
As we can see, it is possible. Does that mean that refrigeration systems are unnecessary for air conditioning processes if we could cool the rooms with plain water? We will find out in the next article, where we will perform a quantitative and qualitative analysis on a specific example, comparing investment and operating costs :)
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