What are psychrometric processes?

What are psychrometric processes?

As part of our series on the basics of HVAC, we want to introduce a key concept: psychrometry and psychrometric processes. Psychrometry is a branch of engineering that deals with the study of moist air, otherwise known as humidity, and its behaviour under various sets of conditions. These conditions are, of course, essential to maintaining building comfort.

Here, we’ll briefly outline the key processes that control these conditions in an HVAC system and how they work: sensible cooling, sensible heating, cooling and dehumidification, cooling and humidification, heating and humidification, and heating and dehumidification.

Sensible cooling and heating

During this process, humidity remains constant but temperature decreases as the air pass over a cooling coil. To keep the moisture content constant, the cooling coil should be dry and its temperature should be greater than the dew point. In theory, the air should exit the system at the same temperature as the cooling coil, but in practice, it tends to be a little higher.

Meanwhile, sensible heating performs the opposite function. As with sensible cooling, the moisture content of air will remain constant, but in contrast, its temperature increases as it flows over a heating coil.

Cooling and dehumidification

The addition of moisture to the air, with a change in temperature, is known as humidification and, as one would expect, dehumidification is the opposite. Cooling and dehumidification processes are generally used in summer air conditioning, where the air is passed over a cooling coil via a cold water spray. Dehumidification is achieved when the effective surface of the cooling coil is less than the dew point temperature of the air entering the coil.

Cooling and humidification

This process is generally achieved via evaporative cooling, where cool water is injected into the flowing stream of dry air. The final humidity level of the water will depend on the amount of evaporation.

Heating and humidification

This process is when air is passed through a humidifier with a spray water temperature higher than the dry temperature of the entering air. The heat of the evaporation of the water is absorbed from the spray water itself, and hence, is cooled. Via this process, the air becomes heated and humidified. This could also be achieved via steam injection into the space, which is more common in industrial settings.

Heating and dehumidification

This can be achieved by using a hygroscopic material that absorbs or adsorbs the water vapour from the moisture. This hygroscopic material can be a solid or a liquid. The absorption of water by the hygroscopic material is an exothermic reaction, which releases heat during this process that is transferred to the air.

Achieve optimum building comfort

This is but a basic outline of what psychrometry and psychrometric processes involve. A good MEP engineer will apply complex equations, charts, knowledge of equipment and years of experience to calculate the dew point and dry air temperatures in order to configure a system to produce the optimum output.

These variables are essential to the proper functioning of an HVAC system and building users’ comfort. Make sure you hire someone with all the relevant knowledge to make sure your system is fully optimised. Ultimately, this will save you time, money and hassle – and make the system more efficient so you can do your bit to save the environment too.



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