How to calculate the cooling coil capacity
In this article guide, I will use an example problem to calculate the cooling coil capacity.
Air at 27 deg dry bulb and at a flow of 1 m3/sec and 60% relative humidity is cooled and dehumidified. Accordingly, the temperature of the exiting air is 16 deg and at 80% relative humidity.
You need to use the psychrometric chart shown below in order to calculate the total cooling coil capacity.
You first add point 1 on the chart at 60% relative humidity line and at 27 degrees as shown in the x-axis (dry bulb temperature). Additionally, you extract the specific enthalpy for point 1 as shown in the chart below. Next, you add point 2 on the chart above at 80% relative humidity and at 16 degrees Celsius. In the same way, you extract the specific enthalpy for point 2.
To calculate the total load of the air handling unit you will use the following thermodynamic rule:
Q = Air mass flow rate x (Enthalpy point 1 – Enthalpy point 2)
Q= Cooling Load in KW
Enthalpy point 1 = 67 Kj/kg
Enthalpy point 2 = 38 Kj/kg
Accordingly, you need to remember that you still don’t have the air mass flow rate, in order to calculate it you need the specific volume of air at point 1. Therefore, you need to use the psychrometric chart again to extract that information:
As shown in the figure above, the green line is between 0.85 and 0.9, it is actually 0.878. Consequently, now you can calculate the air mass flow:
Air Mass Flow Rate= (1 / 0.878) = 0.878 kg/s
Finally, using the total load equation ‘Q‘, you can insert all the values extracted from the psychrometric chart:
Q= 0.878 x (67 – 38) = 25.4 Kw
In the next article, we will learn how to calculate the sensible and latent heat loads using the psychrometric chart.
This article is part of a series of documents discussing HVAC Design, stay tuned for more design guides.