Figure 3.  Space temperature profile of a typical system. 

Looking at Figure 4, the return air temperature for the RTU is 80° and the unit must deliver 55° air to overcome the heat gain from the space. 

Figure 4. Entering and leaving air temperatures of an RTU.

Using a simple sensible heat calculation, Equation 1, the required tonnage for the RTU can be calculated. 

MBHs=CFM×∆t×1.0812,000

Equation 1. Sensible heat.

For this example, we will assume the supply airflow is 10,000CFM. 

MBHs=10,000×25℉×1.0812,000=22.5 tons

Knowing the calculated load equals 22.5 tons, a 25-ton unit is selected for cooling the space.  However, if we look at the actual performance of a 25-ton unit under these conditions, the unit has a decreased unit capacity of 10% to 22.2 tons.  This is due to the additional motor heat produced by the required supply ductwork with a 4-way diffuser and the 80° return air temperature. Not only is the unit capacity reduced, but also the compressor power draw increases as the return air temperature increases.  

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