In looking at the capacity data for the SST you will note that total heat of rejection, gpm, and pressure drop in psi are provided for various Initial Temperature Differentials from 15°F to 40°F. You can now look for a Total Heat of Rejection that exceeds the 225,000 Btu requirement, and read the corresponding flows and gpm. An SST–1500A (2 pass tower) will provide the desired performance with 44 gpm and an ITD of 20°F or, 24 gpm and an ITD of 30°F. You will notice that models through an SST–4505A would also perform well. However, they will cost much more. An SST–1500A, 15 hp condenser, is the ideal choice since the Total Heat of Rejection required falls in the middle of its performance window.

However, matching nominal horsepower can result in over sizing for low and very low temperature applications, and over sizing costs more. While sizing by matching nominal compressor horsepower to condenser horsepower is often accurate, the best practice is to begin by calculating the actual total heat of rejection.

Sizing by Total Heat of Rejection

For example, total heat of rejection for a system with the following performance characteristics would be calculated like this:

Compressor Performance (manufactures published data)
110°F condensing temperature
10°F evaporating temperature
75°F incoming water temperature Refrigerant R–22
Evaporating Watts = 6500
Evaporating Load:
40,200 Btu
watts x 3.4 = Heat of Compression
Heat of Compression + Evaporating Load =
Total Heat of Rejection
6500 watts x 3.4 = 22,100 Btu
Heat of Compression = 22,100 Btu
      Evaporating Load = 40,200 Btu
Total Heat of Rejection = 62,30