Sizing a Condenser

A condenser is properly sized when its capacity to transfer heat from the system is equal to the cooling load, plus the extra heat generated by the work of compressing the gas. This total is called the Total Heat of Rejection.
There are some proven rules of thumb for sizing that can get you in the ball park. For air–conditioning or a high back pressure system, it’s safe and convenient to

Sizing by Nominal hp

Under General Data in our condenser catalog, you’ll note that most Standard condensers are rated by nominal horsepower in a fouled condition. An SST–750A will provide 7.5 hp after being in use for some time and fouled. It will provide 12 hp when new. This means that there is additional capacity available when new.
It is often possible to size a condenser by matching nominal horsepower to compressor horsepower in commercial or high temperature systems when manufacturer’s information is not available. You can estimate the total heat of rejection by multiplying motor horsepower by three thousand to find the heat of compression, and then adding the load. In the following example, the nominal horsepower of the compressor will match the nominal tonnage of the air–conditioning system and the Total Heat of Rejection.
A 15–horsepower compressor in a 15–ton system, produces 225,000 Btu per hour total heat of rejection. That’s 3,000 Btu for heat of compression, plus 12,000 Btu of load for each ton.

Heat of Compression:

15 hp x 3000 Btu/hp = 45,000 Btu

Evaporating Capacity:

15 ton x 12,000 Btu/hr = 180,000 Btu

Estimated total heat of rejection = 225,000 Btu

Once you have determined the total heat of rejection and the corresponding condenser capacity, you are ready to put Standard performance data to work. Looking closely at the waterflow ratings in the condenser catalog, you will note that data is provided for both tower and city.
City means operating conditions where incoming municipal water is at 75°F and condensing temperature is 105°F, a 30°F ITD.

Tower means cooling tower supply water, which is normally at 85°F and condensing temperature is again at 105°F, a 20°F ITD.