When selecting a refrigeration unit for a new installation, select the unit to provide the desired cooling effect to reduce and/or maintain the temperature of the product. This capacity is typically expressed in Btuh. Unit coolers (evaporators), compressors and metering devices are all selected based on the Btuh requirement of the refrigerant selected. The only major component not selected based on this Btuh requirement is the condenser.
The condenser must be selected for the total heat rejection (THR) of the system, which is the sum of the heat energy absorbed by the evaporator and the additional heat energy added to the refrigerant by the compressor. This additional heat energy is called the heat of compression. The heat of compression varies depending on the compressor design, so the compressor manufacturer’s information should be used whenever possible. If this information is not available, many condenser manufacturers will provide a reference table to determine this value.
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It is not always necessary to select an air-cooled condenser; many times, condensing units are selected for installation. Condensing units are selected based on the required Btuh capacity, and their condensers are selected by the condensing unit manufacturer. However, systems that use remote air-cooled condensers require a separate condenser selection.
To begin the selection process, first, determine the THR of the system by adding the heat of compression of the compressor (obtained from the compressor manufacturer’s or condenser manufacturer’s table.) If the condenser is installed above sea level, a correction factor needs to be applied to the THR. The condenser manufacturer will usually provide a table with the required correction factors.
Next, calculate the design temperature difference (TD), which is the design condensing temperature minus the design ambient temperature.
Design TD = condensing temperature – ambient temperature
Next, for the type of refrigerant in the system, use the condenser manufacturer’s selection chart to choose a condenser based on THR and design temperature differences. Some manufacturers’ charts may show the capacity of their condensers at a 1°F temperature differential. If necessary, you can determine the TD required at 1°F by dividing the THR by the design condenser TD as follows.
THR/°TD = THR ÷ calculated design condenser TD
If the required THR is between the two condenser capacities on the selection table, select the higher of the two. For example, you are selecting a remote air-cooled condenser for a system with a net cooling demand of 225,000 Btuh. The condenser will be installed at sea level, so there is no need for a height correction factor. It is a low-temperature system with an R-404A semi-hermetic suction cooling compressor with a design evaporative temperature of -20°F, an ambient outdoor temperature of 90°F, and a design condensing temperature of 110°F.
Using this factor, the THR would be 348,750 Btuh (225,000 Btuh x 1.55). Design the condenser TD to be 20°F (110°F – 90°F).
Selecting the correct air-cooled condenser for the installation is an important part of ensuring the system operates properly and at maximum efficiency.
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