Determining a System's Btu Capacity

August 7, 2006
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Many field-assembled refrigeration systems require the installing contractor or sales engineer to select the system components. This involves choosing the correct condensing unit (compressor), unit cooler (evaporator), metering device, refrigerant, accessories, and piping.

Before any of the system components can be selected, however, the required Btu capacity of the system must be determined. This is accomplished by performing a heat load calculation.

The required Btu capacity is the baseline for selecting the proper components for any installation. Many of the major system components are selected based on the Btu capacity of the system.

After determining the required Btu capacity, the sales engineer or installing contractor will choose the type of refrigerant to use in the system. Today, R-134a is popular for medium-temperature applications; R-404A is popular for low-temperature applications.

FIRST THINGS FIRST

Normally, the first major system component selected is the condensing unit, or the compressor if the system will have a remote condenser. The Btu capacity of any compressor or condensing unit is based on the suction pressure of the refrigerant entering the compressor.

Because of this, most manufacturers will provide the Btu capacity of their compressors or condensing units at a specific entering suction pressure; in particular, at a specific saturation temperature. The saturation temperature of the refrigerant is obtained by using a standard pressure-temperature (P-T) chart and converting the suction pressure to its equivalent saturation temperature.

For example, if R-134a were the refrigerant and the entering suction pressure was 18.5 psig, the corresponding saturation temperature entering the compressor would be 20°F. To determine the saturation temperature of refrigerant entering a compressor, the design evaporator temperature must first be determined.

Many standard medium-temperature applications are designed around an evaporator temperature of 20° to 25°. Many low-temperature applications are designed around an evaporating temperature of -10°.

Once the design evaporator temperature is determined, subtract any suction line pressure losses to determine the approximated saturation temperature at the inlet of the compressor. A pressure loss equivalent to a 2° reduction in saturation temperature is used for most applications (e.g., if the design evaporating temperature were 22°, the assumed saturation temperature entering the compressor would be 20°).

NEXT SELECTIONS

The next component selected is the unit cooler or evaporator. The unit cooler must closely match the design Btu capacity of the compressor or condensing unit. Manufacturers of unit coolers will provide Btu capacity at a specific temperature difference (TD: the difference between the air entering the unit cooler and the evaporating temperature of the refrigerant leaving the unit cooler).

The desired TD of the unit cooler is selected to match the humidity requirements of the product being stored. A TD of 10° is normally used for storage of general packaged products at 85 percent rh. Other types of products may require different relative humidity conditions; this will change the design TD.

Once the unit cooler has been selected, the metering device is next. Most field-installed systems use a thermostatic or electronic expansion valve. Capillary tubes and automatic expansion valves are normally used on self-contained systems, which are completely assembled at the factory.

The thermostatic expansion valve (TXV) is also selected based on the system's Btu capacity, both the capacity of the condensing unit and the unit cooler. The capacity of a TXV is based on several system characteristics. Always follow the TXV manufacturer's selection guidelines.

LINE SIZE

Once all the major system components have been chosen, the line size for the ACR tubing is selected. The size of the tubing is also based on the Btu capacity of the system, as well as the length of run.

It is important to use correctly sized suction, discharge, and liquid lines on any application. Using too small of a suction line will reduce the overall system capacity. Using a suction line that's too large may lead to oil return problems.

Always follow the equipment component manufacturer's guidelines for selecting the correct line sizes.

Selecting the correct components for a refrigeration system is not difficult as long as the installing contractor understands the basic principles of the selection process.

Publication date: 08/07/2006

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