HVAC Refrigerant Flow Capacity

Factors

• Various factors affect the refrigerant flow in air conditioning systems. The flow rate is usually recorded as a mass flow rate that depends on such factors as the size of the orifices through which the refrigerant will pass. Flow rate is also affected by the fluid density of the refrigerant, the gravitational force per mass unit, and the upstream and downstream pressure of the refrigerant through the refrigerant pipes.

Compression and Cooling

• Because the flow capacity is partly a reflection of the fluid density and the pressure within the system, the flow rate is affected by the ability of the compressor to do its job. Compressors with more power to force the refrigerant into liquid form will have a greater flow capacity in terms of cooling efficiency. The compressed refrigerant will return to gas form at a greater rate, causing a greater rate of heat exchange.

Piping Design

• Piping design ultimately plays a role in the flow capacity of the air conditioner. McQuay International notes that the oil flowing back into the compressor must do so at a constant enough rate to keep the refrigeration process occurring at a constant level. The flow rate noted by McQuay is 500 feet per minute (fpm) for lines characterized as horizontal suction lines. For suction and hot gas risers, the rate is 1,000 fpm. When the solenoid closes, the rate drops to less than 300 fpm to prevent the oil from hammering the compressor valves.

Variable Flow Rates

• Many HVAC systems use constant flow rates that are subject to the compressor's capacity. However, some systems use variable flow rates that can be controlled to vary the cooling capacity in each room of the house. Theoretically, this should result in more energy efficient use of the refrigerant. However, as a paper presented at a 2009 meeting of the International Building Performance Simulation Association points out, this is not usually the case. Manufacturers often tout flow capacities without regard to factors such as pipe design and gravity.