Refrigeration Cycle

The compressor is a vapor pump, so it’s important to ensure liquid refrigerants aren’t introduced into the vapor pump’s inlet. Some components are included in the system in order to prevent this from happening as well as safeguard the overall operation of the compressor.

When the compressor starts, it compresses the vapor and discharges high-pressure, high-temperature gas through the discharge line. The gas then enters the condensing coil, where the refrigerant condenses as it gives up — or rejects — heat to the outdoor air. This allows the refrigerant to change state from a high-temperature, high-pressure gas to a medium-temperature liquid.

The medium-temperature liquid leaves the condenser’s outlet and is directed to a liquid receiver, where it is pushed out through a dip tube. It usually also goes through a liquid line filter drier, where any contaminants in the refrigerant are trapped.

The liquid refrigerant continues through the line, and in mechanical systems using a thermostatic expansion valve (TXV), a solenoid valve will open when cooling is needed, allowing refrigerant to flow through the expansion device. In electronic systems, the electronic expansion valve (EEV) does the job of both the solenoid and the TXV.

The expansion device feeds the distributor tubes, which evenly distribute refrigerant through the coil passage on the evaporator coil inlet. As the liquid refrigerant enters the evaporator coil inlet, there is a sudden drop in pressure, which also lowers the refrigerant’s temperature significantly.

The refrigerant is in the path of warmer air that is being drawn across the coil by the evaporator fan motor, and because heat always moves from a hotter source to a colder source, heat is transferred from the air into the colder refrigerant. As the refrigerant continues to pick up the warmth from the air, it changes state and exits the coil as a vapor.

The main job of the expansion device (either TXV or EEV) is to control the amount of superheat, which is the excess heat present in the refrigerant beyond what is needed to transition from a liquid to a vapor state. The refrigerant picks up additional heat from its surroundings as it travels down the suction line on its way back toward the compressor. This superheat is necessary to ensure that only vapor enters the compressor.

Before reaching the compressor, there will likely be a suction filter drier, which sequesters any contaminants and allows the refrigerant to continue through. There may also be a suction accumulator, which is a vessel to hold any remaining liquid so it can safely boil off as a vapor. A tube inside the accumulator then draws the vapor to the compressor’s inlet.

Basic Refrigeration Cycle Diagram Click to enlarge

Control Valves

The refrigeration cycle must be stable and consistent, regardless of the outdoor ambient temperature. For instance, when storing chicken at a store or restaurant at a target of 34°, the proper temperature must be maintained, whether the outdoor temperature is 10° or 110°. Differing temperatures can influence the refrigerant, but head pressure control valves — either single or dual systems — can manage those changes by artificially inflating the discharge pressure, ensuring that the liquid in the receiver always has an adequate supply of liquid to supply to the metering device.

In a system with a single head pressure control valve, when it’s warm outside, refrigerant from the compressor — in the form of a hot gas — makes its way to the condensing coils inlet. Assuming the pressure is high enough, the refrigerant will take the normal path, exiting the condenser outlet to flow from port “C” to port “R.”

Note: The refrigerant goes through the condenser coil, and by the time it reaches the last third of the coil, it has changed from a high-temperature vapor into a liquid.

As outdoor temperatures fall, the pressure falls, too, and there may not be enough liquid refrigerant for the receiver to feed 100% liquid to the expansion device. In this case, the head pressure control valve allows the hot gas to flow from port “D” to port “R” — straight from the discharge valve to the receiver, bypassing the condensing coil (see Figure 1). This helps pressurize what liquid remains in the liquid receiver.

Figure 1 Click to enlarge

Meanwhile, the path to the condenser outlet is closed off. The compressor is still putting hot gas into the top of the condensing coil, so liquid is stacking in the bottom portion of the coil. As that stacking takes place, the refrigerant has nowhere to go, and the pressure inside the coil increases. Once pressure is sufficient, some liquid is let out of the condensing coil outlet and into the liquid receiver.

In systems with dual head pressure control valves, two valves work together to provide a path around the condensing coil. The ORI valve, which opens on a rise in inlet pressure, works in conjunction with the ORD valve, which opens on a rise in differential pressure (see Figure 2). The ORI valve opens when the inlet pressure of the valve itself rises. Typically adjustable, this valve can be fine-tuned during installation to the condensing coil’s outlet. The pressure of the liquid exiting the coil determines if the refrigerant is allowed to flow. If the pressure is high enough, it opens, or it can be held back if the pressure isn’t great enough.

Figure 2 Click to enlarge

Meanwhile, a path is needed for the discharge gas to go directly into the top of the liquid receiver. The ORD opens on a rise in differential pressure between the refrigerant line exiting the ORI valve and the discharge line coming from the compressor. Only then does the hot gas move through the valve into the receiver. In addition, gas coming from the compressor is still filling the top of the condensing coil. The ORI valve holds and stacks the liquid, elevating the pressure, and when it’s high enough, that liquid goes into the liquid receiver. The TXV or EEV will only work if there is enough liquid — and no vapor.

These valve systems ensure that a consistent temperature is maintained, no matter the season or ambient conditions. By artificially inflating the head pressure, the system can do its job and create 100% liquid for the expansion device. Without these valves in place, these systems would only run in warm ambient conditions.