March 1997 Volume 3 Issue 5
How Refrigeration Works
To grasp the energy implications of refrigeration systems it helps to have a clear understanding of how these systems work — how they go about creating temperatures low enough to cool and freeze. Fortunately, the basics are fairly straightforward.
The diagram at the right illustrates a typical mechanical vapour-compression refrigeration system. Such systems include several key pieces of hardware:
· a compressor,
· a condenser,
· an expansion or throttling device such as the TX valve shown, and
· an evaporator.
There is also, of course, a refrigerant fluid that flows in a closed path around the system.
We know that water, propane, and other fluids are sometimes in a vapour or gas state and sometimes in a liquid state, depending on the temperature and pressure to which they are subjected. In this respect, refrigerants are no different. It’s just that refrigerant fluids change from liquid to gas at temperatures and pressures suited to refrigeration purposes. Refrigerants, for instance, “boil” at a much lower temperature than water.
Refrigeration depends on changes of pressure, and two of the devices mentioned above create these changes. The compressor raises refrigerant pressure; the TX valve (or similar device) lowers it. In fact, it is convenient to divide a refrigeration system into two pressure zones or domains: a high pressure zone, and a low pressure one. In our diagram, the dashed line going through both the compressor and TX valve does this. Above and to the right of that line the refrigerant exists at high pressure — typically one hundred to several hundred pounds per square inch gauge (psig). Below and to the left of that line the same refrigerant exists at a much lower pressure — typically a few psig or tens of psig, and sometimes at less than atmospheric pressure (hence the term “suction”).
TYPICAL REFRIGERATION SYSTEM
HAPPENINGS AT THE EVAPORATOR
Because the refrigerant flows around the system in a closed loop, we could pick any point to begin our tour of the system. Let’s start at the input to the evaporator unit — the point on the diagram marked “cold liquid.” In a system designed only for cooling, not freezing, the refrigerant here might be slightly above 0°C (32°F). In a system designed for freezing, it might well be minus 40°C (minus 40°F), or even lower.
Evaporators can have various forms, but the type shown is typical. It consists of a back-and-forth length of copper tubing to which metal fins have been attached. As the cold liquid refrigerant enters the evaporator it cools the tubing and the fins. A fan blows air across the fins. The cold fins remove heat from the air, thereby lowering the air temperature in the cooled space. The removed heat is conducted through the metal to the refrigerant where it is absorbed. As this happens, the refrigerant gradually changes from liquid to gas.
Although the refrigerant gas coming out of the evaporator is only slightly warmer than the liquid refrigerant going in, it is laden with heat — the heat it absorbed as it changed state from liquid to gas. To make the refrigerant ready to do further cooling, it is necessary to get rid of that heat and convert the refrigerant back to a cold liquid again.
In the first step of this process, the low-pressure gas coming out of the evaporator is compressed to the “head pressure” level of roughly one hundred to several hundred psig. A motor-driven mechanical compressor does this — and serves at the same time as a vapour pump that keeps the refrigerant circulating around the loop. As the gas is compressed its temperature goes up, and at the compressor output we have high-pressure very-hot gas.
HAPPENINGS AT THE CONDENSER
To this point we have yet not gotten rid of the heat picked up in the evaporator. In fact, the compressor has added even more heat. It is the condenser that saves the day by letting the system dump that excess heat into the atmosphere. Typical condensers are built much like evaporators, and they, too, have fans that blow air across their fins. Here, the passing air picks up heat from the refrigerant — just the opposite of what happens at the evaporator. In the process, the temperature of the high pressure refrigerant drops to a point where it condenses back into a liquid. Thus, the refrigerant enters the condenser as a high-pressure, high-temperature gas, and leaves in liquid form — cooler (typically 80 to 125°F), but still under high pressure.
In the usual industrial refrigeration system the warm liquid refrigerant coming out of the condenser goes into a reservoir called a receiver (the rectangular box in the diagram), and then through a sight glass (the round device). The sight glass is a troubleshooting aid that allows the liquid refrigerant to be observed, along with any bubbles of gas that might be present.
THE TX VALVE
With the refrigerant finally free of that unwanted heat, the system is ready to create the cold liquid which the evaporator needs for the cooling process to continue. It is the nature of refrigerant fluids that reducing the pressure to which a liquid refrigerant is subjected will cause its temperature to drop sharply. This pressure reduction is accomplished in practice by allowing the liquid to pass through a flow-restricting device — a section of small-bore tubing, or in the present example a special TX valve. As the liquid passes from the high-pressure zone to the low-pressure zone through the constricted passageway, its temperature falls to the level needed to conduct the cooling or freezing activity.
We are now back where we began, with cold liquid refrigerant on hand ready to enter the evaporator. Keep in mind that the refrigeration process is a continuous one, and at each point in the system there is always refrigerant “passing through” in the state described.
ADDITIONAL REFRIGERATION INFORMATION
For more information about refrigeration systems, how they work, and cost saving opportunities, call us at 368-5010 (toll free) or fax us at 368-6582. Request the free booklet Energy Efficient Refrigeration Systems.