Before mechanical refrigeration systems were introduced, people cooled their food with ice and snow, either found locally or brought down from the mountains. The first cellars were holes dug into the ground and lined with wood or straw and packed with snow and ice: this was the only means of refrigeration for most of history.

Refrigeration is the process of removing heat from an enclosed space, or from a substance, to lower its temperature. A refrigerator uses the evaporation of a liquid to absorb heat. The liquid, or refrigerant, used in a refrigerator evaporates at an extremely low temperature, creating freezing temperatures inside the refrigerator. It's all based on the following physics: - a liquid is rapidly vaporized (through compression) - the quickly expanding vapor requires kinetic energy and draws the energy needed from the immediate area - which loses energy and becomes cooler. Cooling caused by the rapid expansion of gases is the primary means of refrigeration today.

 

The first known artificial refrigeration was demonstrated by William Cullen at the University of Glasgow in 1748. However, he did not use his discovery for any practical purpose. In 1805, an American inventor, Oliver Evans, designed the first refrigeration machine. The first practical refrigerating machine was built by Jacob Perkins in 1834; it used ether in a vapor compression cycle. An American physician, John Gorrie, built a refrigerator based on Oliver Evans' design in 1844 to make ice to cool the air for his yellow fever patients. German engineer Carl von Linden, patented not a refrigerator but the process of liquefying gas in 1876 that is part of basic refrigeration technology.

Refrigerators from the late 1800s until 1929 used the toxic gases ammonia (NH3), methyl chloride (CH3Cl), and sulfur dioxide (SO2) as refrigerants. Several fatal accidents occurred in the 1920s when methyl chloride leaked out of refrigerators. Three American corporations launched collaborative research to develop a less dangerous method of refrigeration; their efforts lead to the discovery of Freon. In just a few years, compressor refrigerators using Freon would became the standard for almost all home kitchens. Only decades later, would people realize that these chlorofluorocarbons endangered the ozone layer of the entire planet.

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 vapor-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”).

 

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. 

 

COMPRESSOR ACTION

 

    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 vapor 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  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.