Lesson 10. The Refrigeration Cycle

The Refrigeration Cycle


The refrigeration cycle is the common method used to produce cold air in an air conditioning unit, freezing air in a refrigerator and hot air in a heat pump.  There is a high pressure side of the cooling cycle referred to as the liquid side and a low pressure side referred to the suction side.  Whenever we discuss the temperature of the refrigerant in the system you should understand that the temperatures change whenever the pressure of the refrigerant changes.  When operating in cooling mode the components are:
• Liquid line:  This is the copper tube that connects the inside unit evaporator coil to the outside unit, compressor.  The liquid line is most often copper pipe and is the smaller of the two copper tubes connecting the inside and outside units.  The refrigerant is in a high pressure liquid state.  The liquid line is hot to the touch.  The temperature of the refrigerant in the liquid line is referred to as the subcooling temperature.  The subcooling is the amount of heat above the temperature in which the refrigerant changes from a gas to a liquid or the saturation temperature.  If the liquid refrigerant is 10F above the saturation temperature it has 10F of subcooling.  This is an important concept and we will measure and calculate subcooling any time we connect our gauges to the system.
• Expansion Device:  The expansion device is connected at the inlet of the evaporator coil.  There are two common expansion devices in use today.  The fixed orifice device meters the refrigerant at the inlet to the evaporator coil.  The pressure drop across the fixed orifice allows the refrigerant to vaporize and change state from a liquid to a gas absorbing massive amounts of heat.  The Thermal Expansion Valve, TXV, is a valve that automatically adjusts to regulate the flow of refrigerant and is also installed at the inlet to the evaporator coil.  The TXV sensor is attached to the tubing at the outlet of the evaporator coil and adjusts the TXV and the amount of refrigerant entering the coil based on the outlet temperature.  The expansion device separates the high pressure discharge side of the refrigeration cycle from the low pressure suction side.
• Evaporator Coil:  As the refrigerant travels through the tubing in the evaporator coil it changes from a liquid to a gas.  The change in state of the refrigerant removes the heat in the air.  Air is forced over the aluminum fins attached to the coil tubing.  Aluminum and copper are good conductors of heat.  The evaporator is a heat exchanger that transfers heat in the air to the refrigerant in the tubing.  The evaporator coil is sized to remove the maximum amount of heat on the hottest day of the year.  We want to vaporize all the liquid in order to keep energy efficiency and cooling efficiency at its peak.  The temperature that the refrigerant changes from a liquid to a gas is the saturation temperature.  The saturation temperature changes with the pressure of the refrigerant in the tubing.  Heat is removed from the air on the air side of the evaporator heat exchanger.  This includes both kinds of heat, sensible and latent.  With sensible heat the temperature of the air drops producing lower temperature air.  Any latent heat in the form of moisture in the air condenses on the aluminum fins and is drained off in the form of condensate water.
• Liquid Line:  The tubing leaving the evaporator coil is called the suction line.  The inlet to a compressor which is also a pump is referred to as the suction side. The suction line is the larger copper tube since a larger tube is required to transport a gas than is required to transport a liquid.  The suction line is cold to the touch and is insulated to prevent sweating and to prevent the refrigerant from picking up additional heat on its way to the compressor.  The temperature of the refrigerant leaving the evaporator coil is information we need to diagnose the health of the system.  This temperature/pressure relationship is called superheat.  Superheat is the amount of heat added to the refrigerant above the change of state saturation temperature.  15F of superheat is 15F above the temperature that the refrigerant changes from a liquid to a gas.  We will use the superheat calculation to determine if the fixed orifice system is properly charged with refrigerant.
• Compressor:  There are two types of compressors in common use for split systems today.  The older reciprocating compressor is a positive displacement unit using a piston to compress the gas and restore it to a high pressure.  Most of today’s new units use a scroll compressor to raise the refrigerant gas pressure.  The compressor separates the low pressure suction side of the system from the high pressure discharge side.  Both the pressure and the temperature of the gas are raised.
• Condenser Coil:  This coil is similar to the evaporator coil.  It is a heat exchanger and is used to transfer heat in the refrigerant to the air.  It is typically constructed of copper or aluminum tube and aluminum fins with a fan to force air over the coil.  In cooling mode the heat is disposed of outside.  In heat pump mode the heat is used to heat the building.  As the heat is removed, the gas in the condenser tubes drops below the saturation or change of state temperature and the gas condense to a liquid.  The system is sized to reduce the temperature of the liquid about 10F of subcooling to make sure all the refrigerant is in a liquid state when it gets back to the expansion device.
You should not proceed with other subjects until you fully understand the refrigeration cycle and the terms used to describe it.
Additional information is included in our EPA Study Guide with sample EPA Test questions.  This eBook is available for instant download.

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