How The 4 Principles Of Thermodynamics Keep Your Air Conditioner & Refrigerator Chill

When it comes to using the products, you may not think a central air conditioning system or window air conditioner has much in common with your refrigerator. The HVAC system isn’t much good at preventing milk from spoiling and the refrigerator does a lousy job of keeping the living room and bedrooms at a comfortable temperature. And yet, window and wall air conditioners, heat pumps, and central air conditioning systems all rely on the same refrigeration cycle that keeps appliances like refrigerators, freezers, ice machines, and wine coolers working properly. This is where thermodynamics comes into play…

A primer on the principles of thermodynamics

The refrigeration cycle is governed by the four key principles of thermodynamics (the science of the relationship between heat, work, temperature, and energy):

  • Zeroth law of thermodynamics – If two thermodynamic systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other.
  • First law of thermodynamics – Energy can neither be created nor destroyed. It can only change forms. In any process, the total energy of the universe remains the same. For a thermodynamic cycle, the net heat supplied to the system equals the net work done by the system.
  • Second law of thermodynamics – The entropy of an isolated system not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium.
  • Third law of thermodynamics – As temperature approaches absolute zero, the entropy (the measure of a system’s thermal energy per unit temperature that is unavailable for doing useful work, or the molecular disorder)of a system approaches a constant minimum.

How the refrigeration cycle works

In simple terms, a refrigeration cycle’s mission is heat absorption and heat rejection. Any HVAC technician will tell you (emphatically), you can’t make cold, you can just remove heat. The refrigeration cycle, sometimes called a heat pump cycle, is a means of routing heat away from the area you want to cool. This is accomplished by manipulating the pressure of the working refrigerant through a cycle of compression and expansion.

The refrigeration cycle operates by using four major components:

  • Metering Device -a Capillary Tube on refrigerators, freezers, and room air conditioners, and a Thermal Expansion Valve on most central air or heat pump systems

Refrigerant remains encased in the cooling system which connects these four components and passes through this system in a continuous loop.

The refrigerant begins as a cool gas when heading to the first component: the compressor. The compressor is widely considered the engine of the refrigeration cycle; it consumes the most power of the system’s components as it forces/compresses the refrigerant through the system.

Upon leaving the compressor, the cool, gaseous refrigerant is pumped into the condenser. The condenser’s job is to turn the refrigerant gas into a hot liquid by condensing it. This happens when air is drawn in by a motorized fan and blown across the condenser coil that is filled with the hot, gaseous refrigerant. This allows heat to transfer from the refrigerant to the air surrounding the coil, where the excess heat is rejected to the atmosphere. The condenser coil winds through the condenser to maximize the surface area of the refrigerant tubing, and effectively, the heat transfers to the air. The refrigerant then turns from a gas into a hot liquid due to the high pressure and reduction in temperature.

Next, the refrigerant, now a hot, high-pressure liquid, swiftly moves to the metering device. In most central air and heat pump systems, this device is a thermal expansion valve that is responsible for quickly driving the pressure of the refrigerant down so it can boil (evaporate) more easily in the evaporator. The valve is designed to regulate the rate at which liquid refrigerant flows into the evaporator. This controlled flow is necessary to maximize the efficiency of the evaporator while preventing excess liquid refrigerant from returning to the compressor.

Now that the refrigerant is a cold mix of liquid and gas, it begins to move through the evaporator. The evaporator is responsible for boiling (evaporating) the refrigerant flowing through it. A motorized fan or blower wheel will draw air through the now-cold evaporator coil where heat transfers from the air to the refrigerant, which cools the air directly before the air is sent to the refrigerator and freezer compartments (if we’re talking about appliances), through the product’s front vent (if we’re talking about window air conditioners or heat pumps), or the home’s venting (if we’re talking about central air conditioning). Like the condenser coil, the evaporator coil also winds through the evaporator to maximize heat transfer from the refrigerant to the air. The low-pressure liquid refrigerant is quickly boiled by the air blown across the evaporator and heads back to the compressor as a cool gas. Simply put, the refrigerant is hottest when it leaves the compressor and coldest when it leaves the metering device. 

To summarize — heat is absorbed by the refrigerant (cooling the air) in the evaporator and expelled from the refrigerant in the condenser. Simultaneously, the metering device and compressor help manipulate the pressure of the refrigerant to make the cycle possible.

Keep your refrigerator & air conditioner working great with parts from Repair Clinic

Refrigerator condenser and evaporator fan motors (and fan blades), as well as central air condensing unit fan motors and furnace circulation blower motors (and blower wheels), are all refrigeration cycle components that intrepid do-it-yourselfers can replace on their own. Compressors can be replaced as well, but it is strongly recommended that this only be done by experienced technicians. Repair stocks all of these genuine manufacturer parts direct from such top industry names as Whirlpool, Electrolux, GE, LG, Samsung, Bryant, Lennox, Goodman, and many more. To find the right part for your refrigeration appliance or HVAC system, enter the full model number of the appliance or product in the Repair Clinic website search bar. You can then use the “Part Category” and “Part Title” filters to refine the results to identify the specific component that matches your model.

For further assistance, check out Repair Clinic’s “Repair Help” section as well as its vast library of videos & articles to learn troubleshooting tips and how to install parts through step-by-step procedural guides. A successful repair is only a few clicks away when your repair partner is Repair Clinic.

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