The refrigeration cycle of an HVAC system is governed by the principles of thermodynamics, or the transfer of energy from one place to another. The key concept is that heat is a form of energy corresponding to a definite amount of mechanical work.
Key Concepts
- Boiling or evaporation means changing from a liquid state to a gas state.
- Heat of vaporization is the amount of heat that is required to convert a liquid into a gas (boiling point).
- 212 degrees Fahrenheit is the boiling point of water at sea level.
- Lower pressures (above sea level) make it easier for a liquid to boil, and higher pressures (below sea level) make it more difficult.
- There Is No Such Thing as Cold — Only the Absence of Heat.
- Heat (Energy) Is Neither Created nor Destroyed — Only transferred.
- The purpose of the refrigeration cycle is to remove the heat in a given area and eject it outside.
Vapor-Compression Refrigeration
In vapor-compression refrigeration systems, the refrigerant undergoes several phase changes throughout the cycle. This is the most widely used method for air conditioning of buildings and automobiles. The Vapor Compression Refrigeration Cycle involves four components: compressor, condenser, expansion valve/throttle valve and evaporator.
Refrigerant in the evaporator absorbs heat (cooling the air), and is expelled from the refrigerant to the outdoor air via the condenser. The expansion device and compressor help manipulate the pressure of refrigerant to make the cycle possible.
Key Components
1. Compressor
The compressor is the main driver of the refrigeration cycle and consumes the most power compared to any other component. It’s job is to compresses the cool refrigerant vapor leaving the evaporator, transforming it into a very hot and high-pressure vapor.
- The inlet of a compressor is called the suction line.
- The outlet of a compressor is called the discharge line.
2. Condenser (outside)
At the condenser, warm outdoor air blows across a coil filled with hot, gaseous refrigerant. As the refrigerant cools, it turns from a gas into a liquid, or condenses down. Coils wind through the condenser to maximize the surface area of the piping—and heat transfer to the air. Refrigerant turns from a vapor into a hot liquid due to the high pressure and reduction in temperature.
3. Expansion Valve
The expansion valve is responsible for quickly reducing the pressure of the refrigerant so it can boil (or evaporate) more easily. At this point in the cycle, refrigerant approaches the expansion device as a hot, high-pressure liquid. The pressure drops so rapidly, the refrigerant turns into a combination of a cold liquid and vapor.
4. Evaporator (inside)
The evaporator is responsible for cooling the air going to a space by boiling (evaporating) the refrigerant flowing through it. Warm air blows across the evaporator as cold refrigerant moves through the evaporator coil, whereby heat transfers from the air to the refrigerant, cooing the air directly before being vented to a conditioned space.
Cooling is achieved by transferring (removing) heat from the return air. The coil winds through the evaporator to maximize the surface area (and heat transfer) from the refrigerant to the air. Low-pressure liquid refrigerant is easily boiled by the warm air blown across the evaporator and heads back to the compressor as a cool gas/vapor.
5. Refrigerant (coolant)
Refrigerant travels through the system to the different parts of the refrigeration cycle, changing from a low-pressure gas to a high-pressure liquid. The type of refrigerant in air conditioning will depend on how old a system is.
- R-12 (Chlorofluorocarbons), contribute to greenhouse gas effects, ceased production in 1994.
- R-22 (hydrochlorofluorocarbons) or more commonly known as Freon, harmful to the environment. After 1/1/2020 only reclaimed supplies or recycled R22 can be used.
- R410a, R-32, R134a (Hydrofluorocarbons) (HFCs), also known as Puron. More environmentally friendly than Freon, most popular choice for modern AC systems. Not recommend for recharging systems that run on R22.
Phase Changes of the Refrigerant
- Stage 1 (compressor): low pressure warm gas enters compressor and leaves as high pressure hot gas.
- Stage 2 (condenser): high pressure hot gas enters the condenser from the compressor, cools through the coil fins (convection) and condenses back into a high pressure ambient temperature liquid.
- Stage 3 (expansion valve): refrigerant enters expansion valve from the condenser as a high pressure ambient temperature liquid. The valve regulates flow of refrigerant around the system using a diaphragm and is connected between the valve and evaporator output. This device restricts the flow of refrigerant and allows it to expand into a low pressure cold liquid.
- Stage 4 (evaporator): refrigerant enters the evaporator from the expansion valve as a low pressure cold liquid. Warm air is blown across the coils, which is carried away by the refrigerant as it boils and evaporates into a gas. The boiling point is very low at low pressure.
Temperature Changes
- Low Pressure, Low Temperature, Saturated Vapor (leave evaporator, enter compressor).
- High Pressure, High Temperature, Superheated Vapor (leaves compressor, enter condenser).
- High Pressure, Medium Temperature, Saturated Liquid (leave condenser, enter expansion valve).
- Low Pressure, Low Temperature, Liquid/Vapor Mix (leave expansion valve, enter evaporator).
Vapor-Absorption Refrigeration
Another type of cooling system called vapor-absorption refrigeration is a process driven by heat energy, rather than mechanical energy. The compressor is replaced by an absorber and a generator.
The evaporator allows the refrigerant to evaporate and to be absorbed by an absorbent fluid. This process produces the cooling effect. The combined fluids then go to the generator where it is heated, thus driving the refrigerant out of the absorbent.
The refrigerant vapor then goes to a condenser and gets cooled down to liquid phase, whereas the absorbent is pumped back to the absorber. The cold liquid refrigerant is released through an expansion valve into the evaporator, and the cycle continues.
