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Refrigeration Cycle Diagram: How Does It Work?
A basic refrigeration cycle diagram shows four major components: the compressor, condenser, thermostatic expansion valve (TEV), and evaporator.
Together, these components transfer heat from one location to another, producing the cooling effect needed in the conditioned space.
Stage 1: Compression
The refrigeration cycle begins with the compressor, which drives the refrigerant through the system and raises its pressure and temperature.
How does a Compressor work?
In the refrigeration cycle, the compressor draws in low‑pressure vapor refrigerant from the evaporator and compresses it, raising both its pressure and temperature to the level required by the condenser.
This action drives the refrigerant through the system and ensures continuous circulation.
The most common compressors are positive displacement types, such as reciprocating, screw, or scroll compressors.
Consider the example of a refrigerator: as the refrigerant passes through the evaporator, it absorbs heat from the refrigerated space and changes phase from a liquid to a vapor.
To continue the cycle, the compressor then comes into action, raising the refrigerant’s pressure and temperature and converting it from a low‑pressure, low‑temperature vapor into a high‑pressure, high‑temperature vapor.
In the absence of a compressor, the heat may start to flow from the condenser to the outside air.
Stage 2: Condensation
After leaving the compressor as a high‑pressure, high‑temperature vapor, the refrigerant enters the condenser.
Here, it releases the heat it absorbed from the evaporator into the surrounding air or water.
As the refrigerant loses heat, it changes phase from a vapor back into a high‑pressure liquid.
How does condensation work in hot climates?
In hotter regions the outside air can reach 42–48 °C, yet the refrigerant still condenses back to liquid because of its critical temperature.
What is the critical temperature of a refrigerant?
The critical temperature of a refrigerant is the highest temperature at which it can exist as a liquid, regardless of the pressure applied.
Above this temperature, the refrigerant cannot be liquefied by pressure alone and exists only as a gas.
This property is crucial in designing and operating refrigeration and air conditioning systems, especially in hot climates.
How does a refrigeration condenser function?
The condenser coil receives hot, high‑pressure vapor from the compressor and cools it, first removing the superheat and then rejecting the latent heat.
This process causes the refrigerant to condense back into a high‑pressure liquid, ready for the next stage of the cycle.
What is latent heat?
Latent heat is the energy required to change a substance’s state — such as from solid to liquid or liquid to gas — without altering its temperature.
For example, when ice melts into water or water boils into steam, latent heat is absorbed or released, yet the temperature of the substance remains constant during the phase change.
This concept is fundamental to understanding how materials absorb or release energy during state transitions.
Condenser cooling mediums are usually fan or water
The small condensing surface required by a domestic appliance such as a fridge/freezer uses the outside metal skin of the body itself as a surface heat exchanger.
In such a construction, the condenser tubes are mechanically fixed in close contact with the skin, so that heat gets conducted through to the outside air by natural convection.
Types of condenser:
- AIR-COOLED CONDENSERS
- WATER-COOLED CONDENSERS
- EVAPORATIVE CONDENSERS
Stage 3: Expansion (TEV)
This stage is where the pressure of the refrigerant is suddenly reduced, causing its temperature to drop as well. The expansion process prepares the refrigerant for the evaporator by creating the low‑pressure, low‑temperature conditions needed for effective heat absorption.
What is a thermostatic expansion valve (TXV or TEV) or Metering valve: Throttling device?
A thermostatic expansion valve (TEV), also known as a metering device, is an essential component in a refrigeration or air conditioning system.
Its primary role is to regulate the flow of refrigerant into the evaporator coil.
By maintaining a certain degree of superheat, the TEV ensures that liquid refrigerant does not enter the suction side of the compressor.
Also, it maximizes heat exchange between the evaporator coil and the refrigerated area.
The valve controls refrigerant flow so that the refrigerant leaving the evaporator is a slightly superheated vapor.
This guarantees that the full latent heat has been absorbed, which improves system efficiency and reduces the mass flow rate required.
A sensing bulb with a good thermal contact is fastened at the evaporator outlet to make sure the refrigerant gas leaving the evaporator outlet is superheated.
As the bulb temperature rises, the refrigerant inside it expands, opening the expansion valve and allowing more refrigerant to flow through the evaporator
When the flow becomes excessive, the outlet temperature falls, reducing the bulb pressure and causing the valve to close.
This self‑regulating action maintains stable operation.
In smaller refrigeration and air conditioning systems, a capillary tube is often used instead of a thermostatic expansion valve (TEV).
Another phenomenon takes place across the Thermostatic expansion valve(TEV), ie. the flashing effect.
When the refrigerant passes through the valve, the sudden pressure drop lowers its temperature.
Heat is extracted from the remaining liquid, causing part of it to evaporate or “flash” into vapor. This process is known as flashing.
The expansion valve is designed to handle liquid only, a vapor‑liquid mixture can cause malfunction.
To avoid this, sufficient subcooling must be provided in the condenser so that only liquid refrigerant reaches the valve.
The thermostatic expansion valve (TEV) plays a vital role in controlling and metering the flow of liquid in the system.
To avoid flash gas problems, sufficient subcooling must be provided to the liquid refrigerant in the condenser.
Why must the sensing bulb use the same refrigerant as the system?
The sensing bulb is filled with the same refrigerant as the system (e.g., R134a) for two key reasons:
- To ensure the bulb expands and contracts in the same way as the system refrigerant, providing accurate valve control.
- If the diaphragm ruptures, that bulb refrigerant can leak into the main refrigeration system.
How does a thermostatic expansion valve (TEV) work?
- Bulb pressure on one side of the diaphragm tends to open the valve.
- Evaporator pressure on the opposite side of the diaphragm tends to close the valve.
- Spring pressure applied to the pin carrier gets transmitted through the pushrods to the evaporator side of the diaphragm. This assists in closing the valve.
Stage 4: Evaporation
The evaporator plays a critical role in the refrigeration cycle by converting low‑pressure, low‑temperature liquid refrigerant into vapor.
How the Evaporator Absorbs Heat?
The refrigerant absorbs heat from the surrounding environment inside the refrigerated space. As a result, the evaporator provides the cooling effect that makes refrigeration possible.
The refrigerant then exits the evaporator as a vapour.
How does the evaporator maximize heat absorption?
The evaporator is strategically located within the space that requires cooling and operates at a temperature lower than its surroundings.
This temperature difference allows the evaporator to absorb heat efficiently, making it a critical component of the refrigeration cycle.
To optimize heat transfer, the design of the evaporator coil is crucial.
By incorporating fins and fans, the coil’s surface area is increased, which accelerates airflow over the evaporator and enhances its heat absorption capacity.
In high‑humidity environments, ice can form on the evaporator coils, creating an insulating layer that reduces heat transfer.
To prevent this, defrosting mechanisms such as electric heating coils are installed near the evaporator.
These gently warm the coils, melting accumulated ice and allowing the water to drain away, thereby maintaining consistent efficiency.
Types of evaporators include:
- Air‑cooling evaporators
- Liquid‑cooling evaporators
Key Questions and Answers in Refrigeration:
Why must the critical temperature of a refrigerant be high?
The critical temperature of a refrigerant is the maximum temperature above which it cannot be liquefied, regardless of the pressure applied.
Beyond this point, the refrigerant will remain in the vapor state even if it passes through the condenser or any cooling medium.
If a refrigerant has a low critical temperature, it becomes difficult to condense under high ambient conditions.
For this reason, refrigerants with a high critical temperature are preferred, as they can condense into liquid form even when operating in hotter climates.
This ensures reliable system performance and efficient heat rejection.
What is Heat?
Heat is a form of energy present in all objects on Earth, expressed in terms of both quantity and intensity.
The intensity of heat is measured as temperature, commonly in degrees Fahrenheit (°F) or Celsius (°C).
If all heat is removed from a substance, its temperature falls to –459.6 °F (–273.2 °C), known as absolute zero — the point at which all molecular motion ceases.
It is important to distinguish between the quantity of heat and the intensity of heat.
For example, desert sand contains a large quantity of heat, while a burning candle has a higher intensity of heat despite its smaller size.
Air‑conditioning and refrigeration systems rely on the principles of heat transfer to create cooling and heating effects. These principles are:
- Heat energy cannot be destroyed; it can only be transferred.
- Heat always flows from a higher‑temperature substance to a lower‑temperature substance.
- Heat transfer occurs through one of three processes: conduction, convection, or radiation.
To produce cooling, heat must be removed from a substance and transferred to another medium.
What is a ton of refrigeration?
A ton of refrigeration is a unit of cooling capacity. It is defined as the amount of heat required to melt one ton (2,000 lb or 907.18 kg) of ice in 24 hours.
This equals 288,000 BTU of heat absorbed per day, or 12,000 BTU per hour.
In practical terms, a one‑ton air conditioner has the capacity to remove 12,000 BTU of heat from a space every hour.
What is subcooling?
Subcooling is the process of cooling a refrigerant below its condensing (saturation) temperature so that it becomes a 100% liquid.
Any additional cooling of the liquid refrigerant beyond this point is referred to as subcooling.
What are the advantages of subcooling?
Subcooling provides several important benefits in a refrigeration system:
- Reduces flash gas formation – By cooling the refrigerant below its condensing temperature, subcooling minimizes vapor formation at the expansion valve, ensuring only liquid enters the evaporator.
- Increases evaporator capacity – Subcooled liquid has a lower specific volume, allowing more refrigerant mass to flow through the evaporator, which boosts cooling capacity.
- Improves efficiency – With more liquid refrigerant available for heat absorption, the evaporator extracts more heat from the conditioned space, enhancing overall system performance.
What is superheating?
Superheating is the process of adding heat to a refrigerant vapor after it has completely evaporated inside the evaporator coil.
In this state, the refrigerant absorbs additional heat from the surrounding air, raising its temperature above the saturation point without changing its phase.
This step is critical for protecting the compressor. Most refrigeration systems use positive‑displacement compressors, which can be severely damaged if liquid refrigerant enters the suction line.
Since liquid is incompressible, the refrigerant must enter the compressor only as a vapor.
By ensuring the refrigerant is superheated, the system guarantees safe operation and efficient performance.
Refrigeration cycle P-H diagram
A Pressure–Enthalpy (P‑H) diagram, also called a Mollier diagram, is a graphical tool used to represent the thermodynamic properties of a refrigerant.
It is widely used in analyzing and designing refrigeration cycles. The diagram clearly shows the four main processes of the cycle:
- C → D: Compression Low‑pressure vapor refrigerant enters the compressor at point C. It is compressed to a high‑pressure, high‑temperature vapor and discharged at point D.
- D → A: Condensation At constant high pressure, the refrigerant vapor rejects heat to the environment in the condenser. It condenses into a high‑pressure liquid at point A.
- A → B: Expansion (Throttling) The liquid refrigerant passes through the expansion device at constant enthalpy. The sudden pressure drop causes part of the liquid to flash into vapor (about 25% vapor, 75% liquid). This reduces the refrigerant temperature to the saturation temperature at the low‑side pressure.
- B → C: Evaporation The low‑temperature refrigerant absorbs heat from the environment inside the evaporator. It evaporates completely at constant pressure, returning as low‑pressure vapor to point C, ready to repeat the cycle.
Refrigeration cycle: HVAC system basics and refrigerant charging from r/HVAC
well done explanation for basic refrigeration system
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