As through the metering device into the evaporator.

As we can see from the image above, the path represents the refrigeration cycle comprising of condenser, compressor, evaporator and metering device. The refrigerant flows through the system absorbing heat from the cold-room. Starting from the compressor, the refrigerant goes to condenser and cools down the refrigerant which them goes through the metering device into the evaporator.   

To obtain the desired heat transfer conditions at appropriate points in the cycle the system is divided into a low-pressure side and a high-pressure side. On the low-pressure side the drop in refrigerant pressure reduces its saturation temperature to below that of its surroundings. Heat transfers to the liquid refrigerant causing it to evaporate and absorb energy called the latent heat of vaporization.

On high-pressure side the increase in refrigerant pressure raided its saturation temperature to above that of its surroundings. The refrigerant condenses and gives up the heat collected on the low-pressure side of the system, plus compressor power.

System Description

Heat pumps utilize the same refrigerant and basic components as a cooling only system. However, they also have a 4-way valve and non-return check valves which enable the system to switch from cooling to heating mode.

To do this, the 4-way valve reverses the evaporator and condenser functions:

·        The heat exchanger in the outdoor unit becomes an evaporator and absorbs heat from the outside air.

·        The heat exchanger in the indoor unit becomes a condenser and gives up heat to the air inside the building.


1.      Refrigerant

Both cooling only and heat pump split systems utilize a refrigerant to transfer heat from one place to another. Refrigerant is a substance with the ability to rapidly change from a vapor to a liquid within specific temperature ranges. It absorbs heat in the evaporator by changing from a liquid to a vapor and releases its heat in the condenser by changing from a vapor to a liquid. The two most common refrigerants supplied in new systems are the non-ozone depleting HFCs R410A and R407C.

2.      Evaporation

In the cooling system installed, the refrigerant is admitted into evaporator through a metering device. As this happens it changes from a high-pressure/high-temperature liquid into a slightly vaporized, low-pressure/low-temperature liquid. This pressure reduction means that the refrigerant’s saturation temperature is significantly lower than the indoor temperature. Consequently, when indoor air is drawn across the evaporator’s surface the refrigerant evaporates and absorbs huge amounts of heat energy from air. The cooled air is then distributed in the room.


3.      Compression

After evaporation the refrigerant passes through a copper pipe to the compressor located in the outdoor unit of the system. The most common types of compressor used in split systems are of a rotary scroll design. Refrigerant enters the compressor as a slightly superheated, low-pressure/low-temperature vapor. The compression process then pressurizes the refrigerant vapor and raises its saturation temperature to above that of the outside air.


4.      Condensation

When the refrigerant has been compressed it passes to the condenser. Refrigerant enters the condenser as a superheated, high-pressure/high-temperature vapor with a saturation temperature significantly higher than the outdoor temperature. Consequently, when outdoor air is drawn across the condenser tubes the refrigerant condenses and gives up the heat collected indoors to the air. By the time refrigerant has reached the end of the condensation stage it has transformed into a high-pressure/high-temperature liquid.


5.      Expansion

After condensation the refrigerant passes through a copper pipe to a metering device and onwards to the evaporator located in the indoor unit. The metering device controls the flow of refrigerant into the evaporator and ensures that the amount of refrigerant admitted is sufficient for the cooling load. The refrigerant enters the metering device as a high-pressure/high-temperature liquid and through a process of expansion leaves as a slightly vaporized, low-pressure/low-temperature liquid i.e. in a condition suitable for evaporation.