Operation: The refrigeration cycle begins when a temperature sensor in the cold room detects that the temperature has risen above a set point (e.g., -20°C). This signal activates a solenoid valve, which allows refrigerant to flow. The compressor then draws in low-pressure, gaseous refrigerant from the evaporator and compresses it, raising its pressure to around 18 bar. This high-pressure gas is then cooled and condensed in the condenser, turning it into a liquid. The liquid refrigerant is collected in the receiver and passes through a filter and drier.

Next, the thermostatic expansion valve (TEV) controls the flow of this liquid refrigerant, throttling it down to a low pressure of about 1.7 bar, which corresponds to a saturation temperature of -20°C. This process converts the liquid into a mixture of gas and liquid. This cold mixture then enters the evaporator, where it absorbs heat from the surrounding area, cooling the room to the desired temperature. The refrigerant fully evaporates into a gas by the time it leaves the evaporator and returns to the compressor, completing the cycle. The solenoid valve closes when the room temperature drops below the set point, stopping the refrigerant flow and allowing the compressor to cut off.

Operating temperature is adjusted primarily through two methods:

• A refrigerant flow control valve: (often a solenoid valve) regulates the refrigerant flow rate into the evaporator. Reducing the flow reduces the amount of refrigerant evaporating, thus increasing the evaporator temperature (and therefore the space temperature). Increasing the flow has the opposite effect. A thermostatic expansion valve automatically adjusts the flow rate based on evaporator temperature.
• The thermostatic expansion valve: this includes an "adjusting screw" which allows for manual calibration of the superheat at the evaporator outlet. This fine-tunes the refrigerant flow and thus the temperature.

(i) High Ambient Temperature: If the ambient temperature is too high, the condenser cannot effectively dissipate heat. This means the refrigerant will not completely cool and condense into a liquid. As a result, gaseous refrigerant will accumulate in the condenser, increasing the pressure on the compressor's discharge side. When this pressure becomes excessively high, a high-pressure (HP) cut-out safety switch will activate, shutting down the compressor to prevent damage.

(ii) Gradual Loss of Gas: A gradual loss of refrigerant gas causes the system to become undercharged. Initially, the compressor will begin to short-cycle, meaning the time between its on and off cycles will decrease, and it will run for longer periods to try and maintain the set temperature. If the gas loss continues, the system will become severely undercharged, and the compressor will eventually trip on low suction pressure because there isn't enough refrigerant to create the necessary pressure in the evaporator. Consequently, the cold room temperature will not be maintained.

(iii) Dirty Heat Exchanger: A dirty heat exchanger, whether it's the condenser or evaporator, impairs heat transfer. In the case of a dirty condenser, the system's ability to reject heat is reduced. This leads to an accumulation of gaseous refrigerant, similar to the effect of high ambient temperature, which increases the discharge pressure. If the issue is severe enough, the compressor will trip on high discharge pressure. If the evaporator is dirty, it can't absorb heat from the room efficiently, reducing the cooling capacity and potentially causing the evaporator to ice up.