Chiller Knowledge Base — FAQs, Guides & Technical Resources

First, the condenser brings the high pressure vapour to a saturated condition (de- superheated). Enough heat must be transferred from the refrigerant to lower its temperature to the saturation temperature.

At this point, condensation begins. As heat continues to be transferred from the refrigerant vapour to the air (or water, if it is a water cooled chiller), the quality of the refrigerant (% of the refrigerant in the vapour state) will continue to decrease, until the refrigerant has been completely condensed.

In an ideal system, this occurs at the outlet of the condenser. In the real world, some kind of sub-cooling can be expected at the condenser outlet. Sub-cooled liquid provides insurance against liquid flashing as the refrigerant experiences pressure losses in the tubing and components.

The refrigerant is in the liquid state now, and at a high pressure and temperature. It must undergo one more change before it becomes a useful heat transfer medium; a reduction in temperature. This is accomplished by reducing the pressure.
You can count on the refrigerant’s pressure – temperature relationship to be an infallible law. If the pressure of a saturated liquid is reduced, the law governing its existence requires it to assume the saturation temperature at the new pressure.

So, in order to reduce the temperature, the pressure has to be reduced, and some sort of restriction is required for this to occur. It would be preferable if the restriction could regulate itself as the system load demands change. This is exactly what the thermostatic expansion valve does; it is an adjustable restriction which causes a reduction in liquid refrigerant pressure, yet will modulate in an effort to maintain constant superheat at the evaporator outlet.
The thermostatic expansion valve is a superheat control, and will not maintain a constant vapour pressure. It only provides the restriction necessary to reduce the pressure to some level, which will be determined by compressor size, thermostatic expansion valve, size load, load demand and system conditions.

If a constant evaporator temperature is required, it can be achieved very simply by maintaining the pressure corresponding to the saturation temperature required.
This is accomplished by adding an evaporator pressure regulating valve to the system. Our ideal cycle has experienced a pressure drop in the thermostatic expansion valve. Sub-cooling or superheat cannot exist where there is a mixture of liquid and vapour. Therefore any place in the system where the refrigerant exists in two states, it will be at the saturation temperature for its pressure.
Some of the liquid refrigerant is required to boil as a means of removing the heat necessary to achieve this lower temperature. Yet another heat transfer process, which yields a lower liquid temperature.

The liquid that is sacrificed in the boiling process explains the increase in refrigerant quality. The greater the difference between the liquid temperature and evaporator temperature, the more liquid will have to be boiled in order to achieve the new saturation temperature. This results in an even higher refrigerant quality.

The final portion of the refrigerant’s journey is as a mixture of saturated liquid and vapor, traveling through the evaporator tubing. Warm air is blown across the evaporator, where its heat content is transferred to the boiling refrigerant.
This is a latent heat gain to the refrigerant, causing no temperature increase, while experiencing a change of state. In the ideal cycle, the last molecule of saturated liquid boils off at the evaporator outlet, which is connected to the compressor inlet.
Hence, the vapor at the inlet of the compressor is saturated. The cycle continues this way until the refrigerated space temperature is satisfied, and the equipment cycles off. (Information Courtesy: Sporlan, Inc.)