Chillers – a common system in many industrial and power plants, as well as the average home air conditioner and refrigerator. The same basic process lies at the heart of each of them, commonly referred to as the “refrigeration cycle.” Warm fluid (air or water, generally) goes in, cold fluid comes out. The cold fluid is used to cool power plant components and processes, absorbing heat from them, and the warm fluid returns to the chiller to begin the process all over again. In theory, a fairly simple process. However, conducting power plant training so that plant operators understand the process by which a chiller works can provide multiple benefits to the power plant, including both an increased ability to detect equipment issues and prevention of malfunctions which could result in the release of refrigerant (which can in turn expose the company to substantial fines).
Most chillers, whether for home or industrial use, are typically mechanically driven vapor-compression cooling units. The same basic process lies at the heart of each of them, commonly referred to as the “refrigeration cycle.” Such units are made up of four main components: an evaporator (a heat exchanger, often a shell-and-tube type), a compressor, a condenser (another heat exchanger), and a thermostatic expansion valve, along with a refrigerant that can be evaporated completely to gas by the heat absorbed from the power plant’s cooling medium and condensed completely by the power plant’s heat-rejection system – a cooling tower or a river- or lake-based cooling water system.
In the evaporator, refrigerant in the heat-exchanger tubes is present in a mixture of liquid and vapor form, at saturation conditions (the combination of temperature and pressure at which addition of energy causes boiling of liquid or removal causes condensation of vapor). The power plant’s cooling medium flow through the shell side of the evaporator transfers heat into the refrigerant, adding enough energy to cause it to fully vaporize. To prevent immediate condensation of some amount of refrigerant due to losses in the line to the compressor, which could cause damage to the compressor, a small degree of superheat (warming to a temperature above the saturation temperature for current pressure) is usually designed into the system.
The hot gaseous refrigerant is drawn into the compressor inlet, where it is compressed as it flows to the condenser tubes. As a result of this pressurization, the vapor’s temperature is also raised. It is this compression that permits the chiller to remove heat from a space and reject it to an environment that is possibly warmer than the space or system being cooled. The addition of work to the process puts the refrigerant vapor at a temperature and pressure where it is able to be condensed by normal environmental conditions in the condenser shell.
The condenser is where heat from the power plant’s cooling medium is ultimately rejected to the environment in one way or another. A common arrangement is for cooling-tower water to flow through the condenser’s shell side, absorbing heat from the refrigerant which is inside the tubes. The design of the condenser will be such that all of the refrigerant will be condensed by the time it has traveled through the condenser. The liquid refrigerant, which is still pressurized and rather warm compared to the conditions in the evaporator, then leaves the condenser to head for the evaporator.
On its return to the evaporator to complete the cycle, the liquid refrigerant must be reduced in both pressure and temperature. This is performed by the thermostatic expansion valve (or sometimes with an orifice), which sets up an abrupt restriction to flow in the line. Liquid flowing through the valve will undergo a rapid depressurization, and as a result some of the liquid will flash to vapor, carrying away heat and lowering the temperature of the liquid and vapor refrigerant prior to it entering the evaporator. This cooled, lower-pressure saturated fluid (liquid/vapor mix) is now ready to absorb heat from the power plant’s cooling medium and flows back through the evaporator’s tubes, starting the cycle over again.
This is obviously only a short, simple explanation of an ideal chiller process, but starting with a good understanding of the basics of the refrigeration cycle is essential. Other considerations in the power plant training that your operators may undergo are legal requirements for work on a refrigerant system (most effective refrigerants are ozone-depleting substances, greenhouse-gas substances, or both, and are subject to EPA and state-level regulations), and any specifics about the operation of your particular equipment, as there are a variety of methods by which the basic chiller cycle can be accomplished. For best results in your power plant, a customized description of the process and equipment of your power plant would be required. These customized descriptions are typically obtained through the use of an outside training consultant, or generated in-house if the power plant’s training department can support it.
