Components of an Industrial Chiller
The mechanical compression cycle has four basic components through which the refrigerant passes: (1) the evaporator (2) the compressor (3) the condenser (4) the expansion valve. The evaporator in the chiller will operate at a lower pressure and lower temperature than the condenser.
How an Industrial Chiller Works:
In an ideal cycle, the condenser serves as a two fold component. Before any condensation occurs, the high pressure vapor must be first brought 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 can begin. As heat continues to be transferred from the refrigerant vapor to the air (or water, if a water condenser is used), the quality of the refrigerant (% of the refrigerant in the vapor state) will continue to decrease, until the refrigerant has been completely condensed. In the ideal system, this occurs at the outlet of the condenser. In the real world, some subcooling would be expected at the condenser outlet. Subcooled 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 vapor 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. Subcooling or superheat cannot exist where there is a mixture of liquid and vapor. 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 from Sporlan, Inc.)
Contact Cooling Technology, Inc and one of our technical engineers will help you design one to meet your custom needs.
Evaporators in Industrial Chillers
Shell & Tube Evaporators: Typically configured with the refrigerant flowing through the tube side and the process fluid through the shell side of the unit. Tube construction can be copper, cupronickel, or stainless steel. Shell construction can be cast iron, brass, cupronickel, or stainless steel. This style of evaporator has lower pressure drops than Brazed Plate technology and is available up to 500 tons in standard construction.
Brazed Plate Evaporators: Cooling Technology, Inc was one of the first in the United States to utilize this innovative design in heat transfer. The major advantages of this technology are the supreme efficiency that is achieved, lower costs, and 316L stainless steel or copper construction. This style is available from fractional tonnages to 140 tons.
Compressor Types in Industrial Chillers
Reciprocating Compressor: A reciprocating compressor is a compressor that uses pistons driven by a crankshaft. It is used for delivering a small amount of refrigerant at a very high pressure. Reciprocating compressors may carry a dual voltage and range from three to sixty horsepower. Reciprocating compressors are usually semi – hermetic compressors, which simply means that they are serviceable.
Centrifugal Compressor: Centrifugal compressors have few moving parts which make them a favorite in the industry. They are also very energy efficient and give a higher refrigerant flow than a similarly sized reciprocating compressor. Centrifugal compressors are more suited to higher volume but low pressure applications, such as those that use ventilation fans, cooling units, and air movers. The centrifugal compressor operates by using the centrifugal force applied to an air mass to achieve compression. Typical capacities range from sixty to several hundred tons. These are hermetically sealed, magnetic bearing compressors and are 230 volts OR 460 volts.
Screw Compressor: The screw compressor is almost exactly as it sounds. There are two screws (male & female) in a screw compressor that are fitted together in stationary housing. As the rotors rotate, the gas is compressed by direct volume reduction between the two rotors. These compressors are also semi – hermetically sealed compressors, carry a dual voltage, and range from 40 – 1000 horsepower. There are also single screw compressors which rely on a single rotating screw passing through two star wheels to provide the compression.