Gas Engine Chillers   A gas engine chiller, as shown in the schematic above, mates a natural gas internal combustion engine to a compressor-driven chiller. The Natural Gas Engine Besides providing the energy needed to drive the compressor, the gas engine also generates heat which, if recovered, can be used for other purposes, such as water heating. The heat recovery feature is partly what makes gas engine chillers cost less to operate over electric chillers. The Vapour Compression Cycle The underlying process of any compressor-driven chiller is the vapour compression cycle. The vapour compression cycle relies on two principles. The first principle is that a fluid absorbs heat when changing from a liquid to a gas and releases heat when changing from a gas to a liquid. During this change in state, the temperature of the fluid does not change until all of the fluid is evaporated or condensed. The energy needed to change the state of a fluid is called latent energy or latent heat. A proportionately larger quantity of heat is needed to change state than to raise temperature. This is because when a fluid evaporates the molecular bonds that hold the fluid in a liquid state must be broken, and breaking molecular bonds requires a lot of energy. The second principle is that the boiling point of a liquid changes with pressure. The lower the pressure, the lower the boiling point. In fact, it is possible to boil a liquid without adding heat, but simply by sufficiently lowering the pressure it is under. Following is an explanation of the vapour compression cycle. 1. The Compressor Refrigerant enters the compressor as a gas. The compressor applies pressure to the gas, reducing its volume and raising its temperature. Compressing the refrigerant also raises the temperature at which the refrigerant will condense to liquid. After leaving the compressor the hot, compressed refrigerant gas travels to the condenser. 2. The Condenser In the condenser, cold water is used to cool the refrigerant gas and condense it to liquid. While the temperature of the refrigerant changes, the pressure of the refrigerant is the same as it was when it left the compressor. 3. The Expansion Valve After leaving the condenser, the high-pressure liquid refrigerant passes through an expansion valve that restricts the flow of refrigerant. The flow restriction reduces the pressure of the refrigerant. By reducing the pressure, the boiling point is lowered making the refrigerant more efficient at removing heat. As the pressure is reduced the refrigerant begins to expand and its temperature drops. 4. The Evaporator After flowing through the expansion valve the refrigerant enters the evaporator. Due to the lower pressure in the evaporator, some of the refrigerant immediately evaporates and absorbs heat from the system water, thus chilling it. As the refrigerant flows through the evaporator, more and more of it evaporates as heat is absorbed. By the time the refrigerant leaves the evaporator it is entirely gaseous. It is then piped to the compressor where the cycle begins again. back to "Gas Engine Chillers" article back to PilotLight online