Heat pump and refrigeration cycle – wikipedia gas kansas


In the early years of the twentieth century, the vapor absorption cycle using water-ammonia systems was popular and widely used but, after the development of the vapor compression cycle, it lost much of its importance because of its low coefficient of performance (about one fifth of that of the vapor compression cycle). Nowadays, the vapor absorption cycle is used only where heat is more readily available than electricity, such as waste heat provided by solar collectors, or off-the-grid refrigeration in recreational vehicles.

The absorption cycle is similar to the compression cycle, except for the method of raising the pressure of the refrigerant vapor. In the absorption system, the compressor is replaced by an absorber which dissolves the refrigerant in a suitable liquid, a liquid pump which raises the pressure and a generator which, on heat addition, drives off the refrigerant vapor from the high-pressure liquid. Some work is required by the liquid pump but, for a given quantity of refrigerant, it is much smaller than needed by the compressor in the vapor compression cycle. In an absorption refrigerator, a suitable combination of refrigerant and absorbent is used. The most common combinations are ammonia (refrigerant) and water (absorbent), and water (refrigerant) and lithium bromide (absorbent). Gas cycle

When the working fluid is a gas that is compressed and expanded but does not change phase, the refrigeration cycle is called a gas cycle. Air is most often this working fluid. As there is no condensation and evaporation intended in a gas cycle, components corresponding to the condenser and evaporator in a vapor compression cycle are the hot and cold gas-to-gas heat exchangers.

For given extreme temperatures, a gas cycle may be less efficient than a vapor compression cycle because the gas cycle works on the reverse Brayton cycle instead of the reverse Rankine cycle. As such, the working fluid never receives or rejects heat at constant temperature. In the gas cycle, the refrigeration effect is equal to the product of the specific heat of the gas and the rise in temperature of the gas in the low temperature side. Therefore, for the same cooling load, gas refrigeration cycle machines require a larger mass flow rate, which in turn increases their size.

Because of their lower efficiency and larger bulk, air cycle coolers are not often applied in terrestrial refrigeration. The air cycle machine is very common, however, on gas turbine-powered jet airliners since compressed air is readily available from the engines’ compressor sections. These jet aircraft’s cooling and ventilation units also serve the purpose of heating and pressurizing the aircraft cabin. Stirling engine

The Stirling cycle heat engine can be driven in reverse, using a mechanical energy input to drive heat transfer in a reversed direction (i.e. a heat pump, or refrigerator). There are several design configurations for such devices that can be built. Several such setups require rotary or sliding seals, which can introduce difficult tradeoffs between frictional losses and refrigerant leakage. Reversed Carnot cycle

Since the Carnot cycle is a reversible cycle, the four processes that comprise it, two isothermal and two isentropic, can all be reversed as well. When this happens, it is called a reversed Carnot cycle. A refrigerator or heat pump that acts on the reversed Carnot cycle is called a Carnot refrigerator and Carnot heat pump respectively. In the first stage of this cycle (process 1-2), the refrigerant absorbs heat isothermally from a low-temperature source, T L, in the amount Q L. Next, the refrigerant is isentropically compressed (process 2-3) and the temperature rises to the high-temperature source, T H. Then at this high temperature, the refrigerant rejects heat isothermally in the amount Q H (process 3-4). Also during this stage, the refrigerant changes from a saturated vapor to a saturated liquid in the condenser. Lastly, the refrigerant expands isentropically where the temperature drops back to the low-temperature source, T L (process 4-1). [2] Coefficient of performance

This implies that COP HP will be greater than one because COP R will be a positive quantity. In a worst-case scenario, the heat pump will supply as much energy as it consumes, making it act as a resistance heater. However, in reality, as in home heating, some of Q H is lost to the outside air through piping, insulation, etc., thus making the COP HP drop below unity when the outside air temperature is too low. Therefore, the system used to heat houses uses fuel. [2]