Heat pump and refrigeration cycle

Thermodynamic heat pump and refrigeration cycles are the models for heat pumps and refrigerators. The difference between a heat pump and a normal air conditioner is that a heat pump can be used to heat a home as well as cool it. Even though the heat pump can heat, it still uses the same basic refrigeration cycle to do this. In other words a heat pump can change which coil is the condenser and which the evaporator. In cooler climates it is common to have heat pumps that are able only to heat the house, making the pumps simpler and cheaper, since cooling is rarely necessary.

Heat pumps

A heat pump has the ability to pump heat two ways. This allows the heat pump to both bring heat into a house and take it out. In the cooling mode a heat pump works the same as your typical A/C. It uses an evaporator to absorb heat from inside the house and rejects this heat outside through the condenser. The refrigerant is flowing in the normal direction to where the condenser and compressor are outside of the space to be conditioned, while the evaporator inside. The one key component that makes a heat pump different from an A/C is the reversing valve. The reversing valve allows for the flow direction of the refrigerant to be changed. This allows the heat to be pumped in either direction.

In the heating mode the outdoor coil becomes the evaporator, while the indoor becomes the condenser. The air outside even at 0 degrees has heat in it. With the refrigerant flowing in the opposite direction the evaporator(outdoor coil) is absorbing the heat from the air and moving it inside. Once it picks up heat it is compressed and then sent to the condenser(indoor coil). The indoor coil then rejects the heat into the air handler, which moves the heated air through out the house.

In the cooling mode the outdoor coil is now the condenser. This makes the indoor coil now the evaporator. The indoor coil is now the evaporator in the sense that it is going to be used to absorb the heat from inside the enclosed space. The evaporator absorbs the heat from the inside, and takes it to the condenser where it is rejected into the outside air.

Many heat pumps also use an auxiliary heat source. This means that even though the heat pump is the primary source of heat, but another form is put in place as a back-up. Electric, oil, or gas are the most commons sources. This is put in place so that if the heat pump fails or can't provide enough heat, the auxiliary heat will kick on to make up the difference.

Geothermal heat pumps are another form. They use the ground and water to work as the condensers and evaporators. They work in the same manner as an air to air heat pump, but instead of indoor and outdoor coils they use the earth as natural evaporators and condensers. These are very eco-friendly and are a cheaper alternative in the long run due to lower operating cost.

A heat pump is a very efficient appliance. It has an efficiency rating of 200%. This makes a heat pump one of the cheapest ways to heat. With a heat pump you also get the best of both worlds. Instead of spending money on both a furnace and a central air system you get both in a heat pump.

Heat naturally flows from hot to cold. Work is applied to cool a living space or storage volume by pumping heat from a lower temperature heat source into a higher temperature heat sink. Insulation is used to reduce the work and energy required to achieve and maintain a lower temperature in the cooled space. The operating principle of the refrigeration cycle was described mathematically by Sadi Carnot in 1824 as a heat engine.


Heat pump can be classified as:

Vapor cycle,
Gas cycle, and
Stirling cycle
Vapor cycle refrigeration can further be classified as:

Vapor compression refrigeration
Gas absorption refrigeration

Vapor-compression cycle
(See Vapor-compression refrigeration for more complete technical details)
The vapor-compression cycle is used in most household refrigerators as well as in many large commercial and industrial refrigeration systems. Figure 1 provides a schematic diagram of the components of a typical vapor-compression refrigeration system.


Figure 1: Vapor compression refrigerationThe thermodynamics of the cycle can be analyzed on a diagram[1][2] as shown in Figure 2. In this cycle, a circulating refrigerant such as Freon enters the compressor as a vapor. From point 1 to point 2, the vapor is compressed at constant entropy and exits the compressor superheated. From point 2 to point 3 and on to point 4, the superheated vapor travels through the condenser which first cools and removes the superheat and then condenses the vapor into a liquid by removing additional heat at constant pressure and temperature. Between points 4 and 5, the liquid refrigerant goes through the expansion valve (also called a throttle valve) where its pressure abruptly decreases, causing flash evaporation and auto-refrigeration of, typically, less than half of the liquid.


Figure 2:Temperature–Entropy diagramThat results in a mixture of liquid and vapor at a lower temperature and pressure as shown at point 5. The cold liquid-vapor mixture then travels through the evaporator coil or tubes and is completely vaporized by cooling the warm air (from the space being refrigerated) being blown by a fan across the evaporator coil or tubes. The resulting refrigerant vapor returns to the compressor inlet at point 1 to complete the thermodynamic cycle.

The above discussion is based on the ideal vapor-compression refrigeration cycle, and does not take into account real-world effects like frictional pressure drop in the system, slight thermodynamic irreversibility during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).

More information about the design and performance of vapor-compression refrigeration systems is available in the classic "Perry's Chemical Engineers' Handbook".[3]


[edit] Vapor absorption cycle
(See gas absorption refrigerator for more details)
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 waste heat is available or where heat is derived from solar collectors.

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).


[edit] Gas cycle
When the working fluid is a gas that is compressed and expanded but doesn't 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 in gas cycles.

The gas cycle is less efficient than the vapor compression cycle because the gas cycle works on the reverse Brayton cycle instead of the reverse Rankine cycle. As such the working fluid does not receive and reject 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, a gas refrigeration cycle will require a large mass flow rate and would be bulky.

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 pressurizing the aircraft cabin.