Heat Pump Thermodynamics
All the heat pumps, air conditioners, and refrigerators utilize heat transfer from cold to hot. Basically they use the heat engines that run backward. Here we are saying backward instead of reverse, because apart from Carnot engines, every other heat engine usually runs backwards, and they cannot be reversed. In these cases heat transfer occurs from a cold reservoir Qc into a hot one. This requires work input “W”, which is also converted to heat transfer process. Thus the heat transfer to the hot reservoir is Qh = Qc + W. (remember that Qh, Qc, and W are positive, with their directions indicated on schematics rather than by sign.)
A heat pump’s mission is to transfer heat Qh to occur into a warm environment, such as in any home in winter. The sole mission of air conditioners and refrigerators is for heat transfer Qc to occur from a cool environment, such as chilling a room or keeping food at lower temperatures than the environment. In fact, a heat pump can be used for both purposes, that is to heat and cool any space. It is very important to have an air conditioner and a heating unit all in one.
In these heat exchanges, the transfer can occur through three different processes, which are used by the refrigeration industry or have an impact on its efficiency, these processes include:
• Convection: The convection is basically the most present process in the refrigeration equipment you are dealing with. It usually occurs mostly in the fluids (Liquids and Gas). It is the outcome of a fluid circulation, which may happen either naturally, due to the differences in the temperature of the fluid or may be in the forced way. The heat exchange that happens on the evaporator and the condenser are the examples of convection.
• Conduction: well, the conduction usually happens between two objects that have somewhat different temperatures from each other, or it may also happen in only one object, but it always occurs from the hottest to the coldest area. It is generally related to the thermal conduction of every material. Regarding this process, it is very vital to remember that what defines a heat insulator is its low thermal conductivity, which is considered very essential for an efficient cooling system. For example, all those materials such as polyurethane that are used to insulate cabinets, keeping the internal temperature of the refrigerator lower than the external environment.
The irradiation is not directly connected to refrigeration but has an impact on the performance of the equipment. It occurs through electromagnetic waves, particularly infrared radiation, even without direct contact between bodies or substances. An example is the warming of the Earth by the sun, where there is no direct contact, but there is heat transfer.
• Radiation: Radiation is very important in order to keep refrigeration equipment away from any types of heat sources, so that the heat does not affect its performance in any way.
