Basic refrigeration cycle principles:
1.As refrigerant in the latent state or as vapor refrigerant in the process of changed state to liquid, this is the phase where it absorbs or rejects large quantities of heat. The quantities of heat absorbed or rejected can be managed by controlling the pressure and temperature of the refrigerant.
2.The boiling point of closed-system liquid can be controlled by changing the vapor pressure above it.
3.Gauge pressure is used to determine the pressure inside the closed refrigeration cycle system. It’s expressed in pounds per square inch gauge (psig).
4.Heat flows from a material at a higher temperature to a material at low temperature.
5.Heat energy is not created but converted and transferred.
The vapor compression refrigeration cycle is a common method for transferring heat from a low temperature to a high temperature.
The above figure shows the objectives of refrigerators and heat pumps. The purpose of a refrigerator is the removal of heat, called the cooling load, from a low-temperature medium.
The purpose of a heat pump is the transfer of heat to a high-temperature medium, called the heating load. When we are interested in the heat energy removed from a low-temperature space, the device is called a refrigerator. When we are interested in the heat energy supplied to the high-temperature space, the device is called a heat pump. In general, the term heat pump is used to describe the cycle as heat energy is removed from the low-temperature space and rejected to the high- temperature space.
The performance of refrigerators and heat pumps is expressed in terms of coefficient of performance (COP), defined as
Both COPr and COPHp can be larger than 1. Under the same operating conditions, the COPs are related by
This equation is true , Why because
Refrigerators, air conditioners, and heat pumps are rated with a SEER number or seasonal adjusted energy efficiency ratio. The SEER is defined as the Btu/hr of heat transferred per watt of work energy input. The Btu is the British thermal unit and is equivalent to 778 ft-lbf of work (1 W = 3.4122Btu/hr). An EER of 10 yields a COP of 2.9. Refrigeration systems are also rated in terms of tons of refrigeration. One ton of refrigeration is equivalent to 12,000 Btu/hr or 211 kJ/min.