Vapour Compression Cycle You will Learn: 1 Vapour Compression Cycle Actual Vapour Compression Cycle Components in a Vapour Compression Plant Multistage Vapour Compression System

Vapour Compression Cycle Vapour compression refrigeration systems are most commonly used among all refrigeration systems. Vapour is the working fluid in this system. In this system refrigeration is obtained as the refrigerant evaporates at low temperatures. During evaporation the working fluid absorbs heat from the cold body, which is used as latent heat of vaporisation. Thus cooling effect is obtained. Vapour Compression Cycle 2

The fundamental processes involved in the vapour compression cycle are as follows: Vaporisation of liquid for cooling effect of the body Compression of vapour to increase its pressure Condensation of vapour to liquid at high pressure Expansion of liquid to reduce its pressure Vapour Compression Cycle 3 Simple Vapour Compression Cycle

Vapour Compression Cycle In an actual vapour compression cycle, the liquid refrigerant in the condenser may be sub-cooled before passing through the expansion valve. Undercooling of liquid refrigerant increases the refrigerating effect. Gas leaves the evaporator in superheated condition before it enters the compressor. Compression is assumed to be isentropic, but in actual practice it is very complex that is neither isentropic nor polytropic. Actual Vapour Compression Cycle 4

Vapour Compression Cycle As the refrigerant has to flow through pipes in condenser, evaporator and other connecting pipes, due to frictional resistance to flow there is a pressure drop in pipes. In an actual vapour compression cycle, the cylinder walls of the compressor are hotter than the incoming gases from the evaporator. Actual Vapour Compression Cycle 5

Vapour Compression Cycle Components in a Vapour Compression Plant 6 The components of a vapour compression refrigeration cycle are: 1.Compressor: Isentropic compression of saturated vapour takes place in a compressor. The shaft work / kg input is given by: wc = h2 – h1 where wc is shaft work, h1 and h2 are enthalpies at suction and delivery of compressor. 2. Condenser: Heat rejection at constant pressure takes place in a condenser. The heat removed is given by: q2 = h3 – h2 where q2 is the heat removed in the condenser, h3 is the enthalpy of liquid leaving the condenser h2 is the enthalpy of vapour entering the condenser

Vapour Compression Cycle 3.Throttling in throttle valve: Throttle expansion is a constant enthalpy process. There is no work or heat transfer in this process. 4.Evaporator: Heating at constant pressure takes place in an evaporator. The heat absorbed / kg is given by: q1 = h1 – h4 where q1 is the heat absorbed / kg in the evaporator, h1 is the enthalpy of vapour leaving the evaporator/kg h4 is the enthalpy of liquid entering the evaporator/kg Components in a Vapour Compression Plant 7

Vapour Compression Cycle Single stage vapour compression systems are not recommended, when the evaporator temperature becomes very low and when the condenser temperature becomes high. For overcoming these drawbacks multistage vapour compression systems are used in practice. Generally for fluorocarbon and ammonia based refrigeration system a single stage is used up to an evaporator temperature -30 degree centigrade. Whereas a two stage system is used up to -60 and a three stage system is used for temperatures below -60 degree centigrade. Multistage Vapour Compression System 8

Vapour Compression Cycle Multistage vapour compression systems are used when refrigeration is required at different temperatures. When the temperature lift of a single stage vapour compression refrigeration system increases, thereby work of compression also increases. Input to the vapour compression system is in the form of mechanical energy required to run the compressor. Hence these systems are also called as mechanical refrigeration systems. Summary 9