Ideal Basic Refrigeration cycle

REFRIGERATION :

       

        Refrigeration or cooling process,is the removal of unwanted heat from a selected object,substance or space and its transfer to another object,substance, or space.Removal of heat lowers the temperature and may be accomplished by use of ice,snow,chilled water or mechanical refrigeration.

DEFINITION OF 1 TON OF REFRIGERATION :

         

          A refrigeration system moves heat from a space, fluid or material for the purpose of lowering its temperature. In the past, this was done by collecting ice in the winter and using its specific heat to cool as the ice melted.

1 ton of cooling is the effect of 1 ton of ice melting over a 24 hour period.

         When 1 pound of ice melts, it absorbs 144 Btu, as latent energy. When 1 ton (2000 lbs) melts over a 24-hour period:

                   Q = 2000 lbs × 144 Btu/lb/24 hrs

                       = 12,000 Btu/h

This is the definition of 1 ton of refrigeration.

Ideal Basic Refrigeration Cycle

          The ideal basic refrigeration cycle consists of four components, connected by piping with refrigerant flowing through the system. Figure 1 shows the components in the cycle and Figure 2 shows the basic cycle on the

P-h diagram.

1)Expansion device (or) Metering device

2)Evaporator

3)Condensor

4)Compressor

Figure 1 : Ideal Basic Refrigeration cycle

1)Expansion device (or) Metering device

          The expansion device or metering device is between points 4 and 1. In an ideal cycle, it drops the pressure and temperature of the refrigerant with no heat transfer to the surroundings (adiabatically).Point 4 can be identified by finding the properties of a saturated liquid at the condensing temperature or pressure. In an ideal cycle, the expansion process has constant enthalpy so the enthalpy at point 1 is the same as point 4. This can be used to calculate the quality of the refrigerant entering the evaporator.

2) Evaporator

          The evaporator is between points 1 and 2. In this component, the refrigerant starts as a cold, two-phase substance (part liquid, part vapor) and is boiled to a saturated gas by absorbing heat from the space/fluid/item that needs to be cooled.

          The evaporator introduces energy into the refrigerant from the air that is passing through the coil. The energy balance gives:

          q = m (h2 - h1)

Where:

q = heat absorbed, Btu/h (W)

h2 = enthalpy of the refrigerant at point 2, Btu/lb (kJ/kg)

h1 = enthalpy of the refrigerant at point 1, Btu/lb (kJ/kg)

There is a phase change associated with the evaporation process that occurs at a constant temperature and pressure (in an ideal cycle). To capture the total heat transfer, we use the mass flow rate multiplied by the change in enthalpy. In a real (non-ideal) evaporation process, there will be some pressure drop as the fluid moves through the evaporator which causes some temperature change in the fluid and therefor some sensible heat transfer.

3)Compressor

          The compressor is between points 2 and 3. The compressor does work on the refrigeration system (consumes energy). It raises the pressure, temperature and enthalpy of the refrigerant by compressing the saturated gas, in an isentropic process, to a superheated gas (i.e. entropy is constant – reversible process).

4)Condensor

          The condenser is between points 3 and 4. It cools the refrigerant until it condenses back into a (high-pressure) liquid by rejecting heat from the refrigerant to the surroundings.When complete, the refrigerant is a saturated liquid. The condenser rejects not only the heat gained in the evaporator but also the work of compression added by the compressor.

Low and medium temperature units

Ø  Walk in Refrigerator (or) cooler       = 35^o-37^o

Ø  Reach in Refrigerator (or) cooler     =38^o-40^o

Ø  Walk in freezer                               =10^o

Ø  Reach in freezer                             =0^o

P-H diagram of basic refrigeration cycle

 

Figure 2 : P-H diagram of ideal basic refrigeration cycle

v  In an ideal cycle, the expansion device is indicated as a vertical line on a Ph diagram because there is no change in enthalpy of the refrigerant. In a carnot cycle, the expansion process is both adiabatic and isentropic.

v  The blue line under the line 1-2 in a Ph diagram indicates the cooling effect.