VAPOUR COMPRESSION REFRIGERATION SYSTEMCARNOT REFRIGERATION CYCLE

The Carnot cycle refrigeration performs the reverse effect of the heat engine because it transfers energy from a low level of temperature to a high level of temperature. The refrigeration cycle requires the addition of external work for its operation. The diagram of the equipment and the temperature entropy diagram of the refrigeration cycle.

The process which constitute the cycle are:

1-2 Adiabatic compression.

2-3 Isothermal rejection of heat.

3-4 Adiabatic expansion.

4-1 Isothermal addition of heat.

All the process in the Carnot cycle are thermodynamically reversible .Process 1-2 and 3-4 isentropic.

The withdrawal of heat from low temperature source in process 4-1is the refrigeration step and is the entire purpose of the cycle. All the other processes in the cycle function so that the low temperature energy can be discharged to some convenient high temperature sink.

VAPOUR COMPRESSION CYCLE

The vapour compression cycle is the most widely used refrigeration cycle in practice. In this cycle vapour is compressed, then condensed to a liquid, following which the pressure so that fluid can evaporate at low pressure.

The standard vapour compression cycle is shown in temperature enthalpy diagram.

The process constituting the standard vapour compression cycle are:

1-2 Reversible and adiabatic compression from saturated vapour to condenser pressure.

2-3 Reversible rejection of heat at constant pressure, causing de-superheating and condensation of refrigerants;

3-4 Irreversible expansion at constant enthalpy from saturated liquid to the evaporated pressure;

4-1 Reversible addition of heat at constant pressure causing evaporation to saturated vapour.

PERFORMANCE OF THE STANDARD VAPOUR COMPRESSION CYCLE

With the help of pressure-enthalpy diagram, the significant quantities of the standard vapour compression cycle will be determined. These quantities are the work of compression, heat rejection rate, refrigerating effect, COP, volume rate of flow per KW of refrigeration and power per KW of refrigeration.

The work of compression in KJ per kg is the changes in enthalpy in process 1-2 of (H1-H2).The difference in enthalpy is a negative quantity, indicating the work is done on the system. The heat rejection in KJ/kg is the heat transferred from the refrigerants in process 2-3,which is (H3-H2).The value of H3-H2 is negative, indicating that the heat is transferred from the refrigerant .The value of the heat rejection used in sizing the condenser and calculating the required flow quantities of the condenser cooling fluid.

The refrigerants effect in KJ/kg is the heat transferred in process 4-1, or H1-H4

.

The COP of the standard VCRS is the refrigerating effect is divided by the work of the compression:

COP=(H1 -H4 )/(H2 -H1 )

The volume flow rate in m3 /s is computed at the compression inlet or state point 1, the volume flow rate is the rough indication of physical size of the compressor. The power per KW of refrigerant is the inverse of the COP, and efficient refrigeration system has low value of power per KW of refrigeration but high COP.

ACTUAL VAPOUR COMPRESSION CYCLE

The actual vapour compression cycle suffer from insufficiencies compared with the standard cycle. There are also other changes from standard cycle, which may be intentional or unavoidable.

The essential difference between actual and standard cycle appear in pressure drops condenser and evaporator, in the sub cooling of liquid leaving the condenser, and in the superheating of evaporator leaving the

evaporator .The standard cycle assumes no drop in pressure in condenser and evaporator. Because of friction, however, the pressure of the refrigerants drops in actual cycle. The results of these drops in pressure are that compression process between 1-2 require more work than the standard cycle. Sub cooling of the liquid in the condenser is a normal occurrence and serves the desirable functions of insuring that hundred percent liquid enter the expansion valve. Superheating of the vapour usually occur in the evaporator and is recommended as a precautions against droplet of liquid being carried over in the compressor. The final difference in actual cycle is that the compression is no longer isentropic and there are inefficiencies due to friction and other losses.

DEPARTMENT OF MECHANICAL ENGINEERINGNATIONAL INSTITUTE OF TECHNOLOGY, RAIPUR

CERTIFICATEThis is to certify that the project work titled “DESIGN OF CAPILLARY TUBE FOR AN AIR CONDITIONER” submitted by Nikita Mishra (07119030), Sourabh Das (07119049), Tushar Raj (07119052) , Vikram Kumar Yadav (07119055), Ramlesh students of B Tech final year of Mechanical Engineering during the academic year 2010-11 in partial fulfillment of the requirements for the award of the degree of Bachelor of Technology ,Raipur is a presentation of work done by them. This certification does not necessarily endorse or accept any statement made, opinion expressed or conclusion drawn as recorded in the report. However , it only signifies the acceptance of the report for the purpose for which it is submitted.

APPROVED BY: Under The Guidance of:

Dr. S.Sanyal Mr R.Salhotra

H.O.D Reader,

Deptt. Of Mechanical Engineering, Deptt. Of Mechanical Engineering,

National Institute of Technology, National Institue of Technology,

Raipur Raipur

Acknowledgement

We would like to extend our profound gratitude to our guide, Mr.

R.SALHOTRA for his kind help during initial stages of our work. We are

deeply indebted to him for his valuable guidance, suggestions and constant

encouragements throughout the work. It is our pleasure to acknowledge the

cooperation given by other staff members of Mechanical Engineering

Department.

No work can be completed without the encouragement of family

members and friends. We thank them all for always being there for us. We

thank all people involve directly or indirectly in completion of work.