REFRIGERATOR/HEAT ENGINE/ 

HEAT PUMP CLASS NOTES

Heat engine converts heat into

mechanical motion/electricity.

Refrigerator produces cooling effect.

Heat pump produces heating effect.

 

Q. COMPARE HEAT ENGINE, REFRIGERATOR AND A HEAT PUMP WHEN WORKING ON CARNOT CYCLE OR REVERSE CARNOT CYCLE

Heat Engine

η_{heat engine} = (T_H—T_L)/T_L

COP _refrigerator  = T_L/(T_H–T_L)

COP _{heat pump} =T_h/(T_H–T_L)

In the case of a heat engine, higher temperature is say > 500⁰ C and lower temperature may be around 100 ^o C

Q. What is COP of a Refrigerator?

Coefficient of Performance is the ratio of the amount of cooling provided to the energy consumed by the system

COP _R=N/W

Where N is the cooling effect    k J/kg Or k J/min

W is work input to compressor  k J/kg Or k J/min

In the case of a refrigerator, higher temperature is say > 40⁰ C and lower temperature may be around –15 ⁰ C

 Q. What is COP of a Heat Pump?

Coefficient of Performance is the ratio of the amount of heating provided to the energy consumed by the system

COP _HP= (N+W)/W

In the case of a heat pump, higher temperature is say > 60⁰ C and lower temperature may be around 5⁰ C

 Q. What is a Reverse Carnot Cycle?

REVERSE Carnot Cycle is a refrigeration cycle. Reversed Carnot cycle also consists of four processes i.e. reversible adiabatic, isothermal, reversible adiabatic and isothermal processes.

In this

Input is work and

Output is cooling or Output is heating.

 Q. WHAT ARE THE LIMITATIONS OF REVERSE CARNOT CYCLE?

Iso-thermal process is extremely slow and

isen-tropic process is extremely fast.

This alternate combination of very fast, very slow, is not possible. It becomes the limitations of this cycle.

Q.   How to achieve Reverse Carnot Cycle in actual practice?

The only alternatives for this cycle to achieve are

(i) That there are two iso-thermal phase change processes

(ii) Use two isen-tropic process ( achievable in a high speed compressor and a high speed turbine )

The four processes are in the following sequence.

Iso-thermal phase change process             (Evaporation)

High speed compressor                                (Compression)

Iso-thermal phase change process              (Condensation)

High speed turbine for expansion               (Expansion)

Q.   WHAT IS COEFFICIENT OF PERFORMANCE OF A REVERSE CARNOT CYCLE?

COP _Carnot a measure of refrigeration efficiency of a refrigeration system working on reverse Carnot cycle.

It is also called theoretical Coefficient of Performance of Reversed Carnot Cycle

It depends on  system two key absolute temperatures only.

Evaporator absolute temperature T_e and condenser absolute temperature T_c.

COP _{Reverse Carnot}   = COP _Theoretical

                                  =    T_e / (T_c – T_e)= T_L/(T_H –T_L)

This COP _{Car not} uses only two absolute temperatures, and does not take into account

(i)  The type of compressor/condenser/expansion device/evaporator and

(ii) Properties of the working fluid.

Q. What is EPA?

EPA is Environment Protection Agency. It deals with

(i) All the problems of environment and their overall effect  on living beings

(ii) Suggests the remedial measures for the environment protection.

Q. What is a heat engine?

In a heat engine

INPUT IS HEAT and

OUTPUT IS WORK OR MOTION OR ELECTRICITY

Efficiency of a heat engine (Carnot Cycle)  = (T_H – T_L) / T_H

T_H high temperature is of the order of 500⁰ to 800⁰ C

T_L low temperature is of the order of 60⁰ to 150⁰ C

 Q. WHAT IS A REFRIGERATOR?

In this

Input is work and

     Output is cooling in summer.

COP = T_L/ (T_H -T_L)

Here T_H is around 30⁰ to 40⁰ C

T_L is around   —15⁰ C

 Q. WHAT IS A HEAT PUMP?

In this

Input is work and

Output is heating in winter.

COP = T_H/ (T_H -T_L)

Here T_H is around 60⁰ to 80⁰ C

T_L is around   0 to 5⁰ C

Q. COMPARISON OF HEAT ENGINE/REFRIGERATOR/HEAT PUMP

η of a heat engine =output/input = (heat supplied – heat rejected) / heat supplied

It is less than 1.

 COP of a refrigerator = Cooling effect/work input = N/W

It is normally much greater than 1 for vapor compression refrigeration systems.

COP of a heat pump = heating effect/work input = (N+ W)/W.

=1+N/W=COP _refrigerator + 1

It is still far greater than 1

Q. Describe Carnot Cycle.

Carnot cycle is a heat engine cycle. It is an ideal cycle. Carnot cycle consists of four processes. These are isothermal, reversible adiabatic, isothermal and reversible adiabatic processes. It is most efficient cycle IF ACHIEVED IN ACTUAL PRACTICE. This is not achievable because of alternate very slow (isothermal) and very fast (Isen-tropic) processes. It is thus a reference cycle for judging the performance of other heat engine cycles.