HEAT PUMP CLASS NOTES
Heat engine converts heat into
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
ηheat engine = (TH—TL)/TL
COP refrigerator = TL/(TH–TL)
COP heat pump =Th/(TH–TL)
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
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.
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 Te and condenser absolute temperature Tc.
COP Reverse Carnot = COP Theoretical
= Te / (Tc – Te)= TL/(TH –TL)
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) = (TH – TL) / TH
TH high temperature is of the order of 5000 to 8000 C
TL low temperature is of the order of 600 to 1500 C
Q. WHAT IS A REFRIGERATOR?
Input is work and
Output is cooling in summer.
COP = TL/ (TH -TL)
Here TH is around 300 to 400 C
TL is around —150 C
Q. WHAT IS A HEAT PUMP?
Input is work and
Output is heating in winter.
COP = TH/ (TH -TL)
Here TH is around 600 to 800 C
TL 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.