Dinc Ibrahim. Refrigeration systems and applications 2th edition
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Heat Pumps 371
Study Problems
Heat Pumps – Concept Questions
6.1 What is a heat pump?
6.2 What is the similarity and the difference between a heat pump and a refrigerator?
6.3 What are bivalent and monovalent modes of heat pump operation?
6.4 For which city will a heat pump be more cost-effective during winter season? Chicago or
Istanbul? Why?
6.5 Consider a heat pump operating entirely on back-up electric resistance heater because
the ambient temperature is very low. What is the COP of the heat pump during
this operation?
6.6 Define primary energy ratio (PER) and explain its use.
6.7 What is the relationship between COP and energy efficiency ratio (EER)?
6.8 What is heating season performance factor (HSPF)?
6.9 What is seasonal energy efficiency ratio (SEER)?
6.10 Classify heat pumps for residential heating and cooling depending on their operational
function.
6.11 What are the characteristics of absorption heat pumps?
6.12 What are the commonly used heat sources in heat pump applications?
6.13 What are the basic advantages and disadvantages of using groundwater as a heat source?
6.14 Classify heat pumps according to heat source and type of heat carrier.
6.15 Describe the operation of a solar heat pump with a schematic.
6.16 Provide a list of heat pump cycles.
6.17 What are the two basic types of geothermal heat pump systems?
6.18 What are the four basic GSHP open and closed-loop designs?
6.19 What are hybrid heat pump systems? How does it operate?
Heat Pump Efficiencies
6.20 A heat pump is driven by a natural gas-fired internal combustion engine, which has a
thermal efficiency of 35%. The COP of the heat pump is 2.5. Determine the primary
energy ratio.
6.21 A heat pump is driven by a natural gas-fired internal combustion engine and it supplies heat
to a building at a rate of 130,000 kJ/h. The engine has a thermal efficiency of 42% and it
consumes 2 kg of natural gas in 1 hour. The heating value of natural gas is 50,050 kJ/kg.
Determine the primary energy ratio.
372 Refrigeration Systems and Applications
6.22 Consider the following heating systems and determine the primary energy ratio in each
case.
(a) Heat pump is driven by an internal combustion engine with η = 0.33, COP = 2.6.
(b) Heat pump is driven by a combined power plant with η = 0.50, COP = 3.3.
(c) A natural gas furnace with η = 0.90.
Energy Analysis of Heat Pumps
6.23 A heat pump is used to keep a room at 20
◦
C by rejecting heat to an environment at 8
◦
C.
The total heat loss from the room to the environment is estimated to be 24,000 kJ/h and the
power input to the compressor is 2.2 kW. Determine (a) the rate of heat absorbed from the
environment in kiloJoules per hour, (b) the COP of the heat pump, and (c) the maximum
rate of heat supply to the room for the given power input.
6.24 A heat pump system operates between temperature limits of −5 and 18
◦
C. The heating
load of the space is 48,000 Btu/h and the COP of the heat pump is estimated to be 1.7.
Determine (a) the power input, (b) the rate of heat absorbed from the cold environment,
and (c) the maximum possible COP of this heat pump.
6.25 A room is kept at 25
◦
C by an 18,000 Btu/h (on heating basis) split air conditioner
when the ambient temperature is 6
◦
C. The air conditioner is running at full load
under these conditions. The power input to the compressor is 2.4 kW. Determine (a)
the rate of heat removed from the ambient air in British thermal units per hour, (b)
the COP of the air conditioner, and (c) the rate of heating supplied in British thermal
units per hour if the air conditioner operated as a Carnot heat pump for the same
power input.
6.26 Water is continuously heated by a heat pump from 20 to 40
◦
C. The heat absorbed in the
evaporator is 34,000 kJ/h and the power input is 3.8 kW. Determine the rate at which water
is heated in liters per minute and the COP of the heat pump. The specific heat of water is
4.18 kJ/kg ·
◦
C.
6.27 Water is continuously heated by a heat pump from 70 to 110
◦
F. The heat absorbed in the
evaporator is 32,000 Btu/h and the power input is 4.1 kW. Determine the rate at which water
is heated in gallons per minute and the COP of the heat pump. The specific heat of water
is 1.0 Btu/lbm ·
◦
F.
6.28 Refrigerant-134a enters the compressor of a heat pump system at 160 kPa as a saturated
vapor and leaves at 900 kPa and 85
◦
C. The refrigerant leaves the condenser as a saturated
liquid. The rate of heating provided by the system is 2600 W. Determine the mass flow rate
of R-134a and the COP of the system.
6.29 Refrigerant-22 enters the condenser of a residential heat pump at 1200 kPa and 55
◦
Cata
rate of 0.024 kg/s and leaves at 1200 kPa as a saturated liquid. If the compressor consumes
1.8 kW of power, determine (a) the COP of the heat pump and (b) the rate of heat absorbed
from the outside air. The enthalpies of R-22 at the condenser inlet and exit are 435.62 kJ/kg
and 237.07 kJ/kg, respectively.
Heat Pumps 373
Q
H
Condenser
Evaporator
Compressor
Expansion
valve
1200 kPa
55 °C
Q
L
W
in
1200 kPa
x = 0
6.30 A heat pump with refrigerant-134a as the working fluid is used to keep a space at 23
◦
C
by absorbing heat from geothermal water that enters the evaporator at 60
◦
Catarateof
0.045 kg/s and leaves at 45
◦
C. The refrigerant enters the evaporator at 20
◦
C with a quality of
15% and leaves at the same pressure as saturated vapor. If the compressor consumes 1.3 kW
of power, determine (a) the evaporator pressure and the mass flow rate of the refrigerant
and (b) the heating load and the COP. Take specific heat of water to be 4.18 kJ/kg ·
◦
C.
Q
H
45 °C
Condenser
Evaporator
Compressor
Expansion
valve
20 °C
x = 0.15
Q
L
W
in
Geo water
60 °C
Saturated vapor
6.31 A heat pump operates on the ideal vapor-compression refrigeration cycle with refrigerant-
134a as the working fluid between pressure limits of 200 kPa and 1400 kPa. Determine (a)
the heat absorption in the evaporator, (b) the heat rejection in the condenser, (c) the work
input, and (d) the COP. (e) Also, determine the COP if the cycle operates as a refrigerator
instead of a heat pump.
374 Refrigeration Systems and Applications
6.32 A heat pump operates on the ideal vapor-compression refrigeration cycle with refrigerant-
134a as the working fluid between pressure limits of 30 psia and 200 psia. Determine (a)
the heat absorption in the evaporator, (b) the heat rejection in the condenser (c) the work
input, and (d) the COP. (e) Also, determine the COP if the cycle operates as a refrigerator
instead of a heat pump.
6.33 A heat pump operates on the ideal vapor-compression refrigeration cycle with refrigerant-
22 as the working fluid. The evaporator pressure is 300 kPa and the condenser pressure is
2000 kPa. The flow rate at the compressor inlet is 0.090 m
3
/min. Determine (a) the mass flow
rate in the cycle, (b) the rate of heating supplied by the heat pump in British thermal units
per hour, and (c) the COP. The properties of R-22 at various states are h
1
= 399.18 kJ/kg,
v
1
= 0.07656 m
3
/kg, h
2
= 447.85 kJ/kg, h
3
= 265.17 kJ/kg.
6.34 A heat pump with a heating load of 13.2 kW operates on the vapor-compression refrigeration
cycle with refrigerant-22 as the working fluid. The evaporator pressure is 200 kPa and the
condenser pressure is 1200 kPa. The refrigerant is saturated vapor at the compressor inlet
and saturated liquid at the condenser exit. The isentropic efficiency of the compressor is
84%. Determine (a) the mass flow rate of the refrigerant, (b) the power input, and (c) the
COP. The properties of R-22 at various states are h
1
= 394.67 kJ/kg, h
2s
= 439.97 kJ/kg,
h
3
= 237.07 kJ/kg. (d) Also, determine the mass flow rate, the power input and the COP if
R-134a is used as the working fluid with the same cycle conditions and the heating load.
6.35 A heat pump with a heating load of 36,000 Btu/h operates on the vapor-compression refrig-
eration cycle with refrigerant-22 as the working fluid. The evaporator pressure is 40 psia
and the condenser pressure is 300 psia. The refrigerant is saturated vapor at the compressor
inlet and saturated liquid at the condenser exit. The isentropic efficiency of the compressor
is 82%. Determine (a) the mass flow rate of the refrigerant, (b) the power input in kilo-
watts, and (c) the COP. The properties of R-22 at various states are h
1
= 171.21 Btu/lbm,
h
2s
= 193.53 Btu/lbm, h
3
= 114.90 Btu/lbm. (d) Also, determine the mass flow rate, the
power input and the COP if R-134a is used as the working fluid with the same cycle
conditions and the heating load.
6.36 Refrigerant-134a enters the condenser of a residential heat pump at 800 kPa and 55
◦
Cat
a rate of 0.080 kg/s and leaves at 750 kPa subcooled by 6
◦
C. The refrigerant enters the
compressor at 200 kPa superheated by 10.1
◦
C. Determine (a) the isentropic efficiency of
the compressor, (b) the rate of heat supplied to the heated room, and (c) the COP of the heat
pump. The enthalpy of R-134a at the condenser exit is h
3
= 83.65 kJ/kg. Also, determine
(d) the COP and the rate of heat supplied to the heated room if this heat pump operated on
the ideal vapor-compression cycle between the pressure limits of 200 kPa and 800 kPa.
Q
H
750 kPa
Condenser
Evaporator
Compressor
Expansion
valve
800 kPa
55 °C
Q
L
W
in
1
2
3
4
Heat Pumps 375
6.37 A heat pump with refrigerant-134a as the working fluid is used to keep a space at 24
◦
C
by absorbing heat from geothermal water that enters the evaporator at 50
◦
Catarateof
0.125 kg/s and leaves at 40
◦
C. The refrigerant enters the evaporator at 20
◦
C with a quality
of 23% and leaves at the inlet pressure as a saturated vapor. The refrigerant leaves the
compressor at 1.4 MPa and 65
◦
C. Determine (a) the isentropic efficiency of the compressor
and the degrees of subcooling done on the refrigerant in the condenser, (b) the mass flow
rate of the refrigerant, and (c) the heating load and the COP of the heat pump.
1.4 MPa
65 °C
1
2
3
4
Q
H
20 °C
x = 0.23
Condenser
Evaporator
Compressor
Expansion
valve
Saturated vapor
Q
L
W
in
Water
50 °C
40 °C
6.38 A ground-source heat pump operates on the vapor-compression refrigeration cycle with
refrigerant-134a as the working fluid. The evaporator pressure is 120 kPa and the condenser
pressure is 1600 kPa. The refrigerant is saturated vapor at the compressor inlet and saturated
liquid at the condenser exit. The isentropic efficiency of the compressor is 82%. The refriger-
ant absorbs heat from the ground at a rate of 5.7 kW as it flows in the evaporator. Determine
(a) the volume flow rate at the inlet and exit of the evaporator in liters per second, (b) the
heating load and the COP. (c) Determine the same parameters if the evaporator pressure is
200 kPa. (d) Determine the same parameters if the condenser pressure is 1200 kPa.
6.39 An air source heat pump operates between the ambient air at 0
◦
C and the indoors at
20
◦
C. The working fluid R-134a evaporates at −10
◦
C in the evaporator. The refrigerant
is saturated vapor at the compressor inlet and saturated liquid at the condenser exit. The
refrigerant condenses at 39.4
◦
C. Heat is lost from the indoors at a rate of 18,000 kJ/h.
The isentropic efficiency of the compressor is 78%. Determine (a) the temperature of the
refrigerant at the exit of the compressor and the mass fraction of the refrigerant vapor at
the exit of the throttling valve and (b) the power input and the COP. (c) Also, determine
the COP if a ground-source heat pump is used with a ground temperature of 15
◦
C, and the
evaporating temperature in this case is 5
◦
C. Take everything else the same. What is the
percentage increase on COP when ground source heat pump is used instead of air source
heat pump.
Exergy Analysis of Heat Pumps
6.40 A heat pump is used to keep a room at 23
◦
C by rejecting heat to an environment
at 4
◦
C. The total heat loss from the room to the environment is estimated to be
376 Refrigeration Systems and Applications
24,000 kJ/h and the power input to the compressor is 2.6 kW. Determine (a) the rate
of heat absorbed from the environment in kiloJoules per hour, (b) the COP of the
heat pump, (c) the maximum rate of heat supply to the room for the given power
input, and (d) the second-law efficiency of the cycle. (e) Also, determine the exergy
of the heat transferred to the high-temperature medium and the exergy destruction of
the cycle.
6.41 A house is kept at 24
◦
C by a 45,000 Btu/h (on heating basis) split air conditioner when
the ambient temperature is 2
◦
C. The heat pump is operating at 80% load under the given
conditions. The power input to the compressor is 4.8 kW. Determine (a) the COP and
(b) the second-law efficiency of the heat pump. (c) It is proposed to replace the exist-
ing air source heat pump with a ground source heat pump. This heat pump absorbs heat
from the ground at 12
◦
C under the same conditions. What is the COP of this ground
source system if the second-law efficiencies of the air source and ground source systems are
the same.
6.42 A house is kept at 77
◦
F by a 45,000 Btu/h (on heating basis) split air conditioner when
the ambient temperature is 35
◦
F. The heat pump is operating at 80% load under the given
conditions. The power input to the compressor is 4.3 kW. Determine (a) the COP and
(b) the second-law efficiency of the heat pump. (c) It is proposed to replace the exist-
ing air source heat pump with a ground source heat pump. This heat pump absorbs heat
from the ground at 55
◦
F under the same conditions. What is the COP of this ground
source system if the second-law efficiencies of the air source and ground source systems are
the same.
6.43 A heat pump operates on the ideal vapor-compression refrigeration cycle with
refrigerant-134a as the working fluid. The refrigerant evaporates at −20
◦
Cand
condenses at 1400 kPa. The refrigerant absorbs heat from ambient air at 5
◦
Cand
transfers it to a space at 26
◦
C. Determine (a) the work input and the COP, (b) the
exergy destruction in each component of the cycle and the total exergy destruction
in the cycle, and (c) the minimum work input and the second-law efficiency of
the cycle.
6.44 A heat pump operates between a heated space at 22
◦
C and the ambient air at −2
◦
C.
Refrigerant-134a enters the compressor of a heat pump system at 160 kPa as a satu-
rated vapor and leaves at 900 kPa and 55
◦
C. The refrigerant leaves the condenser as a
saturated liquid. The rate of heating provided by the system is 2600 W. Determine (a)
the mass flow rate of R-134a and the COP of the system, (b) the isentropic and exer-
getic efficiencies of the compressor, (c) the exergy destruction in each component of the
cycle and the total exergy destruction in the cycle, and (d) the second-law efficiency of
the cycle.
6.45 A heat pump with refrigerant-134a as the working fluid is used to keep a space at
25
◦
C by absorbing heat from tap water that enters the evaporator at 15
◦
Catarate
of 0.125 kg/s and leaves at 5
◦
C. The refrigerant enters the compressor at 4
◦
Casa
saturated vapor and leaves at 1600 kPa and 75
◦
C. The refrigerant leaves the condenser
at 1600 kPa and 52
◦
C. Determine (a) the mass flow rate of the refrigerant, the heating
load, and the COP of the heat pump, (b) the isentropic and exergetic efficiencies of
the compressor, (c) the exergy destructions in the expansion valve and the evaporator,
and (d) the exergetic efficiency of the expansion valve and the second-law efficiency of
the cycle.
Heat Pumps 377
1.6 MPa
75 °C
1
2
3
4
Q
H
Condenser
Evaporator
Compressor
Expansion
valve
Saturated vapor
Q
L
W
in
Water
15 °C
5 °C
1.6 MPa
52 °C
6.46 An air source heat pump operates between the ambient air at −5
◦
C and the indoors at
25
◦
C. The working fluid R-134a evaporates at −20
◦
C in the evaporator. The refrigerant
is saturated vapor at the compressor inlet and saturated liquid at the condenser exit. The
refrigerant condenses at 46.3
◦
C. Heat is lost from the indoors at a rate of 45,000 kJ/h. The
isentropic efficiency of the compressor is 83%. Determine (a) the evaporator and condenser
pressures and the temperature of the refrigerant at the exit of the compressor, (b) the power
input and the COP, and (c) the minimum power input, the total exergy destruction, and the
exergy efficiency of the cycle. (d) Determine the COP, the minimum power input, the total
exergy destruction, and the exergy efficiency of the cycle if a ground source heat pump is
used with a ground temperature of 10
◦
C. The evaporating temperature in this case is −5
◦
C.
Take everything else the same.
6.47 An air source heat pump operates between the ambient air at 24
◦
F and the indoors at
77
◦
F. The working fluid R-134a evaporates at −5
◦
F in the evaporator. The refrigerant
is saturated vapor at the compressor inlet and saturated liquid at the condenser exit. The
refrigerant condenses at 109.5
◦
F. Heat is lost from the indoors at a rate of 24,000 Btu/h. The
isentropic efficiency of the compressor is 86%. Determine (a) the evaporator and condenser
pressures and the temperature of the refrigerant at the exit of the compressor, (b) the power
input and the COP, and (c) the minimum power input, the total exergy destruction, and the
exergy efficiency of the cycle. (d) Determine the COP, the minimum power input, the total
exergy destruction, and the exergy efficiency of the cycle if a ground source heat pump is
used with a ground temperature of 50
◦
F. The evaporating temperature in this case is 25
◦
F.
Take everything else the same.
References
ARI (2000) Standard for Variable Capacity Positive Displacement Refrigerant Compressors and Compressor
Units for Air-Conditioning and Heat Pump Applications, Standard 500, Air-Conditioning & Refrigeration
Institute, Arlington, VA.
378 Refrigeration Systems and Applications
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Martinus Nijhoff Publishers, The Hague, pp. 279–301.
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7
Heat Pipes
7.1 Introduction
During the past two decades, heat pipe technology has received a great deal of attention and
miniature and conventional heat pipes can be successfully used in many industrial applications,
especially in the cooling of electronic components and devices.
The idea of heat pipes was first suggested by Gaugler in 1944. However, it was not until
1962 when G.M. Grover invented it that its remarkable properties were appreciated and serious
development began. Since the heat pipe was first patented by Grover in 1963, elementary theories
have been advanced and developments in aerospace and terrestrial applications have progressed to
the point where the heat pipe is now used commercially. While heat pipe technology has reached a
rather high level, its market has not yet met expectations. More recently, increasing environmental
problems have attracted a great deal of attention.
Many countries started their research and development programs on heat pipes and their uti-
lization in the 1970s. Since then, both academic institutions and research organizations have been
involved in research activities while the industry has gained experience in application. Various
laboratories, for example, Los Alamos National Laboratory in the United States, have played a
significant role in the research and development of heat pipe technology. Most of them have estab-
lished the design and manufacturing capability of thermosiphon heat pipes and wicked heat pipes.
Such heat pipes have found applications in waste heat recovery and electronic enclosure cooling.
Recently, several companies have also developed micro heat pipes as cutting edge technology,
particularly for cooling notebook personal computers.
The heat pipe is an efficient heat conductor device for transferring heat from one part to another
part. They are often referred to as the superconductors of heat as they possess an extraordinary
heat-transfer capacity and with almost no heat loss. The heat-transfer rate is thousands of times
greater than that possible with a solid heat conductor of the same size (e.g., solid rod and simple
fin) because of its thermophysical properties. In this device, one end of the heat pipe (or tube) is
filled with a working fluid. Heat is applied to this end, vaporizing the fluid. The vapor comes to
the other end, which is cooler, by the capillary effect there and condenses, releasing heat. A simple
heat pipe consists essentially of a metal tube (a sealed aluminum or copper container whose inner
surfaces have a capillary wicking material) lined with a wick and filled with a fluid. A heat pipe is
similar to a thermosiphon. It differs from a thermosiphon by virtue of its ability to transport heat
against gravity by an evaporation–condensation cycle with the help of porous capillaries that form
the wick. The wick provides the capillary driving force to return the condensate to the evaporator.
The quality and type of wick usually determines the performance of the heat pipe, for this is the
Refrigeration Systems and Applications
˙
Ibrahim Dinc¸er and Mehmet Kano
ˇ
glu
2010 John Wiley & Sons, Ltd
380 Refrigeration Systems and Applications
heart of the product. Different types of wicks are used, depending on the application for which the
heat pipe is being used.
Some of the well-known working fluids (so-called heat pipe liquids) are liquid hydrogen, ammo-
nia, acetone, methanol, water, sodium, potassium, lithium, mercury, and silver. In addition, a number
of heat pipe wall and wick materials are recommended, for example, aluminum, carbon steel, nickel,
copper, tungsten, molybdenum, and refractory metals and alloys. Much research effort has been
carried out to find the most efficient and reliable working fluids and wall/wick materials and the best
configurations of the heat pipes for various ranges of temperature, depending on the applications
(Dincer, 1997).
It is obvious that elimination of the fluid pump and power supply leads to greater reliability of
the heat transport system and reduced weight, in addition to the saving in power consumption.
Heat pipe technology was suggested as a way to transfer solar heat passively and effectively
from a solar absorber plate to the inside of a building. However, heat pipes are now of great
importance in a variety of industrial applications ranging from mechanical engineering to food
process engineering, for example, solar thermal applications using heat pipes as evacuated solar
collectors, food cooking, and cooling applications. Further information on heat pipe technology,
operating characteristics, heat-transfer limits, and heat pipe design technologies for cooling and
heat exchange applications may be found in Peterson (1994) and Faghri (1995).
The main objective of this chapter is to introduce the heat pipes for various thermal applications,
highlight the importance of their utilization for various cooling and heating applications, and discuss
their technical, design and manufacturing, and operational aspects, as well as their benefits, along
with practical examples.
7.2 Heat Pipes
Heat pipes are characterized by their excellent heat-transfer capability, fast heat-transfer rate, uni-
form temperature distribution, simple construction, compactness, high reliability, high efficiency,
small heat losses, low manufacturing costs, environmentally benign nature, and versatile applica-
tions. Their most attractive feature is that they do not require external energy.
The concept of a passive two-phase heat-transfer device capable of transferring large quantities of
heat with a minimal temperature drop was first introduced by Gaugler in 1942. This device received
little attention until 1964 when Grover and his colleagues at Los Alamos National Laboratories
published the results of an independent investigation and first applied the term heat pipe. Since that
time, heat pipes have been employed in many applications ranging from temperature control of the
permafrost layer under the Alaska pipeline to the thermal control of optical surfaces in spacecraft.
A heat pipe is a heat-transfer device with an extremely high effective thermal conductivity. Heat
pipes are evacuated vessels, typically circular in cross-section, which are backfilled with a small
quantity of a working fluid. They are totally passive and are used to transfer heat from a heat source
to a heat sink with minimal temperature gradients, or to isothermalize surfaces.
A heat pipe consists typically of a sealed container with a wicking material. The container is
evacuated and filled with just enough liquid to fully saturate the wick. As illustrated in Figure 7.1,
a heat pipe consists of three distinct regions: an evaporator or heat addition region of the container,
a condenser or heat rejection region, and an adiabatic or isothermal region. When the evaporator
region is exposed to a high temperature, heat is added and the working fluid in the wicking structure
is heated until it evaporates. The high temperature and the corresponding high pressure in this region
cause the vapor to flow to the cooler condenser region where the vapor condenses, giving up its
latent heat of vaporization. The capillary forces existing in the wicking structure then pump the
liquid back to the evaporator. The wick structure thus ensures that the heat pipe can transfer heat
if the heat source is below the cooled end (bottom heat mode) or if it is above the cooled end (top
heat mode).