Dinc Ibrahim. Refrigeration systems and applications 2th edition

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Heat Pipes 411

Figure 7.17 Three conﬁgurations of energy recovery heat pipes: (a) Over and under horizontal air streams

with heat pipes in a vertical plane. (b) Side by side vertical air streams with heat pipe in a horizontal plane. (c)

Side by side horizontal air streams with heat pipe in a vertical plane (Courtesy of Heat Pipe Technology, Inc.).

with the vapor to the cold end of the tube, where it is removed by transference to the cold gas as

the vapor condenses. The condensate is then carried back in the wick to the hot end of the tube by

capillary action and by gravity (if the tube is tilted from the horizontal), where it is recycled.

The heat pipe is compact and efﬁcient because (i) the ﬁnned-tube bundle is inherently a good

conﬁguration for convective heat transfer in both ducts and (ii) the evaporative–condensing cycle

within the heat tubes is a highly efﬁcient method of transferring heat internally. It is also free of

cross-contamination.

The heat pipe sits level in a vertical plane and exchanges heat/cooling in both directions. Energy

recovery heat pipes with over- and under-horizontal airﬂows provide an excellent one-directional

energy transfer with the warmer air stream at the bottom. Some of their beneﬁts can be underlined

as follows:

• made of high quality copper tube for reliability and longevity,

• highest heat-transfer effectiveness,

• bidirectional heat transfer (except for vertical module),

• no tilting necessary,

• no mechanical seasonal changeover necessary,

• high thermal conductivity and great heat transfer,

• better thermal response than any metal,

• equality of thermal distribution,

• small size and lightweight,

• ﬂexible design,

• cost effectiveness,

• reliability,

• no electrical or mechanical power required; maintenance free,

• both ﬂat and tube series available,

• ﬂexible shape and length,

• easy combination with properly designed spreader,

• less cost,

• no need for reheat,

• no mechanical or electrical input required,

• maintenance free,

• lower operating and maintenance costs,

• longer operational time, and

• environmentally benign.

In an air-conditioning system, the colder the air that passes over the cooling coil (evaporator),

more is the moisture that is condensed out. The heat pipe is designed to have one section in the

412 Refrigeration Systems and Applications

warm incoming stream and the other in the cold outgoing stream. By transferring heat from the

warm return air to the cold supply air, the heat pipes create the double effect of precooling the

air before it goes to the evaporator and then reheating it immediately. Activated by temperature

difference and therefore consuming no energy, the heat pipe, because of its precooling effect, allows

the evaporator coil to operate at a lower temperature, increasing the moisture removal capability

of the air-conditioning system by 50–100%. With lower relative humidity, indoor comfort can be

achieved at higher thermostat settings, which results in net energy savings. Generally, for each 1

◦

rise in thermostat setting, there is a 7% savings in electricity cost. In addition, the precooling effect

of the heat pipe allows the use of a smaller compressor.

Design ﬂexibility is achieved because of the heat pipe’s ability to transfer heat efﬁciently. While

traditional heat sinks must be located on the heat source, heat pipes transfer heat away to areas

where dissipation is more convenient or airﬂow is greater. Heat pipes can be bent and formed into

a variety of conﬁgurations while maintaining their heat-transfer properties.

Heat pipes can be used efﬁciently by libraries, restaurants, cold storage facilities, supermarkets,

applications requiring controlled/reduced humidity, and applications where reheat or desiccants are

necessary.

Hill and Lau (1993) studied supermarket air-conditioning systems equipped with heat pipe heat

exchangers. Operation in four different climates was considered. The heat pipe heat exchangers were

used to save refrigeration energy by reducing the humidity of the refrigerated spaces. Rosenfeld

and North (1995) discussed the use of heat pipes in “porous media heat exchangers” for the cooling

of high-heat-load optical components.

7.12 Concluding Remarks

This chapter deals with heat pipes particularly for thermal applications, and discusses related matters

from the structure, features, technical aspects, operational aspects, technical details, heat pipe ﬂuids,

design and manufacturing aspects, heat-transfer limits, energy savings, types and applications points

of view. Some examples are given to highlight the importance of the topic and show the beneﬁts of

the technology for some speciﬁc thermal applications and for refrigeration at large. It is clearly indi-

cated that micro heat pipes are now really essential equipment for electronics cooling applications.

Nomenclature

A cross-sectional area, m

; surface area, m

eva

heat-transfer area of evaporator section, m

ﬁn

total surface area of ﬁns, m

speciﬁc heat, J/kg ·

◦

C volumetric heat capacity, J/m

◦

D diameter, m

h convection heat-transfer coefﬁcient, W/m

◦

k thermal conductivity, W/m ·

◦

L length, m

con

length of condenser section, m

eff

effective ﬁn length, m

eva

length of evaporator section, m

ﬁn

ﬁn factor for uniform cross-sectional area

˙m mass ﬂow rate of cooling air, kg/s

M moisture content, kg/kg

Heat Pipes 413

q speciﬁc heat transfer, kJ/kg

˙q heat ﬂux, W/m

Q heat-transfer rate, kW

R thermal resistance,

◦

C/W

air

thermal resistance of cooling air ﬂow

axial

thermal resistance along heat pipe

con

thermal resistance of the heat pipe condenser

thermal resistance due to convection

eva

thermal resistance of the heat pipe evaporator block

ﬁn

thermal resistance due to ﬁn efﬁciency

thermal resistance of heat pipe

int

thermal resistance of evaporator block heat pipe interface

total

thermal resistance of heat sink to ambient



ﬂux thermal resistance,

◦

C/W · m

T temperature,

◦

T

air

temperature change in cooling air ﬂow,

◦

T

eva

temperature change in evaporator section (block),

◦

T

temperature change due to convection,

◦

T

ﬁn

temperature change due to ﬁn efﬁciency,

◦

T

temperature change in heat pipe,

◦

T

int

temperature change in the interface of evaporator block and heat pipe,

◦

evap

thickness of the evaporator block, m

ﬁn

thickness of the ﬁn, m

ﬁn

ﬁn efﬁciency

Study Problems

Heat Pipes

7.1 What is a heat pipe? Why is it also referred as a superconductor?

7.2 Describe the geometry and operation of a heat pipe.

7.3 What is called as the heart of a heat pipe and why?

7.4 Provide a list of heat pipe liquids used.

7.5 Provide a list of heat pipe wall and wick materials used.

7.6 What are the basic characteristics of a heat pipe? What is the most attractive feature of heat

pipes.

7.7 What are the three main objectives we expect from heat pipes?

7.8 List some of the typical heat pipe applications.

Types of Heat Pipes

7.9 Give a list of the types of heat pipes.

7.10 What are the characteristics of cryogenic heat pipes.

414 Refrigeration Systems and Applications

Heat Pipe Components

7.11 What characteristics should the container material of a heat pipe have?

7.12 What factors should be considered in the selection of the container material of a heat pipe?

7.13 What characteristics should the working ﬂuid of a heat pipe have?

7.14 Why is a high value of surface tension desirable for the working ﬂuid of a heat pipe?

7.15 Why is a high value of latent heat of vaporization desirable for the working ﬂuid of a heat

pipe?

7.16 What are the most commonly used working ﬂuids in heat pipes? What are the useful

temperature ranges for these ﬂuids? Which working ﬂuid is more suitable for subfreezing

temperature application?

7.17 What are the two most important properties of a wick and why are they important?

7.18 What are the most common types of wicks that are used in heat pipes?

7.19 What are the two functions of the wicking structure in heat pipe operation?

7.20 What are the three broad categories of wicking structures? Describe each brieﬂy.

7.21 What are the three improvements accomplished in advanced wicking structure designs?

Heat Pipe Performance

7.22 What does the total thermal resistance of a heat pipe consist of?

7.23 Consider a heat pipe with a heat rejection of 100 W. The effective thermal resistance of the

heat pipe is 0.4

◦

C/W. What is the temperature difference involved in the dissipation of this

heat load?

7.24 Consider a heat pipe with a heat rejection of 35 W with a temperature change of 28

◦

What is the effective thermal resistance of the heat pipe?

7.25 A heat pipe is used as a heat sink for cooling of an electronic component. The temperature

difference across the heat sink is 30

◦

C and the effective thermal resistance of the heat sink

is 1.8

◦

C/W. What is the rate of heat rejected?

7.26 Consider a copper/water heat pipe. The evaporator and condenser heat ﬂux is 4.9 W/cm

and the axial heat ﬂux of the vapor space is 124 W/cm

. What is the total temperature drop?

7.27 Consider a 0.8 cm diameter copper/water heat pipe. It is 22 cm long with a 0.55 cm diameter

vapor space. The heat pipe is dissipating 58 W with a 6 cm evaporator and a 7 cm condenser

length. Determine the total temperature drop.

Design and Manufacture of Heat Pipes

7.28 What are the three operational considerations that govern the design and manufacture of

heat pipes?

7.29 List some of the factors to consider when designing a heat pipe.

7.30 What are the three properties of wicks that are important in heat pipe design?

7.31 What are the common heat pipe diameters and lengths?

Heat Pipes 415

Heat-Transfer Limitations

7.32 What are the physical phenomena that might limit heat transport in heat pipes?

Heat Pipes in HVAC

7.33 What are the beneﬁts of heat pipes in HVAC applications?

7.34 Describe the working principle of the heat pipe used for dehumidiﬁcation in an

air-conditioning system with the help of a schematic of the system.

7.35 Explain how an indoor dehumidiﬁer heat pump provides energy savings.

7.36 List some of the beneﬁts of energy recovery heat pipes.

References

Cotter, T.P. (1984) Principles and Prospects of Micro Heat Pipes. Proceedings of the 5th International Heat

Pipe Conference. Tsukuba, Japan, pp. 328–337.

DeHoff, R. and Grubb, K. (2000) Heat Pipe Application Guidelines, Thermacore Inc., Lancaster, PA.

Dincer, I. (1997) Heat Transfer in Food Cooling Applications, Taylor & Francis, Washington, DC.

Dincer, I. (2003) Refrigeration Systems and Applications, Wiley, 1st ed, London.

Dinh, K. (1996) Heat pipes in HVAC, in Heat Pipe Technology (eds J. Andrews et al.), Pergamon, London,

pp. 357–363.

Faghri, A. (1995) Heat Pipe Science and Technology, Taylor & Francis, Washington, DC.

Faghri, A. (1996) Heat pipe simulation: from promise to reality, in Heat Pipe Technology (eds J. Andrews

et al.), Pergamon, London, pp. 1–21.

Garner, S.D. (1996) Heat pipes for electronics cooling applications. Electronics Cooling, September,1–9.

Gaugler, R.S. (1944) Heat Transfer Devices. US Patent 2, 350,348.

Hill, J.M. and Lau, A.S. (1993) Performance of supermarket air-conditioning systems equipped with heat pipe

heat exchangers. ASHRAE Transactions, 99 (1), 1315–1319.

Narayanan, S. (2001) What Is a Heat Pipe? The Chemical Engineering Resource Page, http://www.

cheresources.com/htpipes.shtml Last Accessed 11 September 2009.

Peterson, G.P. (1994) An Introduction to Heat Pipes, John Wiley & Sons, Ltd., New York.

Rosenfeld, J.H. and North, M.T. (1995) Porous media heat exchangers for cooling of high-power optical

components. Optical Engineering, 34 (2), 335–342.

Xie, H., Aghazadeh, M. and Toth, J. (2001) The Use of Heat Pipes in the Cooling of Portables with High

Power Packages – A Case Study with the Pentium. Processor-Based Notebooks and Sub-notebooks,Intel

Corporation, Chandler, AZ and Thermacore International Inc., Lancaster, PA.

Appendix A

Conversion Factors

Refrigeration Systems and Applications

Ibrahim Dinc¸er and Mehmet Kano

glu

 2010 John Wiley & Sons, Ltd

418 Refrigeration Systems and Applications

Table A. 1 Conversion factors for commonly used quantities.

Quantity SI to English English to SI

Area 1 m

= 10.764 ft

= 1550.0 in.

1ft

= 0.00929 m

1in.

= 6.452 × 10

−4

Density 1 kg/m

= 0.06243 lb

/ft

1lb

/ft

= 16.018 kg/m

1 slug/ft

= 515.379 kg/m

Energy 1 J = 9.4787 × 10

−4

Btu 1 Btu = 1055.056 J

1 cal = 4.1868 J

1lb

·ft = 1.3558 J

1hp·h = 2.685 × 10

Energy per unit mass 1 J/kg = 4.2995 × 10

−4

Btu/lb

1 Btu/lb

= 2326 J/kg

Force 1 N = 0.22481 lb

1lb

= 4.448 N

1 pdl = 0.1382 N

Gravitation g = 9.80665 m/s

g = 32.17405 ft/s

Heat ﬂux 1 W/m

= 0.3171 Btu/h·ft

1 Btu/h·ft

= 3.1525 W/m

1 kcal/h·m

= 1.163 W/m

1 cal/s·cm

= 41870.0 W/m

Heat generation

(volume)

1W/m

= 0.09665 Btu/h·ft

1 Btu/h·ft

= 10.343 W/m

Heat transfer coefﬁcient 1 W/m

·K = 0.1761 Btu/h·ft

◦

F1Btu/h·ft

◦

F = 5.678 W/m

·K

1 kcal/h·m

◦

C = 1.163 W/m

·K

1 cal/s·m

◦

C = 41870.0W/m

·K

Heat transfer rate 1 W = 3.4123 Btu/h 1 Btu/h = 0.2931 W

Length 1 m = 3.2808 ft

= 39.370 in

1km = 0.621371 mi

1ft = 0.3048 m

1in. = 2.54 cm = 0.0254 m

1mi = 1.609344 km

1yd = 0.9144 m

Mass 1 kg = 2.2046 lb

1 ton (metric) = 1000.0 kg

1grain= 6.47989 × 10

−5

1lb

= 0.4536 kg

1slug= 14.594 kg

Mass ﬂow rate 1 kg/s = 7936.6 lb

= 2.2046 lb

1lb

/h = 0.000126 kg/s

1lb

/s = 0.4536 kg/s

Power 1 W = 1J/s = 3.4123 Btu/h

= 0.737562 Ib

·ft/s

1 hp (metric) = 0.735499 kW

1 ton of refrig. = 3.51685 kW

1 Btu/h = 0.2931 W

1 Btu/s = 1055.1 W

1lb

·ft/s = 1.3558 W

1 hp(UK) = 745.7 W

Pressure and stress

(Pa = N/m

)

1Pa = 0.020886 lb

/ft

= 1.4504 × 10

−4

/in.

= 4.015 × 10

−3

in water

= 2.953 × 10

−4

in Hg

1lb

/ft

= 47.88 Pa

1lb

/in

= 1psi = 6894.8 Pa

1 stand. atm. = 1.0133 × 10

1bar= 1 × 10

Speciﬁc heat 1 J/kg·K = 2.3886 × 10

−4

Btu/lb

◦

F 1 Btu/lb

◦

F = 4187.0 J/kg·K

Conversion Factors 419

Table A. 1 (continued)

Quantity SI to English English to SI

Surface tension 1 N/m = 0.06852 lb

/ft 1 lb

/ft = 14.594 N/m

1 dyn/cm = 1 × 10

−3

N/m

Temperature T(K) = T(

◦

C) + 273.15

= T(

◦

R)/1.8

= [T(

◦

F) + 459.67]/1.8

◦

C) = [T(

◦

F) – 32.0]/1.8

◦

R) = 1.8T(K)

= T(

◦

F) + 459.67

= 1.8T(

◦

C) + 32.0

= 1.8[T(K) – 273.15] + 32

Temperature difference 1 K = 1

◦

C = 1.8

◦

R = 1.8

◦

R = 1

◦

F = 1K/1.8 = 1

◦

C/1.8

Thermal conductivity 1 W/m·K = 0.57782 Btu/h·ft·

◦

F1Btu/h·ft·

◦

F = 1.731 W/m·K

1 kcal/h·m·

◦

C = 1.163 W/m·K

1 cal/s·cm·

◦

C = 418.7 W/m·K

Thermal diffusivity 1 m

/s = 10.7639 ft

/s 1 ft

/s = 0.0929 m

1ft

/h = 2.581 × 10

−5

Thermal resistance 1 K/W = 0.52750

◦

F·h/Btu 1

◦

F·h/Btu = 1.8958 K/W

Velocity 1 m/s = 3.2808 ft/s

1km/s = 0.62137 mi/h

1 ft/s = 0.3048 m/s

1 ft/min = 5.08 × 10

−3

m/s

Viscosity (dynamic)

(kg/m·s = N·s/m

)

1 kg/m·s = 0.672 lb

/ft·s

= 2419.1 lb

/fh·h

1lb

/ft·s = 1.4881 kg/m·s

1lb

/ft·h = 4.133 × 10

−4

kg/m·s

1 centipoise (cP) = 10

−2

poise

= 1 × 10

−3

kg/m·s

Viscosity (kinematic) 1 m

/s = 10.7639 ft

= 1 × 10

stokes

1ft

/s = 0.0929 m

1ft

/h = 2.581 × 10

−5

1 stoke = 1cm

Vo l u m e 1 m

= 35.3134 ft

1L = 1dm

= 0.001 m

1ft

= 0.02832 m

1in

= 1.6387 × 10

−5

1gal (US) = 0.003785 m

1 gal (UK) = 0.004546 m

Volumetric ﬂow rate 1 m

/s = 35.3134 ft

= 1.2713 × 10

1ft

/s = 2.8317 × 10

−2

1ft

/min = 4.72 × 10

−4

1ft

/h = 7.8658 × 10

−6

1 gal (US)/min = 6.309 × 10

−5