This article investigates the thermal performance of a heat pipe heat exchanger for heat recovery in air conditioning applications. Experiments were conducted with ratios of return to fresh air mass flow rates of 1, 1.5, and 2.3. The fresh air inlet temperature was varied from 32°C to 40°C while the return air temperature was kept at 26°C. The results show that the temperature changes and effectiveness of both the evaporator and condenser sections increase with higher fresh air inlet temperatures. The enthalpy ratio between heat recovery and conventional air mixing also increases with fresh air inlet temperature, reaching 85% at 40°C.

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Heat pipe heat exchanger for heat recovery in air conditioning
Mostafa A. Abd El-Baky 1
, Mousa M. Mohamed *
Mechanical Power Engineering Department, Faculty of Engineering, Minufiya University, Shebin El-Kom, Egypt
Abstract
The heat pipe heat exchangers are used in heat recovery applications to cool the incoming fresh air in air conditioning applications.
Two streams of fresh and return air have been connected with heat pipe heat exchanger to investigate the thermal performance and effec-
tiveness of heat recovery system. Ratios of mass flow rate between return and fresh air of 1, 1.5 and 2.3 have been adapted to validate the
heat transfer and the temperature change of fresh air. Fresh air inlet temperature of 32–40 °C has been controlled, while the inlet return
air temperature is kept constant at about 26 °C. The results showed that the temperature changes of fresh and return air are increased
with the increase of inlet temperature of fresh air. The effectiveness and heat transfer for both evaporator and condenser sections are also
increased to about 48%, when the inlet fresh air temperature is increased to 40 °C. The effect of mass flow rate ratio on effectiveness is
positive for evaporator side and negative for condenser side. The enthalpy ratio between the heat recovery and conventional air mixing is
increased to about 85% with increasing fresh air inlet temperature. The optimum effectiveness of heat pipe heat exchanger is estimated
and compared with the present experimental data. The results showed that the effectiveness is close to the optimum effectiveness at fresh
air inlet temperature near the fluid operating temperature of heat pipes.
Ó 2006 Published by Elsevier Ltd.
Keywords: Heat pipes; Heat recovery; Heat exchangers; Air conditioning
1. Introduction
Heat pipe heat exchanger for heat recovery equipment
are aimed for recovering sensible heat and they are recom-
mended for systems in which inlet and return air should not
be mixed such as surgery rooms in hospitals and chemical
and biological laboratories. The advantages of using heat
pipes over conventional methods is that large quantities
of heat can be transported through a small cross-sectional
area over a considerable distance with no additional power
input to the system, (except for the fans to drive the air-
streams) together with simplicity of design and ease of
manufacture [1]. Efforts have successfully developed a ser-
ies of heat pipes equipment, such as heat pipes gas to gas
exchangers, heat pipes steam generators, high-temperature
heat pipes hot air furnaces, and progresses have been made
in the fields of metallurgical, petrochemical, chemical,
power and construction material industries on the basis
of experimental and theoretical investigations [2,3]. Also,
heat pipe heat exchangers are suitable for energy recovery
in air conditioning systems in tropical countries where
incoming fresh air at high ambient temperature could be
pre-cooled by the cold exhaust air stream before it enters
the refrigeration equipment [4]. Any study of an air condi-
tioning system in a building should be focused mainly on
indoor air quality, thermal comfort, energy saving and
environmental protection [5].
Numerous investigations have been made to obtain the
thermal performance, ensure efficient and reliable opera-
tion of heat pipe heat exchanger [6–10]. Simple experiment
was carried out for using heat pipe heat exchanger for heat-
ing automobiles using exhaust gas [11]. It is obvious that
the heat transferred by the heat pipe heat exchanger
increased with the rise of exhaust gas temperature. The
effects of input heat transfer rate, the working fluid filling
1359-4311/$ - see front matter Ó 2006 Published by Elsevier Ltd.
doi:10.1016/j.applthermaleng.2006.10.020
*
Corresponding author. Tel.: +20 48 2237117; fax: +20 48 235695.
E-mail addresses: mostahmed2004@yahoo.com (M.A. Abd El-Baky),
mousamohamed@yahoo.com (M.M. Mohamed).
1
Tel.: +20 48 2235520.
www.elsevier.com/locate/apthermeng
Applied Thermal Engineering 27 (2007) 795–801

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ratio and the evaporator length on the thermal perfor-
mance of thermosyphon were investigated [12]. A heat pipe
heat exchanger was designed, constructed and tested under
low temperature of 15–35 °C, operating conditions [13].
The results showed that the minimum heat transfer is well
above the required heat transfer rate, and for increasing the
effectiveness of the heat pipe heat exchanger, the number of
rows should be increased and finned pipes should be used.
A design method by using computational fluid dynamic
simulation of the dehumidification process with heat pipe
heat exchangers was presented [14]. The study suggests that
modeling is able to predict the thermal performance and
optimize the design of the heat pipe fin stack.
The aim of this study is to investigate the thermal per-
formance and effectiveness of heat pipe heat exchanger
for heat recovery in air conditioning applications by mea-
suring the temperature difference of fresh warm and return
cold air through the evaporator and condenser side. The
heat transfer and enthalpy ratio between heat recovery
and conventional air mixing are also targeted. The opti-
mum effectiveness of heat pipe heat exchanger is calculated
and compared with the experimental results.
2. Experimental apparatus and procedure
The experimental apparatus has been designed and con-
structed as shown in Fig. 1. The test section consists of two
air ducts of 0.3 · 0.22 m2
section areas connected together
by finned tubes heat pipe heat exchanger. A square hole of
0.3 · 0.3 m2
was made in one side of the two ducts for heat
pipe heat exchanger installation. A laboratory refrigeration
machine consisting of evaporator; compressor, condenser,
and expansion device beside the measuring instrumenta-
tions were used to supply the return cold air to the con-
denser side of the heat pipe heat exchanger. The unit was
equipped with a blower of variable speed installed before
the cooling coil. The refrigeration unit was charged with
R-134a and the evaporator was made from copper-finned
tubes cooling coil, installed in the duct of 0.3 · 0.3 m2
inside dimensions. The fresh air duct was equipped with
a blower to supply air to the evaporator side of the heat
pipe heat exchanger. The return cold and fresh warm air
ducts were insulated with glass wool of 50 mm thickness
to minimize the heat transfer to surrounding air.
The flow rates of air in both two ducts were measured
with Pitot-static tube. The fresh air was kept constant at
0.4 kg sÀ1
, while the return air was changed from 0.4, 0.6
and 0.933 kg sÀ1
. The ratios between return air and fresh
are 1, 1.5, and 2.333. The air temperature and relative
humidity at inlet and outlet of the two ducts were measured
with Humidity-temperature digital device and the mea-
sured data were conducted in steady state. The refrigera-
tion unit was operated and the two blowers of fresh and
return air were also running. After enough time, the tem-
peratures and humidity of fresh and return air before and
after heat pipe heat exchanger were recorded, when they
became nearly constant. The ratio between return cold
and fresh air mass flow rates was obtained. The recorded
Nomenclature
A constant, Eq. (9)
AHX surface area of HPHE (m2
)
B constant Eq. (10)
Cp specific heat (J kgÀ1
KÀ1
)
CA area dependent first cost of HPHE ($ mÀ1
)
CE cost of heat recovery by HPHE ($ WÀ1
hÀ1
)
CF fixed operational cost for fans ($)
H annual time of operation (h yearÀ1
)
H enthalpy (J kgÀ1
)
hfg heat of vaporization (J kgÀ1
)
I energy price rate in fraction
_m air mass flow rate (kg sÀ1
)
N technical life of the HPHE (year)
P1 ratio of life cycle energy cost, Eq. (11)
P2 ratio of life cycle expenditure incurred, Eq. (12)
Q heat transfer rate (W)
R gas constant (J kgÀ1
KÀ1
)
r radius of heat pipe (m)
T temperature (°C)
Tmax temperature of inlet fresh air (°C)
Tmin temperature of return cold air (°C)
DTmax Tmax À Tmin (K)
DT temperature change of air stream (K)
U overall heat transfer coefficient (W mÀ2
KÀ1
)
e effectiveness
c specific heat ratio for gas Cp/CV
q density (kg mÀ3
)
r surface tension (N mÀ1
)
x humidity ratio (kg kgÀ1
dryair)
n enthalpy ratio of heat recovery to conventional
mixing air
Subscripts
ent entrainment limit
l liquid
M mixing point
O fresh air
O.i fresh air inlet
O.o fresh air outlet
opt optimum
R return cold air
R.i return air inlet
R.o return air outlet
s sound limit
V vapor
796 M.A. Abd El-Baky, M.M. Mohamed / Applied Thermal Engineering 27 (2007) 795–801

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data of the air were represented on the psychrometric
chart. In this study, the thermodynamic properties of moist
air and working fluids were obtained by using Cool Pack
and NIST software [15,16]. The enthalpy and humidity
ratio for each run was calculated from the cools tool aux-
iliary program using Engineering Equation Solver by
knowing the dry bulb temperature and relative humidity
of air at inlet and outlet of heat pipe heat exchanger.
2.1. Test section and heat pipe heat exchanger
The two air ducts of 0.3 · 0.22 m2
section areas and 1 m
length were made of galvanized steel sheets having 0.5 mm
thickness. The length of each duct is 1 m. A nozzle was
installed to connect the outlet of the fan and the fresh air
duct. The nozzle was made of galvanized steel with
0.5 mm thickness. It has inlet section of 0.1 · 0.1 m2
and
outlet section 0.3 · 0.22 m2
with length of 0.3 m. The heat
pipe heat exchanger consists of 25 copper tubes with length
of 0.5 m, and inside and outside diameters of 10.2 and
12.7 mm respectively. The heat pipe consists of three parts
with straight length, evaporator section of 0.2 m, adiabatic
section of 0.1 m and condenser section of 0.2 m. Four lay-
ers of 100 mesh brass screen with wire diameter of
0.125 mm were installed inside the tubes to assist the liquid
return from the condenser section to the evaporator sec-
tion. The heat pipes are closed at both ends and evacuated
from air and charged with R-11 as a working medium at
pressure of 0.127 MPa, which corresponds to saturation
temperature of 30 °C. It is note that this fluid is replaced
now by R-123. The heat pipes were arranged horizontally
in staggered form as indicated in Fig. 2. The evaporator
and condenser sections are finned with 50 square aluminum
sheets of 0.5 mm thickness and area of 0.29 · 0.29 m2
.
2.2. Air processes and data reduction
The sensible cooling of fresh air and sensible heating of
return air processes are represented on psychrometric chart
as shown in Fig. 1. The heat rejected from the air stream in
the evaporator section can be calculated as,
Q ¼ _mOCPðT O:i À T O:oÞ: ð1Þ
The effectiveness of the heat exchanger is defined as the ra-
tio of actual rate of heat transfer by the heat exchanger to
the maximum possible heat transfer rate between the two
air streams [13,17]. Assuming, there is no water condensa-
tion in fresh air stream and also assuming the specific heat
of air passing through the evaporator and condenser sec-
tions to be constant, then the effectiveness of heat pipe heat
exchanger at evaporator side is represented as,
e ¼
TO:i À TO:o
TO:i À TR:i
: ð2Þ
The ratio of utilized heat in the heat recovery process to the
utilized heat in the conventional mixing air process defined
by enthalpy ratio is:
n ¼
hO:i À hO:o
hO:i À hM
: ð3Þ
The above procedures were conducted for each experiment
at various mass flow ratios of 1, 1.5 and 2.33 and fresh air
temperatures of 32–40 °C, while the return cold air temper-
ature was kept constant at about 26 °C.
3. Results and discussion
The temperature change of fresh, hot, and return, cold,
air at various inlet air temperatures and mass flow rate
Fig. 1. Air ducts and measuring instrumentations.
M.A. Abd El-Baky, M.M. Mohamed / Applied Thermal Engineering 27 (2007) 795–801 797

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ratios are illustrated in Fig. 3. It is observed that for fresh
and return cold air, the temperature change increases with
increasing the inlet fresh air temperature. The increase in
temperature change for fresh air with increasing mass flow
rate ratio between return and fresh air is slightly positive.
But, the temperature change of return cold air is going
down with increasing mass flow rate ratio.
The calculated results of effectiveness for fresh and
return air are indicated in Fig. 4. The effectiveness is
increased with increasing the inlet fresh air temperature.
The effect of mass flow rate ratios on the effectiveness of
the heat exchanger is slightly positive for evaporator side
and largely negative for condenser side. It is interesting
to find that the increase in return to fresh air mass flow rate
ratios by about two times leads to increase in the tempera-
ture change of fresh air by about 20% and the effectiveness
in the evaporator side by about 26%. Otherwise, the values
of inlet return cold air are nearly remaining constant for all
experiments. Obviously, it is considered that the inlet fresh
air temperature is the most dominant parameter to enhance
the heat transfer rate in the evaporator side of the heat pipe
heat exchanger.
The utilized heat in the heat recovery process compared
to the conventional mixing air process, n, defined by Eq. (3)
is illustrated in Fig. 5. It is found that the heat recovery
increased with increasing inlet fresh air temperature and
it reached about 85% at inlet fresh air temperature of
40 °C. Also, the heat recovery is decreased by about 10%
with increasing mass flow rate ratio by about two times.
3.1. Heat transfer analysis and optimum effectiveness
The optimum operating conditions of a certain design of
heat pipe heat exchanger is subjected to a number of heat
transfer limitations. These limitations determine the maxi-
mum heat transfer rate of a particular design that can be
achieved under certain working conditions. The sonic and
entrainment limits of the heat transferred for a single heat
Fig. 2. Heat pipe heat exchanger and heat pipe design.
20 25 30 35 40 45
T [ºC]
0
1
2
3
4
5
6
7
8
ΔT[ºC]
Rm / m = 1o
..
RT
ToΔ
Δ
Fresh AirReturn Air
T = 26 [
o
C]R
o
0.8 1.2 1.6 2.0 2.4 2.8
m / m
0
1
2
3
4
5
6
7
8
T[ºC]
R o
Δ
RT = 26 [ C]
T = 40.0 [ C]
Fresh Air
Return Air
. .
o
o
o
Fig. 3. Effect of fresh air temperature and mass ratio on DTO and DTR.
798 M.A. Abd El-Baky, M.M. Mohamed / Applied Thermal Engineering 27 (2007) 795–801

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pipe were used in this research [13]. The sonic limitation
occurs, when the vapor velocity at the evaporator exit is
sonic. The maximum heat transferred can be calculated
as follows:
Qs ¼ pr2
VqVhfg
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
cRT V
2ðc þ 1Þ
s
: ð4Þ
The entrainment limitation occurs, when the liquid and va-
por move in opposite directions in the heat pipe and the va-
por velocity is sufficiently high so that the liquid turned
from the surface of the wick and entrained in the vapor.
The maximum heat transmitted can be calculated as
follows:
Qent ¼ pr2
VqVhfg
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2pqVrl cos h
k
r
; ð5Þ
where k is the characteristic dimension of liquid/vapor
interface and for 100 mesh was taken as 0.036 mm and h
is contact angle and was taken equal to zero [16]. It is nec-
essary to have a heat pipe capable of transferring a mini-
mum heat at the temperature range of 15–55 °C. The
effectiveness of certain heat pipe design can be obtained
[18,19] as follows:
e2
ð2A À BÞ þ 4ðB À AÞe þ ð2A À 4BÞ ¼ 0: ð6Þ
The second degree polynomial equation can be solved to
get the optimum effectiveness value as follows:
eopt ¼
4ðA À BÞ Æ
ffiffiffiffi
D
p
2ð2A À BÞ
; ð7Þ
where
D ¼ ½4ðB À AÞŠ2
À 4ð2A À BÞð2A À 4BÞ; ð8Þ
A ¼ P1CEHðTmax À TminÞ; ð9Þ
B ¼ P2CA=U; ð10Þ
P1 ¼ N=ð1 þ iÞ; ð11Þ
P1CEHQ À P2CAAHX À CF ¼ 0: ð12Þ
The sign concerning the square root of D must be taken as
negative to get a physically correct effectiveness value, since
30 35 40 45
T [ºC]
0.0
0.1
0.2
0.3
0.4
0.5
0.6
ε
o
Rm / m = 1o
. .Evaporator Side
Condenser Side T = 26 [ C]R
o
0.8 1.2 1.6 2.0 2.4 2.8
m / m
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
ε
R o
Evaporator Side
Condenser Side
T = 40 [ C]o
o
. .
Fig. 4. Effect of fresh air temperature and return air mass flow on effectiveness, e.
30 35 40 45
T [ºC]
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
ξ
o
Rm / m = 1o
. .
Evaporator Side
Condenser Side
T = 26 [ C]R
o
0.8 1.2 1.6 2.0 2.4 2.8
m / m
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0ξ
R o
Evaporator Side
Condenser Side
T = 40 [ C]o
o
. .
Fig. 5. Effect of fresh air temperature and return air mass flow on enthalpy ratio, n.
M.A. Abd El-Baky, M.M. Mohamed / Applied Thermal Engineering 27 (2007) 795–801 799

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e must be between 0 and 1. A computational program was
developed to calculate the optimum effectiveness of the
heat pipe heat exchanger. For a typical HPHE problem
as illustrated in [18,20], it is assumed that CE = 10À4
$/
(W h), H = 4000 h yearÀ1
, CA = 100 $/m2
, CF = 10,000$,
and i = 0.085. From our experiments, the average heat
transferred per heat pipe, Q is 50 W, the average overall
heat transfer coefficient, U is 29 W mÀ2
KÀ1, and the heat
transfer area of finned evaporator section, AHX is 8.42 m2
.
The temperature of fresh air, Tmax,is in the range of 32–
40 °C, and the temperature of return air, Tmin is kept con-
stant at about 26 °C (see Figs. 4 and 5).
The values of heat transferred through one heat pipe
compared to the sonic and entrainment limits are illus-
trated in Fig. 6. The data was taken at fresh air tempera-
ture of 32, 36.2 and 40 °C. It can be seen that the heat
transferred at fresh air temperature of 32 and 36.2 °C is
lower than the sonic and entrainment limits. But, at fresh
air temperature of 40 °C, the heat transferred is above
the sonic limit, which means that the vapor is superheated
in the evaporator section. The main parameters affecting
the optimum effectiveness are Q, U, DTmax and CF. Fig. 7
shows the comparison between experimental effectiveness
and calculated optimum effectiveness with neglecting CF.
It is observed that the experimental effectiveness is close
to the optimum effectiveness at low temperature of fresh
air. It is also seen that, the optimum effectiveness of the
heat pipe heat exchanger increases, when the fresh air tem-
perature was increased.
4. Conclusions
The experimental study of heat pipe heat exchanger for
cooling fresh air with return air in air conditioning leads to
the following conclusions:
1. The temperature changes of fresh air, hot, and return
air, cold, are increased with increasing the inlet temper-
ature of fresh air.
2. The heat transfer and effectiveness for both evaporator
and condenser sections are increased with increasing
the fresh air inlet temperature.
3. Increasing the return to fresh air mass flow rate ratios by
about two times leads to increase the temperature
change of fresh air about 20% and effectiveness of the
heat pipe heat exchanger by about 26%.
4. The effect of mass flow rate ratio on effectiveness is
positive for evaporator side and negative for condenser
side.
5. The enthalpy ratio between the heat recovery and con-
ventional air mixing is increased with increasing the inlet
fresh air temperature and decreased with increasing
mass flow rate of return air.
6. The heat recovery is increased with increasing inlet fresh
air temperature and attained about 85%.
7. The calculated data showed that the heat transferred
through the heat pipes at fresh air temperature of 32
and 36.2 °C is lower than the sonic and entrainment lim-
its. But, for fresh air temperature of 40 °C, the heat
transferred is above the sonic limit, which means that
the vapor is superheated in the evaporator section.
8. The maximum deviation between experimental data of
effectiveness and the calculated optimum effectiveness
at the same conditions is less than 3.6%.
9. The main parameters affecting the optimum effectiveness
are Q, U, DTmax and CF and the experimental data of
effectiveness are close to optimum effectiveness at low
temperature of fresh air, which is near the operating
temperature of working fluid inside the heat pipe.
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T [o
C]
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
ε
Present experimental data
Evaporator section
optε
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o
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Fig. 7. Optimum effectiveness and experimental data with neglecting CF.
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Fig. 6. Present heat transfer data, Q compared with Qmax.
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