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International Journal of Mechanical Engineering and Technology (IJMET)
Volume 9, Issue 1, January 2018, pp. 895–904 Article ID: IJMET_09_01_098
Available online at http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=1
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication Scopus Indexed
COILED TUBE HEAT EXCHANGERS -
A REVIEW
Sai Sarath Kruthiventi, N.Govindha Rasu
School of Mechanical Engineering, VIT, Vellore, India
Sai Sarath Kruthiventi and Y.V.Hanumantha Rao
Department of Mechanical Engineering, Koneru Lakshmaiah Education Foundation, India
ABSTRACT
Heat exchangers are used to transfer heat from one fluid stream to another fluid
stream. Coiled TiTHEXs (Tube in tube Heat exchangers) are widely used in
refrigeration, air-conditioning and cryogenic applications. Research has been carried
out extensively on coil tubes to assess their performance. Studies on overall system
level with these heat exchangers, is done by a few while some studies reported the
correlations for Nusselt number and friction factor to estimate the heat transfer
coefficients and pressure drops. Most of the work is done on coiled tubes, but there is
a wide gap in literature in the estimation of the friction factor and rate of heat transfer
in multiple TiTHEXs. The present paper deals with review of coiled tube heat
exchangers over past few decades and also discuss about the importance of multiple
TiTHEXs which would helpful to the researchers to have a precise selection of a
device used in cryogenic applications.
Keywords: Coiled heat exchanger, Friction factor, Nusselt number.
Cite this Article: Sai Sarath Kruthiventi, N.Govindha Rasu and Y.V.Hanumantha
Rao, Coiled tube heat exchangers - A Review, International Journal of Mechanical
Engineering and Technology 9(1), 2018. pp. 895–904.
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=1
1. INTRODUCTION
Heat exchangers are used whenever heat must be transferred from one fluid stream to other
fluid stream (single or multiple streams). The type of heat exchanger used in the system
depends on many factors like availability of space, capital cost, fabrication techniques,
working fluids, application etc. The foremost and widely used heat exchanger from many
decades is the simple TiTHEXis due to the fact that it is simple in construction. The
geometrical parameter of the TiTHEX (tube diameters and length) depends on the amount of
heat to be transferred. To minimize the space required by a TiTHEX in the system, most of
the times they are made in the coiled form. However the nature of flow through a coiled tube
is entirely different from that of the straight tube. Centrifugal forces are developed in the flow
Sai Sarath Kruthiventi, N.Govindha Rasu and Y.V.Hanumantha Rao
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through coiled tubes which in turn generates secondary flow. These secondary flows are
responsible for high turbulence which improves the heat transfer rate in coiled tubes.
In fact Grindley and Gibson [1] were the first one who noticed the effect of curvature on
fluid viscosity in a coiled tube. Later Eustice [2] observed an extra resistance for the fluid
flowing through coiled tube when compared to straight tube. Similar to Reynolds
experiments, Eustice [3] also injected ink into the fluid flowing through the coiled tube and
observed the secondary flow pattern of fluid. Jescheke [4] was the first one to conduct heat
transfer studies in coiled tubes experimentally. His studies were carried out with air in
turbulent regime while. He also proposed an equation to estimate the enhancement in heat
transfer in coiled tubes when compared to that of straight tubes. Secondary flow and mixing
of the fluid are the reasons for enhancement in heat transfer in coiled tubes. So the coiled tube
heat exchangers have considerable advantage over conventional straight tube heat exchangers
and this advantage has been quantitatively investigated. Dean [5] was the first one to propose
a mathematical model for laminar flows in coiled tubes. In coiled tubes, secondary flow
develops and centrifugal forces cause proper mixing of the fluid which is more pronounced in
laminar flows. He developed a mathematical description to describe the flow through coiled
tube, using a perturbation method. His work is aimed at finding out the deviation of flow
pattern from poiseuille flow. He as represented the secondary flow developmentby a
parameter De= Re * d
s
/R called Dean number. However the solution he has developed is
applicable for small Dean numbers which has little practical significance [6].
Few authors have developed correlations for HTC in helical coils [7, 8]. But very few
works are available on estimation of outside heat transfer coefficients and these available
works deal with constant wall temperature or constant wall heat flux boundary condition on
the coil [9].There is a need to study the heat transfer characteristics by considering exact flow
scenario i.e. instead of assuming boundary condition on coil wall, a fluid has to be passed
outside the wall and the boundary parameters to be assessed practically. In addition to heat
transfer, pressure drop is also an important parameter to be considered for optimal functioning
of the overall system. Due to this reason, multiple tubes are replaced with single tube inside a
shell. Further the tube side flow is equally divided to reduce the pressure drop on the tube
side. But, coiled multiple TiTHEX offers other advantages like Compact when compared with
single TiTHEX, High heat transfer coefficients may be achieved on shell side by reducing the
shell size. Due to these advantages, a heat exchanger designer always proposes to use the
multiple TiTHEXs to satisfy their diverse requirements. The present paper discusses the
review of the existing studies on coiled TiTHEX and coiled multiple TiTHEXs.
2. STUDIES ON COILED TITHEXS
White [10] extended the work of Dean by conducting experiments on three different coil Heat
Exchangers with different coil diameters. The tests were performed with two different fluids
i.e. oil and water. In his study, oil was used, for simulating low Reynolds number flows and
water for simulating higher Reynolds number flows. He concluded that the flow in the curved
tubes is more stable than in straight tubes and there is no significant effect of curvature on
pressure drop when the Dean numbers were less than 11.6. He proposed a friction factor
correlation as given below for the coiled tubes as a function of straight tube friction factor and
the validity of correlation is established for D/d = 5,15, 20, and 50 where, D is coil diameter
and d is tube diameter.
(1)
Ito [11] conducted experiments on both curved and straight tubes. All the experiments
were performed for curved tube in the turbulent regimes with curvature ratio (tube diameter to
Coiled Tube Heat Exchangers - A Review
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coil diameter) ranging from 0.00154 to 0.06. He reported that the friction factor in curved
tubes is same as in straight tubes for Re (d/D)
2
values less than 0.034. The friction coefficient
correlation for coiled tube that relates with straight tube for D/d = 16.4 to 648 and
is given in equation 2.
(2)
Seban and McLaughlin [12] experimentally studied flow through coiled tubes with
different curvature ratios and different Reynolds numbers (12 to 5600). They observed that
for the same Reynolds number, Nusselt numbers are found to be significantly high in the case
of coiled tubes compared to that of straight tubes. Later, with the same set up the turbulent
regimes were studied with the Reynolds numbers ranging from 6000 to 65000 and reported
the Nu correlation as below
!
""#$%
!&
''
#
()
''
(3)
*+)##%
!&
#,-#./0. 0/12#%)34#5+,1#6"7
Ozisik and Topakoglu [13] studied the flow pattern of fully developed laminar flows in
curved tubes. They solved the heat transfer equations for a fully developed laminar flow by
assuming uniform heat flux boundary condition and showed that the heat transfer depends on
curvature ratio, Reynolds number and Prandtl number. Kalb and Seader[14] reported the
study of heat transfer and friction factors of fluids with different Prandtl numbers in curved
tubes for uniform heat flux boundary conditions. They observed that the increase in Nusselt
numbers is significant compared to friction factors for all the fluids that were tested and it is
marginal for liquid metals. Later they extended the study with constant wall temperature as a
boundary condition and observed that the increase in average Nusselt number with respect to
curvature to be marginal. The reported correlations are given in Eqn. 4 and Eqn.5. Prasad et
al. [15] conducted a series of experiments to estimate the performance of helical coil type heat
exchanger by developing the new correlations to measure the HTC as given in Eqn.6.
Acharya et al. [16] reported the effect of chaotic particle paths on heat transfer in coiled tubes.
It is found that the flow of fluid in chaotic paths can be used as one of the methods to
improvise the rate of heat transfer in coiled tubes. They proposed two correlations for
predicting Nusselt numbers, as given in Eqn. 7 and Eqn. 8.
88#9
()
:
for ()"""$#13#""$ (4)
";8#9
<
()
for #()#"=#13#$ (5)
""$=
:
(6)
## ";#
'
()
'
(Pr > 1) (7)
"=#
'
()
(Pr < 1) (8)
Xin et al. [17] conducted pressure drop studies in the helical pipes with both vertical and
horizontal orientations. Air and water were used as working fluids in their studies.
Experiments were conducted with helical pipes for various inside and outside tube diameters
and compared their results with the existing correlations [7, 15] and found that the deviations
are within ±15%. The proposed correlation are given in Eqn. 9 and for turbulent flow in Eqn.
10. Nusselt number correlation proposed for laminar flow is given in Eqn. 11 and for
turbulent flow is given in Eqn. 12.
Sai Sarath Kruthiventi, N.Govindha Rasu and Y.V.Hanumantha Rao
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>
?
@
AB
:
C
D
EF
GH
I
JJ
'
::''
K
L
M
(9)
Here m =2 for Dean number less than 20, m = 1 for Dean Number lies between 20 and 40,
m = 0 for Dean Number greater than 40 and f
st
= 16/Re.
%
"""=N$J""=#
O
O
(10)
8J""PQJ"PR
S
T
U
V
I
()
D
(11)
5+)W4"$J"N;"8Q2
X
9V
U
Y
ξ
Z:[\]^
C<#_
ξ
`
Yab
ZD
O#[
(12)
ξ#"8c
J#""82
X
9
M. A. Petrakis et al. [18] fabricated and conducted experiments on two heat exchangers
which differ only in inner tube diameter, keeping outer tube diameter, coil diameter and wall
thickness same whose values are 15.9 mm, 235.9 mm and 0.8 mm respectively. They
concluded that for identical mass flow rates in annulus and tube side, overall HTC values
increases with increase in inner tube size of a heat exchanger. Prabhanjan et al. [19] reported
the advantages of helical coiled heat exchangers as compared to the straight tube heat
exchangers. The experimental results clearly show the increase in HTC by changing the
geometry of the heat exchanger. Finally, they stated that helical coiled type heat exchangers
possess higher HTC as compared to the straight tube heat exchangers. Few authors [20, 21]
developed a computer based numerical technique to estimate the HTC values of a helical
coiled heat exchangers. The results obtained were matches with the present experimental
information. Tests were conducted for different flow rates by assuming operating fluid as
milk. The amendment in the flow pattern of a heat exchanger with annular contact leads to
increase of overall HTC values by 20 percent. The performance of these heat exchangers
(inner tube is coaxial with outer tube) were tested and compared by Louw and Meyer
[22].They also observed that the Nusselt numbers in the inner tube are reduced by 40% for the
same heat exchanger and it is due to the low wall temperatures at the point of contact but this
is more than compensated by the increase in annulus HTC, which is approximately 96%. The
flow condition in the annulus is strongly influenced by the overall HTC as predicted by the
numerical model of Timothy et al. [23] for double pipe helical heat exchanger. They
correlated annulus Nusselt number with a modified dean number.
3. SHELL AND COIL HEAT EXCHANGERS
Somchai and Maitree [24] carried out experiments to investigate the HTC and pressure drop
characteristics during evaporation process in both helical coiled and straight tube heat
exchangers. Results show that average HTC values are higher in helical coiled with annular
contact (concentric tube heat exchanger) as compared with straight tube. The proposed
correlation is given in Eqn. 13. Somchai and Maitree [25] and Garimella et al. [26] carried out
experiments to investigate the HTC and pressure drop characteristics during condensation
process in both helical coiled and straight tube heat exchangers. They observed that there is
Coiled Tube Heat Exchangers - A Review
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fractional increase in HTC values of 33-53% and pressure drop of 29.46%. The proposed
correlation is given in Eqn. 14.
P;$9
'
de
O
Yf#g##"
[0.132g£-0.0238 (13)
"8$N9
<
de
:
he
O'
Yf#g##"
[0.112g£-0.0422 (14)
Few authors [27, 28] compared completely different studies of curved tube heat
exchangers. Entire studies were categorised into two sets like helical coil and spiral coil tubes.
He explicit the benefits and effects of helical coiled and spiral coil heat exchangers mostly
based upon the HTC values. Paisarn et al. [29] carried out experiments and also numerical
simulation in a horizontal spiral coiled heat exchangers. Their studies declared that there is a
valid agreement between the experimental and simulation data. Experiments were extended
by varying the curvature ratios. They concluded that there is a better heat transfer rate in case
of spiral coiled as compared to straight tubes.
Salimpor [30] experimentally investigated the heat transfer characteristics in shell and
coiled tube heat exchanger. It was clearly seen that coil side heat transfer coefficients with
larger pitches are less than the heat transfer coefficients with smaller pitch. Variation of
Nusselt numbers with Dean number for the coiled heat exchangers tested by Salimpor with
different oil inlet temperatures. It can be clearly understood that the Nusselt numbers
decreases for all the three coils with increase in oil inlet temperatures. Finally he has proposed
a new correlation and it is given in Eq. 15.
&ij
"$$c9
U
de
k
O'':
l
'
(15)
For, 35<De<410, 0.058<ɤ<0.095, and 0.34<φ<0.6
Shokouhmand et al. [31] conducted experiments on shell and coiled tube heat exchangers
by varying the coil pitches and curvature ratios. Experiments were carried out for heat
exchangers with three different coil pitches. The Dean numbers are varying from 50 to 200.
He reported that the shell-side heat transfer coefficients of the coils with smaller pitches are
less than the ones with larger pitches. It is evident that HTC values are proportional to Re as
well as curvature ratio. It is very clear that higher HTC values are predicted with counter flow
configuration but at low Reynolds number, while the Nusselt numbers for both the flow
configurations (parallel/counter) are predicted to be same at higher Reynolds numbers
(Re>10000). Few authors [32-34] performed both experimental and theoretical analysis on
helical coiled type heat exchanger. The results show reasonable match between experimental
values and CFD predictions. Based upon the obtained results a different correlation was
developed to estimate the inner HTC of the helical coil.
""N$9
U
de
(16)
m3)9N"""#13#N"""
Vimalkumar et al. [35] conducted his studies on numerically designed helically coiled
heat exchangers. The studies were performed by developing three dimensional governing
equations for mass, momentum, and heat transfer equations and solved by applying control
volume finite difference method. Moreover, the studies were extended by varying the mass
flow rate ranging starting from 200 to 600kg/hr. The results obtained were compared with
existing experimental information and ascertained that % error was within the permissible
limit. Wenzhicui et.al [36] conducted experiments in a micro finned helically coiled tube with
R134a as operating fluid. They categorised the flow as 3 regimes such as stratified-wavy
flow, annular flow and intermittent flow. Friction factor studies were done in a two phase
region and accordingly the correlations were developed [37]. Ivan oi piazza et al. [38] studied
flow through on helically coiled pipes numerically for both laminar and turbulent type of
flows by using k- Ɛ, k-ω, RSM models. They observed that application of RSM- model in the
fully turbulent region (Re>1.4×10
4
) for different values of Coil curvature ratio#Yn) gives
Sai Sarath Kruthiventi, N.Govindha Rasu and Y.V.Hanumantha Rao
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accurate results compared with experimental data while all the models predicted more or less
the same in laminar region. Gupta et.al [39] experimentally measured pressure drops of an
incompressible Newtonian fluid in helical coils of circular cross section for understanding the
result of varying coil pitch and coil diameter upon friction factor using five coils with twelve
completely different combinations of radii. They classified their study based on Germano
number Y
op
[ which is a function of Reynolds number and coil curvature. Based on these
studies, correlations were developed for estimating friction factors in coiled tubes as given in
equations 19 and 20 and found that the correlation is capable of predict the experimental data
with 10% deviation.
%
!&
%
q&
rJ";"8
op
<
s,t)4/u3#u 4v)w=" (17)
%
!&
%
q&
rJ"$N$
op
sWt)4/u3#u 4v)=" (18)
Pimenta and Campos [40] developed friction factor correlations for constant wall
temperature condition in a helical coil when it is placed vertically and concluded that friction
factors for Newtonian fluids with simultaneous heat transfer can be calculated using
correlations of isothermal flows that are available in the literature. They further observed that
the friction factors of non-Newtonian fluids are identical to Newtonian fluids for the Dean
numbers less than 80. Pawar and Sunnapwar [41] experimentally investigated heat transfer
characteristics of Newtonian and non-Newtonian fluids in helical coils for both laminar and
turbulent regimes. Water-glycerol is used as Newtonian fluid whereas polymers of carboxy-
methyl cellulose as non-Newtonian fluid. It is observed that Nusselt number decreases with
the increase in helix diameter for the same mass flow rate. They concluded that that overall
heat transfer coefficients of water are high when compared to water-glycerol and non-
Newtonian fluids. At the end, they developed a correlation for Nusselt number for Newtonian
fluids as givenin equation 19, 20 and 21. The results are compared with the previous works of
Xin et al. [18]and found that results were satisfactory. A new correlation for Nusselt number
is developed by Pimento and Campos [41] which is applicable for all Newtonian and non-
Newtonian fluids for finding heat transfer characteristics and is given in Eqn. 22.
"""$x
<U:
;"=wxw88cPy#""$wz#w""=$ (19)
""";x
PP"wxwccNy""$w#z#w""=$ (20)
";8t{
:<<
()
;wt{w$Ny#""$w#z#w""=$ (21)
Where z|
\
}
~
E
`
• M = Molecular weight of the fluid
Y"$9
:
"c$[()
<
(22)
For $w9w"N"y/u2#
W""N8
Hardik et al. [42] investigated experimentally flow in helical tubes with water as working
medium to studythe effect of coil curvature and Re on local Nusselt number. They measured
the wall temperatures at different locations both in axial and circumferential direction.
correlations were proposed for heat transfer for inner side and the outer side of the helical
coil, respectively given in Eqn. 23 and 24.
Xp#
""c
O'
:
()
(23)
€i&
""N
O::
:
()
(24)
%3)##=""#13#c"""
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4. STUDIES ON COILED MULTIPLE TITHEXS
Martynov and Krasnikova [43] performed experiments on three coiled multiple tube heat
exchangers with wire fins wound over the inner tubes. These heat exchangers differ only in
terms of the coil diameters while the other geometrical parameters were the same. HTC and
pressure drop characteristics were measured for the Reynolds number range 3700 to 100000.
It is concluded that the effect of coiling is not that significant on heat transfer if the ratio of
hydraulic diameter on shell side to coil diameter is less than 0.026.
Boiarski et al. [44] proposed a methodology for designing the cryogenic refrigerator. Two
TiTHEXs of lengths 2 m and 4 m were used with mixed refrigerants to study the performance
of the system. The homogeneous model is used to predict the overall heat transfer coefficients
and two-phase pressure drops in the heat exchanger to obtain an optimal design of the system.
These studies were carried out by applying correction factors on heat transfer and pressure
drops while the methodology that was used to estimate the correction factor was not
discussed. Later in 2000, the same authors fabricated three different multiple tube in tube heat
exchanger and experiments were conducted in a single stage and two stage systems with both
optimum and non-optimum mixtures. Better performance is observed with the single stage
system for the particular refrigerants tested. They also estimated the exergy efficiency of the
heat exchangers used in their work and reported that estimated efficiency range is about 0.42
to 0.62. Gong [45, 46] studied the variation of temperatures of the fluids in TiTHEXs. The
mixed refrigerants used in this work are classified based on the lowest temperature achieved
by the system. They stated that pinch in temperatures is a strong function of the mixture
composition and may occur at the warm end, middle or at the cold end of the heat exchanger.
Walimbe [47] performed experiments in a J-T (Joule-Thompson) refrigerator with different
mixtures. The refrigerator built by them uses a recuperative heat exchanger that cools the
mixed refrigerant charged in their system to temperatures as low as 65 K. They stated that
their system is capable of absorbing a heat load of 6 W at a refrigeration temperature of 80 K.
5. CONCLUSIONS
The studies so far conducted on coiled tubes focused mainly on determination of Heat transfer
and friction fall experimentally under constant temperature and constant wall heat flux
conditions using different working fluids. There are occasional numerical studies on helical
coils as well. It is clear that there are few studies on coiled multiple TiTHEXs that are stated,
whereas these studies are carried out on overall system level, in which the heat transfer and
flow characteristics of coiled tube- in- tube heat exchangers are extended for estimating their
heat transfer and flow characteristics. It can be of great interest for heat exchanger engineers
to understand the heat transfer and flow characteristics of coiled multiple TiTHEXs which are
widely used in refrigeration, air-conditioning and cryogenic applications. For achieving this,
further studies need to be carried out to estimate the heat transfer and friction factors in coiled
multiple TiTHEXs.
Nomenclature
c
p
Specific heat (J/Kg K)
De
Dean number
De
i
Inner Dean number
Nu Nusselt number
Nu
t
Tube side Nusselt number
Pr
Prandtl number
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Re
Reynolds number
Gz
Graetz number
BN Boiling Number
d
inner tube diameter (m)
D
Coil diameter(m)
f
Frictional factor
f
ct
Frictional factor for curved tube
f
st
Frictional factor for straight tube
HTC
heat transfer coefficient (W/m
2
K)
K
Thermal conductivity (W/m K)
q
Heat flux (W/m
2
)
r
Inner tube radius (m)
R
Helical coil radius (m)
TiTHEX Tube-in-tube heat exchanger
U
0
Over all heat transfer coefficient (W/m
2
K)
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Coiled Tube Heat Exchangers - A Review
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