effect of inserting coiled wires on pressure drop of r-404a condensation
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i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 4 0 ( 2 0 1 4 ) 2 4e3 0
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Effect of inserting coiled wires on pressure dropof R-404A condensation
Mohammad Reza Salimpour*, Hesam Gholami
Department of Mechanical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e i n f o
Article history:
Received 3 June 2013
Received in revised form
12 October 2013
Accepted 27 October 2013
Available online 7 November 2013
Keywords:
Pressure drop
Turbulator
R-404A
Condensation
* Corresponding author. Tel.: þ98 311 391521E-mail address: [email protected] (M
0140-7007/$ e see front matter ª 2013 Elsevhttp://dx.doi.org/10.1016/j.ijrefrig.2013.10.013
a b s t r a c t
In this study, an experimental investigation was carried out to determine the increase of
pressure drop during convective condensation of R-404A vapor inside coiled wire inserted
tubes. Experiments were performed for a plain tube and five coiled wire inserted tubes.
Data were collected for four mass velocities. For each mass velocity, six different vapor
qualities were considered. From the results, it is found that the coiled wire inserts
increased the pressure drop up to 1200% compared to plain tube values. An empirical
correlation is also developed to predict the pressure drop of coiled wire inserted tubes
during R404-A condensation with accuracy of �20%.
ª 2013 Elsevier Ltd and IIR. All rights reserved.
Effet de l’insertion de fils torsades sur la chute de pression decondensation de R404A
Mots cles : Chute de pression ; Turbulateur ; R404A ; Condensation
1. Introduction
Accurate prediction of two-phase flow pressure drop in
evaporators, condensers and pipelines is of paramount
importance for the design and optimization of refrigerant, air-
conditioning and heat pump systems. Themethods which are
used to enhance the heat transfer will often increase the
pressure drop too. One of these methods is using tube inserts
like twisted tapes, brushes, and coiled wires. Due to low cost
and their capability for easy installing or removing inside the
tubes (for cleaning purposes), the coiled wire inserts usage is
0; fax: þ98 311 3912628..R. Salimpour).ier Ltd and IIR. All rights
growing. The coiled wire insert causes increase in both heat
transfer and pressure drop; therefore, the calculation of
pressure drop in such systems is of vital importance.
Some researchers have studied two-phase flow heat
transfer and pressure drop during forced convective conden-
sation in horizontal tubes with inserts. Kumar et al. (2005)
studied heat transfer enhancement during condensation of
R-22 inside a horizontal twisted tape inserted tube. Hejazi
et al. (2010) studied the condensation of R-134a in horizontal
tubes equipped with twisted tapes and developed a new cor-
relation for prediction of pressure drop. Salimpour and
reserved.
Nomenclature
x vapor quality
d diameter, m
e wire diameter, mm
L length of test section, mm
p coil pitch, mm
f friction factor
Re Reynolds number
Y2 gas to liquid pressure gradient ratio
G mass velocity, kg(m2 s)�1
P Pressure, Pa
Greek symbol
a helix angle
ε void fraction
m dynamic viscosity, Pa s
r density, kg m�3
F2Ch Chisholm two phase multiplier
Subscripts
in inlet
G vapor
L liquid
out outlet
i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 4 0 ( 2 0 1 4 ) 2 4e3 0 25
Yarmohammadi (2012a, 2012b) reported both enhanced heat
transfer and increased pressure drop of twisted tape inserted
tubes during condensation of R-404A, experimentally. They
observed that the insertion of twisted tape inside horizontal
tubes increases the condensing pressure drop up to 239%
compared to plain tubes on a nominal area basis.
Agrawal et al. (1998) used coiled wires to augment heat
transfer coefficient in condensation of R-22 vapor. Akhavan-
Behabadi et al. (2005, 2008) performed two experimental
studies on heat transfer enhancement and pressure drop
increase of R-134a condensation inside horizontal tubes
with spring inserts. They observed that the insertion of
helically coiled wires inside horizontal tubes augments the
pressure loss from 260% to 1600% compared to the plain
tube values.
Despite the large amount of research devoted to the effect
of tube inserts on the condensation pressure drop, it is noted
that only a limited number of studies have been carried out on
R-404A condensation. Moreover, it is noteworthy that there is
no investigation on the effect of the coil inserts on the pres-
sure drop of this kind of refrigerant. R-404A is a non-
azeotropic and zero ODP refrigerant designed to serve as a
long-term alternative to R-502 and R-22 in low and medium
temperature commercial refrigeration applications. Due to
the increasing application of R-404A in different systems the
present experimental workwas conducted to assess the effect
of inserting coiled wires on the pressure drop during
condensation of R-404A in horizontal tubes.
Fig. 1 e Schematic vie
2. Experimental setup
The schematic view of the experimental setup is illustrated in
Fig. 1. Test section is a double pipe counter-flow heat
exchanger. Refrigerant flows inside inner tube while cold
water flows in counter flow through annulus. The internal
tube is made of copper with 14.1 mm ID and 0.9 mm thickness
and 1.00 m length. The outer carbon steel tube is insulated
with glass wool to minimize heat loss to surrounding. Using J-
type thermocouples with precision of 0.1 �C, outside wall
temperature of the inner tube of test section is measured at
four locations. At each location, at the top, sides and bottom of
the tube, the sensors were used in order to achieve the lon-
gitudinal and circumferential temperature distribution of the
tube wall. The average value of four circumferential readings
is taken at each longitudinal location. The cooling water
temperatures are measured at inlet and outlet of test and pre-
condensers with the same accuracy. Refrigerant absolute
pressure is measured by pressure gauge at three locations: at
the inlet of pre-condenser and at the inlet and outlet of the
test condenser with precision of 0.1 kPa. Pressure drop be-
tween the inlet and outlet of the test condenser is measured
by differential pressure transmitter which is calibrated in the
range of 0e7.4 kPa with accuracy of 3 Pa. Pre-condenser is
used to obtain the desirable range of vapor quality at the test
condenser inlet by regulating the pre-condenser coolingwater
flow rate. Refrigerant temperature ismeasured at the inlet and
w of the set-up.
Table 2 e Range of operating parameters of theexperiments.
Parameters Range
Refrigerant mass velocity 71.2e142.4 kg (m2 s)�1
Average condensing temperature 27.8e33.2 �CAverage cooling heat flux 4.33e15 kW m�2
Cooling water mass flow rate 0.036e0.122 kg s�1
Inlet temperature of cooling water 12e13.5 �CCooling water temperature increase 1.1e5.0 �CInlet vapor quality 0.2e0.82
Table 1 e Characteristic parameters of the coiled wires.
Tube set di (mm) do (mm) de (mm) L (mm) p (mm) e (mm) a (degree)
A 14.1 15.9 11.9 1000 10 0.5 73
B 14.1 15.9 10.4 1000 10 1 73
C 14.1 15.9 9.1 1000 10 1.5 73
D 14.1 15.9 8.4 1000 8 1.5 77
E 14.1 15.9 9.9 1000 13 1.5 69
F 14.1 15.9 e 1000 Plain Plain e
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 4 0 ( 2 0 1 4 ) 2 4e3 026
outlet of pre-condenser. Leaving the refrigerant from the test
condenser, it is cooled down to sub-cooled liquid using a
secondary condenser. The secondary condenser is actually
used to deliver completely condensed refrigerant to the rota-
meter. The pre- and secondary condensers are insulated by
glass wool to minimize heat loss to surrounding. Two appro-
priate calibrated rotameters with flow ranges of 0e15 L min�1
and accuracy of 0.1 L min�1 are used to measure the cooling
water flow rate through the pre- and test condensers. A cali-
brated rotameter with the range of 30e100 L h�1 and accuracy
of 10 L h�1 installed downstream of the secondary condenser
is used to measure the refrigerant mass flow rate. Character-
istic parameters of the used coiled wires such as thickness, e,
coil pitch, p, tube length, L, internal tube diameter, di, and
external tube diameter, do, are given in Table 1. Moreover,
geometrical parameters of the coils are illustrated in Fig. 2.
3. Data collection
In this experimental study, 150 test runs were conducted to
measure the pressure drop during the condensation of R-404A
vapor with five refrigerant mass velocities, 71.2, 89, 106.8, 124.6
and 142.4 kg(m2 s)�1 and different vapor qualities inside plain
tube and tubes with coiled wire inserts. To have a reasonable
comparison among different geometries, mass velocities are
calculated based on the plain tube case. The range of operating
conditions is given in Table 2. At first, data were collected for
condensation of R-404A vapor inside plain tube. Plain tube data
were collected in order to verify integrity of the experimental
setup and to have a reference data for contrasting the perfor-
mance of different coiled wire inserts. After that, data were
collected for condensation of R-404A vapor inside coiled wire
inserted tubes. The vapor qualitywasmeasured at the inlet and
outlet of test condenser by performing energy balance along
the pre- and test condensers, respectively. It was noted that
since the changes of vapor quality throughout the test
condenser were less than 0.2, vapor quality was assumed to be
the average of these two values.
The uncertainty of the measured refrigerant mass velocity
and saturation temperature were less than 4.5% and 0.5 �C,respectively; while, the uncertainty of the measured total
pressure drop was less than 1.5 Pa.
Fig. 2 e Dimensional parameters of the coils.
4. Results and discussion
4.1. Plain tube results
Fig. 3 shows the variation of pressure drop at different vapor
qualities for plain tube. From this figure, it is revealed that
with increasing mass flow rate and vapor quality, pressure
drop grows. The reason lies in this fact that higher mass ve-
locity augments turbulence intensity in the liquid film and
vapor core which results in the increase of shear stress. Also,
higher vapor quality enhances the vapor core volume. Hence,
vapor core velocity causes more velocity difference between
vapor core and liquid film. To validate the experimental set-
up, the plain tube pressure drop results were compared with
an existing correlation.
Fig. 3 e Frictional pressure drop variations with vapor
quality for plain tube.
i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 4 0 ( 2 0 1 4 ) 2 4e3 0 27
The experimental pressure drop is calculated as the sum of
static, momentum and frictional pressure drops as,
DPtot ¼ DPfric þ DPmom þ DPsta (1)
For horizontal tube, DPsta ¼ 0; hence,
DPtot ¼ DPfric þ DPmom (2)
Momentum pressure drop is obtained from the following
correlation (Collier and Thome, 1994).
DPmom ¼ G2total
("ð1� xÞ2rLð1� εÞ þ
x2
rGε
#out
�"ð1� xÞ2rLð1� εÞ þ
x2
rGε
#in
)(3)
where, Gtotal, x and ε are total mass velocity, vapor quality and
void fraction, respectively. Void fraction is calculated from
Steiner (1993) version of Rouhani’s (1969) correlation as
follows.
ε¼ xrG
(½1þ0:12ð1�xÞ�
�xrG
þ1�xrL
�þ1:18ð1�xÞ½gsðrL�rGÞ�0:25
Gtotalr0:5L
)�1
(4)
Frictional pressure drop is achieved from subtracting the
momentum pressure drop from the experimental data. From
the experiments it is seen that the ratio of the calculated
momentum pressure drop to the total measured pressure
drop range is 5e12%.
The results for the plain tube are then compared with the
predicted values from correlations proposed by Friedel (1979),
Lockhart and Martinelli (1979), Gronnerud (1972), Chisholm
(1973) and Muller-Steinhagen and Heck (1986); and it is
found that the present experiment is best agreed with the
proposed correlation by Chisholm (1973). As is seen from
Fig. 4, this correlation predicts the experimental pressure drop
of present study within error band of �20%. The Chisholm
correlation to calculate the frictional pressure drop is:
Fig. 4 e Comparison of the plain tube experimental
frictional pressure losses with the values predicted by
Chisholm correlation.
�dpdz
�¼
�dpdz
�F2
Ch (5)
frict Lwhere,F2Ch is Chisholm two-phase flow correlation factor
defined as
f2ch ¼ 1þ �
Y2 � 1�hBx
ð2�nÞ=2ð1� xÞð2�nÞ=2 þ x2�ni
(6)
In Eq. (6), n ¼ 0.25, B ¼ 4.8, and Y is calculated from liquid
and gas frictional pressure drop gradients as
Y2 ¼ ðdp=dzÞGðdp=dzÞL
(7)
where,
�dpdz
�L
¼ fL2G2
total
dirL(8)
and
�dpdz
�G
¼ fG2G2
total
dirG(9)
As coiled wires turbulate the flow, single-phase friction
factors are defined as,
fL ¼ 0:079
Re0:25L
; fG ¼ 0:079
Re0:25G
(10)
where,
ReL ¼ Gtotaldi
mL
; ReG ¼ Gtotaldi
mG
(11)
4.2. Coiled wire inserted tubes results
It is observed that by using coiled wires inside horizontal
tubes, pressure drop is increased. The amount of this increase
depends on the mass velocity, vapor quality and geometric
dimensions of coiled wire inserts. Figs. 5e7 show the varia-
tions of pressure drop versus vapor quality for tubes with
Fig. 5 e Variation of frictional pressure loss with vapor
quality for tube set ‘A’.
Fig. 6 e Variation of frictional pressure loss with vapor
quality for tube set ‘B’.
Fig. 8 e Variation of frictional pressure loss with vapor
quality for tube set ‘D’.
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 4 0 ( 2 0 1 4 ) 2 4e3 028
coiledwire inserts of 10mmpitch and 0.5, 1.0 and 1.5mmwire
diameter, respectively. Considering these figures, it is
concluded that the increase of pressure drop is due to the
increase of coiled wire diameter. For example, tube set C
which holds the thickest coiled wire has the highest pressure
drop compared to other tube sets. Akhavan-Behabadi et al.
(2008) referred this trend to the fact that the rise in wire
thickness increases the frictional surface which results in
more frictional pressure loss. Also, thicker coils promote tur-
bulence of the vapor core and liquid film which causes
increased frictional pressure loss. For these coils, momentum
pressure loss is also increased because of the increased ve-
locity resulted from reduced flow cross-section area. Figs. 7e9
present the variations of condensation pressure drop with
Fig. 7 e Variation of frictional pressure loss with vapor
quality for tube set ‘C’.
vapor quality for tubes with coiled wire inserts of 1.5 mmwire
diameter and 10, 8 and 13 mm coil pitches, respectively. From
these figures, it is seen that with reducing the coil pitch,
pressure drop increases. This trend can be explained as for
smaller coil pitches, frictional surface per length is increased
which results in higher frictional pressure drop.
Fig. 10 shows the variations of the ratio of coiled wire
inserted tube to plain tube pressure drops for different mass
velocities. From this figure, it is obvious that the ratio usually
increases with mass velocity. From this figure, it is also seen
that maximum value of pressure drop increase is occurred in
tube set D which has the thickest wire diameter and the
smallest coil pitch. The pressure drop of this tube set is
increased by about 1300% in comparison with plain tube
Fig. 9 e Variation of frictional pressure loss with vapor
quality for tube set ‘E’.
Fig. 11 e Comparison of the present experimental frictional
pressure drops with the values predicted by Eq. (15).
i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 4 0 ( 2 0 1 4 ) 2 4e3 0 29
values at mass velocity 142.4 kg (m2 s)�1. Tube set A which
exhibits the minimum increase, augments the pressure drop
by 131% in comparison to the plain tube at mass velocity 89 kg
(m2 s)�1.
5. Development of a correlation
Literature review on forced condensation of refrigerant
revealed the need for a suitable correlation to predict the
condensation pressure drop of R-404A vapor in tubes with
coiled wire inserts. To develop a new correlation, the Chis-
holm’s correlation that predicted plain tube results within the
Fig. 10 e Ratio of coiled wire inserted tube to plain tube
frictional pressure losses for different mass velocities.
error band of �20%, was used as the basic correlation to
consider the effects of coiled wire inserts. The combination of
two parameters (p/d) and (e/d) in the form of (e2/pde) was used
in the proposed relation. For this purpose, the following form
of functional relationship which was previously proposed by
Akhavan-Behabadi et al. (2008) is used,
DPc=DPs ¼�c1 þ c2
e2
pde
�c3
(12)
where DPs and DPc are total pressure drops in plain tube and
coiled wire inserted tube, respectively and de is equivalent
diameter calculated as follows,
de ¼�d2 � ge
��ðdþ gÞ (13)
where,
g ¼ peðd� eÞ=ðp sinaÞ (14)
In Eq. (14), a is coil helix angle. The following correlation is
derived by using least square regression analysis to predict
the present experimental data of coiled wire inserted tubes:
DPc=DPs ¼�2þ 450
e2
pde
�0:81
(15)
Fig. 11 contains a comparison between the data predicted
by our proposed correlation and experimental data of pres-
sure drop. From this figure, it is observed that this correlation
predicts the experimental data within an error band of �20%.
6. Conclusions
From the present investigation, the following conclusions can
be drawn.
1. The insertion of coiled wire inside horizontal tubes causes
increase of condensing pressure drop up to 1300%
compared to plain tube values.
2. Pressure drop is increased with the increase of coiled wire
diameter.
3. It is also seen that with reducing the coil pitch, pressure
drops more.
4. A new correlation is proposed to predict the forced
convective condensation pressure drop of R-404A inside
horizontal coiled wire inserted tubes.
Acknowledgment
The authors wish to acknowledge Isfahan University of
Technology for its financial support for the setup construction
and implementing this research.
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