Purandare, P. S., et al.: Experimental Investigation on Heat Transfer and Pressure Drop …
THERMAL SCIENCE, Year 2016, Vol. 20, No. 6, pp. 2087-2099
2087
EXPERIMENTAL INVESTIGATION ON HEAT TRANSFER
AND PRESSURE DROP OF CONICAL COIL HEAT EXCHANGER
by
Pramod S. PURANDARE
a*
, Mandar M. LELE
b
, and Raj K. GUPTA
c
a
Department of Mechanical Engineering, Thapar University, Patiala, India
b
Department of Mechanical Engineering, Maharashtra Institute of Technology, Pune, India
c
Department of Chemical Engineering, Thapar University, Patiala, India
Original scientific paper
DOI: 10.2298/TSCI140802137P
The heat transfer and pressure drop analysis of conical coil heat exchanger with
various tube diameters, fluid flow rates, and cone angles is presented in this pa-
per. Fifteen coils of cone angles 180° (horizontal spiral), 135°, 90°, 45°, and 0°
(vertical helical) are fabricated and analysed with, same average coil diameter,
and tube length, with three different tube diameters. The experimentation is car-
ried out with hot and cold water of flow rate 10 to 100 L per hour (Reynolds
range 500 to 5000), and 30 to 90 L per hour, respectively. The temperatures and
pressure drop across the heat exchanger are recorded at different mass flow
rates of cold and hot fluid. The various parameters: heat transfer coefficient,
Nusselt number, effectiveness, and friction factor, are estimated using the tem-
perature, mass flow rate, and pressure drop across the heat exchanger. The anal-
ysis indicates that, Nusselt number and friction factor are function of flow rate,
tube diameter, cone angle, and curvature ratio. Increase in tube side flow rate
increases Nusselt number, whereas it reduces with increase in shell side flow
rate. Increase in cone angle and tube diameter, reduces Nusselt number. The ef-
fects of cone angle, tube diameter, and fluid flow rates on heat transfer and pres-
sure drop characteristics are detailed in this paper. The empirical correlations
are proposed to bring out the physics of the thermal aspects of the conical coil
heat exchangers.
Key words: helical, spiral, conical coil, heat exchanger, secondary flow
Introduction
Heat exchangers are considered as an important engineering systems of energy gen-
eration and energy transformation in many industrial applications such as power plants, nu-
clear reactors, refrigeration and air-conditioning systems, heat recovery systems, chemical
processing, and food industries. Extensive use of heat exchanger in industries necessitates not
only performances, but also size of the heat exchanger. Hence, selection of proper heat trans-
fer enhancement technique has a prime importance. In industrial applications, various tech-
niques are used for heat transfer enhancement. These techniques are classified in two groups:
active and passive techniques [1]. The techniques which require external forces for enhance-
ment are known as active techniques like fluid vibration, electric field, and surface vibration,
whereas passive techniques are the techniques which are due to special surface geometries or
various tube inserts.
––––––––––––––
* Corresponding author; e-mail: purandareps@gmail.com
Purandare, P. S., et al.: Experimental Investigation on Heat Transfer and Pressure Drop …
2088 THERMAL SCIENCE, Year 2016, Vol. 20, No. 6, pp. 2087-2099
Shell and coil tube configurations are the important passive techniques, frequently
used in industry. Helical coiled configuration is very effective for heat transfer equipment
such as heat exchangers and reactors because of enhanced heat transfer, compact structure,
and accommodate large heat transfer surface area in a small space [2, 3].
Several studies have indicated that helically coiled tubes are superior to straight
tubes when employed in heat transfer applications [4, 5]. The secondary developed in the
curved helical coiled tubes due to centrifugal force observed in the fluid flowing, enhances the
heat transfer in coiled tube heat exchanger. The intensity of secondaries [6-8] developed in the
tube are the function of tube diameter, d, and coil diameter, D. For the smaller coil and tube
diameter the intensity of secondaries developed is high. This increase in intensity of secondar-
ies allows proper mixing of the fluid, which enhances heat transfer coefficient for the same
flow rate. Increase in tube and coil diameter reduces the seconderies developed which reduces
heat transfer coefficient [9].
In helical coil heat exchangers coil diameter remains same, hence the intensity of
seconderies developed does not change which effects in constant heat transfer coefficient. In
spiral coiled geometry the diameter of the coil continuously changes from innermost to the
outermost section [10]. This indicates continuous alteration of the local heat transfer coeffi-
cient from innermost to the outermost section. This indicates, in case of helical coils the heat
transfer coefficient is same throughout its dimensions, whereas in case of spiral coil heat ex-
changer overall heat transfer coefficient calculated with respect to mean diameter of the coil is
slightly different than of local coil heat transfer coefficient.
In literature, numbers of correlations are available for helical coil which shows that
Nusselt number, is a function of the Dean number, Prandtl number. Numerical studies of uni-
form wall heat flux for Dean number in the range of 80-1200 and curvature ratios, δ, of 1/10
to 1/100 for fully developed velocity and temperature field were performed by Kalb and
Seader [11]. The analysis presented results in Nusselt number relationship:
Nu = 0.836 De
0.5
Pr
0.1
for De ≥ 80 and 0.7 < Pr < 5 (1)
Xin and Ebadian [12] has analysed the effect of Prandtl number and geometric pa-
rameters on the performance of on helical coil heat exchanger, which results in correlation:
Nu = (2.153+0.318De
0.643
) Pr
0.177
(2)
for
0.7 < Pr < 175; 20 < De < 1200; 0.0267 < d/D < 0.0884
Cengiz et al. [13] have provided the analysis of heat transfer and pressure drops in a
heat exchanger with a helical pipe containing inside springs. The results indicated that the
Nusselt number increased with decreasing pitch/wire diameter ratio. On the basis of the ex-
perimental data the empirical correlation was presented for Nusselt number:
Nu = 0.055 De
0.864
Pr
0.4
for
70 < De < 1200; 0.7 < Pr < 5 (3)
Experimental analysis of heat transfer enhancement in shell and helical coil heat ex-
changer was analysed by Jamshidi et al. [14]. Tube and shell side heat transfer coefficients are
determined using Wilson plots. Experimental and Taguchi method are used to investigate the
effect of fluid flow and geometrical parameters on heat transfer rate. The analysis indicate that
the increase in coil diameter, coil pitch, and mass flow rate in shell and tube can increase the
heat transfer rate in these types of heat exchangers.
Purandare, P. S., et al.: Experimental Investigation on Heat Transfer and Pressure Drop …
THERMAL SCIENCE, Year 2016, Vol. 20, No. 6, pp. 2087-2099
2089
An investigation on the shell-side flow and heat transfer performances of multilayer
spiral-wound heat exchanger under turbulent flow was studied by experimentally and numeri-
cally by Lu et al. [15]. The heat exchanger was analysed by Wilson plot method and correla-
tion for the shell-side Nusselt number outside the tube is obtained:
Nu = 0.179 Re
0.862
for 500 ≤ Re ≤ 3500 (4)
The third configuration in the coiled tube heat ex-
changer is the conical coil configuration, which is the inter-
mediate configuration of helical and spirally coiled configu-
rations. The conical coil configuration is shown in fig. 1.
The conical coil is considered as spiral coiled when the
coil cone angle is 180° and it is considered as helical coiled,
when the coil cone angle is 0°.
In literature, it is observed that sufficient experimental
and numerical data is available for the analysis of helical
and spiral coil heat exchanger whereas conical coil configu-
ration with respect to different cone angles is not explored
thermodynamically and hydro-dynamically by the research-
ers for the heat transfer applications. This fact created the
motivatation to undertake the current work. The objective of
this work is to analyse the conical coil heat exchanger ther-
modynamically from spiral to helical coil configuration at
different intermediate cone angles, spiral (180°), 135°, 90°, 45
°
, helical (0°). The analysis is
extended to establish the relationship between Nusselt number (thermal parameter), Dean
number (flow parameter), Prandtl number (fluid parameter), and δ (geometric parameter)
from thermal analysis, and f (friction factor), Dean number (flow parameter), and δ (geometric
parameter) form pressure drop analysis. In this paper the thermal and pressure drop analysis
of conical coil configurations is presented.
Experimentation
Experimental set-up
For the experimentation the exper-
imental set-up was developed as
shown in fig. 2.
The experimental set-up consists of
heat exchanger with conical coil. Coni-
cal coils are fabricated on the wooden
formers of specified cone angles and by
hot rolling process. The care has been
taken for dimensional stability of the
conical coils in fabrication process. The
shell is designed to accommodate all
coils, one by one, in the same shell. The
specially designed closed loop constant
temperature hot water supply system
Fig
ure 1.
Conical coil used in heat
exchanger
; θ – cone angle,
D
1
– minimum diameter,
D
2
– maximum diameter,
D
m
– mean diameter
D
m
+ (D
1
+ D
2
)/2,
d – tube diameter
Fig
ure 2. Experimental set-up; T
1
– cold water inlet,
T
2
– cold water outlet, T
3
– hot water inlet,
T
4
– hot water outlet
Purandare, P. S., et al.: Experimental Investigation on Heat Transfer and Pressure Drop …
2090 THERMAL SCIENCE, Year 2016, Vol. 20, No. 6, pp. 2087-2099
supplies hot water to the tube side of the heat exchanger by a constant discharge pump. Constant
temperature cold water is supplied to shell side by pump (from separate storage of 500 L capaci-
ty installed and maintained at constant temperature in test run). Two rotameters (accuracy of
±1% of the maximum flow rate) are used to measure the cold and hot water flow through the
heat exchanger. The constant temperature of inlet cold and hot water flowing through the heat
exchanger are maintained with the help of thermostat controlled system. Heat exchanger is insu-
lated by polyurethane foam layer of 6 mm thickness (designed on the basis of available data) to
avoid the heat loss due to convection from outer surface. Temperature measurements are carried
out using calibrated k-type thermocouples (Make-Kristake Instruments and transducers, with ac-
curacy of 0.1 K) which are mounted at various locations. Pressure drop across the heat exchanger
is measured with micro-manometer with an accuracy of 0.5 mm of water column. To reduce the
heat loss, extensions of the heat exchanger coils are well insulated to ensure that the temperature
measured should have minimum error. All properties were assessed at the mean bulk tempera-
ture of the fluids for both tubes as well as shell side fluid (average of inlet and outlet tempera-
tures). The uncertainty in the experimental set-up are identified and taken into considerations for
the uncertainty analysis.
Parameters for experimentation
For the experimentation and the analysis of the conical coiled heat exchanger the pa-
rameters considered are listed in tab. 1.
Table 1. Parameters considered for analysis
Experimentation and analysis
Thermal analysis
The experimentation is carried out by keeping the cold water flow rate to 30 L per
hour and varying the hot water flow rate 10-100 L per hour by the interval of 5 L per hour.
For each reading, steady-state is achieved with constant hot and cold water flow rate. The
same procedure is repeated for the cold water flow rate of 45, 60, 75, and 90 L per hour. To-
tally about 125 test runs are recorded for each coil.
All properties were assessed at the mean bulk temperature of the fluids for both
tubes as well as shell side fluid (average of inlet and outlet temperatures).
Heat transfer to the cold water in the heat exchanger is calculated by the equation:
Parameter
Details
Total number of coils
15
Coil mean diameter [mm]
200
Coil pitch [mm]
Outer diameter of the tube
Coils
Helical (0°), 45°, 90°, 135°, and spiral (180°)
Tube size inner diameter × outer diameter [mm]
8 × 10, 10 × 12, 12 × 15
Tube side fluid [°C]
Hot water of temperature 70±1%
Shell side fluid [°C]
Cold water of temperature 22±1%
Hot water flow rate [L per hour]
10 to 100, Re = 500-5000, De = 120-1200)
Cold water flow rate [L per hour]
30 to 90
Purandare, P. S., et al.: Experimental Investigation on Heat Transfer and Pressure Drop …
THERMAL SCIENCE, Year 2016, Vol. 20, No. 6, pp. 2087-2099
2091
c c w c,o c,i
()
p
qm t tc= −
(5)
Heat transfer to the hot water in the heat exchanger is calculated by the equation:
h h w h,o h,i
()
p
qm t tc= −
(6)
The average heat transfer rate used in the calculation is determined from the hot wa-
ter-side and cold waterside:
ch
avg
2
qq
q
+
=
(7)
The overall heat transfer coefficient was calculated from the inlet and outlet temper-
atures:
avg
ov
i
q
U
A LMTD
=
(8)
The calculation of LMTD is considered as per the coulter flow condition [16]:
12
1
2
ln
TT
LMTD
T
T
D −D
=
D
D
(9)
where ∆T
1
is the temperature difference between inlet temperature of hot water and outlet
temperature of cold water, and ∆T
2
is the temperature difference between outlet temperature
of hot water and inlet temperature of cold water.
Heat transfer coefficients for the outer tube, h
o
, and for the inner tube side, h
i
, are
calculated using traditional Wilson plots method [7]. Wilson plots allow the heat transfer coef-
ficients to be calculated based on the overall temperature difference and the rate of heat trans-
fer, without the requirement of wall temperatures. This method is chosen to avoid the disturb-
ance of flow patterns and heat transfer while attempting to measure wall temperatures. The
analysis is extended to evaluate the Nusselt number and furthermore to evaluate the relation-
ship between Nusselt number and Reynolds number or Dean number (flow parameter),
Prandtl number (fluid parameter), and δ (geometric parameter).
Pressure drop measurement
In order to evaluate f-Re relationship for the conical coil configuration, the pressure
drop, Δp, and average velocity, U, are measured. The friction factor is defined [17]:
i
2
1
2
d
p
f
L
U
ρ
D
=
(10)
where the average velocity is evaluated from U (Q/A
c
) and nominal cross-sectional area A
c
(πd
i
2
/4). In order to ensure complete steady-state, 15-20 minutes is allowed to elapse when the
flow rate was changed.
Uncertainty analysis
Least counts and the sensitivities of the measuring instruments used in the present
investigation contribute the errors in the analysis. Coleman and Steele [18] and Measure-
ment uncertainty ANSI/ASME [19] has proposed the uncertainty analysis for all experi-