Chapter 14
Topics: Double-Pipe Heat Exchangers
Objective :
The prime objective in the design of an exchanger is to determine the surface
area required for the specified duty (rate of heat transfer) using the temperature
differences available.
Heat Exchanger Standards and Codes
British Standard BS-3274
TEMA standards are universally used.
TEMA standards cover following classes of exchangers:
Class R – designates severe requirements of petroleum and other related
processing applications
Class C – moderate requirements of commercial and general process applications
Class B – specifies design and fabrication for chemical process service.
Basic design equations for heat exchangers
Th − Tc Peter Chapter 14
Rtot = − − − 14.1
q
q=heat flux, R tot =total resistance
and Th -Tc =temperature driving force.
For a heat exchanger, heat exchanging between two fluids:
1 1 x 1
Rtot = = + Rh , f + + Rc , f + − − − 14.2
U hh kw hc
The three overall heat transfer coeeficients are related :
1 1 1
= = − − − 14.3
U i Ai U 0 A0 U m Am
For outside wall area A 0 is used :
1 A0 A0 A0 xw 1 1
= + + + + − −14.4
U 0 hi A i hi ,d A i k w Am , w h0 h0,d
When inside wall area is used :
1 1 1 A i xw Ai Ai
= + + + + − − − 14.4a
U i hi hi ,d k w Am , w A 0 h0 h0,d A 0
L( D0 − Di )
Ai = Di L; A0 = D0 L; Am , w =
D0
ln
Di
Common Heat Exchanger Types
1. Double-pipe exchanger: 5. Scrapped surface exchangers:
the simplest type, used for cooling and heating highly viscous or crystallization systems
with fouling tendency (peanut butter, ice
2. Shell and tube exchangers: cream, chocolate industry)
used for all applications 6. Air cooled:
3. Plate and frame exchangers: used for coolers and condensers
used for heating and cooling 7. Direct contact:
used for cooling and quenching
4. Plate-fin exchangers
Compact heat transfer system
8. Spiral plate and Tube exchanger:
good choices for small-scale applications
with variety of fluids
Selection of Heat Exchanger
The selection process normally includes a number of factors, all of which are
related to the heat-transfer application. These factors include, but are not limited
to, the following items:
1. Thermal and hydraulic requirements
2. Material compatibility
3. Operational maintenance
4. Environmental, health, and safety considerations and regulations
5. Availability
6. Cost
Double pipe Heat Exchanger
• Fluid flows through tube/pipe
• Outer one acts like a Jacket of the inner one
• Hairpin construction mostly important to make practical one where one hairpin is connected with other one.
• Opposite ends are joint by U-bends
• Bending tube thickness is comparatively larger
• For low heat duty application
Application of Fin
Enhanced annular-side heat transfer is often achieved by attaching
longitudinal fins to the outer surface of the inner tube. The fins increase the
surface area for heat transfer while reducing the cross-sectional flow area on
the annular side, resulting in an increase in the fluid velocity. The net effect
is an increase in both heat transfer and pressure drop.
Since a temperature gradient normally is present along the extended height
of the fin, the heat-transfer effectiveness of the fin surface is less than that
observed for the bare surface of the tube.
The effectiveness ηf of the fin area for straight longitudinal and pin fins is
given by:
tanh ML f
f = − − − 14.26
ML f
1/2
h
where M =
k
and h is the heat-transfer coefficient,
f
k f the thermal conductivity of the fin,
, the ratio of the volume of the fin to the surface area
of the fin, that is, = V f / Af , and L f , the height of the fin.
The effective heat transfer area 0 A 0 may then be defined as
0 Ao = Ap + f Af = A0 - Af (1- f ) − − − 14.27
Where η0 is the overall surface effectiveness, A0 is the total heat-
transfer area, Ap is the outside area of the inner pipe not covered with
fins, and Af is the fin area.
The surface effectiveness results in:
Af
0 =1- (1 − f ) − − − − − 14.28
A0
The overall heat-transfer coefficient for the finned double-tube heat exchanger may
be obtained by modifying Eq. (14-4) to include the correction for the effective area of
the fins, as given by
1 A1 D1 ln( D1 / D2 ) D1
= + + − − − 14.29
U1 h1 ( A f f + Ap ) 2k w h2 D2
where the subscripts 1 and 2 refer to the outside and inside areas and diameters
of the tube, respectively, rather than the total area of the tube and fins.
Example 14.5: Peter and Timmerhaus
It is desired to preheat 4 kg/s of Dowtherm from 10 to 70 0C with a hot water condensate that is to
be cooled from 95 to 60 0C. Two double-pipe heat exchangers, one without fins and one with fins
on the outer surface of the inner tube, are being considered for this heat-transfer process.
Determine the area and exchanger length anticipated for these two exchangers fabricated with
copper tubes and fins.
Dimensions are as follows:
Tube dimension: Tube outer diameter, D1 = 0.0483 m
Tube inner diameter, D2 = 0.0408 m
Shell dimension Do,s = 0.0889 m; Di,s = 0.0779 m
Fin height Lf= 0.0127 m
Fin thickness tf= 0.0027 m
Number of fins Nf = 24 longitudinal fins
To simplify the calculations, assume that each copper fin is attached individually to the outside
surface of the inner copper tube, and thus, each fin occupies 0.0027 m2 of the bare surface tube area
for each 1-m length of tube.
Solution:
✓ Determine the heat duty and the mass flow rate of the water.
✓ Obtain the individual heat-transfer coefficients for the two fluids and the overall heat-
transfer coefficient.
✓ With this information calculate the area and length of the exchanger required.
For the exchanger with fins, the latter calculation can be made only after the effectiveness of
the fin area has been established.
D1
D2
Assume that the fluid properties at the average bulk temperature for each fluid (A more
precise calculation would require using the mean temperature between the average fluid
temperature and the surface temperature of the wall).
Also, to further simplify the calculation, assume negligible resistance on the hot and cold
surfaces due to fouling. Consider first the double-pipe exchanger without the finned
surface.
Table. The fluid properties of each stream at the average temperature
Property Dowtherm at 40 °C Water at 77 °C
Heat capacity, Cp 1.622 x 103 J/kg-K 4.198 x 103 J/kg-K
Viscosity, µ 2.70x 10-3 Pa-s 3.72x 10-4 Pa-s
Density, ρ 1.044 x 103 kg/m3 9.74 x 102 kg/m3
Thermal conductivity, k 0.138 J/s-m-K 0.668 J/s-m-K
The total rate of heat transfer is obtained with Eq. (14-6) using the flow rate
of Dowtherm through the annulus.
q = (mCp ΔT)C = 4(1.622 x 103)(70 - 10) = 3.89 x 105 W
The mass flow rate of water, after rearranging Eq. (14-6), is
Fin height Lf= 0.0127 m
Remarks:
The addition of the 24 copper fins reduced the required length
of the exchanger by a factor of 4.7. However, it is still too long
to be accommodated in one exchanger and will require several
exchangers in series.
The calculations have been simplified by neglecting possible
fouling effects. Inclusion of these effects will increase the
lengths of both exchangers since the overall heat-transfer
coefficient will decrease with time due to surface fouling. Note
that the fluid velocity of the water in both exchangers is high,
and this will contribute to considerable pressure loss in the
tubes. This deficiency can be corrected by selecting a slightly
larger tube diameter.
