HT 305

Finned Tube Heat Exchanger

Objective:

To determine the efficiency of given longitudinal fin and compare it with the theoretical value for
the given fin.

Apparatus:

1. Longitudinal finned tube heat exchanger.

2. Bare pipe heat exchanger.

3. Steam generator provided with temperature indicator, pressure indicator and a feed valve.

4. Stop watch

Theory:
Industrial processes involve transfer of heat energy and the equipment employed to transfer heat
between two or more fluids is called the heat exchanger. This equipment is used for both cooling
and heating purposes and is of interest to process engineers. Fluids may be separated by walls or
they may be in direct contact. In this particular experiment, the fluids are separated by a metal
wall.

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Heat transfer from a hot solid surface to a flowing fluid is governed by the Newton’s law of cooling
shown by Eq. (1).
Q = hAS (TS − T ) (1)

where TS is the surface temperature and T is the temperature of the fluid far from the surface.

Therefore, heat transfer rates may be enhanced by:

i. Increasing the temperature difference (TS − T ) between the surface and the fluid.

ii. Increasing the heat transfer coefficient h.

iii. Increasing the contact surface area AS .

The overall heat transfer coefficient is given by Eq. (2) shown below:
1 1 x 1
= + + (2)
Ui Ai hi Ai km Al ho Ao
where
Ui is the overall heat transfer coefficient based on inside surface area in units of Kcal/hr m2 C
hi and ho represent the inside and outside heat transfer coefficients in units of Kcal/hr m2 C
Ao is the outside surface area in m2 and Ai is the inside surface area in m2
k m is the thermal conductivity of the wall in Kcal/hr mC

Al is the logarithmic mean of inside and outside area of the tube in m2

x is the wall thickness in m

In a double pipe heat exchanger or any standard tubular heat exchanger, a challenging heat
exchange problem arises when one of the two fluid streams has a much lower heat transfer
coefficient than the other; a typical example is heating a fixed gas,such as air, by means of
condensing steam. The individual coefficient of steam may be ~100 to ~200 times higher than the
corresponding value for air and hence, the overall heat transfer coefficient will be approximately
the individual heat transfer coefficient of air. Since it may not be feasible and economical to
increase the individual heat transfer coefficients and temperature difference, one alternative for
enhancing the heat flow rate is to increase available area for heat transfer and for this, certain types
of heat exchange surfaces called extended surfaces are employed. In this experiment, the type of
extended surface used is the fin.

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Two types of fins are used commonly:

1. Longitudinal Fins: These fins are used when direction of flow of fluid is parallel to the axis
of tube.

2. Transverse Fins: These fins are used when direction of flow of fluid is perpendicular to
the axis of tube.

The outside area of a finned tube consists of two parts: the area of fins and the area of bare tube
not covered by the bases of the fins.
The expression for theoretical fin efficiency can be derived by solving the general differential
equation of heat transfer with suitable boundary conditions.
Generally, three boundary conditions are used based on assumptions made:
1. Fin of infinite length: No heat dissipation from tip i.e. temperature at the tip of fin is same
as that of surrounding fluid
2. Insulated tip: Temperature gradient is assumed to be zero at the tip (tip area is negligible
as compared to total fin area so heat dissipated from tip can be neglected)
3. Finite heat dissipation from the tip: Fin is finite in length and loses heat by convection from
its tip

Fin Efficiency:
The fin efficiency is defined as ratio of actual heat transfer rate through fin to the maximum
possible heat transfer rate that could occur through fin. It is denoted by η.

For longitudinal fin with insulated tip, the theoretical fin efficiency is given by Eq. (3) below

 fin =
( )
tanh mL fin
(3)
mL fin
Where Lfin is length along the only direction where temperature is assumed to vary in the fin.

hC
m=
kA

h = heat transfer coefficient from the fin surface in Kcal /hr m2 C

C = circumference of the fin in m

k = thermal conductivity of fin material in Kcal /hr mC

A = cross-sectional area of fin in m2

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Procedure:

The steps to be followed in this experiment are as given below. Precautions are noted in italics.

1. Start-up procedure
a. Plug in coil from heater to power supply.
b. Open drain valve provided at the bottom of steam generator and drain out the water
from Steam Generator completely.
c. Close the drain valve.
2. Add 4 litres of distilled water through feed valve at the top of steam generator and close it.
Ensure dead weight safety valve is free.
3. Check water level in the steam generator and make sure that water level is such that the
heating coil remains completely immersed in water throughout the experiment.
4. Start the electrical heater of steam generator. Initially supply full voltage to the electrical
heater. Set the operating temperature to 105°C. Temperature should not exceed 110° C
during the experiment.
5. Steam will start forming at approximately 20 min. after switching on the heater.
6. Once steam starts forming, Finned Tube Heat Exchanger (FTHX) and Bare Tube Heat
Exchanger (BTHX) will start getting heated up and condensate will begin forming inside
the tube.
7. Drain valve is opened at 15 min. intervals for Finned Tube Heat Exchanger to collect
condensate in plastic containers and at intervals of 30 min. collect the condensate for the
Bare Tube Heat Exchanger.
Condensate should be completely drained out, but steam should not escape during the
collection of the condensate. Hence, do not open drain valve completely.
8. If the quantity of condensate collected is same for 2-3 consecutive readings, note down the
volume of condensate collected and time interval.
9. Note the observations in the table format given below and note readings till the experiment
is completed.
10. On completion of experiment, the shutdown procedure is:
a) Reset temperature values to room temperature, i.e., 25°C-27°C.
b) Remove the plug from power supply after switching off.
c) Discard all water collected during the experiment in a distilled water can.
d) Any water that may have spilled or leaked should be cleaned up.

Table Format:

Table 1: Observation Table

S.No. Time (min.) Volume of condensate collected in
FTHX (ml) BTHX (ml)

Total duration of experiment is up to 120 min.

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Data required for calculations

Data for finned tube and bare tube is specific to Set Up B on which experiment (shown in the
video) was performed:

1. Finned Tube:
1. Height of fin (L) : 580 mm

2. Width of fin (W) : 70 mm

3. Thickness of fin (b) : 3 mm

4. Number of fins (N) : 4

5. O.D. of fin tube : 25 mm

6. Thermal conductivity of fin material (k) : 13.97 Kcal /hr m°C

2. Bare Tube:

1. Length of tube (Lt) : 580 mm.

2. O.D. of tube : 25 mm.

Temperature of surroundings is 30°C

Latent heat of vapourisation of steam, 𝜆 = 557.97 Kcal/Kg at 0.3 bar pressure

Density of water ρ = 1 g/cm3

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Calculations:

1. Circumference of fin (C) = 2 (L+b) = …………m

2. Cross sectional area of fin (A’) = L x b = ………. m2

3. Fin area available for heat transfer (AF) = C x W x N =……………. m2

4. Tube area available for heat transfer in finned tube heat exchanger (AB) = π x D x Lt –

N x b x L =…………. m2

5. Total area of finned tube heat exchanger (At) = (AF) + (AB) = ………. m2

Steady state values at min are considered for the following calculations.

6. Heat given out by steam through finned tube heat exchanger (Q1) = m1 x λ =

… … … … … … … 𝐾𝑐𝑎𝑙/ℎ𝑟

7. Heat given out by steam through bare tube heat exchanger (Q2) = m2 x λ =

… … … … … … . . 𝐾𝑐𝑎𝑙/ℎ𝑟

𝐾𝑐𝑎𝑙
8. Heat transfer coefficient from bare tube exchanger, h = … … … … … . . ℎ𝑟 𝑚2 °𝐶

9. m = … … … … … … . . 𝑚−1

10. mLfin = m x W = ……………

11. 𝜂𝐹𝑖𝑛(𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙) = ⋯ . . %

12. Amount of heat actually dissipated by the fin, QFin = Q1 – (AB X h X ΔT) =…. Kcal/hr

13. Amount of heat dissipated by ideal fin, Qideal = AF x h x ΔT = ………… Kcal/hr

𝑄 114.17
14. Observed value of fin efficiency, 𝜂𝑂𝑏𝑠𝑒𝑟𝑣𝑒𝑑 = 𝑄 𝐹𝑖𝑛 = 379.4182 = ⋯ … . . %
𝑖𝑑𝑒𝑎𝑙

𝜂𝐹𝑖𝑛 −𝜂𝑂𝑏𝑠𝑒𝑟𝑣𝑒𝑑
15. % Error = 𝑋100 = ⋯ … … . %
𝜂𝐹𝑖𝑛

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Results, Discussion and Conclusions:

Results, discussion and conclusions are to be added in the report and the following should be
included: -

1. Schematic diagram with steam flow directions shown should be included in report
2. An observation table, clearly mentioning the steady state values that are used for final
calculations
3. Sample calculations (excel spreadsheet or matlab code, etc. used for calculations also
to be submitted along with the report) and calculation table as shown below:

Table 2: Calculation Table

S.No. Q1 Q2 hBTHX m mLfin theory obs Error
2
(Kcal/hr) (Kcal/hr) (Kcal/hrm °C) %

4. Results should be explained briefly and discussion should include the following
aspects
a. Comparison of performance of the finned tube heat exchanger with the bare
tube exchanger and subsequent discussion on the same.
b. Comparison of the efficiency of the longitudinal fin : efficiency as obtained
from observations noted during the experiment with that from theory;
followed by discussion on the same.
c. Possible sources of errors

Precautions:

1. Check for any leakage in the tubing, connectors, heat exchangers, steam generator and
for any cuts in or worn out electric wires
2. Check to ensure water is able to flow through the apparatus.
3. Check for any damage to the overall apparatus
4. Check that the dead weight safety valve is free and working
5. Use gloves while collecting the condensate in plastic containers
6. Regularly check for water level to ensure coil is immersed in water
7. Do not let the steam escape while collecting the condensate
8. Do not open the drain valve completely for condensate collection
9. Keep the measuring jar on a flat rigid surface while taking the reading
10. Any water that may have spilled during the experiment should be cleaned up on
completion of the experiment