HEAT the experiment. APPARATUS AND EQUIPMENT 1. HT30XC

HEAT TRANSFER OF DOUBLE
PIPE HEAT EXCHANGER

INTRODUCTION

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The heat transfer principles
in fluids is often used in industrial applications to ensure safe and effective
control in heating and cooling applications. Heat exchangers are devices that
transfer heat energy to heat or cool the incoming or outgoing fluid. The two
streams have different temperatures and heat characteristics and are in 2
different pipes. Heat transfer in heat exchangers is through radiation,
convection and conduction. Heat conduction occurs through the pipe wall from
the fluid of high temperature to the fluid of low temperature. The major heat
transfer method is through convection from one fluid pipe to the other. The
fluid in the pipe wall is combined and mixed with into one fluid stream hence
losing the heat.

AIMS AND OBJECTIVES

The major aim of
this experiment is to use a double sided heat exchanger to investigate heat
transfer of water with different temperature gradients. The flowing water
stream is either co-current or counter-current configurations.

The following are
the specific objectives of the experiment:

1.     To
study the basic principles of heat transfer in heat exchanger with a solid wall
separating the 2 fluids streams.

2.     To
operate a heat exchanger running with either co-current or counter-current flow
in different hot or cold water flowrates.

3.     To
carry out mass balances and overall efficiency

4.     To
calculate the heat transfer efficiencies at the investigated flow rates

 

THEORY

Fig
1: Schematic diagram of HT30XC heat exchanger

The
HT30XC double sided heat exchanger module comprises of a controlled heater
section, heat exchanger rig, control valves, feedback circuits and user
interface section. Water from the reservoir is heated at 50 then pumped via the inner stainless tube while
the pipe with cold water at ambient temperature flows between the inner hot
stream tube and the outer acrylic tube to enable heat transfer. The heat is
transferred from the hot stream to the cold stream leading to temperature
change between the 2 water tubes. The valves helps to control the flow length
of the heat exchanger allowing several heat exchanger tube configurations to be
used in the experiment.

APPARATUS AND EQUIPMENT

1.     HT30XC
double pipe heat exchanger

2.     Flow
control valves

3.     Feedback
circuitry and sensory components

4.     Heater

5.     Pump

6.     The
Armfield Armsoft software

PROCEDURE

Equipment setup

a)     The
Armfield Armsoft software was loaded for the HT36 fitted to the HT30XC and the
counter-current option of the flowing water streams selected from the start up
screen

b)    The
‘power on’ icon on the software’s mimic diagram screen was clicked to switch on
the unit from stand by to on. The green light illuminated on the front of
HT30XC

c)     To
control the cold water flow in the heat exchanger, the flow rate was set using
a control valve adjusted from 0% that is fully closed to 100%, fully open in
equal intervals of 1% as shown below.

Fig
2: Adjusting the cold water flow

d)    To
control the hot water flow in the inner stainless tube, the ‘Flow’ button on
the software display was clicked to enable access to the PID controller of the
Heat exchanger while setting hot water flow rate.

e)     To
set the hot water temperature, the ‘Heater’ button of the Heat exchanger
software to enable access to the PID controller that sets the hot water flow
rate.

f)     To
configure the heat exchanger for counter current flow, the “Counter-software exercise
for counter- or co- current flow was selected to ensure that cold water inlet
supply always enters the heat exchanger in the same end in concurrent flow.

i)              
For counter current flow,
the hot water is configured to enter at the opposite direction using the hot
water pump in the heat exchanger module.

g)    The
number of active heat exchanger tubes were chosen by closing each manual valves
on the heat exchanger. The valve taps were closed by turning the handle at a
right angle in the clockwise direction. The appropriate valves were opened
byturning the valves’ knobs as shown in the figure below.

Fig 3: Opened valve
configuration for counter current operation

EXPERIMENT PROCEDURE

a)     The
number of active tubes in the heat exchanger were selected in the software’s “Number
of Tubes” icon in the left of the display box.

b)    The
cold water flow rate was set according to 0.25 by adjusting the arrows on the
side of  the cold water flow rate display
box

c)     The
cold water flow inlet temperature T6 was checked on the display box on  the mimic diagram screen

d)    The
hot water temperature controller was set to a set point at approximately 30 above the cold water inlet temperature

e)     The
following control parameters were set as follows: proportional band was set to
5, integral time was set to 200s and the derivative action was set to 0. The
values were changed accordingly if the control parameters did not match the
above control values.

f)     The
icons ‘Apply’ and ‘Ok’ on the display box were clicked to close the window

g)    The
hot water flow was set to 1L/min then the controller box icon ‘flow’ was
clicked and a set point was typed. The control parameters were set and checked
to be as follows: Proportional band was set to 100%, integral time was set to
3s while the derivative time was set to 0s.

h)    The
‘Automatic’ icon on the top right of the display window was clicked followed by
‘Apply’ and ‘Ok’ to close the window.

i)      The
heat was allowed to stabilize while monitoring the temperatures on the mimic
diagram display.

j)      The
‘Go’ icon on the top toolbar was selected to record data after the temperature
is stabilized. The data of T1, T2, T3, T4, T5, T6, T7, T8, T9, T10  and the actual cold and hot water flow rates  and  were recorded.

k)    The
cold water control valves were adjusted to give 0.5L/min using the arrow
buttons on the side of the display box.

l)      A
new results icon was created.

m)   The
correct number of tubes selected was checked and confirmed on the mimic diagram

n)    The
heat was allowed to stabilize while monitoring the temperatures on the mimic diagram.

o)    The
process was repeated as the first experiment using the ‘Go’ icon

p)    to
investigate other tube configurations of the hot and cold flow rates, the
valves were appropriately set and a new results sheet was created

q)    The
heater controller window was opened and the controller adjusted from
‘automatic’ to ‘Off.’

r)     The
hot water pump controller window was opened and the set point changed to 0L/min

s)     The
cold water flow control was set to 0%, fully open.

t)      The
manual flow rate for the number of tubes under investigation were  closed and the valves for the next number of
active tubes configurations were opened for investigation

u)    All
data was saved in an excel sheet.

RESULTS

Tube 1

Temperature change

 

 

   Calculating hot fluid
reynolds number

Calculating
cold fluid reynolds number

 

 

 

 

 

Calculating energy
emitted (Qe) and energy absorbed (Qa)

Calculating heat power,
mean temperature efficiency

 

Table 2:

Temperature values

 

 

 

 

 

 

 

Calculating temperature
change

 

Calculating Qe, Qa and
heat power

Calculating
efficiencies and transfer coefficients

Calculating hot fluid
Reynolds number

 

Calculating cold fluid
Reynolds number

 

DISCUSSION

Fig 4: The graph of
Temperature change  with change in flow rate

 

Fig 5: The graph of
Temperature change against change in flow rate

From the graphs above,
as the flow rate increase, the temperature change decreases. This is because there is no enough
time for the water in the tubes to exchange heat for both countercurrent and
co-current flow.

The increase in the
number of tubes in the heat exchanger increases the effective area of heat
transfer in the HT30XC. However, in an ideal situation, the heated water
flowing in the heat exchanger may loss energy to the surrounding by radiation.
This energy loss is neglected such that there’s an assumption that negligible
heat energy is lost from the already cooling water in the long tubes
configurations. From the data sheet, the effective heat transfer area

Where

 is the mean diameter

L is the length of the
tube

N is the number of
tubes used

The effect of
circulating the hot water in the outer annulus would lead to an increase in
area of heat transfer and hence an increased heat transfer co-efficient since
the hot water flows through both tubes.

Since heat energy is
transferred from the hot water tube to the cold water acrylic tube, Qe is the
Heat energy lost to the cold water while Qa is the heat energy gained/absorbed
by the cold water. Qe is negative since it is energy required to absorb heat
energy in the heat exchanger. Theoretically, for an ideal heat exchanger the
amount of heat emitted by hot fluid should be equal to the amount of heat
absorbed by cold fluid. However this is not the case in our experiment due to
heat losses and gain in the tubes by radiation.

A positive change in flow
rate in the hot region decrease the surface area for heat exchange since the
hot water travels at a greater velocity hence increasing the volume covered. A
decrease in flow rate leads to slow heat transfer speed.

Comparing the overall
heat transfer co-efficient  with the heat efficiency of cold and hot water
streams.

u

Efficiency hot stream

Efficiency cold stream
 

418.7541

11.195652

-91.63043

429.71782

20.875421

-44.78114

405.24385

24.94382

-14.1573

488.71838

30.156951

-3.475336

277.13957

4.2025862

-87.60776

259.32109

3.9173014

-89.44505

532.24847

18.130631

-16.89189

677.88177

20.022497

-9.786277

240.98171

2.8784648

-89.23241

283.17427

2.792696

-89.90333

615.98064

13.218391

-23.90805

586.26647

12.984055

-23.12073

The overall heat
transfer co efficient is greater than the percentage efficiency of the hot
stream. The efficiency in the cold region is negative since it gains heat
energy from the hot stainless steel tube. The overall heat transfer
co-efficient increases with increase in the number of tubes in the heat
exchanger.

After determining the
overall heat transfer co efficient, the Reynold’s number is calculated which is
the dimensionless measure of the inertial and viscous flow forces in the
streams. In the inner stainless tube, if the Reynolds number is less than 2000,
the flow is laminar flow. If the Reynolds’s number is 10,000, the flow is
predicted to be turbulent.

Heat transfer
coefficient increases with increase in Reynolds number. This is due to the fact
that localized secondary flow is formed in the tube heat exchanger. Secondary
flow increases turbulence evidenced from increased Reynolds number resulting
into increased heat transfer between tubes.

CONCLUSION

The experiment on the
double-pipe heat exchanger was a success. It was experimentally shown that hot
water or any fluid flowing through via the inner tube transfers heat energy to
the cold water flowing in the double outer pipes until the temperature of the 2
regions stabilizes. This can be altered by change in the inlet temperature or
the fluid’s flow rate. The stabilization process starts again and with time, a
new stabilized value of the final temperature is recorded. It was evident that the
inlet and outlet temperatures can never be constants, but with significant
changes in the stream temperatures and flow rates in an experimental study
leads to a relative temperature stability.

An increase in the
number of u shaped tubes in the heat exchanger increases the effective area for
heat transfer. More so, the increase in flow rate increases the speed taken for
heat transfer.

 

REFERENCES

1.     Denver’s,
N., Fluid Mechanics, McGraw Hill, (1991).

2.     Incropera,
F.P., D.P. DeWitt, Fundamentals of Heat and Mass Transfer, John Wiley &
Sons, Inc., pp. 460, 582-612. (1996).

3.     Redding,
Alyssa M., Shell-and-Tube Heat Exchanger, Project 1, and Laboratory Manual. Sept.
21, 2001.

4.     Standards
of the Tubular Exchange Manufacturers Association, 6th ed., Tubular
Exchanger Manufacturers Association, New York, 1978.