Lab 3

ME495

Lab 3: Tubular Heat Exchanger

Group D:

David Elting

Christopher Goulet

Gerardo Espinoza

Rodolfo Gonzalez

Professor Sam Kassegne

11-21-07

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Table of Contents

1. Title Page...............................................................................................................1

2. Table of Contents...................................................................................................2

3. Objective of the Experiment (David Elting)........................................................3-4

4. Equipment (Rudy Gonzalez)................................................................................5-6

5. Experimental Procedure (Gerardo Espinoza).......................................................7-8

6. Experimental Results (Christopher Goulet)........................................................9-12

7. Discussion of Results (Christopher Goulet).......................................................13-14

8. Lab Guide Questions (Christopher Goulet) .........................................................15

9. Conclusion (David Elting)....................................................................................16

10. References (David Elting).....................................................................................17

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ObjectiveThe objective of this laboratory exercise is to familiarize the student with heat transfer in heat exchangers. The student will also learn about the different transducers used to detect and measure the physical properties used when calculating the heat transfer between the hot and cold media in the heat exchanger. A HT30X Heat Exchanger Unit and HT31 Tubular (tube-in-tube) Heat Exchanger are used for this laboratory. The HT30X and HT31 are test devices created for use in physics and engineering laboratories by Armfield Limited, Ringwald, Hampshire England.

IntroductionThe tubular heat exchanger is the simplest form of heat exchanger. It consists of two concentric (coaxial) tubes. The inner metal tube carries a hot fluid and the outer acrylic annulus carries the cold fluid, such that the inner tube’s outer surface is in direct contact with the cold fluid. Any temperature difference across the metal tube wall will result in the transfer of heat between the two fluid streams. The hot water flowing through the inner tube will be cooled and the cold water flowing through the outer annulus will be heated. A thermocouple is placed at the center location along the heat exchanger length and at entrance and exit of both the hot and cold fluid streams. The temperature of the hot fluid and the flow rate of the cold and hot streams are controlled by the student during each exercise. Tubular heat exchangers may be configured where the flow of the two

fluids enter at the same side of the exchanger and flow in the same direction (parallel flow) or made to flow in opposite directions (counter flow).

Figure1: Parallel flow heat exchanger

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Figure 2: Counter flow heat exchanger

Counter flow is preferred, as the difference in temperature between the hot and cold fluids is relatively constant along the full length of the heat exchanger. This is a benefit because extreme differences in temperature are eliminated that can thermally stress the heat exchanger material.The following relationships will be used in this lab exercise.Mass flow rate: mdot = Volume flow rate (Vdot) density of the fluid ()Heat power: Q = mdot cp T

cp constant specific heatHeat emitted from the hot fluid: Qe = mhcp,h(T1 T3)Heat absorbed by the cold fluid: Qa = mccp,c(T6 T4)The temperature efficiency (countercurrent flow) for hot fluid: h = [(T1 T3)/(T1 T4)] 100%The temperature efficiency (countercurrent flow) for cold fluid: c = [(T6 T1 T4) ] 100%

The mean temperature efficiency: c = (h c)/2 %Overall Efficiency for the system: = (Qa / Qe) 100%Theoretically, Qe Qa but do not due to heat loss given by Qf = Qe Qa

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Equipment

Fig.3 HT31 Tubular Heat Exchanger Fig.4 HT30X Heat Exchanger service unit

HT31 Tubular Heat Exchanger

-Height: 0.16m

-Width: 0.51m

-Depth: 0.39m

-Volume 0.05m3

-Gross weight 4kg

-Max heat transfer area: 0.02m2,0.08m2

-Maximum 6 temperature measurement points at a time:

1 Hot fluid inlet

2 Hot fluid mid-position

3 Hot fluid outlets

4 Cold fluid inlet

5 Cold fluid- mid position

6 Cold fluid outlet

7 Hot fluid internal positions

8 Cold fluid internal positions

HT30X Heat Exchanger service unit

- Volume: 0.33m³

-Gross weight: 33kg

1. Cold water supply stream

2. Hot water supply stream

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3. Hot water vessel with an electrical heater

4. Gear pump

5. Variable flow valves

6. Pressure regulator

7. Flowmeters calibrated from 0.2 to 5 L/min

8. Digital displays

9. Conditioning circuit outlets for the Heat Exchanger

10. Drain

Lab table

Latex gloves

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Procedure

Using the lab manual as a guide to do the experiment, the lab was started by

mounting the heat exchanger to the HT30X heat exchanger service unit. Then all the

sensors were connected to the main console in order to record the temperature and set the

hot and cold water flow rates. Also the hoses were connected to flow water through the

heat exchanger and the heater. Before the experiment was started, the hot and cold water

circuits had to be primed in order to remove any air bubbles that could give false

readings.

The priming was done by connecting the heat exchanger hot water inlet to the

heat exchanger cold water outlet. Next, the heat exchanger hot water outlet was

connected to the HT30X hot water inlet. The hot water bypass valve had to be closed, and

the cold water pressure regulator was set to a minimum setting by pulling the control

knob out (to the right) and turning full counter clockwise. Then the cold water flow

control valve was fully opened and gradually adjusted the cold water pressure regulator

control knob clockwise until water was seen flowing through the hot water circuit

flexible tubing and into the clear plastic priming vessel. When the priming vessel was full

and there were no more air bubbles in the lines, the cold water flow control valve was

closed. Afterwards the tube from the heat exchanger cold water outlet was disconnected

and reconnected to the HT30X hot water outlet. The hot water circulating pump and

heaters were switched on. Finally the hot water bypass valve was opened and closed

several times until all of the air bubbles were expelled from the tubing.

After the hot and cold water circuits were primed, the cold water pressure had to

be set. First the cold water pressure regulator was set by connecting the heat exchanger

cold water inlet to the HT30X cold water outlet. Then the heat exchanger cold water

outlet tube was routed to the center drain area of the HT30X. Next the flow indicator

switch on the main console was set to Fcold and the cold water flow control valve was

fully opened. The cold water pressure regulator control knob was adjusted until the flow

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display on the main console indicated 3 liters/min. Finally, the cold water pressure

regulator control knob was locked into position by pressing to the left on the tip of the

knob and then the cold water flow control valve was closed. Now the experiment was

ready to begin.

The experiment began by connecting the fluid supply tubing so the flow was

countercurrent. All the thermocouple plugs were connected to their respective sockets in

the console display. Then 45C was added to the reading and the temperature controller

was set to this value by momentarily pressing the setpoint key. Next the increase key or

decrease key was pressed until the desired setting was indicated. The flow indicator

switch on the main console was set to Fcold and then the cold water control valve Vcold was

adjusted to read 1 liter/min. The flow indicator switch was set to Fhot and then the hot

water control valve Vhot was adjusted to read 3 liter/min. The heat exchanger was allowed

to stabilize by monitoring the temperatures using the console display. When the

temperatures were stable, the thermocouple selector knob was rotated to different

temperatures in order to record the values for T1, T2, T3, T4, T5, T6, Fcold, and Fhot. Next

the flow indicator switch was set to Fcold and the cold water control valve Vcold was

adjusted to read 2 liter/min. After the heat exchanger was stabilized, the new values from

the sensor outputs were recorded. Finally the flow indicator switch was set to Fcold and the

cold water control valve Vcold was adjusted to read 3 liter/min. After the heat exchanger

was stabilized, the new values from the sensor outputs were recorded.

Once all the values were recorded, it was time to clean up the area. The heat

exchanger was removed from the HT30X heat exchanger service unit and put away in its

rightful location. The HT30X was shut down and disconnected and returned to its

location. Finally, all the water that was spilled on the units and on the floor was picked up

in order to keep the area safe and clean.

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Experimental Results

1. Calculate the following for each set of data:

Sample Calculation

Experiment 1

a. The heat emitted from the hot fluid

b. The heat absorbed from the cold fluid

c. Mass flow rate for the hot fluid

d. Mass flow rate for the cold fluid

e. The heat lost from the system

f. The temperature efficiency of the hot fluid

g. The temperature efficiency of the cold fluid

h. The mean temperature efficiency

i. The overall efficiency for the system

2. Hot fluid volume flow rate

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3. Cold fluid volume flow rate

4. Midpoint hot water specific heat and density

5. Midpoint cold water specific heat and density

6. The calculations in a through i use flowmeter and thermocouple data that is subject to

bias and precision error. The density of water decreases by about 0.7% between 0 and 38

ºC. If the standard deviation of temperature is ±0.1ºC, the temperature deviation is

around ±0.1% density standard deviation is 0.002% (at 95% confidence). The bias error

in the temperature reading can be estimated as ±1ºC, resulting in a bias density

uncertainty of ±0.02% (at 95% confidence).

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Table 1 Experimental values

Table 2 Calculated values

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7.

Heat Rate vs Cold-Water Flow Rate

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0 0.5 1 1.5 2 2.5 3 3.5

Cold water flow rate, Fcold (L/min)

Hea

t ra

te, Q

_do

t (k

W)

Heat emitted Cold absorbed Poly. (Heat emitted) Poly. (Cold absorbed)

Figure 5 Heat rate vs. cold water flow rate

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Discussion of Results

The data obtained was consistent with the hypotheses with regards to temperature

variation at the heat exchanger outlets. In both experiments, the hot and cold water

streams approached an equilibrium temperature. When the cold water flow was

increased while keeping the hot water flow the same, both outlet temperatures decreased.

More heat was transferred from the hot stream to the cold stream at the higher cold water

flow rate, resulting in a greater hot stream temperature drop. The higher cold water flow

resulted in lower cold water outlet temperatures because the cold water was exposed to

the hot water in the heat exchanger for a shorter period of time.

At T1 (hot water inlet) the temperature dropped 9.1° C between experiment one and

three. This is possibly due to the larger cold water flow, which may have cooled the

tubular heat exchanger exterior. The hot water at the inlet may have been cooler as a

result of a larger temperature difference at contact with the heat exchanger, resulting in

heat transfer between the hot water stream and the exterior of the tubular heat exchanger.

Additionally, the tubes carrying the hot water towards the inlet may have been in contact

with the tubes carrying the cold water, resulting in heat loss from the hot stream before it

entered the heat exchanger.

There were a few noticeable sources of experimental error. The hot water

temperature at T1 (hot water inlet) was measured at 88.6° C in experiment one despite the

heater being set to 60° C. This is probably a consequence of not allowing the system to

warm up for long enough before beginning the experiment. Small air bubbles

(approximately 2mm diameter) trapped in the hot and cold water circuits may have

altered the volume flow measurements. Air pockets may have also caused temperature

variations and affected the speed of the water pump. Also, as the system warmed up, the

variables in the system probably became a little more consistent and efficient. Examples:

water density, temperature, flow rate, air bubbles, and pressure.

Despite the fact that the temperatures changed in accordance with the hypotheses, the

calculated efficiency for the heat exchanger was greater than 100%, indicating that the

cold water was absorbing more heat than the hot water was emitting. The efficiency of

the heat exchanger ranged from 148% to 160%. One possible explanation is that the cold

water experienced frictional heating as it entered the heat exchanger, resulting in an

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increased outlet temperature. However, it seems unlikely that frictional heating would

result in such a large increase in cold water temperature at the low flow rates of this

experiment. Thus, it seems more likely that these impossible efficiencies were the result

of faulty flow sensors or thermocouples, incorrect setup, or incorrectly primed flow. If

the cold water flow was measured as being higher than it actually was by a faulty flow

sensor, the calculated heat absorption of the cold water would also be higher than it

actually was, resulting in a higher calculated efficiency.

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Lab Guide Questions

1. Did the heat exchanger remove more or less heat from the hot stream as the flow

rate of the cold water increased?

More heat was removed from the hot stream at the higher cold water flow rates

than at the lower cold water flow rates. The higher cold water flow results in a larger

average temperature difference in the heat exchanger, resulting in increased heat transfer

from the hot stream to the cold stream.

2. Did the system efficiency increase or decrease as the cold water flow rate increased?

The system efficiency decreased from experiment one to two, but increased from

experiment two to three, suggesting that the higher cold water flow does not have much

effect on the system efficiency. However, all overall efficiencies were over 100%,

making it difficult to make any definitive claims about the effect of flow on efficiency.

The data suggests that there is some other factor adding significant heat to the cold water

flow stream, thus increasing the apparent efficiency.

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Conclusion

This lab went exactly as expected with regards to trend in temperature readings.

Add more cold water, and the outlet temperature of the hot water will go down. Cold

water temperature was increased by flowing next to hot water. The lab was very straight

forward, however, we have very little experience with running heat exchangers. It was a

great learning experience to get a hold on how much temperature change will take place

with varying opposing flows of water.

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Works Cited

1. Kassegne, Sam. "Plate Heat Exchanger." Blackboard. SDSU Engineering. 21 Nov 2007 <https://blackboard.sdsu.edu>.

2. Armfield Engineering Education. 21 Nov. 2007 <http://www.armfield.co.uk/ >.

3. Beckwith, T., Marangoni, R., Lienhard, J. Mechanical Measurements 5th. Addison-Wesley, New York, 89-91.

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