ME495
Lab 3: Tubular Heat Exchanger
Group D: David Elting Christopher Goulet Gerardo Espinoza Rodolfo Gonzalez
Professor Sam Kassegne
11-21-07
1
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
2
Objective The 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. Introduction The 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 co ntact 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 T
δQ T
h
,
=
h
U
( Tc
-
T
T
A
h
∆ T h
T
∆ T 2
∆ T
∆ T 1
T c ,
h ,
o u
c ,
o u
o
l d
T
∆ T c T
) d
i n
t
t
c
i n
1
2 A
d A
C
d A
H
o T
h
t ,
F
l u
i d
T
c ,
o u
F
H
o T
i n
C
o l d T c, in
F
l u
l u
t
h ,
t o u
i d
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
3
T
T
h
,
i n
T
h
∆ T T T
c ,
T
h ,
o u
t
T
c ,
o u
t
c
i n
A
d A
H
o T
h
t ,
F
l u
C
o l d T c , in
i d
F
l u
H
o T
i n
C
o T
l d c ,
o u
F
l u
h ,
i
t o u t
i d
t
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 (V dot ) × density of the fluid ( ρ ) Heat power: Q = mdot × c p × ∆ T c p ∴ constant specific heat Heat emitted from the hot fluid: Qe = mhc p,h(T 1 − T 3) Heat absorbed by the cold fluid: Qa = mcc p,c(T 6 − T 4) The temperature efficiency (countercurrent flow) for hot fluid: η h = [(T 1 − T 3)/(T 1 − T 4)] × 100% The temperature efficiency (countercurrent flow) for cold fluid: η c = [(T 6 − Τ 4 )/( T 1 − T 4) ] × 100% The mean temperature efficiency: η c = (η h − η c)/2 % Overall Efficiency for the system: η = (Qa / Qe) × 100% Theoretically, Qe = Qa but do not due to heat loss given by Q f = 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
6
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 c onnecting 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 45°C was added to the reading and the temperature controller was set to this value by momentarily pressing the setpoint key. Nex t 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
= m hot c p,hot (T 3 −T 1 ) = 0.0493
Q hot
kg
kJ (68 .6 4.18 kg ⋅ C
s
C − 64 .9 C )
= 0.763 kW
b. The heat absorbed from the cold fluid
= m cold c p,cold (T 4 −T 6 ) = 0.0169
Q cold
kg s
kJ ( 40 .4 4.18 kg ⋅ C
C − 23 .1 C )
= 1.22 kW
c. Mass flow rate for the hot fluid
m 3 kg kg m 979 3 = 0.0493 hot = hot ρ hot = 5.03e − 5 s s m V
d. Mass flow rate for the cold fluid
m = V cold ρ cold = 1.70e − 5 s
3
m cold
kg = 0.0169 995 m
kg
3
s
e. The heat lost from the system Q lost
=
Q hot
−
Q cold
=
1.26 kW
1.55 kW
−
0.290 kW
= −
f. The temperature efficiency of the hot fluid η hot
=
T 3
−
T 1
T 1
−
T 4
× 100 % =
68.6 C − 64.9 C 64.9 C − 40.4 C
× 100% = 15.1%
g. The temperature efficiency of the cold fluid η cold
=
T 4
−
T 6
T 1
−
T 4
× 100% =
40.4 C − 23.1 C 64.9 C − 40.4 C
× 100% =
70.6%
h. The mean temperature efficiency η m
=
η hot
+ η cold
2
=
15.1% + 70.6% 2
=
42.9%
i. The overall efficiency for the system η =
Q cold Q hot
=
1.22kW 0.763kW
= 160%
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2. Hot fluid volume flow rate V hot
L m3 = ( F hot ) (1.667 x10 −5 ) = 3.02 (1.667 e − 5) = 5.03 e − 5 s min
3. Cold fluid volume flow rate
= ( F cold ) (1.667 x10
V cold
−5
L m3 ) = 1.02 (1.667 e − 5) = 1.70 e − 5 s min
4. Midpoint hot water specific heat and density T 2
= 67.7°C → c p ,hot = 4.19
kJ kg ⋅ C
kg
= 979
, ρ hot
m3
5. Midpoint cold water specific heat and density T 5
= 31.9
C → c p ,cold
= 4.18
kJ kg ⋅ C
= 995
, ρ cold
kg m3
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).
1
1
2 1 B 2 1 2 = × 0.002 = 0.001 % B = 2 ρ 2 2
ρ
1
1
2 1 σ 2 σ T 2 2 1 + ( 0.1) 2 2 = 0.1% + = × σ = 0 . 02 2 ρ T 2 P = 1.96σ = (1.96)( 0.001 ) = 0.196 % ρ
[
U = ( B )
2
+ ( P )
2
]
1 2
= [( 0.00001 ) + ( 0.00196 ) 2
2
]
1 2
= 0.196 %
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Table 1 Experimental values Hot water flow rate, Fhot (ltr/min) 3.02 3.02 3.02
Cold water flow rate, Fcold (ltr/min) 1.02 2.02 3
Hot outlet temp, T1 (deg C) 64.9 60.3 54.5
Hot midpoint temp, T2 (deg C) 67.7 63.4 57.8
Hot inlet temp, T3 (deg C) 68.6 65.2 59.5
Cold oultet temp, T4 (deg C) 40.4 33.5 30.2
Cold midpoint temp, T5 (deg C) 31.9 27.5 25.5
Cold inlet temp, T6 (deg C) 23.1 22.8 22.5
Table 2 Calculated values Hot specific heat, cp,hot (kJ/kg.K) 4.19 4.19 4.18
Hot density rho_hot (kg/m^3) 979 981 984
Cold specific heat, cp,cold (kJ/kg.K) 4.18 4.18 4.18
Cold density rho_cold (kg/m^3) 995 996 997
Hot fluid volume flow, V_dot_hot (m^3/s) 0.0000503 0.0000503 0.0000503
Hot fluid mass flow, m_dot_hot (kg/s) 0.0493 0.0494 0.0495
Cold fluid volume flow, V_dot_cold (m^3/s) 0.0000170 0.0000337 0.0000500
Cold fluid mass flow, m_dot_cold (kg/s) 0.0169 0.0335 0.0499
Heat emitted Q_dot_hot (kW) 0.763 1.013 1.036
Cold absorbed Q_dot_cold (kW) 1.22 1.50 1.60
Heat loss Q_dot_lost (kW) -0.46 -0.49 -0.57
Hot temp eff, eta_hot (%) 15.10 18.28 20.58
Cold temp eff, eta_cold (%) 70.61 39.93 31.69
Mean temp eff, eta_m (%) 42.86 29.10 26.13
Overall efficiency, eta (%) 160.19 148.11 154.87
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7.
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 an d 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 .
2.
Armfield Engineering Education. 21 Nov. 2007 .
3.
Beckwith, T., Marangoni, R., Lienhard, J. Mechanical Measurements 5th. Addison-Wesley, New York, 89-91.
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