EXPERIMENT B3 HEAT LOSSES IN BARE AND LAGGED PIPES AND FINNED TUBE INTRODUCTION Industrial processes usually require steam for operations such as heating. This medium is usually transported via metal pipes. However, it is inevitable to encounter heat losses in this arrangement because of the inherent temperature difference existing between the hot pipes and the surroundings. This can instead be minimized through insulations placed on bare pipes. On the other hand, if a process requires enhancing heat losses then the use of fins would be more appropriate. This experiment will involve students in determining the effectiveness of the apt use of these heat transfer accessories and also quantify necessary parameters such as the overall effective heat transfer coefficients.
OBJECTIVES 1. To determine the overall effectiveness of industrial insulating materials as compared with unlagged pipe and finned tube by solving for the lagging efficiency. 2. To compare experimental and theoretical heat losses by conduction, convection, and radiation from bare and lagged pipes. 3. To measure effective overall heat transfer coefficient of bare and lagged pipes and finned tubes.
THEORY A. Bare and Lagged Pipes When a pipe, bare or lagged, is used to carry saturated steam under pressure, heat will be lost to the surroundings because of temperature gradient existing between the steam and the surroundings. The rate of heat transferred naturally will depend on the magnitude of the temperature difference, the thermal resistance, and the heat transfer area. The most common method of minimizing heat losses to the surroundings is the use of insulation to increase the resistance and therefore lower the heat transfer rate. If our purpose is to increase the rate of heat transfer, we use finned tubes which expose more area per unit length compared to a similar pipe of the same size.
The rate of heat lost from a pipe carrying steam can be measured simply by determining the rate of condensation of steam, m, which can be collected at a certain pointerval of time.
By heat balance,
where
Under controlled conditions, the condensed steam can be collected as saturated liquid, thus Equation (1) simplifies to,
To determine therefore the effectiveness of an insulation, it is just a matter of comparing the heat lost from the pipe with an insulation with that from a bare pipe. Since heat lost is proportional to the rate of condensation, and the weight of condensate is proportional to the volume of condensate v, assuming temperatures and pressures of condensates are the same, then the lagging efficiency may be determined using the equation
where
To determine the theoretical heat lost, let us consider a pipe of length L insulated as shown
carrying steam at a temperature Th and exposed to surrounding air at Ta and surrounding walls of the room at Tw.
Before heat is transferred to the surroundings, it travels first from the bulk of the steam through the steam film condensate, then through the metal pipe, then through the insulation by conduction until it reaches the surface of the insulation where part of the heat is transferred to the surrounding air by convection and part by radiation to the surrounding walls. That is,
where
hc = Heat transfer coefficient by convection hr = Heat transfer coefficient by radiation Ts = Surface temperature of insulation Ao = Outside area of insulation
For practical purposes, Ta =Tw , therefore Equation (5) becomes
By definition, assuming surrounding area to be large compared to the area of insulation and gray surfaces, hr is given by
where
Î = Emissivity of surface T = Absolute temperature
The convection heat transfer coefficient, hc, will depend on the mechanism involved when heat is transferred from the surface to the air. Under normal conditions, we can consider this transfer as natural convection since no appreciable movement of air due to mechanical agitation is encountered. The data of heat transfer from horizontal pipes to air for X from 10^3 to 10^9 is represented by the dimensionless equation,
where
The subscript f indicates that the corresponding property is to be evaluated based on average film temperature
For air at ordinary temperature and at atmospheric pressure, the simplified dimensional equation for X from 10^3 to 10^9 may be employed as
where
T = Ts - T Do = Outside temperature of cylinder
The calculation for the simultaneous heat loss by convection and radiation as given by Equation (6) is straightforward if the surface temperature Ts is known. However, in most systems this value is not known or cannot be measured with reasonable accuracy. Since Ts is needed in the evaluation of both hc and hr, then this temperature will have to be evaluated by trial. Assuming a value of Ts (you may use measured Ts as a guide), hr is evaluated using Equation (7) and h solved using either Equations (8) or (9). To check the validity of Ts, we use Equation (4) by expressing this in terms of temperature gradient and resistances, that is
where
Using Equation (10), solve for Ts and compare this with the assumed Ts. Repeat iteration until close agreement is achieved. With Ts known, calculate the theoretical heat lost using Equation (6). For bare pipes, trial and error calculation for Ts may be eliminated. Since the thermal resistance of the metal pipe and the steam film condensate are small, it is safe to assume that the surface temperature of the pipe is nearly the same as the temperature of the steam. With Ts known, evaluation of hc, hr and q becomes straightforward. To evaluate the effective overall heat transfer coefficient from steam to air, we use the equation
which can be compared with the actual or experimental U using the equation
B. Finned Tubes In this particular experiment, the integral finned tube is made of brass and fabricated by extruding the fins that are attached to the surface of the tube. The fins are radially extruded from thin walled tube to a height of 1 mm with 16 fins per inch (25.4 mm). External surface of the fins is approximately 2 mm wider than the outside surface of the bare tube whose outside diameter is 16.8 mm. Below is the simplified dimensional figure of the finned tube.
Solve first for the heat transfer coefficient, f h¢ , by assuming that the transfer of heat is by natural convection. Hence,
where
T = Temperature difference between fin surface and air Bf = Outside diameter of circular fin
Determine the fin efficiency,
, using (P Fig. 10 39). i.e., determine
Compare the fin efficiency,
, obtained from (P Fig. 10 39) with the equation
such that
where
Bf = Outer diameter of circular fin Do = Outside diameter of the tube Sf = Thickness of fin
Compute for the heat losses per foot using the equation
where
q ' f = Heat losses per foot L f = Height of the fin Tb = Surface temperature of the fin Ta = Temperature of the air
Then solve for the theoretical heat lost using the equation
where
q = Theoretical heat lost L = Total length of the tube, ft
EQUIPMENT
A. Actual Equipment
B. Schematic Diagram of the Equipment
C. Description of the Equipment
The equipment set-up consists of the following: six graduated cylinders of 5500 to 5000 ml capacity; one stopwatch; six beakers of 1000 to 3000 ml capacity; two pairs of asbestos gloves; pair of pliers; 10 mercury thermometers; a digital surface thermometer; a meter stick; and compressed air supply line.
The test equipment consists of a pipe insulated with asbestos (Pipe A), a bare pipe coated with silver paint (Pipe B), a bare pipe coated with black paint (Pipe C), a GI pipe without any insulation or coating (Pipe D), a finned tube (Pipe E), and a pipe insulated with styrofoam (Pipe F).
These pipes, which are slightly inclined, are rigidly connected to a large horizontal and properly insulated pipe which in turn is connected to an insulated steam supply line leading to the steam boiler. In the supply line, there is a pressure gage that indicates the pressure of the steam coming from the boiler. The pressure within the test pipes is indicated by another gage that is located just after the manually controlled valve. Each pipe is equipped with three thermometer wells that are used to approximately determine the surface temperature by means of a mercury thermometer. The digital surface thermometer may be used to verify these readings. Located at the side of the supply line is a set of throttling calorimeter which can be used to determine the quality of steam entering the distribution tube.
On the other side, the end of these pipes are connected to an insulated cylindrical condensate collector provided with a stopcock on top, a sight glass at the sides with valves, and a control valve at the discharge pipe connected at the bottom of this collector. The discharge pipe goes inside a column in a form of a U-tube. The exit pipe can be turned forward for collecting the condensate or sideward for draining the condensate. The cylindrical coolers are provided each with cooling system in parallel where cooling water can be controlled by a valve located at the main water supply line. The used cooling water from these coolers is discharged directly to the drain. See Figures 2 and 3 for the equipment set up.
PROCEDURE
1. Preheating. Before starting a run, it is necessary to preheat the tubes to a temperature as near as possible to the prescribed temperature for the run. This is achieved by partly opening all the condensate discharge valves and allowing the steam to pass through the tubes by opening the steam pressure control valve to maintain approximately the same pressure as that to be used for the particular run. This procedure will also remove noncondensable gases inside the tubes. Perform this operation for about 5 minutes. During this period, you may check the temperature recorded by the thermometers placed on each well to determine whether the system has already stabilized. Note: To avoid burns always wear asbestos gloves when handling hot metallic parts.
2. Start of Run. Before starting a timed run, make sure that the condensate collector is empty. To check, open fully the valves on top and bottom of the sight glass. If water is indicated, this can be removed by fully opening the discharge valve. To start the timed run all the six discharge valves are closed simultaneously if possible. It is important that somebody must be stationed to control the steam supply valve to watch the pressure gauge since closing the discharge valves might suddenly raise the steam pressure inside the pipes to a dangerous level. It is recommended that the supply valve be partly closed while the discharged valves are being closed. At this point start the time and adjust the control valve to maintain the desired pressure constant throughout the run.
3. Timed Run
a. Method I. This timed run should last not less than 25 minutes. Get temperature readings from time to time from the thermometers from each well, or by using the digital thermometer. If the condensate collector is about to be filled up as indicated in the sight glass, collect some of the condensate using a beaker by carefully opening the discharge valve just to allow part of the condensate out. Do not discharge completely the condensate or steam will escape. If there are leaks encountered, collect these to be added later to the condensate collected from each pipe. When collecting condensate, make sure that the cooling system is on.
b. Method II. After closing all the discharged valves simultaneously, if possible, adjust the steam valve to a certain pressure and maintain it constant throughout the timed run. Open the valve at the main water supply line for the cooling water. Allow the condensate to reach a certain level as indicated in the sight glass. Mark this level and start timing the run. Open the discharge valve to collect some of the condensate from the collector in a beaker. The level of the condensate may rise or fall during the run but adjust the discharged valve so that the level of the condensate will not be far from the marked level.
4. End of Run
a. Method I. When the prescribed time is reached, close completely the steam supply valve then open slowly one, two or three stopcocks on top of the condensate collector to remove the residual steam inside the pipes. Be careful when opening these valves, bear in mind that the steam is initially at high pressure. When the pressure in the pipes reaches atmospheric, collect the condensate in a beaker or graduated cylinder one at a time or simultaneously. Draining will not remove all the condensate because some will stay inside the U-tube within the cooler. One way of removing the condensate completely is to use compressed air. First, close the stopcocks and connect the compressed air line to one of the stopcocks. Adjust the air regulator to indicate an air pressure of about 55 psig. Then slowly open the stopcock to allow air to enter the collector. Because of the pressure, residual condensate will be driven out from the U-tube. Combine the condensate collected from each pipe and record the volume.
b. Method II. The steam supply valve is closed completely at the end of the timed run. But a few minutes before closing the supply valve, let the condensate level be higher than the marked level in step 3 by partly closing the discharge valve. Then right after closing the steam supply valve, slowly drain the condensate and stop draining when the level is on the mark.
Note: To start another run, repeat the procedure by first preheating the systems at least three runs must be performed. The recommended pressures are 15, 20, 25 psig, although you can choose the pressure you want as long as it does not exceed 60 psig. To determine the quality of steam, use the throttling calorimeter provided near the set up.
DATA
RUN 1
RUN 2
TIME
Pipe #1 (°C)
Pipe #2 (°C)
Pipe #3 (°C)
TIME
Pipe #1 (°C)
Pipe #2 (°C)
Pipe #3 (°C)
0
33
88
88
0
38
98
99
112
5
39
120
116
110
10
39
119
118.5
113.7
15
39
120
118
112
20
39.5
121
118
109.5
25
40
125
121
114
30
40
121
118
5
38
10
115
37.5
15
111
38
20
115.8
38.3
25
115.8
38
30
112
38
117
MID SECTION RUN 1 TIME
Pipe #1 (°C)
Pipe #2 (°C)
Pipe #3 (°C)
Pipe #4 (°C)
Pipe #5 (°C)
Pipe #6 (°C)
0
38
90
90
56
72
34
5
38
112
110
106
90
36
10
39
106
108
104
92
38
15
39
114
108
108
94
38
20
39
114
110
108
94
38
25
39
111
109
105
92
38
30
39
112
112
110
94
38
MID SECTION RUN 2 TIME
Pipe #1 (°C)
Pipe #2 (°C)
Pipe #3 (°C)
Pipe #4 (°C)
Pipe #5 (°C)
Pipe #6 (°C)
0
38
90
90
94
94
38
5
38
112
110
111
94
37
10
39
106
108
112
95
3
15
39
114
108
112
96
38
20
39
114
110
113
97
38
25
39
111
109
116
98
38
30
39
112
112
38
38
38
ANALYSES AND CALCULATIONS Bare and Lagged Pipes 1. Using the bare pipe (without any coating) as the reference, determine the lagging efficiency of each insulation for each run. Explain.
𝐿𝑎𝑔𝑔𝑖𝑛𝑔 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
(𝑉𝑏 − 𝑉𝑙 ) 𝑉𝑏
Where: Vb= volume of condensate collected for bare pipe Vl= volume of condensate collected for lagged or insulated pipe.
Table 1. Lagging efficiency for pipes in the first run Pipe
A
B
C
D
E
F
Condensate Volume (ml)
298
642
790
720
480
240
Lagging Efficiency
0.58611111
0.10833
-0.0972
0
0.33333
0.66667
Table 2. Lagging efficiency for pipes in the second run Pipe
A
B
C
D
E
F
Condensate Volume (ml)
422
616
942
845
555
390
Lagging Efficiency
0.50059172
0.27101
-0.1148
0
0.3432
0.53846
Based from the data, the pipe insulated with Styrofoam/polystyrene (Pipe D) gives the highest lagging efficiency. Pipe A insulated with asbestos also shows a high lagging efficiency for both runs.
2. Is there a trend in terms of the pressure of steam and the amount of condensate collected? Plot values to support your explanation.
Table 3. Amount of condensate collected for each pipe at different steam pressure Pipe A
Pipe B
Pipe C
Pressure Volume 298 20 422 30 Pipe D Pressure Volume 720 20 845 30
Pressure Volume 20 642 30 616 Pipe E Pressure Volume 20 480 30 555
Pressure Volume 20 790 30 942 Pipe F Pressure Volume 20 240 30 390
Condensate volume (mL)
Pressure vs. Condensate volume 1000 800
Pipe A
600
Pipe B
400
Pipe C Pipe D
200
Pipe E 0 0
10
20
30
40
Pipe F
Pressure (psi)
From the graph, the majority of the pipes show a directly proportional relationship between the steam pressure and the amount of condensate collected. Excluding Pipe B (which shows a rather inverse relationship), it can be attributed to the fact that at a higher pressure, the steam supply to the experimental equipment also increases, which also results to a higher volume of condensable steam.
3. Calculate the theoretical heat lost from each pipe and the surface temperature of the pipe for each run. Compare these with experimental values. Determine the percentage difference. Explain your findings.
q = (hc + hr )Ao (Ts −Ta )
Table 4. Experimental and theoretical heat losses for pipes for run 1
Heat loss (experimental) Heat loss (theoretical) % difference
Pipe A
Pipe B
Pipe C
Pipe D
Pipe E
Pipe F
384.82
829.05
1020.17
929.77
619.85
309.92
930.2
660.25
410.07
6.12
24.42
364.89 5.46
787.25 5.34
1206.94 15.47
0.056
Table5. Experimental and theoretical heat losses for pipes for run 2
Heat loss (experimental) Heat loss (theoretical) % difference
Pipe A
Pipe B
Pipe C
Pipe D
Pipe E
Pipe F
490.64
716.2
1095.22
982.45
645.28
453.44
495.98
965.54
1320.72
972.48
737.41
410.07
1.08
25.82
17.07
1.02
12.49
10.58
From tables 4 and 5, the differences in values can be attributed to either errors in part of the experimenters in performing the experiment, the conditions of the environment and their impact with the surface temperature readings and also with the heat losses induced by the friction within the pipes as the steam travels through the line.
4. Based on the actual heat lost measured, determine the effective overall heat transfer coefficients for all the pipes.
𝑈0 =
𝑚𝜆𝑠 𝐴𝑜 (𝑇ℎ − 𝑇𝑠 )(1800 𝑠)
Table6. Experimental overall heat transfer coefficient for each pipe for run number 1
Condensate volume (mL) mλs Uo (experimental)
Pipe A
Pipe B
Pipe C
Pipe D
Pipe E
Pipe F
298
642
790
720
480
240
1761313
1605247
1070165
535082
99.9882
91.1258
54.2893
12.0929
664393.98 1431345
15.0153
81.2562
Table7. Experimental overall heat transfer coefficient for each pipe for run number 2
Condensate volume (mL) mλs Uo (experimental)
Pipe A
Pipe B
Pipe C
Pipe D
Pipe E
Pipe F
422
616
942
845
555
390
2103561
1886953
1239359
870901.2
99.9882
91.1258
54.2893
12.0929
942359.76 1375577 15.0153
81.2562
From tables 6 and 7, pipes which are insulated have the lowest values for Uo. In principle, the lower the value for the over-all heat transfer coefficient, the better the performance of an insulator in avoiding heat losses.
Conclusion: It can be concluded that in determining the lagging or insulating efficiency, the thermal conductivities of the insulating material are very important. On the other hand, the determined experimental and theoretical heat losses for each pipes shows reasonably same results through the use of the heat transfer coefficient for convection and radiation in computing for heat losses.
REFERENCE: Ronderf C. Bolo and Servillano Olano, Jr., “Spreadsheet Calculations for Unit Operations Laboratory Experiments” Proceedings of the 2002 Chemical Engineering Congress, De La Salle University, December, 2002
