Bare and Lagged

5/20/2018 Bare and Lagged

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EXPERIMENT B3HEAT 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 necessaryparameters 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.


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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 theheat 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


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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 insulationby 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 convectionhr = 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


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where = 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 forX 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


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For air at ordinary temperature and at atmospheric pressure, the simplified dimensional

equation forX 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


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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.


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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


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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


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EQUIPMENT

A. Actual Equipment


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B. Schematic Diagram of the Equipment


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C. Descr ipt ion of the Equipm ent

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 whichcan 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.


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PROCEDURE

1. Preheatin g. 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 byopening 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 o f 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 tomaintain 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.


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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 ofthe 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 o f 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 drivenout from the U-tube. Combine the condensate collected from each pipe and record the

volume.

b. Metho d 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.


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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

5 38 115 112 5 39 120 116

10 37.5 111 110 10 39 119 118.5

15 38 115.8 113.7 15 39 120 118

20 38.3 115.8 112 20 39.5 121 118

25 38 112 109.5 25 40 125 121

30 38 117 114 30 40 121 118

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


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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.

=( )

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.


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Table 3. Amount of condensate collected for each pipe at different steam pressure

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 pipefor each run. Compare these with experimental values. Determine the percentage difference.

Explain your findings.

q = (hc + hr )Ao (Ts Ta )

0

200

400

600

800

1000

0 10 20 30 40

Condensatevolume(m

L)

Pressure (psi)

Pressure vs. Condensate volume

Pipe A

Pipe B

Pipe C

Pipe D

Pipe E

Pipe F

Pipe A Pipe B Pipe C

Pressure Volume Pressure Volume Pressure Volume

20 298 20 642 20 790

30 422 30 616 30 942

Pipe D Pipe E Pipe FPressure Volume Pressure Volume Pressure Volume

20 720 20 480 20 240

30 845 30 555 30 390


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Table 4. Experimental and theoretical heat losses for pipes for run 1

Table5. Experimental and theoretical heat losses for pipes for run 2

Pipe A

Pipe B

Pipe C

Pipe D

Pipe E

Pipe F

Heat loss(experimental)

490.64 716.2 1095.22 982.45 645.28 453.44

Heat loss(theoretical)

495.98 965.54 1320.72 972.48 737.41 410.07

% difference 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 theirimpact 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.

=

( )(1800 )

Pipe A

Pipe B

Pipe C

Pipe D

Pipe E

Pipe F

Heat loss(experimental)

384.82 829.05 1020.17 929.77 619.85 309.92

Heat loss(theoretical)

364.89 787.25 1206.94 930.2 660.25 410.07

% difference 5.46 5.34 15.47 0.056 6.12 24.42


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Table6. Experimental overall heat transfer coefficient for each pipe for run number 1

Pipe A Pipe B Pipe C Pipe D Pipe E Pipe F

Condensate

volume (mL)298 642 790 720 480 240

ms664393.98 1431345

1761313 1605247 1070165 535082

Uo(experimental)

15.0153 81.2562 99.9882 91.1258 54.2893 12.0929

Table7. Experimental overall heat transfer coefficient for each pipe for run number 2

Pipe A

Pipe B

Pipe C

Pipe D

Pipe E

Pipe F

Condensate

volume (mL)422 616 942 845 555 390

ms 942359.76 1375577 2103561 1886953 1239359 870901.2

Uo

(experimental)15.0153 81.2562 99.9882 91.1258 54.2893 12.0929

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.

Conc lus ion :

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.


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REFERENCE:

Ronderf C. Bolo and Servillano Olano, Jr., Spreadsheet Calculations for Unit Operations

Laboratory ExperimentsProceedings of the 2002 Chemical Engineering Congress, De

La Salle University, December, 2002

Bare and Lagged

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