Experiment 2 Tubular Heat Exchanger

Experiments in Chemical Engineering,2 nd ed

Performance of a Tubular Condenser

by Servillano S.B. Olaño, Jr.

EXPERIMENT B2 PERFORMANCE OF A TUBULAR CONDENSER INTRODUCTION

In a modern industrial plant, many heating processes require steam. Key ingredients are heated to a desirable temperature required for efficient processing. Reactions usually require a definite operating temperature and more importantly, final products must be set to conditions that are most convenient for handling and selling. More often that not, heat exchange among various plant process streams transpire in heat exchanger equipment such as a tubular condensers. It is therefore essential that future plant engineers be exposed to this key industrial equipment, be learned of the concepts and theories of condensation and most importantly be trained with the operation of this type of heat transfer equipment.

OBJECTIVES

1. To determine the capacity of the tubular condenser as as a function of the flow rate of the cooling water used. 2. To determine the experimental overall heat transfer coefficient for a vertical tubular condenser. 3. To calculate theoretical surface coefficients of steam condensing inside the tubes and of the cooling water flowing upwards the shell side of the condenser. 4. To compare experimental and theoretical values of the overall heat transfer coefficients as obtained in (2) and (3). 5. To determine the heat lost to the surroundings.

THEORY

The capacity of the tubular heat exchanger may be expressed in terms of the amount of steam condensed per unit time which is dependent upon the conditions of the cooling water supplied. The maximum capacity, however, can be considered as the amount of steam condensed when an infinite flow rate of cooling water is supplied. This also means that there is practically no heat transfer resistance in the cooling water side such that its temperature throughout the operation remains constant.

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To determine the experimental heat transfer coefficient, one measures first the amount of steam condensed per unit time, (mh )measured . In order to account for the losses in the amount of steam condensed due to the flashing of steam when the condensate leaves the steam trap due to sudden reduction in pressure, a correction known as the flashing effect, (mh )correction correction is added. The total amount of steam is therefore mh = (mh )measured

+

(mh )correction .

By applying heat balance, the total heat given off by the steam is qs

where

=

m h λ s

+

c p (T h

−

T c )

(1)

qs = Total amount of heat transferred by steam, Btu/hr

λ s = Latent heat of vaporization, Btu/lb m mh = Total amount of steam used, lbm °

c p = Specific heat, Btu/lb m - F °

T h = Condensing temperature of steam, F °

T c = Temperature of condensate, F

The amount of heat absorbed by the water is given by qw

where

=

mwc p (T w1 − T w 2 )

(2)

qw = Amount of heat gained by the water, Btu/lb mw = Mass flow rate of cooling water, lbm /hr °

cooling water, F T w1 w1 = Outlet temperature of cooling °

T w2 w2 = Inlet temperature of cooling water, F °

c p = Specific heat, Btu/lb m- F

The heat lost to the surroundings, q L, is the difference between the two which is expected to be small since the cooling water is flowing through the shell side. Hence,

q L

=

qs

−

qw

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(3)




Based on the heat transferred by the steam, the experimental U o is calculated by

(U o )exp erimental

=

mh λ Ao ∆T o

(4)

Ao = Total heat transfer area of the tubes

where

N = Number of tubes

The determination of the surface coefficients of steam condensing inside the tubes is done by assuming that film type condensation occurs. This is most likely since the condenser is old and the tubes are positioned vertically. The heat transfer coefficien t hi for the tube side fluid is estimated by

 k f 3 ρ f 2 g c λ  hi = 0.943  T L ∆ µ  f  

0.25

(6)

O

The subscript f refers refers to the average film temperature evaluated by (MS 13-11) T f

where

=

T h

−

3 4

3

(∆T o ) = T h − (T h − T w )

(7)

4

condensate, °F T f = Film temperature of condensate, T h = Temperature of condensing vapor, °F T w = Assumed temperature of the tube wall, wall, °F

However, Equation (5) is derived on the assumption that the condensate flow is laminar. This limits its use to cases where 4Γ µ is less than 2100. For long tubes, the condensate film becomes sufficiently thick and its velocity sufficiently large to cause turbulence. Even when the flow becomes laminar, coefficients calculated are increased by 20%. This is due to the effect of the ripples on the surface of the falling film. For turbulent flow, the coefficient h increases with an increase in the Reynolds number. Refer to Figure 13-2 (MS) in case such a condition is encountered. The heat transfer coefficient of water flowing in the shell side may be estimated using Equation 15-4 (MS).

 DoGo   = 0.20  k  µ 

ho Do

where

GO

=

0.66

mw S min

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 Cp µ     k 

0.33

 µ     µ w 

0.14

(8)

(9)




S min

π

=

4

( Dis − NDot )

(10)

N = Number of tubes Dis = Inside diameter of the shell Dot = Outside diameter of the tube

The theoretical U o is calculated using 1 U o

=

1 ho

+

Do kD L

+

Do hi Di

(11)

Check for the assumed T w using resistance form of heat transfer, T h

−

T c

1 U o Ao

=

T h

−

T w

1

ho Ao

Compare the theoretical U o with the experimental U o.

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(12)




EQUIPMENT A. Schematic Diagram Diagram of the Equipment

Figure 2: Tubular Heat Heat Exchanger

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Figure 3 Cross-Sectional Cross-Sectional View of a Tubular Heat Exchanger

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B. Description of the Equipment

The equipment used for this experiment is a vertical tube condenser. The tubular condenser has 38 tubes within the shell. These tubes are 85 inches long and made of steel. Consequently, the tubes have outside diameters of 18 mm or 0.7086 inch. The inside diameter of these tubes are 0.4375 inch each. The shell is also made of steel with the outside diameter measuring 8.52 inches. Moreover, the shell's thickness is 0.332 inch with the computed inside diameter measuring 7.856 inches. The steam enters the bottom of the condenser through the header then rises inside the tubes where the steam condenses on the inside surface of the tubes. The condensate flows downwards as thin film and collects at the bottom where it is discharged through the steam trap then to the condensate collection tank. The cooling water is introduced at the bottom of the shell and rises outside the bundle of tubes and exits on top, flows through the double pipe heat exchanger and is discharged to the hot water collection tank. This tank gives a direct reading in pounds of water.

PROCEDURE Note: Note: This experiment should be performed in close coordination with Experiment B1.

1. Drain the residual steam condensate by opening the drain valve. 2. Allow cooling water to flow through the condenser by opening fully the water water supply valves. 3. Stabilize the equipment by allowing small amount of steam inside to escape (this will also remove residual condensate). Close the drain valve and make sure that the condensate line with the steam trap is fully open. Then increase pressure to the desired level (say 30 psig). Allow equipment to heat to a stable temperature. Read temperature occasionally until the system has reached an almost steady state condition. The condensate collected must already be clean. 4. Operate the system for about 10 to 15 minutes at constant steam pressure. Within this interval determine temperature and pressure readings, flow rates of condensate and cooling water used. Note that the condensate collection tank has a gauge for direct reading of mass collected. Since the amount of condensate is usually small, it is suggested that one measures the total condensate collected within the time interval of the run. For the cooling water, (used as the hot fluid in the experiment involving double pipe heat exchanger), the flow rate is measured in the collection tank of Experiment No. 1.

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5. Repeat the procedure making use of various water flow rates and various steam pressures. The water pressure gauge may be used as a guide in varying the water flow rate. It is suggested that three different steam pressures (say 30, 40, & 50 psig) and three water flow rates for a total of nine runs be conducted to complete the experiment.

DATA SHEET A. Shell and Tube Specifications

Shell Outside Diameter

__________

Tube Sheet Outside Diameter __________

Shell Inside Diameter

__________

Tube Length __________

Shell Thickness

__________

Test Pressure __________

Shell Length

__________

Number of Tubes __________

Tube Outside Diameter

__________

Number of Baffles __________

Tube Inside Diameter

__________

Tube Thickness

__________

B. First Run

Steam Pressure

__________

Water Pressure

__________

Time (min)

Temperature (°C) Steam

Hot Water Condensate

0 3 6 9 12 15

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Flow Rate (kg/s) Cooling Cold Water Condensate Cond ensate Water




C. Second Run

Steam Pressure

__________

Water Pressure

__________

Time (min)




0 3 6 9 12 15 D. Third Run

Steam Pressure

__________

Water Pressure

__________

Time (min)




0 3 6 9 12 15

ANALYSES AND CALCULATIONS

1. For each run, calculate the heat supplied by the steam, the heat absorbed by the cooling water and the difference. This difference should indicate the heat lost to the surroundings. Present results as Table 1. 2. For each run, calculate the heat transfer coefficient of steam condensing inside the tubes, hi; the coefficient of water outside the tubes, ho; and the theoretical U o. Compare the experimental U o and determine the percentage deviation. Present as Table 2.

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3. For the six runs, plot the value of h( µ f k f ρ f 2 g )

1

3

versus N Re

=

4Γ µ f for the

condensing steam and compare this with Fig. 13-2 (MS). 4. For the six runs, plot the mass of steam condensed versus the mass flow rate of water used. Is there any correlation obtained?

GUIDE QUESTIONS

1. With the aid of a diagram, describe the operating principles of at least four types of steam traps. 2. Using data gathered, determine theoretically the maximum capacity of the condenser. Compare this with the capacity obtained using the maximum flow rate of cooling water.

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Experiment 2 Tubular Heat Exchanger

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