Shell and Tube Heat Exchanger

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Shell and Tube Heat Exchanger

MECH595 – Introduction to Heat Transfer

Professor M. Zenouzi

Prepared by:

Andrew Demedeiros, Ryan Ferguson, Bradford Powers November 19, 2009

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2 Abstract

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Contents

Discussion of Theory: ... 4

Experimental Apparatus and Procedure ... 7

Experimental Data ... 8

Results ... 10

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Discussion of Theory:

A heat exchanger is a device that is used to transfer energy in the form of heat from one fluid to another. They take two input fluids of different temperatures and as the two fluids run near each other the fluids transfer heat between each other. The heat exchanger looks like a large pipe that consists of 37 small tubes. They are used in various configurations for all sorts of applications such as space heating, refrigeration, air conditioning, power plants, chemical plants, and petrochemical plants.

Heat exchangers can be used in two different configurations parallel flow, Figure 1, or counter flow, Figure 2.

Figure 1 ~ Cocurrent flow

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Each configuration refers to how the fluid moves through their respective flow passages relative to each other. If each fluid is flowing in the same direction such as in figure 1 it is termed a parallel flow. On the other hand if the fluids flow in opposite directions as in figure 2 it is termed counter flow.

Parallel flow in heat exchangers happens when both fluids enter the heat exchanger at their largest temperature difference. The temperature difference becomes less over the length of the heat exchanger. In the counter flow heat exchanger, the fluids enter at opposite ends and therefore at different ends of the temperature scale Figure 2. As the fluids move through the exchanger, they both warm up or cool down at roughly the same rate. The temperature differential between the two fluids is relatively constant over the length of the exchanger.

The heat transfer process which occurs in any basic heat exchanger can be summarized by the following equations.

(

)

( )(

)

LMTD

UA

F

R

LMTD

F

Q

T

c

m

Q

T

c

m

Q

T c pc c c h ph h h

=

=

=

=

Where in the last equation F is the correction factor which equals 1 for this experiment. LMTD is the Log Mean Temperature Difference which is described latter in this section. Q is the heat transferred

between the hot water and cold water.

The overall resistances can be calculated using:

cf w hf

T

R

R

R

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6 Where

[

]

c cf w w h hf

h

A

R

Lk

D

D

R

h

A

R

2 1 2 1

1

2

ln

1

=

=

=

π

hc and hh from the above equations can be found using the appropriate Nusselt number for hot and cold

water.

For the hot water (fluid inner tubes)

Cooling For Pr Re 023 . 0 0h.8 h0.3 h h h h h Nu D K Nu h =       =

For the cold water

Heating For Pr Re 36 . 0 0c.55 c0.33 c c c c c Nu D K Nu h =       =

The log-mean temperature difference is given by the following equation where a and b represent the ends of the heat exchanger. The LMTD is used because the heat must pass through four resistances the hot tube to the cold water.

(

)

(

a b

)

b a

T

T

T

T

=

ln

Tm

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Heat exchanger effectiveness is defined as the ratio of the actual heat transfer rate of the praticlar heat exchanger to the maximum possible heat transfer rate for the same unit.

max

Q

Q

=

ε

(

T

hi

T

ci

)

C

Q

max

=

min

Where Cmin is equal to either Cc or Ch, whichever is smaller and are defined as, Ch = mh cph and Cc = mccpc.

(

)

(

hi ci

)

ho hi h

T

T

C

T

T

C

=

min

ε

Experimental Apparatus and Procedure

For this experiment a HT30X Heat exchanger services unit was used along with an HT33 shell and tube heat exchanger. This device included four K-type thermocouples at the hot and cold inlet and outlets.

The exchanger consisted of seven stainless steel tubes 6.35 mm in diameter with a 0.6 mm wall thickness. The outer annulus was constructed from clear acrylic tubing 39.0 mm inner diameter with a 3.0 mm wall thickness. The length of the tube bundle is 144 mm giving a total heat transfer area of 20,000 m2.

The procedure for the laboratory is listed below.

1. Set the cold water pressure regulator. Adjust the knob until a flow rate of 3.00 liters per minute is established. Lock down this setting.

2. Prime the hot water circuit. Switch on the hot water circulating pump and expel any air bubbles. Do not let the water level fall below the height of the priming vessel to prevent air from entering the system.

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3. Set the computer software to countercurrent flow and maintain a hot water temperature of 60 °F.

Experimental Data

The results of the experiment are displayed in the tables below.

RUN m_cw m-ht Th in Th out Tc in Tc out l/s l/s ˚C ˚C ˚C ˚C 1 1 3 60.7 56.5 15.9 29.7 2 1.5 3 60.5 55.2 15.1 25.1 3 2 3 60.2 54.5 14.5 22.9 4 2.5 3 60.5 53.7 14.8 21.3 5 3 3 60.5 53.8 14.3 20.5

Table 1 ~ Parallel flow temperature data

RUN m_cw m-ht Th in Th out Tc in Tc out l/s l/s ˚C ˚C ˚C ˚C 1 1 3 61.3 56.6 15.1 28.3 2 1.5 3 60.4 55 15 24.8 3 2 3 60.5 54.5 16 23.5 4 2.5 3 60.5 54 15.6 22 5 3 3 60.8 53.9 15.7 21.2

Table 2 ~ Counter current flow temperature data

Sample Calculations:

Calculating hot water heat rate:

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9 Calculating Reynolds Number:

Calculating Nusselt Number:

Calculating heat transfer coefficient:

Calculating overall heat transfer coefficient:

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10 Calculating heat transfer rate:

Calculating maximum heat transfer:

Calculating Efficiency:

Results

Hot Water Heat Rate Cold Water Heat Rate

Run Qh Run Qc 1 52.03833 W 1 56.99436 W 2 65.66742 W 2 61.9504 W 3 70.62345 W 3 69.38444 W 4 84.25254 W 4 67.11293 W 5 83.01353 W 5 76.81849 W

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Hot Water Heat Rate Cold Water Heat Rate

Run Qh Run Qc 1 58.23337 W 1 54.51635 W 2 66.90643 W 2 60.71139 W 3 74.34048 W 3 61.9504 W 4 80.53551 W 4 66.08042 W 5 85.49155 W 5 68.14544 W

Table 4 ~ Counter Current Heat Rates

Figure 3 ~ Total Thermal Resistance 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 0 0.5 1 1.5 2 2.5 3 3.5 To ta l T her m al Res is ta nc e ( K/ m )

Cold Water Flow Rate (kg/s)

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Figure 4 ~ Hot Water Heat Rate

Figure 5 ~ Cold Water Heat Rate 0 10 20 30 40 50 60 70 80 90 0 0.5 1 1.5 2 2.5 3 3.5 H ea t Ra te (W )

Cold Water Flow Rate (kg/s)

Hot Water Heat Rate(Cocurrent) Hot Water Heat Rate (Concurrent)

0 10 20 30 40 50 60 70 80 90 0 0.5 1 1.5 2 2.5 3 3.5 H ea t Ra te (W )

Cold Water Flow Rate (kg/s)

Heat Rate Cold Water (Cocurrent) Heat Rate Cold Water (Concurrent)

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Figure 6 ~ Total Heat Rate

Figure 7 ~ Exchanger Efficiency 0 5 10 15 20 25 30 35 40 45 50 0 0.5 1 1.5 2 2.5 3 3.5 H ea t Ra te (W )

Cold Water Flow Rate (kg/s)

Total Heat Rate (Cocurrent) Total Heat Rate (Concurrent)

0 1 2 3 4 5 6 7 8 0 0.5 1 1.5 2 2.5 3 3.5 Ef fic ien cy

Cold Water Flow Rate (kg/s)

Cocurrent Efficiency Concurrent Efficiency

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Discussion of Results

The results of this laboratory show that the effectiveness of the heat exchanger is related to the cold-water flow rate. This is due to the decrease of thermal resistance decreases with increased cold water flow. Each trend in the Figures 4 through 7 above increases with flow rate.

There was no noticeable advantage to using counter current versus concurrent flow in the data. For each run the data collected for heat transfer rate did not vary greatly.

Conclusions

The data presented in this report shows that heat exchanger performance increase linearly with increasing cold water flow rate. This follows logically since more cold water is delivered to carry away heat per unit of time. Additionally increased flow rate results in more turbulent flow. This also increases the heat transfer rate.

Contrary to heat exchanger theory however, there was no noticeable difference in the heat transfer rate between parallel flow and counter current flow. The counter current flow should show enhanced heat transfer ability.

Figure

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