THIẾT BỊ TRUYỀN NHIỆT ỐNG LỒNG ỐNG

Heat exchangers are a device that exchange the heat between two fluids of different temperatures that are separated by a solid wall. The temperature gradient, or the differences in temperature facilitate this transfer of heat. Transfer of heat happens by three principle means: radiation, conduction and convection. In the use of heat exchangers radiation does take place. However, in comparison to conduction and convection, radiation does not play a major role. Conduction occurs as the heat from the higher temperature fluid passes through the solid wall. To maximize the heat transfer, the wall should be thin and made of a very conductive material. The biggest contribution to heat transfer in a heat exchanger is made through convection.

DOUBLE-PIPE HEAT EXCHANGER

by Jeffrey B Williams

Project No 1H Laboratory Manual

Assigned: August 26, 2002 Due: September 18, 2002 Submitted: September 18, 2002

Project Team Members for Group B:

Thomas Walters, Group Leader

Dong-Hoon Han Jeffrey B Williams

_

Jeffrey Williams

TABLE OF CONTENTS

III EQUPMENT SPECIFICATIONS 6

B USING DATA IN EXCEL 15

E CALIBRATION OF THE FLOW METERS 18

7 Process Flow Diagram of Double-Pipe Heat Exchanger 11

8 Photo of Double-Pipe Heat Exchanger 12

9 Loading the Heat Exchanger Software 13

10 Selecting the Double-Pipe Heat Exchanger 13

12 Opening the Double-Pipe Data 15

SUMMARY

Double-Pipe Heat Exchanger, Project No 1H

Group B Jeffrey B Williams (report author), Thomas Walters, Dong-Hoon Han

Report Date: September 18, 2002

A Procedures Manual was created for the double-pipe heat exchanger The theories of transient heat transfer in double-pipe heat exchangers were explained and followed by literature correlations All of the instrument specifications were defined A procedure for use of the equipment and the

software was outlined Safety and other concerns during operation were discussed This manual will serve to direct anyone in how to start up and run the double-pipe heat exchanger

It is recommended that this Procedures Manual be filed with Robert Cox in MEB 3520 It is recommended that students be given access to the following manual, to aid them in their understanding

of the use of the equipment It is also recommended that students begin with a calibration of the flow meters and possibly the thermocouples before beginning use of the equipment All calibration data, where possible, should be coordinated with the computer-generated data

I INTRODUCTION

Temperature can be defined as the amount of energy that a substance has Heat exchangers are used to transfer that energy from one substance to another In process units it is necessary to control the temperature of incoming and outgoing streams These streams can either be gases or liquids Heat exchangers raise or lower the temperature of these streams by transferring heat to or from the stream

Heat exchangers are a device that exchange the heat between two fluids of different temperatures that are separated by a solid wall The temperature gradient, or the differences in temperature facilitate this transfer of heat Transfer of heat happens by three principle means: radiation, conduction and convection In the use of heat exchangers radiation does take place However, in comparison to conduction and convection, radiation does not play a major role Conduction occurs as the heat from the higher temperature fluid passes through the solid wall

To maximize the heat transfer, the wall should be thin and made of a very conductive material The biggest contribution to heat transfer in a heat exchanger is made through convection

In a heat exchanger forced convection allows for the transfer of heat of one moving stream to another moving stream With convection as heat is transferred through the pipe wall it

is mixed into the stream and the flow of the stream removes the transferred heat This maintains

a temperature gradient between the two fluids

The double-pipe heat exchanger is one of the simplest types of heat exchangers It is called a double-pipe exchanger because one fluid flows inside a pipe and the other fluid flows between that pipe and another pipe that surrounds the first This is a concentric tube construction Flow in a double-pipe heat exchanger can be co-current or counter-current There are two flow configurations: co-current is when the flow of the two streams is in the same direction, counter current is when the flow of the streams is in opposite directions

As conditions in the pipes change: inlet temperatures, flow rates, fluid properties, fluid composition, etc., the amount of heat transferred also changes This transient behavior leads to

change in process temperatures, which will lead to a point where the temperature distribution becomes steady When heat is beginning to be transferred, this changes the temperature of the fluids Until these temperatures reach a steady state their behavior is dependent on time

In this double-pipe heat exchanger a hot process fluid flowing through the inner pipe transfers its heat to cooling water flowing in the outer pipe The system is in steady state until conditions change, such as flow rate or inlet temperature These changes in conditions cause the temperature distribution to change with time until a new steady state is reached The new steady state will be observed once the inlet and outlet temperatures for the process and coolant fluid become stable In reality, the temperatures will never be completely stable, but with large enough changes in inlet temperatures or flow rates a relative steady state can be experimentally observed

II THEORY

The theory behind the operation of a double-pipe heat exchanger is covered in Incropera and Dewitt (1996) Also in this same textbook is the derivation of how transient behavior is treated with respect to heat transfer

As with any process the analysis of a heat exchanger begins with an energy and material balance Before doing a complete energy balance a few assumptions can be made The first assumption is that the energy lost to the surroundings from the cooling water or from the U-bends in the inner pipe to the surroundings is negligible We also assume negligible potential or kinetic energy changes and constant physical properties such as specific heats and density These assumptions also simplify the basic heat-exchanger equations

The determination of the overall heat-transfer coefficient is necessary in order to determine the heat transferred from the inner pipe to the outer pipe This coefficient takes into account all of the conductive and convective resistances (k and h, respectively) between fluids separated by the inner pipe, and also takes into account thermal resistances caused by fouling (rust, scaling, i.e.) on both sides of the inner pipe For a double-pipe heat exchanger the overall heat transfer coefficient, U,can be expressed as

In a heat exchanger the log-mean temperature difference is the appropriate average temperature difference to use in heat transfer calculations The equation for the log-mean temperature difference is

( )1

1ln

2

11

1

, ,

i i i fi i

o i o

fo o

R d

d l k A

R h A A

=

2

ln

, ,

, ,

, , , ,

i o o i

o i i o o i LM

T T

T T

T T T T T

Fluid properties such as density, viscosity and heat capacity are evaluated at the average temperatures The average is found by using the inlet and outlet values

=

a i

T ,

2

, ,o T o i

Thermal conductivity, k, can be evaluated at the average of the average temperatures In a

double-pipe heat exchanger the inner pipe is made of a conductive metal and is thin

The problem can be further simplified if the equipment is assumed to be clean, which means that no scaling exists This is a poor simplification with the double-pipe heat exchanger

in the laboratory, because it is many years old The fouling factors Rfo and Rfi can be looked up

from various sources, including Standards of the Tubular Exchange Manufacturers Association,

or lumped together and determined experimentally

The only part of the overall heat-transfer coefficient that needs to be determined is the convective heat-transfer coefficients Correlations are used to relate the Reynolds number to the heat-transfer coefficient The Reynolds number is a dimensionless ratio of the inertial and viscous forces in flow

i i

i i i

a

m d

( )

.3

.04

.0

6,

160Pr6.0,10,

000,10RePr

Re023.0

,

, 5

s m m

s

i i

i n

i i i

T T for n

or T T for n

d

l Nu

where flow developed fully

turbulent for

Pr

i

i i i

k

Cp µ

=

This gives a Nusselt number that can then be use to find hi

The convective heat transfer coefficient in the annulus is more difficult to determine The hydraulic diameter is used to find the Reynolds number The hydraulic diameter is defined

as the cross-sectional area perpendicular to flow divided by the wetted perimeter With the Reynolds number calculated the same correlations apply and with these ho can be determined

Once all the separate heat-transfer coefficients are calculated an overall heat transfer coefficient is calculated Now everything that was necessary for an energy balance is available With the previous assumptions made earlier the dynamic equations would be

With the transient data taken experimentally an overall heat-transfer coefficient can be determined at each time step This can be solved numerically

( )8

,

i

i i i

k

d h

Nu =

( )9

)( , ,

,

LM o

i i i i i a i i

dt

dT Cp

( )10

)( , ,

, 0

LM o

i o o o o a o

dt

dT Cp

III EQUIPMENT SPECIFICATIONS

The following is a list of all pieces of equipment and their specifications for the double-pipe heat

exchanger

1) Pump

Manufactured by: Dayton Electric Manufacturing Model: Teel Industrial Series (see Figure 1) Horsepower 2

RPM: 3485 Efficiency 80 Incoming pipe diameter: 2 in, Schedule 40 stainless steel Outlet pipe diameter: 1 1/2 in, Schedule 40 stainless steel

Figure 1 – This Pump is used to pump the fluid from the

tank to double-pipe exchanger

2) Double-Pipe Heat Exchanger

Material: Schedule 40 stainless steel Length: 14 ft

Inside Pipe Diameter: 1 1/4 in Outside Pipe Diameter: 2 in Steam Pass 1 Cooling Water Pass 4

3) Valves

• Gate Valves

Manufactured by: Stockham Location: Steam Valves (see Figure 2)

Figure 2 – This valve allows steam to enter the steam pipe

in the annulus of the double-pipe heat exchanger

• Disk Globe Valve

Manufactured by: Nibco Location: Cold Water Valves (see Figure 3)

Figure 3 – When this valve is open the cold water can

enter the double-pipe heat exchanger

• Ball Valves

Location: Process Valves, Tank Valve, Drain Valve

(see Figure 4)

Figure 4 – When the valve (Apollo) on the left is open it allows the cooling water to travel to the drain When the valve (Watts Regulator)

on the right is open, the process fluid can travel to the drain

• Computer Controlled Valves

Manufactured by: Scott Johnson Model: Valtek (see Figure 5) Operating Temperature: 0 to 55°C

Maximum Air Pressure: 30 psig

Figure 5- Control valve used to control amount

of coolant flow to the heat exchanger

4) Flow meters

Manufactured by: Brooks Instruments Model: MT 3810 (see figure 6) Accuracy: ±5% full scale from 100% to 10% of scale

reading Repeatability: 0.25% full scale Operating Temperature: -39 to 215°C Flow Range: 4.1 to 41.6 gpm for inner-pipe flow meter

2.6 to 26.4 gpm for outer-pipe flow meter

Figure 6 – Meter measures the flow of process

fluid coming from the pump

5) Thermocouples

Manufactured by: Omega Model: Type T Sheath Material: 304 Stainless Steel Sheath Length: 12 in

Temperature Range: -60 to 100°C Accuracy: 1.0°C or 0.75% above 0°C (whichever is

greater) 1.0°C or 1.5% below 0°C (whichever is greater)

6) Low Pressure Steam

Pressure: 27 psia Temperature: 118°C

7) Computer

Manufactured by: Dell Systems Operating System: Windows NT Software: Opto-22 electronics and computer based

software, Version R3.16 Copyright

1996-2000 Opto-22

IV SYSTEM OVERVIEW

The double-pipe heat exchanger used in experimentation is located in MEB 3520 Figure 7 describes the setup of double-pipe heat exchanger Fluid from the tank is first heated in the by steam that is condensing in the annulus and is then cooled by the four cooling-water passes In all instances low-pressure steam is used to heat the fluid and water is used to cool the fluid Once cooled the fluid is then returned to the tank

There are six thermocouples that record temperature at six different points that can be seen in the following figure The first records the temperature of the inlet process fluid, the second records the process fluid temperature after heating with steam, the third records the temperature after cooling with the water, the fourth records the cooling-water temperature at the inlet, the fifth records at the outlet and the sixth records the steam temperature at the inlet There is a control valve that controls the steam inlet, the process fluid inlet and the cooling-water outlet There are manual valves that also need to be opened before the process could begin, even

if the control valves were open to 100% Once the proper valves are opened the pump can be manually activated

Figure 7- Process flow diagram for the double-pipe heat exchanger

Figure 8 is a full overview of the double-pipe heat exchanger taken from the south side The disk included with this manual includes this picture, as well as other pictures

Figure 8- Picture of the double-pipe heat exchanger from the southwest corner

V PROCEDURE

A OPTO-22 Software

The valves on the double-pipe heat exchanger are electronically controlled, and the data from the thermocouples and the flow meters are taken via computer The following steps will explain how to start the software, and what the various sections of the software mean

1 Turn on the computer

2 Click on the icon reading “Shortcut to Heat Exchanger MMI.” This will load

the Opto-22 software

3 The first screen that appears will look like this:

Figure 9- Loading the heat-exchanger software

4 Click on the Heat-Exchanger Menu button At this time, another menu will

come up

Figure 10- Selecting the double-pipe heat exchanger

5 Select the “Double Pipe”

6 Figure 11 opens up next This is the Opto-22 interface screen All the work

that is done while the heat exchanger is operating will be done here For both

percent opening of the respective control valves 100% means that the valve is fully opened and 0% means that the valve is completely closed The valve setting is changed by opening up the green MV and inputting a value of 0-100

Figure 11- Opto-22 interface and control screen

7 The lower portion of Figure 11 shows values for the six different

thermocouple readings, for the coolant and process flow meters and also for the control valves themselves The colors in these boxes correspond with the colors of the lines in the graphs This Opto screen provides numerical values and plots the numerical values to the graph A new reading is taken and recorded at least every 5 seconds Old data are saved to a file and are accessible in this screen

B USING DATA IN EXCEL

1 Open the file in Excel It is found in the C drive in “double-pipe data”

Figure 12 – Opening the double-pipe data

2 The data are saved in comma-delimited form So Excel has to convert this to

rows and columns

Figure 13 – Text wizard Step one indicates that the data are in delimited form

Figure 14 – The data are delimited using commas, this

is apparent from the preview

Figure 15 – The last step in the import wizard is

to specify the data format of the columns and create the spreadsheet

3 Once in Excel the data can then be studied and used in any necessary

calculations All the data (thermocouple readings, flow measurements and % control valve opening) given on the Opto-22 interface are recorded in this style for later use Data such as this are very useful in the study of transient behavior