Heat Exchanger Analysis-3

Experiment3Experiment3 INME 4032INME 40321

Table of ContentsTable of Contents

PrinciplePrinciple 33

ObjectiveObjective 33

GoalsGoals 33

BackgroundBackground 44

a)a) Experimental approach Experimental approach 44

Overall efficiencyOverall efficiency 44

Temperature efficienciesTemperature efficiencies 55

Overall heat transfer coefficient UOverall heat transfer coefficient U 66

b)b) Analytical approachAnalytical approach 77

Experimental SetupExperimental Setup 99

a)a) Tubular Heat ExchangerTubular Heat Exchanger 1010

Description of the Tubular Heat ExchangerDescription of the Tubular Heat Exchanger 1010

Technical DataTechnical Data 1111

b)b) Plate Heat ExchangerPlate Heat Exchanger 1111

Description of the Plate Heat ExchangerDescription of the Plate Heat Exchanger 1111


c)c) Shell & tube heat exchangerShell & tube heat exchanger 1212

Description of the Shell & Tube Heat ExchangerDescription of the Shell & Tube Heat Exchanger 1212


ProcedureProcedure 1414

DiscussionDiscussion 1414


University of Puerto RicoUniversity of Puerto RicoMayagüez CampusMayagüez Campus

Department of Mechanical EngineeringDepartment of Mechanical EngineeringINME 4032 - LABORATORY IIINME 4032 - LABORATORY II

Spring 2004Spring 2004 Instructor: Guillermo ArayaInstructor: Guillermo Araya

Experiment 3Experiment 3: : Heat Exchanger AnalysisHeat Exchanger Analysis

PrinciplePrinciple

This experiment is designed to acquire experience on heat exchangersThis experiment is designed to acquire experience on heat exchangers

(being the most usually found in industrial applications: Tubular, Plate and(being the most usually found in industrial applications: Tubular, Plate and

Shell & Tube heat exchangers) and to understand the factors andShell & Tube heat exchangers) and to understand the factors and

parameters affecting the heat transfer rates.parameters affecting the heat transfer rates.

Objective Objective

To acquire experience on three basic heat exchangers (Tubular, Plate andTo acquire experience on three basic heat exchangers (Tubular, Plate and

Shell & Tube) and to understand the factors and parameters affecting theShell & Tube) and to understand the factors and parameters affecting the

rates of heat transfer.rates of heat transfer.

GoalsGoals

For co-current and counter-current operation of the equipment and flowFor co-current and counter-current operation of the equipment and flow

rates (hot and cold fluids) specified by the instructor, determine:rates (hot and cold fluids) specified by the instructor, determine:

a)a) The heat lost to the surroundings.The heat lost to the surroundings.

b)b) The overall efficiency.The overall efficiency.

c)c) The temperature efficiency for the hot and cold fluids.The temperature efficiency for the hot and cold fluids.

d)d) The overall heat transfer coefficient U determined experimentally.The overall heat transfer coefficient U determined experimentally.

e)e) The overall heat transfer coefficient U determined theoretically.The overall heat transfer coefficient U determined theoretically.

Compare with the experimental one.Compare with the experimental one.


BackgroundBackground

The process of heat exchange between two fluids that are at differentThe process of heat exchange between two fluids that are at different

temperatures and separated by a solid wall occurs in many engineeringtemperatures and separated by a solid wall occurs in many engineering

applications. The device used to implement this exchange is called a heatapplications. The device used to implement this exchange is called a heat

exchanger, and specific applications may be found in space heating and air-exchanger, and specific applications may be found in space heating and air-

conditioning, power production, waste heat recovery and chemicalconditioning, power production, waste heat recovery and chemical

processing. Heat exchangers are typically classified according to flowprocessing. Heat exchangers are typically classified according to flow

arrangement and type of construction. In the first classification, flow can bearrangement and type of construction. In the first classification, flow can be

countercurrent or cocurrent (also called parallel). On the other hand,countercurrent or cocurrent (also called parallel). On the other hand,

according to their configuration, heat exchangers can be labeled as tubular,according to their configuration, heat exchangers can be labeled as tubular,

plate and shell & tube heat exchangers.plate and shell & tube heat exchangers.

a)a) Experimental approach Experimental approach

Overall efficiencyOverall efficiency

To design or predict the performance of a heat exchanger, it isTo design or predict the performance of a heat exchanger, it is

essential to determine the heat lost to the surrounding for theessential to determine the heat lost to the surrounding for the

analyzed configuration. We can define a parameter to quantify theanalyzed configuration. We can define a parameter to quantify the

percentage of losses or gains. Such parameter may readily be obtainedpercentage of losses or gains. Such parameter may readily be obtained

by applying overall energy balances for hot and cold fluids. If Qby applying overall energy balances for hot and cold fluids. If Qee is the is the

heat power emitted from hot fluid, meanwhile Qheat power emitted from hot fluid, meanwhile Qa a the heat powerthe heat power

absorbed by cold fluid (neglecting potential and kinetic energyabsorbed by cold fluid (neglecting potential and kinetic energy

changes);changes);


Where,Where,

mass flow rate of hot and cold fluid, respectively.mass flow rate of hot and cold fluid, respectively.

inlet and outlet enthalpies of hot fluid, respectively.inlet and outlet enthalpies of hot fluid, respectively.

inlet and outlet enthalpies of cold fluid, respectively.inlet and outlet enthalpies of cold fluid, respectively.

inlet and outlet temperatures of hot fluid, respectively.inlet and outlet temperatures of hot fluid, respectively.

inlet and outlet temperatures of cold fluid, respectively.inlet and outlet temperatures of cold fluid, respectively.

specific heats of hot and cold fluid, respectively.specific heats of hot and cold fluid, respectively.

Heat power lost(or gained): Heat power lost(or gained):

Percentage of losses or gains Percentage of losses or gains

If the heat exchanger is well insulated, QIf the heat exchanger is well insulated, Qee and Q and Qaa should be equal. In should be equal. In

practice these differ due to heat losses or gains to/from thepractice these differ due to heat losses or gains to/from the

environment.environment.

The above formulas were deducted taking into account that hot fluid isThe above formulas were deducted taking into account that hot fluid is

rounded by cold fluid. If the average cold fluid temperature is aboverounded by cold fluid. If the average cold fluid temperature is above

the ambient air temperature then heat will be lost to the surroundingsthe ambient air temperature then heat will be lost to the surroundings

resulting in P < 100%. If the average cold fluid temperature is belowresulting in P < 100%. If the average cold fluid temperature is below

the ambient temperature, heat will be gained resulting P> 100%.the ambient temperature, heat will be gained resulting P> 100%.

Temperature efficienciesTemperature efficiencies

A useful measure of the heat exchanger performance is the temperature efficiency of each

fluid stream. The temperature change in each fluid stream is compared with the

maximum temperature difference between the two fluid streams giving a comparison


with an exchanger of infinite size.

Temperature efficiency for hot fluid Temperature efficiency for hot fluid

Temperature efficiency for cold fluid Temperature efficiency for cold fluid

Mean temperature efficiency Mean temperature efficiency

Subscripts h and c stand for hot and cold, respectively.Subscripts h and c stand for hot and cold, respectively.

Overall heat transfer coefficient UOverall heat transfer coefficient U

Because the temperature difference between the hot and cold fluidBecause the temperature difference between the hot and cold fluid

streams varies along the length of the heat exchanger it is necessarystreams varies along the length of the heat exchanger it is necessary


Fig 1: Countercurrent and Cocurrent operation for a shell and tube heatFig 1: Countercurrent and Cocurrent operation for a shell and tube heat exchangerexchanger

to derive an average temperature difference (driving force) from whichto derive an average temperature difference (driving force) from which

heat transfer calculations can be performed. This average temperatureheat transfer calculations can be performed. This average temperature

difference is called the Logarithmic Mean Temperature Differencedifference is called the Logarithmic Mean Temperature Difference

(LMTD) (LMTD) ttlmlm..

Where,Where,

tt1 1 = T= T11-T-T44

tt2 2 = T= T22-T-T33

Note: See FIG 1. to identify temperatures in cocurrent and counterflowNote: See FIG 1. to identify temperatures in cocurrent and counterflow

operation.operation.

We can define an overall heat transfer coefficient U as: We can define an overall heat transfer coefficient U as:

Where,Where,

QQee = Heat power emitted from hot fluid = Heat power emitted from hot fluid

A = Heat transmission areaA = Heat transmission area

b)b) Analytical approachAnalytical approach

Up to now, a methodology to evaluate the performance of aUp to now, a methodology to evaluate the performance of a

determined heat exchanger has been developed. Here, an analyticaldetermined heat exchanger has been developed. Here, an analytical

study will be explained in order to understand the initial steps ofstudy will be explained in order to understand the initial steps of

thermal and sizing design. thermal and sizing design.

Analytical methods are only approximate in order to get an idea of theAnalytical methods are only approximate in order to get an idea of the


heat exchanger size. The overall heat transfer coefficient is calculatedheat exchanger size. The overall heat transfer coefficient is calculated

assuming that is constant along all the heat exchanger and can beassuming that is constant along all the heat exchanger and can be

predicted with convection correlations. Nevertheless, there are manypredicted with convection correlations. Nevertheless, there are many

factors that affect this value, for instance, the influence of bubbles,factors that affect this value, for instance, the influence of bubbles,

corrosion, etc. Manufacturers provide manuals that contain informationcorrosion, etc. Manufacturers provide manuals that contain information

more precise regarding the heat exchangers they trade. Then, it ismore precise regarding the heat exchangers they trade. Then, it is

expected that the theoretical values differ from the experimental ones,expected that the theoretical values differ from the experimental ones,

fundamentally due to the presence of bubbles. Of course, experimentalfundamentally due to the presence of bubbles. Of course, experimental

results are mandatory because they reflect real conditions ofresults are mandatory because they reflect real conditions of

operation. However, for heat exchanger selection it is convenience tooperation. However, for heat exchanger selection it is convenience to

have a methodology in order to estimate the overall heat transferhave a methodology in order to estimate the overall heat transfer

coefficient or the size according to given temperature range and flowcoefficient or the size according to given temperature range and flow

specifications.specifications.

Before setting the equation that determines the Overall Heat TransferBefore setting the equation that determines the Overall Heat Transfer

Coefficient, let’s take some assumptions. The conduction resistanceCoefficient, let’s take some assumptions. The conduction resistance

between hot and cold fluid could be neglected, also resistance due tobetween hot and cold fluid could be neglected, also resistance due to

fouling.fouling.

Where,Where,

hhhh : Heat transfer coefficient of hot fluid [W/m : Heat transfer coefficient of hot fluid [W/m22K]K]

hhcc : Heat transfer coefficient of cold fluid [W/m : Heat transfer coefficient of cold fluid [W/m22K]K]

In order to calculate hIn order to calculate hhh and h and hcc, the appropriate correlation will be used., the appropriate correlation will be used.

For flow in circular tubes:For flow in circular tubes:

NuNuDD : 4.36 (Laminar flow, Re : 4.36 (Laminar flow, ReDD < 2300) < 2300)


Colburn equationColburn equation

NuNuDD : 0.023 Re : 0.023 ReDD4/54/5 Pr Pr1/31/3 (Turbulent flow, Re (Turbulent flow, ReDD > 2300) > 2300)

D: Diameter of tubeD: Diameter of tube

k: Conductivity of fluidk: Conductivity of fluid

If the tube is non circular, hydraulic diameter is used, instead.If the tube is non circular, hydraulic diameter is used, instead.

Where Ac and P are the cross-sectional area and the wetted perimeter,Where Ac and P are the cross-sectional area and the wetted perimeter,

respectively.respectively.

Experimental setupExperimental setup

There are three optional There are three optional small-scale heat exchangers that can be installed tosmall-scale heat exchangers that can be installed to

illustrate the principles and different techniques of heat transfer betweenillustrate the principles and different techniques of heat transfer between

fluid streams. The heat exchangers are individually mounted on a commonfluid streams. The heat exchangers are individually mounted on a common

bench-top bench-top Heat Exchanger Service Unit. The unit supplies hot and cold waterHeat Exchanger Service Unit. The unit supplies hot and cold water

streams to the different heat exchangers installed on it.streams to the different heat exchangers installed on it.

The following parameters can be modified for each The following parameters can be modified for each small-scale heatsmall-scale heat

exchangerexchanger: volumetric flow rates of hot and cold fluids, hot fluid temperature: volumetric flow rates of hot and cold fluids, hot fluid temperature

and flow arrangements (countercurrent or cocurrent).and flow arrangements (countercurrent or cocurrent).


Fig. 2: Heat Exchanger Service Unit with the Tubular Heat Exchanger installed.

a)a)Tubular Heat ExchangerTubular Heat Exchanger


Description of the Tubular Heat ExchangerDescription of the Tubular Heat Exchanger::

Please refer to figures 3, 4, and 5Please refer to figures 3, 4, and 5

The tubular heat exchanger consists of two concentric (coaxial) tubesThe tubular heat exchanger consists of two concentric (coaxial) tubes

carrying the hot and cold fluids. The tubes are separated into twocarrying the hot and cold fluids. The tubes are separated into two

sections.sections.

The accessory consists of two concentric tube heat exchangers arrangedThe accessory consists of two concentric tube heat exchangers arranged


Fig. 3: Tubular Heat Exchanger

Fig. 4: Diagram of tubular heat exchanger

under countercurrent operation.

Fig. 5: Diagram of tubular heat exchanger

under co-current operation.


Heat Exchanger Analysis-3

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