How is the heat transfer?

As we discussed early in the first chapter that heat can transfer through materials and the surrounding medium whenever temperature gradient exists until thermal equilibrium is reached.

Heat transfer by:

Radiation is often categorized as either ionizing radiation or non-ionizing radiation depending on the energy of the radiated particles.

Conduction is the transfer of heat through materials by the direct contact of matter.

Convection is the transfer of heat by the motion of the fluid (liquids and gases).

Natural convection

Forced Convection

Natural and forced Convection

Natural convection occurs whenever heat flows between a solid

and fluid, or between fluid layers.

As a result of heat exchange, change in density of effective fluid

layers taken place, which causes upward flow of heated fluid.

If this motion is associated with heat transfer mechanism only, then it

is called Natural Convection

If this motion is associated by mechanical means such as pumps

or fans, the movement of the fluid is enforced.

And in this case, we then speak of Forced convection.

Forced Convection

Types of Heat Exchangers

• Usually involves convection in each fluid and conduction through the

wall of separating the two fluids.

• The overall heat transfer coefficient U contributes to all these factors.

• Usually work with the LMTD.

• LMTD = Logarithmic Mean Temperature Difference

Heat Exchanger is a device that provide the flow of thermal energy

between 2 or more fluids at different temperatures..

They are used in a wide variety of applications. These include power

production process, chemical, food and manufacturing industries,

electronics, environmental engineering, waste heat recovery, air

conditioning, reefer and space applications.

Heat Exchangers may be classified according to the following criteria.

Recuperators/ regenerators

Transfer process: direct and indirect contact

Geometry of construction; tubes, plates, and extended surfaces.

Heat transfer mechanism: single phase and two phase

Flow arrangement: Parallel, counter, cross flow current.

Classification of Heat Exchangers

Transfer process

As mentioned in the previous slight, according to transfer process heat

exchangers are classified as direct contact type and indirect contact type.

In direct contact type, heat is transferred between cold and hot fluids

through direct contact of the fluids (e.g. cooling towers, spray and tray

condensers)

In indirect heat exchanger, heat energy is transferred throw a heat transfer

surface.

Applications of Heat Exchangers

Heat Exchangers

prevent car engine

overheating and

increase efficiency

Heat exchangers are

used in Industry for

heat transfer

Heat exchangers are

used in AC and

furnaces

Heat Exchanger

Basic Principles

Double pipe heat exchanger

Concurrent\Parallel Countercurrent

Double Pipe Heat Exchangers

Advantages of Double Pipe Heat Exchanges:

1. Simplest type of heat exchangers

2. Can be easily assembled

3. Relatively low cost

4. Small sizes

Disadvantages of Double Pipe Heat Exchanger:

1. Leakages are very common

2. Requires a lot of time in dismantling and cleaning

3. Small surface area of heat transfer/pipe

4. Space requirements are large

Double pipe heat exchangers should be considered first in design. The

heat transfer surface should not exceed 200 ft2.

If several double pipes are required, their weight increases and thus the

shell and tube heat exchangers is better.

Shell and Tube Heat Exchangers

A shell and tube heat exchanger is a class of heat exchanger designs. It is

the most common type of heat exchanger in oil refineries and other large

chemical processes.

Shell and tube heat exchangers normally consist of a bundle of tubes

fastened into holes, drilled in metal plates called tube sheets.

The Tubular Exchanger Manufacturers Association (TEMA) provides a

manual of standards for construction of shell and tube heat exchangers,

which contains designations for various types of shell and tube heat

exchanger configurations.

The most common types are summarized below.

Shell and Tube Heat Exchangers

The E-type shell and tube heat exchanger, illustrated in Fig. 2, is the

workhorse of the process industries, providing economical rugged

construction and a wide range of capabilities.

E-Type

The E-type shell is usually the first choice of shell

types because of lowest cost, but sometimes requires

more than the allowable pressure drop, or produces a

temperature, so other, more complicated types are used.

Tubular Exchanger

Manufacturers Association

F-Type

The F-type shell can be effective in some cases if well designed, but has a

number of potential disadvantages, such as :

Thermal and fluid leakage around the longitudinal baffle.

High pressure drop.

Tubular Exchanger

Manufacturers Association

When an F-type shell cannot be used because of high pressure drop, a J-type

or divided flow exchanger, shown in Fig. 4, is considered.

J-Type

Tubular Exchanger

Manufacturers Association

When a J-type shell would still produce too high a pressure drop, an X-type

shell, shown in Fig. 5, may be used.

This type is especially applicable for vacuum condensers, and can be

equipped with integral finned tubes to counteract the effect of low shellside

velocity on heat transfer.

X-Type

Tubular Exchanger

Manufacturers Association

This shell type, shown in Fig. 6, is used especially for boiling range mixtures

and provides better flow distribution than would be the case with the X-type

shell.

The G-type shell also permits a larger temperature cross than the E-type shell

with about the same pressure drop.

G-Type

If a G-type is being considered but pressure drop would be too high, an H-

type may be used. This configuration is essentially just two G-types in

parallel, as shown in Fig. 7.

H-Type

This type is used exclusively for kettle re-boilers and vaporizers, and is

characterized by the oversized shell intended to separate vapor and liquid

phases, Fig. 8.

K-Type

Baffles are used to increase velocity of the fluid

flowing outside the tubes (shellside fluid) and to

support the tubes. Higher velocities have the

advantage of increasing heat transfer and decreasing

fouling (material deposit on the tubes), but have the

disadvantage of more energy consumption.

Baffle-Type

Baffle types commonly used are shown in Fig. 9, with pressure drop

decreasing from Fig. 9a to Fig. 9c.

Shell and Tube Heat Exchangers

w,cp,t1

w,cp,t2

• Non-baffled Heat Exchangers W,Cp,T1

W,Cp,T2

IDs

do

di

w,cp,t1

w,cp,t2

• Baffled Heat Exchangers

W,Cp,T1

W,Cp,T2

Shell

passes

Tube

passes

m - n Heat Exchanger 1 - 2 Heat Exchanger

w,cp,

t1

w,cp,t2

• Baffled Heat Exchangers

W,Cp,

T1

W,Cp,

T2

Shell

passes Tube passes

m - n Heat Exchanger 1 - 2 Heat Exchanger

w,cp,t1 w,cp,t2

• Baffled Heat Exchangers

W,Cp,T1

W,Cp,T2

B

I

D

s

c

1 - 1 Heat Exchanger

Shell-side flow

TEMA Designations

Tubular Exchanger

Manufacturers Association

The heat transfer surface consists of a number of thin

corrugated plates pressed out of a high grade metal.

The pressed pattern on each plate surface induces turbulence

and minimizes stagnant areas and fouling.

Unlike shell and tube heat exchangers, which can be custom-

built to meet almost any capacity and operating conditions, the

plates for plate and frame heat exchangers are mass-produced

using expensive dies and presses.

Plate heat exchangers

Hot and Clod Flows through Plate Heat Exchangers

Hot and Clod Flows through

Plate Heat Exchangers

Performance of Plate HXs

Superior thermal performance is the hallmark of plate heat exchangers.

Compared to shell-and-tube units, plate heat exchangers offer overall heat

transfer coefficients 3 to 4 times higher.

These values, typically 4000 to 7000 W/m2 ºC (clean), result in very

compact equipment.

This high performance also allows the specification of very small approach

temperature (as low as 2 to 3 ºC) which is sometimes useful in geothermal

applications.

Selection of a plate heat exchanger is a trade-off between U-value (which

influences surface area and hence, capital cost) and pressure drop (which

influences pump head and hence, operating cost).

Classification of Plate HXs

Casketed plate heat exchangers (plate and frame heat

exchangers)

Brazed plate heat exchangers

Welded plate heat exchangers

Gasketed plate heat exchangers

41

Cross-Flow Heat Exchangers

Finned - Both Fluids Unmixed

Finned - Both Fluids Unmixed

Unfinned - One Fluid Mixed the Other Unmixed

Unfinned - One Fluid Mixed the Other Unmixed

42

Compact Heat Exchangers

Widely used to achieve large heat rates per unit volume, particularly when one

or both fluids is a gas.

Characterized by large heat transfer surface areas per unit volume (>700

m2/m3), small flow passages, and laminar flow.

Heat Exchangers Chee 318 43

Heat Exchangers Chee 318 44

Baffle Arrangement

Tube sizes

Tubes

Standard tube lengths are 8, 12,

16 and 20 ft.

Tubes are drawn to definite wall

thickness in terms of BWG

(Birmingham Wire Gauge) and

true outside diameter (OD), and

they are available in all common

metals.

The spacing between the tubes

(center to center) is referred to as

the tube pitch (PT). Triangular or

square pitch arrangements are

used. Unless the shell side tends

to foul badly, triangular pitch is

Used.

Tube Pitch

•Heat Exchanger (HEX) Rating

• Checking the existing design for compatibility with the user

requirements (outlet temperature, heat load etc.)

• given: flow rates, inlet temperatures, allowable pressure drop;

thus HT area and passage dimensions.

• find: heat transfer rate, fluid outlet temperatures, actual pressure

drop.

•HEX Sizing

• Thermal and pressure drop considerations, maintenance

scheduling with fouling consideration.

• given: inlet and outlet temperatures, flow rates, pressure drop

• find: dimensions -type and size of HEX. 48

Assumptions for Basic Design Equations for Sizing

• steady-state, steady flow

• no heat generation in the HEX

• negligible ΔPE, ΔKE

• adiabatic processes

• no phase change (later)

• constant specific heats and other physical properties.

49

Overall heat transfer coefficient (U):

Because the temperature difference between the hot

and cold fluid streams varies along the length of the

heat exchanger, it is necessary to derive an average

temperature difference from which heat transfer

calculations can be performed. This average

temperature difference is called the Logarithmic

Mean Temperature Difference (LMTD) ΔTlm.

)/ln( io

iolm

TT

TTT

Where, ΔTo = T1 – T4

ΔTi = T2 – T3

Another way to determine LMTD

Log Mean Temperature Difference (LMTD) is the heat flows between the hot and cold

streams due to the temperature difference across the tube acting as a driving force. As seen in

the Figure below, the difference will vary with axial position within the HX.

2

1

21

ln

LMTD

Where, θ1 = T1-t2

θ2 = T2-t1

52

Concentric Tube Construction

Parallel Flow CounterflowParallel Flow Counterflow

• : • :

Parallel Flow CounterflowParallel Flow Counterflow

53

Heat Exchanger Analysis LMTD Method

Expression for convection heat transfer for flow of a fluid inside a tube:

)( ,, imompconv TTcmq

lms TAUq )/ln( io

iolm

TT

TTT

U = heat exchanger coefficient, Q = heat transfer rate

In a two-fluid heat exchanger, consider the hot and cold fluids separately:

)(

)(

,,,

,,,

icoccpcc

ohihhphh

TTcmq

TTcmq

lmTUAq and

Need to define U and Tlm

Heat Exchanger Analysis

55

Where:

qh is the heat power emitted from hot fluid.

qc the heat power absorbed by cold fluid.

ṁh , ṁc : mass flow rate of hot and cold fluid, respectively.

hh,i , hh,o : inlet and outlet enthalpies of hot fluid, respectively

hc,i , hc,o inlet and outlet enthalpies of cold fluid, respectively.

Th,i , Th,o : inlet and outlet temperatures of hot fluid, respectively.

Tc,i , Tc,o : inlet and outlet temperatures of cold fluid, respectively.

Cph , Cp

c specific heats of hot and cold fluid, respectively

)(

)(

,,,

,,,

icoccpcc

ohihhphh

TTcmq

TTcmq

With the LMTD method, the task is to select a heat exchanger that will meet the

prescribed heat transfer requirements. The procedure to be followed by the

selection process is:

Example 1