HEAT EXCHANGER SLIDE NOTE

7/28/2019 HEAT EXCHANGER SLIDE NOTE

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HEAT

EXCHANGERS



• A heat exchanger is used to exchange heat between two fluids of different temperatures, which are separated by a solid wall.

• Applications in heating and air conditioning, power production, waste heat recovery, chemical processing, food processing,

sterilization in bio-processes.

• Heat exchangers are classified according to flow arrangement andtype of construction.

•

In this chapter we will learn how our previous knowledge can beapplied to do heat exchanger calculations, discuss methodologies fordesign and introduce performance parameters.

HEAT EXCHANGERS



Heat Exchanger Types

1) Parallel Flow – hot and cold fluids enter at the same end, flow in the samedirection and leave at the same end.

Parallel FlowParallel Flow




2) Counter Flow – hot and cold fluids enter at opposite ends, flow inopposite directions and leave at opposite ends.

CounterflowCounterflow



3) Cross Flow – hot and cold fluids perpendicular to each other

Finned Tubular Heat Exchanger (Both fluids unmixed)the fins inhibit motion in a direction (y) that is transverse to the main flow in a direction (x)fluids temperature varies with x and y.

Unfinned Tubular Heat Exchanger (one fluid mixed and the other unmixed)Fluid motion mixing in the transverse directiontemperature variations are primarily in the main-flow direction


- Both Fluids Unmixed

Finned - Both Fluids Unmixed

- One Fluid Mixed the Other Unmixed

Unfinned the Other Unmixed



4) Shell and Tube Heat Exchangers


One Shell Pass,Two Tube Passes Two Shell Passes,Four Tube Passes




6) Compact Heat Exchangers

• Widely used to achieve large heat rates per unit volume, particularly when one orboth fluids is a gas.

• Characterized by large heat transfer surface areas per unit volume (>700 m2/m3),small flow passages, and laminar flow.



Overall Heat Transfer Coefficient

• The overall heat transfer coefficient defined in terms of the totalthermal resistance to heat transfer between two fluids

• The coefficient was determined by accounting for conduction andconvection resistance between fluids separated by composite planeand cylindrical walls, respectively

• The overall heat transfer coefficient can be expressed as

1 = 1 = 1 c = coldUA Uc A c Uh A h h = hot

• For tubular heat exchanger:

– Conduction resistance in the wall

– Convection resistance of the fluids at the inner and outer tube

surfaces

i = inner

0 = outer



Fouling

• Heat exchanger surfaces are subject to fouling by fluid impurities,

rust formation, or other reactions between the fluid and the wallmaterial.

• The deposition of a film or scale on the surface can greatly increasethe resistance to heat transfer between the fluids.

• An additional thermal resistance : The Fouling factor, Rf .

–

Depends on operating temperature, fluid velocity and length of service of the heat exchanger.

– Typical values in Table 11.1.

• For unfinned tubular heat exchangers :

R”f,i = fouling factor of inner surface

R”f,o = fouling factor of outer surface

ooo

o f io

i

i f

ii Ah A R

kL D D

A R

AhUA1

2)/ln(11

",

",



Fin (extended surface) effects

• Fins reduce the resistance to convection heat transfer, by increasingsurface area.

• For finned tubular heat exchanger :

ηo = overall surface efficiency / temp. effectiveness



OVERALL SURFACE EFFICIENCY

• The quantity ηo is termed the overall surface efficiency or temp.

effectiveness of a finned surfaces• The heat transfer rate is :

q = ηo h A (T b - T∞) , T b = base surface temperature

ηo = 1 – A f (1 – ηf ) , A f = fin surface area

A h = N (2L + t)

A h = A f + (πDo – 16t)

L = fin length

ηf = efficiency of single fin

ηf = tanh (mL) , m = (2h/kt)1/2

mL t = fin thickness

Overall surface efficiency:

Efficiency of a single fin:



HEAT EXCHANGER ANALYSIS : LOG MEANTEMPERATURE DIFFERENCE (LMTD)

•

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

• For case involving constant surrounding

fluid temperature:

• In a two-fluid heat exchanger, considerthe hot and cold fluids separately:

)T T ( cmq i ,mo ,m pconv =

lm s T AU q )/ln( io

iolm

T T

T T T

)()(

,,,

,,,

icocc pcc

ohihh phh

T T cmqT T cmq

lmT UAq

Need to define U and Tlm



PARALLEL – FLOW HEATEXCHANGER

COUNTER – FLOW HEATEXCHANGER

ΔT1 ΔT2 ΔT2 ΔT1

ΔT2

ΔT1 Thi

ThoTco

Tci

ΔT1 ΔT2

Thi

Tho

Tco

Tci

)/ln( 12

12

T T

T T T lm

ocoh

icih

T T T

T T T

,,2

,,1

lmT UAq lmT UAq

)/ln( 12

12

T T

T T T lm

icoh

ocih

T T T

T T T

,,2

,,1



HEAT EXCHANGER ANALYSIS : THE EFFECTIVENESSNTU METHOD (ξ - NTU METHOD)

• LMTD is used when the fluid inlet temperatures are known and the

outlet temperature are specified or readily determined.• ξ – NTU is used when only the fluid inlet temperature are known

• Effectiveness of a heat exchanger, ξ is defined as the ratio of the actualheat transfer rate for a heat exchanger to the maximum possible heattransfer rate

ξ = qqmax

• qmax is the maximum heat transfer rate that could possibly be delivered by the heat exchanger

qmax

= Cmin

(Thi

– Tci

)

Cmin is equal to Cc or Ch whichever is smaller

Cc < Ch , qmax = Cc (Thi – Tci)

Ch < Cc , qmax = Ch (Thi – Tci)

Cc = mcCPc

Ch = mhCPh Heat capacity rates



ξ = Ch (Thi – Tho) , if Cc < Ch

Cmin (Thi – Tci)

ξ = Cc (Tco – Tci) , if Ch < Cc

Cmin (Thi – Tci)

ξ must be in the range 0 ≤ ξ ≤ 1

• For any heat exchanger it can be shown that

ξ = f (NTU , Cmin/Cmax) NTU = f (ξ , Cmin/Cmax)

where Cmin/Cmax is equal to Cc/Ch or Ch/Cc

•The number of transfer units (NTU) is a dimensionless parameterthat is widely used for heat exchanger analysis

NTU = UA

Cmin



• Effectiveness – NTU relations for a variety of heat exchangers aresummarized in Table 11.3 and Table 11.4, where Cr is the heat capcity ratio

Cr = Cmin/Cmax

• Effectiveness, ξ – NTU relations for a variety heat exchangers alsoare represented graphically in Figures 11.10 through Figure 11.15



Multipass and Cross-Flow Heat Exchangers

• To account for complex flow conditions in multipass, shell and

tube and cross-flow heat exchangers, the log-mean

temperature difference can be modified:

• The appropriate form of ΔTlm is obtained by applying a

correction factor to the value of ΔTlm that would be computed

under the assumption of counterflow conditions.

where F = correction factor (can find from figures 11.10-11.13)

icoh

ocih

T T T

T T T

,,2

,,1

CF lmlm T F T ,




“Variable t always assigned to the tube side fluid”




• To design a shell and tube heat exchanger

with one shell and any multiple of tube passes

(two, four, etc.).

• If F < 0.8, shell and tube heat exchanger with

two shell and any multiple of tube passes

(four, eight, etc.) can be used instead.

F > 0.8



PROBLEM 11.14

A shell and tube exchanger (two shells, four tube passes) is used toheat 10,000 kg/h of water (cp = 4195 J/kg.K) from 35°c to 120°c with 5000 kg/h pressurized water (cp = 4660 J/kg.K) entering theexchanger at 300°c. If the overall heat transfer coefficient is 1500 w/m2.K, determine the required heat exchanger area. (Assume

counter flow).

a) LMTD method (A = 4.61 m2)

b) ε-NTU method (A = 4.75 m2)



Special Operating Conditions

Condenser:

Hot fluid iscondensing vapor(eg. steam)

Evaporator/boiler:

Cold fluid isevaporating liquid

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HEAT EXCHANGER SLIDE NOTE