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Heat Transfer Operations

How to heat/cool fluids (gas/liquid) in a plant?

What are HT equipment? How do they look?

How many types? Advantages & disadvantages?

How do they operate? How much heat transfer?

How to select & design (size or specify) them?

Useful referencesProcess heat transfer, McGraw Hill, 1964, by DQ Kern

Perrys handbook, McGraw Hill.A guide to ChE process design & economics, John Wiley,

1984, by GD Ulrich

Process design principles, John Wiley, 1999 by WD Seider,JD Seader, DR Lewin

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Select Process Utilities for Various Ts

Media external to process vs. process fluid Refrigerants: 60 - 5 C

Freon, ammonia, methane, ethane, ethylene, CO2, etc.How do we use them? Recall refrigeration cycle?

Brine or chilled water: 5 - 10 C (e.g. NaCl solution)

Any liquid cooled by a refrigerant and used as a cooling agent

Cooling water (CW) & air: 20 - 40 CNatural water (sea, pond, river) at 20-30 C, discharge 45 C

Closed-circuit CW at 40 C, discharge at 50-60 C is air-cooled

Cooling tower CW at 30 C, discharge 45 C max (Why?)

Process steam: 100 - 300 CHigh P (HP): 45 bar, 400 C (Superheated)

Medium-High P (MHP): 33 bar, 239 C (Saturated)

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Select Process Utilities for Various Ts

Medium P (MP): 17 bar, 204 C (Saturated)

Medium-Low P (MLP): 9 bar, 175 C (Saturated)

Low P (LP): 4.5 bar, 148 C (Saturated) HT oils or heavy oils: 250 - 400 CDowtherm A, Dowtherm E, mineral oil, silicon oil, etc.

Stable, noncorrosive, low vapor pressure fluids

Liquid metals, molten salts (Why?): 250 - 600 CSodium, potassium, mercury, alkali nitrates, etc.

No appreciable vapor pressure

Boiler feed water (BFW) or demineralized water

(DMW): 100 - 300 C30-40 ppm solids for BFW, 320 ppm solids for cooling tower

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Three Types of HT Equipment

Direct fired equipment burn fuel, generate hot flue gas &use the gas to heat

Coal, heavy oil, natural gas, solid waste, etc.Boiler produces steam at different levels from BFW

Furnace heats or vaporizes a process fluid to high T

Indirect fired equipment burn fuel, use hot flue gas toheat a thermal fluid, which in turn heats the process fluid

Dowtherm, mineral oil, liquid metals, etc.

Unfired equipment do not burn fuel, but heat or cool viaa process fluid or utility

With phase change: Change sensible & latent heat contentsWithout phase change: Change sensible heat content only

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Two Types of Unfired HT Equipment

Unfired equipment without phase changeCooler cools by indirect HT using CW or air

Cooling tower cools hot CW by direct air contactChiller cools by brine or refrigerant

Heater heats by a process utility (steam / oil)

Exchanger exchanges heat between two process fluids

Unfired equipment with phase changeCondenser liquefies a vapor (Total vs. partial)

Vaporizer vaporizes a liquid other than water (e.g. reboiler)

Evaporator concentrates a solution by evaporating water

Waste heat boiler (WHB) produces steam by evaporatingBFW

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Specification of HT Equipment

Total HT rate or heat duty Q (J/s or Btu/h)

Total HT area A (m2 or ft2)

HT media and/or process fluidsOperating conditions (inlet/outlet T & P)

Equipment type, size, material, mechanical detailsTravel paths of hot and cold fluids Sufficient for preliminary cost estimation

Rating (vs. Design): Given a HT equipment, analyze itsperformance for a given application

Identify outlet fluid temperatures & pressures

Simulation vs. design

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Steps in Designing a HT Equipment

Primary goal: Find HT area A across which heattransfers in the equipment

Step 1: Total heat duty Q (W or Btu/h)Steady state rate of total heat transfer between streams

Step 2: Select equipment type, assign fluid paths

Step 3: Overall temperature driving force T (K or F)Average T between streams, which varies with fluid contact

pattern and temperature profiles Step 4: Overall HT coefficient U (W/m2-K)Coefficient measuring the rate of heat transfer as a function of

fluid flows and properties

Step 5: Compute HT area A (m2) = Q / [U T]

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Step 1: Compute Heat Duty Q

FIX inlet and outlet temperatures of streamsProblem specifies or use thumb rules (CW 30-45 C, 40-60 C)

Select appropriate for economic design

Overall energy balance (No heat loss or gain)

HT

Equipment

Hot Fluid

Cold Fluid

Tha ThbTcb Tca

General: Q = mh(Hha Hhb) = mc(Hcb Hca)No phase change: Q = mhcph(Tha Thb) = mccpc(Tcb Tca)

Step 2: Select equipment type: Four main types of HE

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Double Pipe Exchanger (DPE)

Simplest, but large & cumbersome heat exchangerTwo long, concentric pipes; one (the inner pipe) inside the

other (the outer pipe)One fluid flows in the inner pipe, the other in the outer

Heat transfers across the wall of inner pipe.

Counter (countercurrent): Fluids flow in opposite directionsParallel (cocurrent): Fluids flow in the same direction

Heat transfer A < 10 m2, P < 1000 bar, T < 150 C

Outside Fluid

Outer PipeAnnulus

Inner Pipe

Inside Fluid

Outside Fluid HT

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Double Pipe Exchanger (DPE)

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Shell & Tube (S&T) Heat Exchanger

The most common, very compact heat exchangerOuter pipe of DPE becomes shell and a bundle of small tubes

replaces the inner pipeDouble pipe exchanger is a special case of S&T HE

One fluid flows in the tubes, the other in the shell

Heat transfers across tube wallsA < 1000 m2, P < 140 bar, T < 350 C

Larger area, lower cost per unit area, but lower pressure

Wide variety of types, configurations & sizes

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Shell & Tube Exchanger (S&T)

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Air Cooled Exchanger

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Stpes 3 & 4: Overall T & U

Most HEs use indirect HT across metal (tube) wall

Small tube element of length x with rate of HT = dq

Cold fluid inside with bulk T = Tc, hot fluid outside with bulkT = Th

How many resistances for heat transfer? What are they?

Metal Tube (ID = Di & OD = Do)

Di Dox

dq

0

( )

( )

( ) /

i i wi c

o o h wo

w w wo wi w

i i

o

dq h dA T T

dq h dA T T

dq k dA T T t

dA D x

dA D x

=

=

=

=

=

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Film Heat Transfer Coefficient h

Measures conductance to heat transfer: Higher the betterEstimated from empirical correlations obtained from

experimental measurements Rough ranges of h (W/m2-K). What does h depend on?Steam condensation: 6,000-100,000

Boiling water: 1,700-50,000

Water (heating/cooling): 300-20,000

Condensing organic vapors: 1,000-2000

Oils (heating/cooling): 50-1,500

Steam (superheating): 30-100

Air (heating/cooling): 1-50

How can we avoid dealing with wall temperatures?

h varies

from point

to point

along

flow!

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Overall Heat Transfer Coefficient U

Define local overall HT coefficient (U) to relate localHT rate dq to local bulk temperatures of fluids

Defined based on outside tube area

1 1 1( ) & o w o

o h c

o i i w w

dA t dAdq UdA T T

U h h dA k dA

= = + +

Metal resistance often neglected (How to justify?)1 1 1

o

o i i

D

U h h D

= +

Limiting or controlling coefficient? (Smaller of hi & ho)hi

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Resistance from Fouling is Important

Fluid velocity at metal wall is zeroSome fluid spends a long time at the wall

Dirt, scale, corrosion, etc. deposit on tube wallDeposit grows thicker with time; becomes an additional and

significant resistance with low k

Resistance much greater than metal wall, not negligible This is called dirt or scale or fouling factor (Rd)Depends on nature of fluid (content, fouling, clean, corrosive,

side reactions), fluid temperature, and time between clean-ups

or maintenance etc.

Normally based on yearly clean-up or maintenance, Rd =

0.001-0.006 Btu/ft2-F-h/Btu (resistance) for most industrialliquids.

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Must Account for Fouling Resistances

Ensure satisfactory service over time by using UD =dirty U or design U that includes fouling resistances

1 1 odi do

D C i

DR RU U D

= + +

Local rate of HT: dq(x) = UD(x) [Th(x) Tc(x)] dAoUD, Th, Tc vary with position dAo, as fluids heat/coolHow to get Q or total rate of HT for the equipment?

0 0

( )

Q A

D h c oQ dq U T T dA= = dA = D

odx

dq

Do

dx

Tc+dTcTcTube:

Tedious, trial & error, integration: How to simplify?

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Overall Temperature Difference T

Simplify the general integral HE design equation as:

0

( )[ ( ) ( )]

A

D h c o

Q U x T x T x dA UA T = =

Assume U constant along the length of tube

Fluid enthalpy H vs. T linear for both fluids (constant heatcapacities and no phase change)

Negligible heat exchange with the ambient

Steady state flow (parallel or counter)No heat generation/work input in the unit (e.g. reaction)

Do the simplification for counter flow DPE

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LMTD for a DPE Cooler

Process fluid 120 C to 40 C, let CW 30 C to 45 C

Counter flow: T1 = 10 C, T2 = 75 C

LMTD = (1075) / ln(10/75) = 65/ln 7.5 = 32.3 C

Parallel flow: T1 = 90 C, T2 = 5 C

LMTD = 29.4 C? Is this OK?

Double Pipe Exchanger40 C120 C

30 C45 C

Double Pipe Exchanger 40 C120 C

45 C30 C

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Impact of Parallel Flow

What can you do to the parallel flow cooler?Process fluid cannot cool to 40 C or CW cannot leave at 45 C

What are the corresponding min and max Ts? What if CW leaves at 35 C & process fluid at 40 C?LMTD = 29.4 C More HT area vs. counter flow, why?

More CW required for the same Q

What if CW leaves at 45 C & process fluid at 50 C?

Less heat recovered from the process fluid vs. counter flowLMTD = 29.4 C, will HT area go up or down? How can you

estimate?

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Range, Approach & Cross

Range: T for a fluidHot fluid: ThaThb & Cold fluid: TcbTca

Approach: T between fluids at an endCounter flow: T1 = ThaTcb & T2 = ThbTca

Parallel flow: T1 = ThaTca & T2 = ThbTcb

Minimum approach = min[T1, T2]

Cross: T by which cold fluid heats above the exittemperature of hot fluid or TcbThb

Heat

Exchanger

Hot Fluid

Cold Fluid

Tha ThbTcb Tca

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What if We Decrease Approach?

LMTD & area for a given Q? Heat recovery Q?

What if min approach 0? How much min approach is economical?

5-10 K for liquids or systems with high HT coefficients50 K for gases or systems with low HT coefficients, why?

2-5 K for refrigeration systems.

Select inlet-outlet temperatures as per the above thumb rules

Heat

Exchanger

Hot Fluid

Cold Fluid

Tha ThbTcb Tca