# HEAT EXCHANGER DESIGN - che192.weebly.com

LOGO 1985 HEAT EXCHANGER DESIGN LOGO Heat Transfer Equipment Types 1985 Type Double pipe exchanger Shell and tube exchanger Plate heat exchanger Plate-fin exchanger

Spiral heat exchanger Air cooled Direct contact Agitated vessel Fired heaters Service Heating and cooling All applications Heating and cooling Cooler and condensers Cooling and quenching Heating and cooling Heating

LOGO Double Pipe Heat Exchanger 1985 Consists of two concentric pipes with one fluid flowing through the inner pipe while the other fluid flowing through the annular space LOGO Shell and Tube Heat Exchanger 1985 Consists of tube bundles enclosed in a cylindrical shell with one fluid flowing through

the tubes and the other flowing outside of the tubes LOGO 1985 Heat Transfer Equipment in Industries Exchanger: heat exchanged between two process streams Heaters and coolers: where one stream is plant service Vaporiser: if a process stream is vaporised Reboiler: a vaporiser associated with distillation column Evaporator: if concentrating a solution Fired exchanger: if heated by combustion gases

Unfired exchanger: not using combustion gases LOGO 1985 Heat Transfer Equipment in Industries MODES of HEAT TRANSFER 1. Conduction Transfer of heat from one part of a body to another part of the same body or between two bodies in physical contact, without significant displacement of the particles of the two bodies

2. Convection Transfer of heat from one point to another within a fluid or between a fluid and a solid or another fluid, by the movement or mixing of the fluids involved 3. Radiation Transfer of heat by the absorption of radiant energy LOGO BASIC THEORY

1985 General equation for heat transfer across a surface for DPHE is: Q UATlm Q =heat transferred per unit time, W U=the overall heat transfer coefficient, W/m2oC A= heat-transfer area, m2 Tm= the mean temperature difference,oC

LOGO BASIC THEORY 1985 General equation for heat transfer across a surface for STHE is: Q UAYTlm

Q =heat transferred per unit time, W U=the overall heat transfer coefficient, W/m2oC A= heat-transfer area, m2 Tm= the mean temperature difference,oC Y = geometric correction factor LOGO 1985 Tube-Side Passes One tube pass Two tube pass

Three tube passes LOGO 1985 Geometric Correction Factor Also refer to Figure 11-4, Perry 7th Edition LOGO LOGO 1985 Geometric Correction Factor

Z Y 2 1 1 2 1 X ln 1 ZX

1 2 2 X Z 1 ( Z 1) 2 Z 1 ln 1 2 2 X Z 1 ( Z 1) 2 For design to be

practical, Y 0.85 LOGO 1985 Logarithmic Mean Temperature Difference TT1 TT2 T2 T1 Tlm T2

ln T1 If TT1 < TT2 and (TTT2/TT1) 2, then TTlm is the arithmetic mean temp difference LOGO 1985 Overall Heat Transfer Coefficient Rearranging the General Equation in terms of driving force and total resistance: Driving Force Q UATlm

Total Resistance Tlm Q 1 UA LOGO 1985 Overall Heat Transfer Coefficient The overall coefficient is reciprocal of the overall resistance to heat transfer, which is the sum of several individual resistances. Individual

resistance is the reciprocal of individual HTC. 1 Rtot UA LOGO Total Resistance 1985 the sum of several individual resistances Individual resistance is the reciprocal of individual HTC. Convection

Conduction Convection inside 1 Rtot sum of individual resistances from convection and conduction UA LOGO 1985 Total Resistance Conduction Heat Transfer is governed by Fouriers Law! dQ

dT kA dt dx k = thermal conductivity of the Solid (TBTU/hr-ft2-(TOF/ft)) A = Area perpendicular to the direction of heat transfer x = distance of heat flow LOGO Total Resistance 1985 At Steady State:

dQ dT time invariant kA dt dx dT q kA dx LOGO 1985 Total Resistance If k is constant:

(T1 T2 ) q kA ( x2 x1 ) (T1 T2 ) q ( x2 x1 ) kA Define R = x/kAx/kA Thus, q= - TT/R LOGO Total Resistance 1985

If k is not constant: x2 q x1 If k varies slightly with Temp: T2 dx kdT A T1

(T1 T2 ) q ( x2 x1 ) km A **km is evaluated at the mean temperature LOGO Total Resistance 1985 If k is not constant:

x2 q x1 If A varies slightly with Thickness: T2 dx kdt A T1 (T1 T2 ) q

( x2 x1 ) k m Am LOGO 1985 Total Resistance Convection Heat Transfer q = hcA (TT1 T2) Where: hc- convection heat transfer coefficient, Btu/hrft 2F -similar to k/x A Heat transfer Area T1 temperature at surface 1

T2 temperature at surface 2 LOGO 1985 Total Resistance Convection Heat Transfer: Rearranging q = (TT1 T2)/(T1/hcA) Where: hc- convection heat transfer coefficient, Btu/hrft 2F -similar to k/x A Heat transfer Area T1 temperature at surface 1 T2 temperature at surface 2

LOGO 1985 Total Resistance Convection Conduction Convection inside 1 1 1 x 1

1 Rtot UA hi Ai hi ,d Ai k m Am ho Ao ho ,d Ao LOGO Total Resistance 1985 in

si d e 1 1 1 1 UA U o Ao U i Ai U m Am Ao Ao Ao x 1 1

1 U o hi Ai hi ,d Ai k m Am ho ho ,d Ai x Ai Ai 1 1 1

U i hi hi ,d k m Am ho Ao ho ,d Ao LOGO 1985 Typical Fouling Factor (TFoust, 1980) LOGO 1985 Heat Transfer Without Phase Change LOGO 1985

DOUBLE PIPE HEAT EXCHANGER LOGO 1985 Invidual Heat Transfer Coefficient HT w/o Phase Change: DPHE For Long Tubes (L/D) > 50, Tube-side 1 hi di 0.8 Nu 0.023N RE N Pr 3

k w Applicabilty: 1. Non-metallic fluid 2. 0.5 < NPr < 100 3. NRE > 10,000 0.14 d 0.7 1 L

LOGO Invidual Heat Transfer Coefficient 1985 HT w/o Phase Change: DPHE For Long Tubes (L/D) > 50, Annular Space Nu ho deq k 0.8

0.023N RE NPr 1 Applicabilty: 1. Non-metallic fluid 2. 0.5 < NPr < 100 3. NRE > 10,000 3 w

0.14 d 0.7 1 L LOGO 1985 Invidual Heat Transfer Coefficient HT w/o Phase Change: DPHE For Short Tube (L/D < 50)

his D 1 hi L 0 .7 LOGO 1985 Invidual Heat Transfer Coefficient HT w/o Phase Change: DPHE

Laminar Flow, Forced Convection N NU 2 N GZ N GZ 1 3 mc p kL w

0 . 14 LOGO SHELL AND TUBE HEAT EXCHANGER LOGO 1985 Invidual Heat Transfer Coefficient HT w/o Phase Change: STHE, ho

LOGO 1985 Invidual Heat Transfer Coefficient HT w/o Phase Change: STHE, hi LOGO 1985 Heat Transfer WITH Phase Change LOGO 1985

Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Vertical Surface Assumptions: 1. Pure vapor is at its saturation temperature. 2. The condensate film flows in laminar regime and heat is transferred through the film by condensation. 3. The temperature gradient through the film is linear. 4. Temperature of the condensing surface is constant. 5. The physical properties of the condensate are constant and evaluated at a mean film temperature. 6. Negligible vapor shear exists at the interface

LOGO 1985 Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Vertical Surface, Laminar k l 3 l l v H v g h 0.943 l LTv T1

1 4 LOGO 1985 Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Vertical Surface, Turbulent kl 3 l l v H v g h 1.13

l LTv Tl 1 4 LOGO 1985 Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Horizontal Surface

3 kl l l v H v g h 0.725 l DTv Tl 1 4 If the amount of condensate is unknown

For Nre > 40, h is multiplied by 1.2 LOGO 1985 Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Horizontal Surface 3 kl l l v gL h 0.95

lW 1 3 If the amount of condensate is known For Nre > 40, h is multiplied by 1.2 LOGO 1985 Invidual Heat Transfer

Coefficient HT w/ Phase Change: STHE Film-type Condensation on Horizontal Surface, Banks of Tubes 3 kl l l v H v g h 0.725 l NDTv Tl For Nre > 40, h is multiplied by 1.2 1

4 LOGO Invidual Heat Transfer Coefficient 1985 HT w/ Phase Change: STHE Film-type Condensation on Horizontal Surface, Banks of Tubes h N hN N 1

4 1 4 N 1 N 2 ... N n 3 3 3 4 4 N 1 N 2 ... N n 4 LOGO

1985 Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Horizontal Surface, Banks of Tubes 3 kl l l v H v g h 0.725 l NDTv T1 1

4 w/o splashing LOGO 1985 Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Horizontal Surface, Banks of Tubes 3

kl l l v H v g h 0.725 2 3 l N DTv T1 1 4 w/ splashing LOGO 1985

Invidual Heat Transfer Coefficient Film Temperature Condensate Properties are evaluated at the Film Temperature Tf = (TTsv + Tw) Tf = Tsv - 0.75TT 3rd. Ed. TT = Tsv - Tw by Kern, D.Q., Process HT by McAdams, W.H., Heat Transmission,

LOGO 1985 Invidual Heat Transfer Coefficient Film Boiling on Submerged Horizontal Cylinder or Sphere qC pl h 0.225 A 0.69 Pkl

0.31 l 1 v 0.33 LOGO 1985 Invidual Heat Transfer

Coefficient Film Boiling on Submerged Horizontal Cylinder or Sphere 3 gk v v l v 0 .4 C p v Ts Tsat q C A v D Ts Tsat

LOGO 1985 Invidual Heat Transfer Coefficient Film Boiling on Submerged Horizontal Cylinder or Sphere Nusselt-type Equation by Rohsenow: DG hD C r

k 2 3 C p k Cr varies from 0.006 to 0.015

0 .7 LOGO 1985 Invidual Heat Transfer Coefficient Film Boiling on Submerged Horizontal Cylinder or Sphere Nusselt-type Equation by Forster and Zuber: DG hD

0.0015 k 0.62 Cp k 1 3 LOGO

1985 HE DESIGN SPECS LOGO 1985 TOTAL HEAT TRANSFER AREA Q A U T lm A N T DL A compromise between NT and L is chosen based on (TL/Dshell) between 5 to 10

LOGO 1985 HE DESIGN SPECIFICATION No. of Tubes in Conventional Tubesheet Layout LOGO 1985 TOTAL HEAT TRANSFER AREA With an appropriate pitch to diameter ratio and optimum pipe diameter chosen and the total

HT area, Shell Diameter N C DO N C 1C LOGO 1985 HE DESIGN SPECIFICATION LAYOUT AND PITCH ARRANGEMENT LOGO 1985 HE DESIGN SPECIFICATION

LAYOUT AND PITCH ARRANGEMENT LOGO 1985 HE DESIGN SPECIFICATION LAYOUT AND PITCH ARRANGEMENT Optimum Pitch to Diameter Ratio: 1.25 to 1.50 Suggested clearance: 6.4 mm Tube layout normally follows symmetrical arrangement having the largest number of tubes at the center

LOGO 1985 HE DESIGN SPECIFICATION BAFFLES Used to support tubes against sagging and vibrations Direct the flow of fluid and control velocities Types: Segmental Disk and Doughnut Type LOGO 1985

HE DESIGN SPECIFICATION BAFFLES Segmental Baffles Baffle Cut: 25 to 45% of disk diameter Baffle Spacing: 20 to 100% of Shell Diameter LOGO 1985 HE DESIGN SPECIFICATION

BAFFLES Disk and Doughnut Baffles Reduces pressure drop by 50-60% LOGO 1985 HE DESIGN SPECIFICATION BAFFLES LOGO 1985 HE DESIGN SPECIFICATION

BAFFLES Minimum unsupported tube span (Tin.) acc. to Perry = 74d 0.75 LOGO 1985 HE DESIGN SPECIFICATION BAFFLES THICKNESS: BENDING LOGO 1985 HE DESIGN SPECIFICATION

BAFFLES THICKNESS: SHEARING LOGO 1985 HE DESIGN SPECIFICATION BAFFLES THICKNESS LOGO 1985 Pressure Drop Tube-Side Pressure Drop (Coulson and Richardson, 2005)

Basic Equation for isothermal system Tube friction losses only jf = dimensionless friction factor L = effective tube length Di = inside tube diameter = density of fluid at bulk/film temperature ut = velocity of fluid LOGO 1985 Pressure Drop Tube-Side Pressure Drop (Coulson and Richardson, 2005) For non-isothermal

systems Tube friction losses only LOGO 1985 Pressure Drop Tube-Side Pressure Drop (Coulson and Richardson, 2005) W/ pressure losses due to contraction, expansion and flow reversal Suggestions for the Estimation of these Losses: 1. Kern (T1950) suggests adding 4 velocity heads per pass 2. Frank (T1978) considers this to be too high, and recommends 2.5 velocity heads 3. Butterworth (T1978) suggests 1.8 4. Lord et al. (1970) take the loss per pass as equivalent to a

length of tube equal to: a. 300 tube diameters for straight tubes b. 200 for U-tubes 5. Evans (T1980) appears to add only 67 tube diameters per pass. LOGO 1985 Pressure Drop Tube-Side Pressure Drop (Coulson and Richardson, 2005) W/ pressure losses due to contraction, expansion and flow reversal The loss in terms of velocity heads can be estimated by: 1. counting the number of flow contractions, expansions and reversals, and;

2. using the factors for pipe fittings to estimate the number of velocity heads lost LOGO Pressure Drop 1985 Tube-Side Pressure Drop (Coulson and Richardson, 2005) W/ pressure losses due to contraction, expansion and flow reversal For two tube passes, there will be: 1. two contractions (T0.5) 2. two expansions (T1.0) 3. one flow reversal (T1.5)

LOGO 1985 Pressure Drop Tube-Side Pressure Drop (Coulson and Richardson, 2005) W/ pressure losses due to contraction, expansion and flow reversal LOGO 1985 Pressure Drop Shell-Side Pressure Drop (Coulson and Richardson, 2005) LOGO

1985 Pressure Drop Shell-Side Pressure Drop (Coulson and Richardson, 2005) Shell Equivalent Diameter (Hydraulic Diameter) Square-Pitched Tube Arrangement, de in meter Triangular-Pitched Tube Arrangement, de in meter LOGO

Pressure Drop 1985 Shell-Side Pressure Drop (Coulson and Richardson, 2005) Shell-Side Friction Factor??? LOGO LOGO Pressure Drop 1985 Shell-Side Pressure Drop (Coulson and Richardson, 2005)

Shell-Side NOZZLE Pressure Drop 1 velocity heads for the inlet for the outlet LOGO 1985 Pressure Drop RULES OF THUMBS (Silla, 2003) LOGO 1985 Pressure Drop

RULES OF THUMBS (Silla, 2003) LOGO 1985 Pressure Drop RULES OF THUMBS (Coulson and Richardson, 2005) LOGO 1985 Pressure Drop RULES OF THUMBS (Couper, Penny, Fair & Wallas, 2010)

vacuum condensers be limited to 0.51.0 psi (T2550 Torr) In liquid service, pressure drops of 510 psi are employed as a minimum, and up to 15% or so of the upstream pressure LOGO 1985 Heat Exchanger Temperature Limits RULES OF THUMBS At high temperature, water exerts corrosive action on steel and scaling is increased

To minimize scale formation, water temperature should not be more than 120F To protect against fouling and corrosion, water temperature (Toutlet) should not be more than158F LOGO 1985 Heat Exchanger Temperature Limits RULES OF THUMBS For the cooling water, on an open circulation systems, the temperature of the cooled water is 813F above the wet bulb temperature When using cooling water to cool or condense a process stream, assume a water inlet temperature

of 90oF (Tfrom a cooling tower) and a maximum water outlet temperature of 120oF LOGO 1985 Heat Exchanger Temperature Limits RULES OF THUMBS the greatest temperature difference in an exchanger should be at least 36 degF, and; the minimum temperature difference should be at least 10 degF

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