HEAT EXCHANGER DESIGN

Heat Transfer Equipment Types Service Double pipe exchanger Heating and cooling Shell and tube exchanger All applications Plate heat exchanger Plate-fin exchanger Spiral heat exchanger Air cooled Cooler and condensers Direct contact Cooling and quenching Agitated vessel Fired heaters Heating

Double Pipe Heat Exchanger Consists of two concentric pipes with one fluid flowing through the inner pipe while the other fluid flowing through the annular space

Shell and Tube Heat Exchanger Consists of tube bundles enclosed in a cylindrical shell with one fluid flowing through the tubes and the other flowing outside of the tubes

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

Heat Transfer Equipment in Industries MODES of HEAT TRANSFER 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 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 Radiation Transfer of heat by the absorption of radiant energy

BASIC THEORY General equation for heat transfer across a surface for DPHE is: 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

BASIC THEORY General equation for heat transfer across a surface for STHE is: 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

Tube-Side Passes One tube pass Two tube pass Three tube passes

Geometric Correction Factor Also refer to Figure 11-4, Perry 7th Edition

Geometric Correction Factor For design to be practical, Y ≥ 0.85

Logarithmic Mean Temperature Difference If ΔT1 < ΔT2 and (ΔT2/ΔT1) ≤ 2, then ΔTlm is the arithmetic mean temp difference

Overall Heat Transfer Coefficient Rearranging the General Equation in terms of driving force and total resistance: Driving Force Total Resistance

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.

Total Resistance the sum of several individual resistances Individual resistance is the reciprocal of individual HTC. Convection Conduction Convection inside

Total Resistance Conduction Heat Transfer is governed by Fourier’s Law! k = thermal conductivity of the Solid (BTU/hr-ft2-(OF/ft)) A = Area perpendicular to the direction of heat transfer x = distance of heat flow

Total Resistance At Steady State:

Total Resistance If k is constant: Define R = Δx/kA Thus, q= - ΔT/R

If k varies slightly with Temp: Total Resistance If k is not constant: If k varies slightly with Temp: **km is evaluated at the mean temperature

If A varies slightly with Thickness: Total Resistance If k is not constant: If A varies slightly with Thickness:

q = hcA (T1 – T2) Total Resistance Convection Heat Transfer Where: hc- convection heat transfer coefficient, Btu/hrft2°F -similar to k/∆x A – Heat transfer Area T1 – temperature at surface 1 T2 – temperature at surface 2

q = (T1 – T2)/(1/hcA) Total Resistance Convection Heat Transfer: Rearranging q = (T1 – T2)/(1/hcA) Where: hc- convection heat transfer coefficient, Btu/hrft2°F -similar to k/∆x A – Heat transfer Area T1 – temperature at surface 1 T2 – temperature at surface 2

Total Resistance Convection Conduction Convection inside

Total Resistance inside

Typical Fouling Factor (Foust, 1980)

Heat Transfer Without Phase Change

Double Pipe Heat Exchanger

Invidual Heat Transfer Coefficient HT w/o Phase Change: DPHE For Long Tubes (L/D) > 50, Tube-side Applicabilty: Non-metallic fluid 0.5 < NPr < 100 NRE > 10,000

Invidual Heat Transfer Coefficient HT w/o Phase Change: DPHE For Long Tubes (L/D) > 50, Annular Space Applicabilty: Non-metallic fluid 0.5 < NPr < 100 NRE > 10,000

Invidual Heat Transfer Coefficient HT w/o Phase Change: DPHE For Short Tube (L/D < 50)

Invidual Heat Transfer Coefficient HT w/o Phase Change: DPHE Laminar Flow, Forced Convection

Shell and tube heat exchanger

Invidual Heat Transfer Coefficient HT w/o Phase Change: STHE, ho

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

Heat Transfer WITH Phase Change

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

Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Vertical Surface, Laminar

Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Vertical Surface, Turbulent

Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Horizontal Surface  If the amount of condensate is unknown For Nre > 40, h is multiplied by 1.2

Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Horizontal Surface  If the amount of condensate is known For Nre > 40, h is multiplied by 1.2

Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Horizontal Surface, Banks of Tubes For Nre > 40, h is multiplied by 1.2

Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Horizontal Surface, Banks of Tubes

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

Invidual Heat Transfer Coefficient HT w/ Phase Change: STHE Film-type Condensation on Horizontal Surface, Banks of Tubes  w/ splashing

Invidual Heat Transfer Coefficient Film Temperature Condensate Properties are evaluated at the Film Temperature Tf = ½(Tsv + Tw) by Kern, D.Q., Process HT Tf = Tsv - 0.75ΔT by McAdams, W.H., Heat Transmission, 3rd. Ed. ΔT = Tsv - Tw

Invidual Heat Transfer Coefficient Film Boiling on Submerged Horizontal Cylinder or Sphere

Invidual Heat Transfer Coefficient Film Boiling on Submerged Horizontal Cylinder or Sphere

Invidual Heat Transfer Coefficient Film Boiling on Submerged Horizontal Cylinder or Sphere Nusselt-type Equation by Rohsenow: Cr varies from 0.006 to 0.015

Invidual Heat Transfer Coefficient Film Boiling on Submerged Horizontal Cylinder or Sphere Nusselt-type Equation by Forster and Zuber:

HE DESIGN SPECS

TOTAL HEAT TRANSFER AREA A compromise between NT and L is chosen based on (L/Dshell) between 5 to 10

HE DESIGN SPECIFICATION No. of Tubes in Conventional Tubesheet Layout

TOTAL HEAT TRANSFER AREA With an appropriate pitch to diameter ratio and optimum pipe diameter chosen and the total HT area,

HE DESIGN SPECIFICATION LAYOUT AND PITCH ARRANGEMENT

HE DESIGN SPECIFICATION LAYOUT AND PITCH ARRANGEMENT

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

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

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

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

HE DESIGN SPECIFICATION BAFFLES

HE DESIGN SPECIFICATION BAFFLES Minimum unsupported tube span (in.) acc. to Perry = 74d0.75

HE DESIGN SPECIFICATION BAFFLES THICKNESS: BENDING

HE DESIGN SPECIFICATION BAFFLES THICKNESS: SHEARING

HE DESIGN SPECIFICATION BAFFLES THICKNESS

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 ρ = density of fluid at bulk/film temperature ut = velocity of fluid

Pressure Drop Tube-Side Pressure Drop (Coulson and Richardson, 2005) For non-isothermal systems Tube friction losses only

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: Kern (1950) suggests adding 4 velocity heads per pass Frank (1978) considers this to be too high, and recommends 2.5 velocity heads Butterworth (1978) suggests 1.8 Lord et al. (1970) take the loss per pass as equivalent to a length of tube equal to: 300 tube diameters for straight tubes 200 for U-tubes Evans (1980) appears to add only 67 tube diameters per pass.

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: counting the number of flow contractions, expansions and reversals, and; using the factors for pipe fittings to estimate the number of velocity heads lost

Pressure Drop 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: two contractions (0.5) two expansions (1.0) one flow reversal (1.5)

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

Pressure Drop Shell-Side Pressure Drop (Coulson and Richardson, 2005)

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

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

1 ½ velocity heads for the inlet ½ for the outlet Pressure Drop Shell-Side Pressure Drop (Coulson and Richardson, 2005) Shell-Side NOZZLE Pressure Drop 1 ½ velocity heads for the inlet ½ for the outlet

Pressure Drop RULES OF THUMBS (Silla, 2003)

Pressure Drop RULES OF THUMBS (Silla, 2003)

Pressure Drop RULES OF THUMBS (Coulson and Richardson, 2005)

Pressure Drop RULES OF THUMBS (Couper, Penny, Fair & Wallas, 2010) vacuum condensers be limited to 0.5–1.0 psi (25–50 Torr) In liquid service, pressure drops of 5–10 psi are employed as a minimum, and up to 15% or so of the upstream pressure

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 120ºF To protect against fouling and corrosion, water temperature (outlet) should not be more than158F

Heat Exchanger Temperature Limits RULES OF THUMBS For the cooling water, on an open circulation systems, the temperature of the cooled water is 8-13ºF above the wet bulb temperature When using cooling water to cool or condense a process stream, assume a water inlet temperature of 90oF (from a cooling tower) and a maximum water outlet temperature of 120oF

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