LOGO HEAT EXCHANGER DESIGN LOGO Heat Transfer Equipment

LOGO HEAT EXCHANGER DESIGN

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

LOGO Double Pipe Heat Exchanger v 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 v 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 Heat Transfer Equipment in Industries v Exchanger: heat exchanged between two process streams v Heaters and coolers: where one stream is plant service v Vaporiser: if a process stream is vaporised v Reboiler: a vaporiser associated with distillation column v Evaporator: if concentrating a solution v Fired exchanger: if heated by combustion gases v Unfired exchanger: not using combustion gases

LOGO Heat Transfer Equipment in Industries MODES of HEAT TRANSFER 1. Conduction v 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 v 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 v Transfer of heat by the absorption of radiant energy

LOGO BASIC THEORY v 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/m 2 o. C A= heat-transfer area, m 2 Tm= the mean temperature difference, o. C

LOGO BASIC THEORY v 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/m 2 o. C A= heat-transfer area, m 2 Tm= the mean temperature difference, o. C Y = geometric correction factor

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

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

LOGO

LOGO Geometric Correction Factor For design to be practical, Y ≥ 0. 85

LOGO ΔT 1 Logarithmic Mean Temperature Difference ΔT 2 If ΔT 1 < ΔT 2 and (ΔT 2/ΔT 1) ≤ 2, then ΔTlm is the arithmetic mean temp difference

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

LOGO Overall Heat Transfer Coefficient v 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.

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

LOGO Total Resistance Conduction Heat Transfer is governed by Fourier’s Law! k = thermal conductivity of the Solid (BTU/hr-ft 2 -(OF/ft)) A = Area perpendicular to the direction of heat transfer x = distance of heat flow

LOGO Total Resistance At Steady State:

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

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

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

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

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

LOGO Total Resistance Convection Conduction Convection inside

LOGO Total Resistance in si d e

LOGO Typical Fouling Factor (Foust, 1980)

LOGO Heat Transfer Without Phase Change

LOGO DOUBLE PIPE HEAT EXCHANGER

LOGO Invidual Heat Transfer Coefficient HT w/o Phase Change: DPHE For Long Tubes (L/D) > 50, Tube-side Applicabilty: 1. Non-metallic fluid 2. 0. 5 < NPr < 100 3. NRE > 10, 000

LOGO Invidual Heat Transfer Coefficient HT w/o Phase Change: DPHE For Long Tubes (L/D) > 50, Annular Space Applicabilty: 1. Non-metallic fluid 2. 0. 5 < NPr < 100 3. NRE > 10, 000

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

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

LOGO SHELL AND TUBE HEAT EXCHANGER

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

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

LOGO Heat Transfer WITH Phase Change

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

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

LOGO 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

LOGO 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

LOGO 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

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

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

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

LOGO Invidual Heat Transfer Coefficient Film Temperature Condensate Properties are evaluated at the Film Temperature Tf = ½(Tsv + Tw) by Kern, D. Q. , Process HT

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

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

LOGO 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

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

LOGO HE DESIGN SPECS

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

LOGO HE DESIGN SPECIFICATION No. of Tubes in Conventional Tubesheet Layout

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

LOGO HE DESIGN SPECIFICATION LAYOUT AND PITCH ARRANGEMENT

LOGO HE DESIGN SPECIFICATION LAYOUT AND PITCH ARRANGEMENT

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

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

LOGO HE DESIGN SPECIFICATION BAFFLES

LOGO HE DESIGN SPECIFICATION BAFFLES Minimum unsupported tube span (in. ) acc. to Perry = 74 d 0. 75

LOGO HE DESIGN SPECIFICATION BAFFLES THICKNESS: BENDING

LOGO HE DESIGN SPECIFICATION BAFFLES THICKNESS: SHEARING

LOGO HE DESIGN SPECIFICATION BAFFLES THICKNESS

LOGO 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

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

LOGO 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 (1950) suggests adding 4 velocity heads per pass 2. Frank (1978) considers this to be too high, and recommends 2. 5 velocity heads 3. Butterworth (1978) 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 (1980) appears to add only 67 tube diameters per pass.

LOGO 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 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 (0. 5) 2. two expansions (1. 0) 3. one flow reversal (1. 5)

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

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

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

LOGO

LOGO 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

LOGO Pressure Drop RULES OF THUMBS (Silla, 2003)

LOGO Pressure Drop RULES OF THUMBS (Silla, 2003)

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

LOGO 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

LOGO 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 than 158 F

LOGO Heat Exchanger Temperature Limits RULES OF THUMBS • For the cooling water, on an open circulation systems, the temperature of the cooled water is 813ºF above the wet bulb temperature • When using cooling water to cool or condense a process stream, assume a water inlet temperature of 90 o. F (from a cooling tower) and a maximum water outlet temperature of 120 o. F

LOGO Heat Exchanger Temperature Limits RULES OF THUMBS • the greatest temperature difference in an exchanger should be at least 36 deg. F, and; • the minimum temperature difference should be at least 10 deg. F