# HEAT TRANSFER, HEAT EXCHANGERS, CONDENSORS AND REBOILERS, AIR COOLERS

## Presentation on theme: "HEAT TRANSFER, HEAT EXCHANGERS, CONDENSORS AND REBOILERS, AIR COOLERS"— Presentation transcript:

HEAT TRANSFER, HEAT EXCHANGERS, CONDENSORS AND REBOILERS, AIR COOLERS
Reyad Awwad Shawabkeh Associate Professor of Chemical Engineering King Fahd University of Petroleum & Minerals Dhahran, 31261 Kingdom of Saudi Arabia

Contents HEAT TRANSFER LAW APPLIED TO HEAT EXCHANGERS 2
Heat Transfer by Conduction 3 The Heat Conduction Equation 9 Heat Transfer by Convection 12 Forced Convection 12 Natural Convection 14 Heat Transfer by Radiation 15 Overall heat transfer coefficient 18 Problems 22 DESIGN STANDARDS FOR TUBULAR HEAT EXCHANGERS 23 Size numbering and naming 23 Sizing and dimension 27 Tube-side design 32 Shell-side design 33 Baffle type and spacing 33 General design consideration 35 THERMAL AND HYDRAULIC HEAT EXCHANGER DESIGN 37 Design of Single phase heat exchanger 37 Kern’s Method 45 Bell’s method 49 Pressure drop inside the shell and tube heat exchanger 57 Design of Condensers 65 Design of Reboiler and Vaporizers 72 Design of Air Coolers9 85 MECHANICAL DESIGN FOR HEAT EXCHANGERS10 88 Design Loadings 88 Tube-Sheet Design as Per TEMA Standards 90 Design of Cylindrical shell, end closures and forced head 91 References 95

HEAT TRANSFER LAW APPLIED TO HEAT EXCHANGERS

Heat Transfer by Conduction
W/m2 W/m.K

Thermal Conductivity of solids

Thermal Conductivity of liquids

Thermal conductivity of gases

Example Calculate the heat flux within a copper rod that heated in one of its ends to a temperature of 100 oC while the other end is kept at 25 oC. The rode length is 10 m and diameter is 1 cm.

Example An industrial freezer is designed to operate with an internal air temperature of -20 oC when external air temperature is 25 oC. The walls of the freezer are composite construction, comprising of an inner layer of plastic with thickness of 3 mm and has a thermal conductivity of 1 W/m.K. The outer layer of the freezer is stainless steel with 1 mm thickness and has a thermal conductivity of 16 W/m.K. An insulation layer is placed between the inner and outer layer with a thermal conductivity of 15 W/m.K. what will be the thickness of this insulation material that allows a heat transfer of 15 W/m2 to pass through the three layers, assuming the area normal to heat flow is 1 m2?

The Heat Conduction Equation
Rate of heat conduction into control volume Rate of heat generation inside control volume Rate of heat conduction out of control volume Rate of energy storage inside control volume = + +

The Heat Conduction Equation

Heat Transfer by Convection

Reynolds and Prandtl Numbers
Re < Laminar flow Re > Turbulent flow Values of Prandtl number for different liquids and gases

Flow through a single smooth cylinder
This correlation is valid over the ranges 10 < Rel < 107 and 0.6 < Pr < 1000 where

Flow over a Flat Plate Re < 5000 Laminar flow
Re > Turbulent flow

Natural Convection

q = ε σ (Th4 - Tc4) Ac Th = hot body absolute temperature (K) Tc = cold surroundings absolute temperature (K) Ac = area of the object  (m2) σ = (W/m2K4) The Stefan-Boltzmann Constant

Emissivity coefficient for several selected material
Surface Material Emissivity Coefficient - ε - Aluminum Commercial sheet 0.09 Aluminum Foil 0.04 Aluminum Commercial Sheet Brass Dull Plate 0.22 Brass Rolled Plate Natural Surface 0.06 Cadmium 0.02 Carbon, not oxidized 0.81 Carbon filament 0.77 Concrete, rough 0.94 Granite 0.45 Iron polished Porcelain glazed 0.93 Quartz glass Water Zink Tarnished 0.25

Overall heat transfer coefficient
For a wall For cylindrical geometry

Typical value for overall heat transfer coefficient
Shell and Tube Heat Exchangers Hot Fluid Cold Fluid U [W/m2C] Water Organic solvents    Organic Solvents    Light oils Heavy oils Reduced crude Flashed crude Regenerated DEA Foul DEA Gases (p = atm) 5 - 35 Gases (p = 200 bar) Coolers Organic solvents Brine Gases

Heat Exchangers Hot Fluid Cold Fluid U [W/m2C] Heaters Steam Water Organic solvents Light oils Heavy oils Gases Heat Transfer (hot) Oil Flue gases Hydrocarbon vapors Condensers Aqueous vapors Organic vapors Refinery hydrocarbons Vapors with some non condensable Vacuum condensers Vaporizers Aqueous solutions Light organics Heavy organics Heat Transfer (hot) oil

DESIGN STANDARDS FOR TUBULAR HEAT EXCHANGERS
Size of heat exchanger is represented by the shell inside diameter or bundle diameter and the tube length Type and naming of the heat exchanger is designed by three letters single pass shell The first one describes the stationary head type The second one refers to the shell type The third letter shows the rear head type TYPE AES refers to Split-ring floating head exchanger with removable channel and cover.

Heat exchanger nomenclatures

The standard nomenclature for shell and tube heat exchanger

Removable cover, one pass, and floating head heat exchanger
Removable cover, one pass, and outside packed floating head heat exchanger

Channel integral removable cover, one pass, and outside packed floating head heat exchanger

Removable kettle type reboiler with pull through floating head

Tube sizing: Birmingham Wire Gage
Gauge (B.W.G.) (inches) (B.W.G.) (mm) 00000 (5/0) 0.500 12.7 23 0.025 0.6 0000 (4/0) 0.454 11.5 24 0.022 000 (3/0) 0.425 10.8 25 0.020 0.5 00 (2/0) 0.380 9.7 26 0.018 0.340 8.6 27 0.016 0.4 1 0.300 7.6 28 0.014 2 0.284 7.2 29 0.013 0.3 3 0.259 6.6 30 0.012 4 0.238 6.0 31 0.010 5 0.220 5.6 32 0.009 0.2 6 0.203 5.2 33 0.008 7 0.180 4.6 34 0.007 8 0.165 4.2 35 0.005 0.1 9 0.148 3.8 36 0.004 10 0.134 3.4 11 0.120 3.0 12 0.109 2.8 13 0.095 2.4 14 0.083 2.1 15 0.072 1.8 16 0.065 1.7 17 0.058 1.5 18 0.049 1.2 19 0.042 1.1 20 0.035 0.9 21 0.032 0.8 22 0.028 0.7

Tube sizing: Birmingham Wire Gage

Arrangement of tubes inside the heat exchanger
Tube-side design Arrangement of tubes inside the heat exchanger

Shell-side design types of shell passes one-pass shell for E-type,
split flow of G-type, divided flow of J-type, two-pass shell with longitudinal baffle of F-type double split flow of H-type. types of shell passes

Shell-side design Shell thickness for different diameters and material of constructions

Baffle type and spacing

General design consideration
Factor Tube-side Shell-side Corrosion More corrosive fluid Less corrosive fluids Fouling Fluids with high fouling and scaling Low fouling and scaling Fluid temperature High temperature Low temperature Operating pressure Fluids with low pressure drop Fluids with high pressure drop Viscosity Less viscous fluid More viscous fluid Stream flow rate High flow rate Low flow rate

THERMAL AND HYDRAULIC HEAT EXCHANGER DESIGN
Design of Single phase heat exchanger Design of Condensers Design of Reboiler and Vaporizers Design of Air Coolers

Design of Single phase heat exchanger

Typical values for fouling factor coefficients

Temperature profile for different types of heat exchangers

For counter current For co-current

one shell pass; two or more even tube 'passes

two shell passes; four or multiples of four tube passes
divided-flow shell; two or more even-tube passes

split flow shell, 2 tube pass
cross flow heat exchanger

Shell-side heat transfer coefficient

Shell diameter

Bundle diameter clearance

Tube-side heat transfer coefficient

Tube-side heat transfer factor

Shell and Tube design procedure
Kern’s Method This method was based on experimental work on commercial exchangers with standard tolerances and will give a reasonably satisfactory prediction of the heat-transfer coefficient for standard designs. Bell’s method This method is designed to predict the local heat transfer coefficient and pressure drop by incorporating the effect of leak and by-passing inside the shell and also can be used to investigate the effect of constructional tolerance and the use of seal strip

Kern’s Method

Bell’s method

Figure 34 Baffle cut geometry

Pressure drop inside the shell

Pressure drop inside the tubes

For reactor off-gas quenching Vacuum condenser De-superheating
Design of Condensers For reactor off-gas quenching Vacuum condenser De-superheating Humidification Cooling towers Direct contact cooler

Condensation outside horizontal tubes
For Laminar flow For turbulent flow,

Condensation inside horizontal tubes
stratified flow annular flow

Design of Reboiler and Vaporizers
Suitable to carry viscous and heavy fluids. Pumping cost is high Forced-circulation reboiler The most economical type where there is no need for pumping of the fluid It is not suitable for viscous fluid or high vacuum operation Need to have a hydrostatic head of the fluid Thermosyphon reboiler It has the lower heat transfer coefficient than the other types for not having liquid circulation Used for fouling materials and vacuum operation with a rate of vaporization up to 80% of the feed Kettle reboiler

Boiling heat transfer and pool boiling
Nucleate pool boiling Critical heat flux Film boiling

Nucleate boiling heat transfer coefficient

Critical flux heat transfer coefficient
Film boiling heat transfer coefficient

Convection boiling Effective heat transfer coefficient encounter the effect of both convective and nucleate boiling

Design of air cooler

Mechanical Design for HE
A typical sequence of mechanical design procedures is summarized by the flowing steps Identify applied loadings. Determine applicable codes and standards. Select materials of construction (except for tube material, which is selected during the thermal design stage). Compute pressure part thickness and reinforcements. Select appropriate welding details. Establish that no thermohydraulic conditions are violated. Design nonpressure parts. Design supports. Select appropriate inspection procedure