02 BOPs / Woods D.R 2008 rules-of-thumb-in-Engineering-practice (epdf.tips)

2.6 Solids 61

Pneumatic conveying: dilute phase: pressure: continuous: nominal gas velocity 5–35 m/s with usual 11–25 m/s. Solids loading 3.5–15 kg solid/kg air with usual 6–15 kg solids/kg air or 1–7 m3 solids/m3 air. Power 7–11 kJ/kg. Problems: about 30 % air leakage out of the system. Rotary/star valve problem/bridging: overcome with bin agitation, astute bin design and vent star valve to prevent the back flow of gas through the valve into the bin. Minimize bends. Requires a minimum of air but increasing the air velocity i minimum flow does not change the capacity; it increases the Dp. Capacity increases linearly with increase in pressure of air supplied. Reduce the pipe length by 10 %; increases capacity by 10 %. Increase the inlet pressure 10 %; increase capacity by 10 %. Pipe diameter at least 3–5 times i the maximum particle diameter. Powders become more difficulty to handle as the particle size decreases. When sizing for air flowrate, allow for air leakage across rotary valve. Caution, with friable powders, the conveying process can change the particle size distribution and hence cause receiver hopper discharge problems. An increase in gas velocity at constant solids flowrate give higher Dp.

Pneumatic conveying: dilute phase: vacuum: continuous: nominal gas velocity 20–35 m/s and must be i 15 m/s. Solids loading 2.8–11 kg solids/kg gas with usual 3–4 kg solids/kg air. Power 11–18 kJ/kg. Rate and distance sensitive. Maximum vacuum 35 to 55 kPa. When sizing for air flowrate, allow for air leakage across rotary valve. Conveying velocity increases along the pipeline as approach vacuum pump suction. An increase in gas velocity at constant solids flowrate gives higher Dp.

Pneumatic conveying: dense phase: pressure: pressure, batch; Cycle time 1–4 min; charge 10 s; convey 0.5–3 min; 20–50 batches/h. nominal gas velocity: horizontal 4.5–35 m/s with usual 6–10 m/s; vertical 1.5–27 with usual 3 m/s. Solids loading: horizontal 12–130 kg/kg air; vertical 10–800 with usual 250–400 kg solids/kg air. Blow tank I 15 m3. Line diameter 3–10 cm. Power 3–5.5 kJ/kg. Watch for humid air and line plugging. Pressures 0.17–0.45 MPa abs.

The air supplied is split between air to fluidize and air to convey. An increase in gas velocity at constant solids flowrate gives decrease in Dp.

The longer the distance, the lower the solids loading or the more gas needed to convey.

Options include top discharge or bottom discharge.

Top discharge: can achieve the highest feed rates; flowrate must exceed the minimum for fluidization.

Bottom discharge is preferred for granular materials.

Enclosed conveyor: 0.09–9 dm3/s; conveyor size 7.5–25 cm; travel speed 0.02–0.3 m/s.

x Good Practice

Use a vibratory, screw or apron feeder to unload the particles onto the conveyor. Prefer dense phase to dilute phase pneumatic conveying (H).

62 2 Transportation

x Trouble Shooting

Bucket elevators: major difficulties are unloading and loading: jamming of materials between the buckets and the side of the boot.

Rotary/star valve: use amps as guide to solids throughput. Keep air velocity high enough to prevent plugging of the air-vent line.

Pneumatic conveying: dilute phase: for vacuum: air leakage and powder bridging in hopper are the major threats. “No flow or flow I design”: air leaks/powder bridging in feed hopper, see Section 10.3/low solids flow because of increased air loss in rotary valve/wrong type of rotary valve used/insufficient air/line too long/vacuum pump problems, see Section 2.2. “Pressure (vacuum) at suction to blower i design (vacuum I design)”: air leaks/failure of discharge valve to seal on the receiver. “Erratic pressure readings”: irregular feed. “Explosion”: moisture too low/ lines not grounded. “Does not sound “tinny” when listening with stethoscope”: material accumulated inside pipe at this location.

Pneumatic conveying: dilute phase: for pressure: use pressure at the outlet of the blower as prime indicator. “Dp across blower i design or 2:1 ratio”: restriction in downstream conveying line/check valve jammed closed/dirty intake filter/ plugged discharge silencer/increase in feed to the system/length of pipe i design. “Dp across blower I design”: slipping v-belts/air loss at the rotary valve. “No flow”: [plugged line]* “No flow or flow I design”: overfed fan system/insufficient air/insufficient solids/line too long/inlet air pressure too low. “Erratic pressure readings”: irregular feed. “Amps on rotary valve I usual”: solids flow

I design/air loss through the rotary valve/increased clearances. “Does not sound “tinny” when listening with stethoscope”: material accumulated inside pipe at this location. “Gradual decrease in performance”: wear on the blower caused by dusty air.

[Plugged line]*: within the first couple of metres of the beginning of the system: material feed problems/air supply problems. [Plugged line]* after the first couple of metres: air leak with the plug occurring about 10 m downstream of leak/erosion of rotary valve causing increase in air leakage.

Pneumatic conveying: dense phase:“No flow or flow I design”: plugged line/malfunction of line boosters because of stuck check valve/high humidity. “Solids fed to conveying line I design”: ratio of air to fluidize in the blow tank relative to convey is too small/fault in control system.

“Solids fed to conveying line i design”: ratio of air to fluidize in the blow tank relative to convey is too large/fault in control system.“Solids flow = 0”: top discharge and the ratio of air to fluidize to convey is too small. “Solids flow gradually decreases”: restriction in the discharge pipe/blinding of the fluidizing membrane.

Feeder: volumetric for extruder: “Does not run”: no power/jammed. “Stalls”: material jam/current limit set too low. “Erratic speed control”: controller poorly tuned/ sensor malfunction/material jam. “Feed rate variable”: particles bridging in the hopper/moisture level too high/overheated polymer (prematurely fused) feed polymer.

Feeder: screw conveyor: “Shear pins on feeder drive break”: screw diameter I exit hole from bin. “Motor overload on feeder drive”: screw conveyor diameter I exit

2.7 Ducts and Pipes 63

hole from hopper. “Screw feeder initially OK then motor overloads”: screw flight spacing in the direction of sold flow decreases markedly/difference between FDI and BDI I 5 % suggests a moderately incompressible solid whose flow is very sensitive to screw flight spacing.

Feeder from bottom of hopper: “Feeder motor overloads immediately:” wrong wiring/foreign material in feeder/hopper is full and solids give excessive solids pressure because of particle characterization and hopper design/FDI large and large

HI. “Feeder exit flowrate suddenly I expected”: blockage in hopper outlet/lumps of particles forming in hopper/large RI and small HI possibly caused by temperature cycles.

“Feeder exit flowrate gradually I design:” solids builup in the feeder/large CI, large

AI and RI/wrong materials of construction in feeder. (Often happens with vibrating feeder).

Feeder: belt feeder from the bottom of a hopper: “Belt feeder initially starts but suddenly stops with motor overload”: gap between the belt and hopper interface edge is too small/belt sags between pulleys/large FDI and small % difference between FDI and BDI.

2.7

Ducts and Pipes

x Guidelines

Pressure drops: see Gas, Section 2.1; liquid Section 2.3. For control valves on the lines where flow is because of Dp between an upstream and downstream vessel, allow a Dp across the valve of 10 % of the pressure of the lower terminal vessel or 50 % of the dynamic loss, whichever is greater.

Velocities, see Gas, Section 2.1; and liquid, Section 2.3. Keep the velocity of compressible gases I 0.6 sonic velocity.

Sonic velocity, m/s = 1.97 [(cp/cv) pressure, kPa, q density, kg/m3]0.5

Sewer pipes have maximum flowrate when liquid level is 93 % of the diameter; flowrate when full = flowrate when liquid level is 80 % of the diameter.

Schedule 40 = usual specification; schedule 80 is heavy duty.

x Good Practice

Use vent breaks on high points of syphon. Include high pressure water purge or blowout for pipes with slurries; include steam blowout/purge lines for pipes with edible oils, foodstuffs.

For small diameter pipes consider using schedule 80, even if schedule 40 would satisfy internal pressure requirements, to eliminate need to pipe support over short runs.

3

Energy Exchange

The fundamentals for thermal energy exchange are that heat flows from a high temperature to a low temperature. Thermal forms of energy are not always available to do work. Overall energy is conserved; often we write expressions for the mechanical energy balance (on the macroscopic level as Bernoulli’s equation) and the thermal energy balance (on the macroscopic level as q = UA LMTD). The transfer of heat is resisted and we define heat transfer coefficients, h and U, to describe the rate of heat transfer. The heat transfer coefficients are usually correlated as the Nusselt no. which is proportional to the (Reynolds no.)0.8 (Prandtl no.)0.33. This equation illustrates how the heat transfer coefficient varies with flowrate and with changes in the properties of the fluids. Fluids to watch are water and hydrogen; both have extremes in thermal properties.

In this chapter in Section 3.1 we consider mechanical drives. In Sections 3.2 and 3.3 furnaces and exchangers, condensers and reboilers are considered followed by fluidized bed with coil in the bed, Section 3.4 and static mixers, Section 3.5. Direct contact systems are considered next: liquid–liquid, Section 3.6; gas– liquid cooling towers, Section 3.7; gas–liquid quenchers, Section 3.8; gas–liquid condensers, Section 3.9, and gas–gas thermal wheels, Section 3.10. Heat loss to the atmosphere is described in Section 3.11. Refrigeration, steam generation and high temperature heat transfer fluids are presented in Sections 3.12 to 3.14, respectively. Tempered heat exchange systems are considered in Section 3.15.

3.1 Drives

x Area of Application

Gasoline–diesel engines: 200 to 800 rpm; i 80 kW ; efficiency: 28 to 38 %. Electric motors (synchronous): I 500 rpm; 35 to 500 kW; use 480 V for motors up to 115 kW; 4160 V for i 115 kW.

Electric motors (induction): i 500 rpm; 10 to 15 000 kW.

Steam turbine: single stage, single valve: 1000 rpm to 12 000 rpm; 50 to 1500 kW. single valve, multistage: 1500 to 2800 kW.

multivalve, multistage: 2800 to 30 000 kW.

Rules of Thumb in Engineering Practice. Donald R. Woods

Copyright c 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 978-3-527-31220-7

3.1 Drives 65

Steam turbine generation of electricity:

Gas combustion turbine: cogeneration.

x Guidelines

For less than 75 kW select motor or turbine.

Gasoline–diesel engines usual application: 200 to 400 rpm; 500 to 1200 kW. Electric motors: select synchronous for low speed; usual application for either synchronous or induction: 500 to 2100 rpm; 150 to 500 kW. Usually use Total Enclosed Fan Cooled, TEFC, enclosure. efficiency: 84 to 95 %.

Induction: available for large power requirements, relatively low efficiency, power factor is low if rpm I 500 and at starting and fractional loads.

Synchronous: high efficiency at any speed, suitable for direct coupling for I 1000 rpm. Power factor j 1; constant speed without slip. Power consumption, kW = amps ( 0.001 V q PF q 0.95 3).

Power, kW	volts, V
0.1–1	110
1–75	220–440 3-phase
50–200	440–2300
175–2000	2300–4160
i 2000	11 000–13 200

Steam turbines: competitive above 75 kW; condensing: 2 kg/h steam/kW with 1.8 m2 condenser surface area per kg/h steam. Multivalve, multistage efficiency: 42 to 78 %.

Steam turbine generation of electricity: for example, via condensate-type steam turbine with simple automatic extraction and two-pole synchronous generators with a cylindrical rotor and brushless exciter system.

x Good Practice

Substitute variable speed drives when operation is frequently below capacity. Engines: Use high efficiency motors when replacing or repairing existing installations.

Steam turbine: Consider extracting energy via a steam turbine for any pressure reduction in steam service. Use high pressure steam for energy; low pressure steam for heating (P, F). Don’t operate with wet steam.

x Trouble Shooting

Engines: “Hammering/knocking”: loose parts/seized parts. “Pre-ignition”: fuel with unstable hydrocarbons/incorrect timing. “Detonation”: wet fuel/incorrect timing/ intake air too hot/glowing carbon on the piston/leaking valve stem/worn valve guides. “Misfiring”: incorrect timing/faulty ignition elements/wrong gap in the spark plugs/wet fuel/spark-plug gap coated or filled with carbon or oil. “Overheat”: lubrication failure/inadequate cooling/poor quality fuel/fuel to air ratio too lean. “Sooty exhaust”: incorrect fuel/air–fuel ratio too rich/inadequate cooling/wrong valve adjustment. “Valve leaking”: inadequate cooling/valve angle incorrect/wrong metallurgy. “Piston blow-by:” over lubrication/inadequate oil filtration/

66 3 Energy Exchange

inadequate air filtration/worn piston rings. “Worn bearings”: misaligned crankshaft/wet fuel/unstable fuel.

Electric motor: “Won’t start”: overload trip/loose connection/grounded winding/ grounded stator. “Runs backwards”: reversed phase sequence. “Excessive noise”: 3-phase machine single phased/unbalanced load between phases. “Synchronous motor fails to come up to speed”: faulty power supply or overload trip/windings grounded. “Overheat”: unbalanced load between phases/wrong line voltage/ short circuit in stator winding/single phasing.

DC motors:“Won’t start”: weak field/low armature voltage/open or short circuit in armature or field. “Runs too slow”: low armature voltage/overload/brushes ahead of neutral. “Runs too fast”: high armature voltage/weak field/brushes behind neutral. “Brushes sparking”: brushes worn/brushes poorly seated/incorrect brush pressure/dirty, rough or eccentric commutator/brushes off neutral/short circuited commutator/overload/excessive vibration.“Brush chatter”: incorrect brush pressure/high mica/incorrect brush size. “Bearings hot”: belt too tight/misalignment/shaft bent/damaged bearings.

Steam turbine: “Turbine fails to start”: too many hand valves closed/nozzles plugged or eroded/dirt under carbon rings. “Slow startup”: throttle valve travel restricted/steam strainer plugged/load i rating. “Insufficient power”: throttle valve travel restricted/too many hand valves closed/oil relay governor set too low. “Speed increases as load decreases”: throttle valve travel restricted/throttle assembly friction/valve packing friction. “Governor not operating/excessive speed variation”: governor droop adjustment needed/governor lubrication problem/throttle valve travel restricted. “Overspeed trip on load changes”: trip valve set too close to operating speed/throttle valve travel restricted/throttle assembly friction. “Overspeed trip on normal speed”: excessive vibration/dirty trip valve/trip valve set too close to operating speed. “Leaking glands”: dirt under carbon rings/worm or broken carbon rings/scored shaft.

Steam turbine used for the generation of electricity: “Turbine overspeeding”: [load disconnection suddenly]*/[trip throttle valve stuck]*/control valve fault//[extraction valve fault]*. “Bearings damaged”: [turbine overspeeding]*/[lube oil]*/excessive vibration/no lube oil/bearing temperature too hot/insufficient oil because of clogged lines/flow of parasitic currents.

[Clogging]*: [lube oil]*/long time without operating.

[Electronic pin clogging]*: [lube oil]*/long time without operating. [Extraction valve fault]*: wear on valve bearing/loss of hermetic seal.

[Load disconnection suddenly]*: operator error/automatic bus bar protection because of downstream changes in electric system.

[Lube oil]*: low pressure/oil temperature too hot/oil too old/oxidation/water contaminates oil.

[Solenoid valve malfunction]*: [electronic pin clogging]*/[clogging]*/solenoid shorted coil/faulty control signal/sensor error.

[Trip Throttle Valve stuck]*: [clogging]*/[solenoid valve malfunction]*.

Gas turbine: consists of a compressor, combustor and turbine sections.“Combustion noise”: fouled or clogged combustor/loose or cracked lining in combustor.

3.2 Thermal Energy: Furnaces 67

“Vibration”: bearing failure in compressor or turbine/blade damage in compressor or turbine/surging compressor/fouled turbine. “Exhaust temperature i design”: combustor fouling. “Exhaust temperature I design”: combustor clogged. “Thermal efficiency I design”: fouled turbine/turbine blade damage/turbine nozzle distortion. “Mass flow I design”: compressor fouling/compressor filter clogged/ compressor blades damaged.

3.2

Thermal Energy: Furnaces

Multi-use including heating, boiling, reactions. Related topics distillation, Section 4.2, reactors, Section 6.5.

x Area of Application

250 to 1300 hC; I 30 MPa; thermal efficiency: 70 to 75 %.

x Guidelines

Heat flux in radiant section: 10–60 kW/m2 based on outside tube area with fluid velocity inside tubes 0.1 to 3 m/s. Use 1.5 m/s. In the convection section: 12 kW/m2.

Equate heat duties in the radiant and convection sections. 80 % efficiency based on net heating values.

Size radiant section to absorb 50 % of the radiant energy with 1.22 m3 chamber per m2 tube area.

Field fabricated for sizes above 5 MW total heat load.

Gas catalytic endothermic reaction inside tube, heat transfer coefficient U = 0.045 kW/m2. K.

Flue gas temperature is usually 650–820 hC leaving the radiant section and 260–425 hC leaving the convection section. To minimize the thermal cracking inside tubes in a fired furnace, use 600 kg/s.m2 for lube oils, crude and vacuum desulfurizer and 1200 kg/s.m2 for delayed coker and visbreaker. Keep tube temperatures I 760 hC. Specify the highest fuel gas pressure that is consistently available.

x Good Practice

For fired furnaces: monitor CO and O2 to reduce rejected energy and improve efficiency, consider the installation of economizers and air preheaters to recover additional heat from the flue gas.

For steam generation: preheat boiler feed water with available low temperature process streams, maximize the use of heat transfer surfaces by optimizing sootblowing frequency and decoking of tubes, flash blowdown to produce low pressure steam if required.

68 3 Energy Exchange

x Trouble Shooting

“Gas temperature i design”: instrument wrong/insufficient excess air/process side coking of tubes/leak of combustible material from process side/over-firing because of high fuel gas pressure. “Gas temperature I design”: instrument fault/fouling/too much excess air/insufficient area/fuel gas pressure I design. For convection furnace: “Exit process gas temperature I design”: excess air/decrease in flame temperature/damper has failed to close. “Pressure inside furnace i design”: instrument wrong/fouling on the outside of the tubes in the convection section/exhaust fan failure. “Faint blue-gray smoke rising from top of furnace”: fouling outside tubes in the convection section/pressure in furnace i atmospheric. “Puffing, rhythmic explosions”: burners short of air for short period causing minor over-firing/wind action/start up too fast. “Tube failure”: localized overheating/burning acid gases as fuel/free caustic in water and dryout/dry out and attack by acid chloride carried over from water demineralization/breakthrough of acid into water from demineralizer. “High fuel gas pressure”: failure of pressure regulator. “Tube dryout”: tubeside velocity too low. “Low furnace efficiency”: high combustion air flow/air leak into the firebox/high stack temperature/heat leaks into the system. “Equipment suddenly begins to underperform”: fouling/bypass open. “Temperature control problems”: missing or damaged insulation/poor tuning of controller/furnace not designed for transient state/unexpected heat of reaction effects/contaminated fuel/design error. “Furnace tube blistering”: flame impingement/dirty or worn oil burner tips/tangential firing gun misalignment.

3.3

Thermal Energy: Fluid Heat Exchangers, Condensers and Boilers

Exothermic processes should supply all the heat requirements for the process. Related topic thermal pinch, Section 1.11. Based on the conservation of energy, the heat acquired/lost by a stream = the heat transferred to/from the stream. For sensible heat, q = mass flowrate (F) q heat capacity per unit mass (cp) q DT = heat transferred = UA MTD. This is sometimes rearranged to define a thermal heat transfer unit, THTU, = DT/MTD = UA/F cp.

x Area of Application

Shell and tube: –200 to 600 hC; I 30 MPa; 0.15 to 4 THTU/pass and 5 to 150 kPa/ THTU; fluid viscosities I 100 mPa.s. area: 2–2000 m2.

Fixed tube sheet limited to low thermal expansion or if DT I 30 hC. Usually need U-tube or floating head.

Spiral: –100 to 400 hC; I 1.8 MPa; use with sludges, slurries, high viscosity materials (especially 4 q 104 mPa s to 4 q 105 mPa.s .) i 90 % heat recovery. High heat transfer coefficients; low pressure drop. Ratio of flowrates being handled I 3.5. Not when DT1 i i DT2. area: 4–100 m2.

Lamella: –200 to 500 hC; I 3 MPa. Ratio of flowrates 1 to 1.8. area: 100–10 000 m2.

3.3 Thermal Energy: Fluid Heat Exchangers, Condensers and Boilers 69

Gasket/plate: –30 to 180 hC; I 2.5 MPa depending on the gasket material; areas I 2000 m2; 0.2 to 3 THTU/pass and 15 to 200 kPa/THTU; fluid viscosities I 4 q 104 mPa.s. Holdup low, 1.5 L/m2. Ratio of flowrates 0.7 to 1.3. Not when DT1 i i DT2. area: 10–600 m2.

Double pipe: –200 to 600 hC; I 30 MPa; usually I 20 m2. use for high pressure. area: 0.3–200 m2.

Air cooled: OK when air can be used as coolant. Low maintenance. Area: 20–2000 bare tube m2. Finned area: bare tube area = 16:1.

Options are forced draft or induced draft. Use forced draft with louvers when temperature control is critical. Forced draft has less fan power; easy access for maintenance; easy to use hot air recirculation; but has greater susceptibility to air maldistribution and to inadvertent hot air recirculation; low potential for natural circulation; and the tubes are exposed to the elements. Induced draft: high fan power needed, not easy access for maintenance; limitation on exit air temperature; less chance of air maldistribution or unwanted hot air recirculation; better protection from the elements; process stream temperatures I 175 hC.

Cubic/monolithic: corrosive liquids, acids, bases or used as catalyst/heat exchanger for reactors. Usually made of graphite or carbon that has high thermal conductivity. Area: 1–20 m2. Ceramic monoliths are used as solid catalyst for highly exothermic gas-catalyst mass transfer-controlled reactions.

Agitated falling film: usually to concentrate slurry; see evaporation, Section 4.1. Scraped surface, Votator: usually to condition foodstuffs, crystallize and react. Especially for viscous feed. OK for foaming, for fouling, crystal formation and suspended solids. Viscosities i 2000 mPa.s. Relative to agitated film retention time of 1:1 and volume 1:1. Overall heat transfer coefficient 2 kW/m2. hC decreasing with increasing viscosity. 3 to 12 kW/m2. See also evaporation Section 4.1, crystallization as scraped surface crystallizer, Section 4.6 and as a reactor, Section 6.20.

Coil in tank: area: 1–30 m2.

Jacketed: usually to exchange heat in a reactor or storage tank. Cooling finger: added to tanks to increase the exchange area.

x Guidelines

Media for Heating and Cooling

Water: 18 hC to maximum outlet 50 hC with velocity i 1.2 m/s Air: 18 C to maximum outlet 50 hC

Steam: 1.7 MPa: 203 to 220 hC; 4.3 MPa 260 hC

for pressures 0.1 to 0.2 MPa: 100 hC consider ethane

50 hC consider propane, propylene

30 hC consider ammonia

0 hC consider butane

175 hC consider Dowtherm J

200 to 400 hC consider molten salt

70 3 Energy Exchange

275 hC consider Dowtherm A

310 hC consider Dowtherm G.

Heating: to increase the temperature of 1 L of water by 75 hC requires 0.12 kg steam at 200 kPa-g.

Shell and tube: (Design issues illustrated in Fig. 3.1). The heat loads, q, are identified first based on the heating/cooling media selected; the sensible, latent heat and reaction heat requirements; the pressure and temperature conditions and the flowrate capacities. This is shown top central of Fig. 3.1.The ultimate choice of area and configuration, shown at the bottom RHS, are based on an economic tradeoff with the “approach temperature”. The tradeoff is between the pumping costs and the heat exchange costs. The details between the heat load and the selection are the following issues. The temperature and pressures for the system are specified, as shown in the top LHS. For the mean temperature driving force for the heat transfer, MTD, we start with the hot and cold streams the temperature profiles and the rule-of-thumb “approach temperature”. These lead to the local and ultimately the overall temperature driving force. A visual plot of the temperature profile assists. Usually a variation on the LMTD is selected with corrections made to account for the configuration ultimately selected. At the LHS central the shell and tube side fluids are selected, based on pressure, materials of construction, characteristics of the fluids (corrosivity, viscosity, cleanliness) and function (condensation, boiling, heat exchange). The rate at which the heat is transferred is based on the overall heat transfer coefficient U. Trial rules-of-thumb values for U are given in this section. U depends on the configuration which dictates the individual coefficients on the shell and tube sides, the tube thickness and material of construction and on the allowance for dirt. The configuration depends on the tube and shell side fluids, temperature and pressure, materials of construction, the area required and the prevention of temperature crossover. Central to the design process is the velocity of fluids which directly affects both the pressure drop and the heat transfer coefficients. This sizing map interacts with the sizing maps for pumps, Fig. 2.1; and perhaps for distillation columns, Fig. 4.1 and for reactors, Fig. 6.1.

Use shell and tube exchangers for gas–gas and for low viscosity liquid–liquid systems (I 200 mPa.s).

Use floating head if the temperature difference between shell and tube fluids exceeds 30 hC (to minimize impact of thermal expansion).

Surface compactness: for ordinary tubes: 70–500 m2/m3; for finned tubes

65–3300 m2/m3.
Number of shell passes affected by temperature			Number of shell passes
ratio	(thot, insthot, out)S(tcold, outstcold, in)
	(thot, instcold, in)
0–0.8			1
0.8–1.1			1	or 2
1.1–1.3			2
1.3–1.4			2	or 3
1.4–1.5			3