Page 315 - Fluid mechanics, heat transfer, and mass transfer

SHELL AND TUBE HEAT EXCHANGERS
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occur for heating and cooling of single-phase ➢ Shorter lengths may require multipass designs but
fluids. cleaning becomes difficult.
& Fouling Nature of the Fluid: ➢ Materials selection involves corrosion resistance,
➢ How much fouling allowance is to be provided and initial cost, expected life, maintenance, and per-
what type of fouling-resistant materials is to be formance. A 10-year life should be considered.
specified. ➢ Proper condensate drainage, preferably by gravity,
- Fluid properties such as C p , m, k, and r at the is essential while considering installation. Con-
anticipated range of operating temperatures densate drainage must have sufficient pipe length,
and pressures, for both tube side and shell side typically 0.5 m vertical and not more than 0.2 m to
fluids. steam trap inlet is required.
& Condition of Steam or Other Condensing Medium: ➢ Sizing and length of pipe from control valve outlet
➢ Pressures, flows, and temperatures upstream and to inlet nozzle to heat exchanger is crucial.
downstream of control valve. Minimum 10 pipe diameters length from control
valve to inlet connection is required.
➢ Design conditions, maximum, minimum, and nor-
mal operating conditions. ➢ Air venting and vacuum breaking provisions must
be made for the heat transfer equipment.
& Expected Turndown Ratio: This information is
required for designing proper control strategies . Give a checklist incorporating installation guidelines
and should be known to avoid oversizing the for heat transfer equipment.
exchanger. & Provide a condensate drip leg with steam trap before
& Materials of Construction: Some applications stip- steam control valve.
ulate special materials to ensure compatibility with & Use ball valves with locking handles for all pipe sizes
the fluids to be handled. less than 5 cm in diameter. This provides the best lock
& Special Requirements: Occasionally, applications out/tag out safety procedure.
require special construction requirements such as & Install a strainer in front of the control valve.
removable tube bundles or double tube sheets. & Select a proper control valve based on the
. Under what conditions, ASME standards are necessary application.
for a heat exchanger? & Install pressure gauges before and after the control
& When operating pressures exceed 1 barg. valve.
. What are the maintenance, service, layout, and eco- & Control valve outlet piping must be more than or
nomic considerations involved in the use of heat transfer equal to inlet connection to the exchanger.
equipment? & Use a steam trap for condensate capacities of 5000
& Maintenance, Servicing, Layout, and Economic kg/h.
Considerations: & Use a control valve with a level controller for con-
➢ Design and layout must permit access to heat densate capacities of 5000 kg/h.
transfer area (tube bundle) for cleaning and tube
replacement on failure. Cleaning is easier for a 1–1
10.1.3 Fouling
and plate exchangers.
➢ For high-pressure applications (above 8 barg), . What are thegeneral effects of heat exchanger fouling in
shell and tube exchangers are preferred over plate the chemical industry?
heat exchangers. & Estimates have been made of fouling costs primarily
➢ Higher steam pressures reduce area requirements
due to wasted energy through excess fuel burnt that
due to larger DT obtainable. However, higher are as high as 0.25% of the gross national product
pressures involve higher costs in terms of equip- (GNP) of the industrialized countries.
ment, control, and safety. & Millions of tons of carbon emissions are the result of
➢ Use of highest possible DP reduces exchanger area this inefficiency. Costs associated specifically with
requirements. crude oil fouling in the preheat trains of oil refineries
➢ Using long and small diameter shell is cost effec- worldwide are estimated to be in terms of billions of
tive, provided horizontal/vertical space require- dollars.
ments accommodate such design. & With oil prices at record highs, the payback from
➢ Overall space requirement should be at least dou- fouling reduction by increased throughput and less
ble the length of the shell. wasted fuel increases year after year.