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Steam Engineering Principles and Heat Transfer

2.10 HeatingwithCoilsandJackets

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Vessels can be heated in a number of different ways. This module will deal with indirect heating. In thesesystems, the heat is transferred across a heat transfer surface. Options include:

Submerged steam coils ! widely used form of heat transfer in"ol"es the installation inside a tan#of a steam coil immersed in a process fluid.

Steam $ac#ets Steam circulates in the annular space between a $ac#et and the "essel walls, and

heat is transferred through the wall of the "essel.

Submerged steam coilsThe use of tan# coils is particularly common in marine applications where cargoes of crude oil, edible oils,tallow and molasses are heated in deep tan#s. %any of these li&uids are difficult to handle at ambienttemperatures due to their "iscosity. Steam heated coils are used to raise the temperature of these li&uids,lowering their "iscosity so that they become easier to pump.

Tan# coils are also e'tensi"ely used in electroplating and metal treatment. (lectroplating in"ol"es passingarticles through se"eral process tan#s so that metallic coatings can be deposited on to their surfaces. One of

the first stages in this process is #nown as pic#ling, where materials such as steel and copper are treated bydipping them in tan#s of acid or caustic solution to remo"e any scale or o'ide )e.g. rust* which may ha"eformed.

Steam coil si+inga"ing determined the energy re&uired )pre"ious %odule*, and with #nowledge of thesteam pressure-temperature in the coil, the heat transfer surface may be determined using (&uation ./.0:

Eqation 2.!."

The heat transfer area calculated is e&ui"alent to the surface area of the coil, and will enable an appropriate

si+e and layout to be specified.

1etermining the 232 "alueTo calculate the heat transfer area, a "alue for the o"erall heat transfer coefficient, 3, must be chosen. Thiswill "ary considerably with the thermal and transport properties of both fluids and a range of other conditions.

On the product side of the coil a thermal boundary layer will e'ist in which there is a temperature gradientbetween the surface and the bul# fluid. If this temperature difference is relati"ely large, then the naturalcon"ecti"e currents will be significant and the heat transfer coefficient will be high.

!ssisted circulation )such as stirring* that will induce forced con"ection, will also result in higher coefficients.

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!s con"ection is partially dependent on the bul# motion of the fluid, the "iscosity )which "aries withtemperature* also has an important bearing on the thermal boundary layer.

!dditional "ariations can also occur on the steam side of the coil, especially with long lengths of pipe. Thecoil inlet may ha"e a high steam "elocity and may be relati"ely free from water. owe"er, further along thelength of the coil the steam "elocity may be lower, and the coil may be running partially full of water. In "erylong coils, such as those sometimes found in seagoing tan#ers or in large bul# storage tan#s, a significant

pressure drop occurs along the length of the coil. To achei"e the mean coil temperature, an a"erage steampressure of appro'imately 4/5 of the inlet pressure may be used. In e'treme cases the a"erage pressureused may be as low as 675 of the inlet pressure.

!nother "ariable is the coil material itself. The thermal conducti"ity of the coil material may "aryconsiderably. owe"er, o"erall heat transfer is go"erned to a large e'tent by the heat resistant films, and thethermal conducti"ity of the coil material is not as significant as their combined effect. Table .87.8 pro"idestypical o"erall heat transfer coefficients for "arious conditions of submerged steam coil application. 93"alues for steam pressures between bar g and ; bar g should be found by interpolation of the data in thetable.

The range of figures shown in Table .87.8 demonstrates the difficulty in pro"iding definiti"e 232 "alues.

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('ample .87.8 ;/7 @-mAB 8.7 mA

; mA of heat transfer area is still re&uired, and must be pro"ided by theconnecting pipes.

!rbitrarily selecting 0 mm pipe as a good compromise between robustness and wor#ability:

The lengths of the connecting pipes are ./ m.

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On the basis of proportionality of heat transfer area, the steam header will condense:

This lea"es ?;5 of the ?/7 #g-h > 408 #g-h of steam which must pass through the 8? connecting pipes andalso into the lower )condensate* manifold.

Other steam coil layoutsThe design and layout of the steamcoil will depend on the process fluidbeing heated. @hen the process fluidto be heated is a corrosi"e solution, it

is normally recommended that the coil inlet and outlet connections are ta#en o"er the lip of the tan#, as it isnot normally ad"isable to drill through the corrosion resistant linings of the tan# side. This will ensure that

there are no wea# points in the tan# lining, where there is a ris# of lea#age of corrosi"e li&uids. In thesecases the coil itself may also be made of corrosion resistant material such as lead co"ered steel or copper,or alloys such as titanium.

owe"er, where there is no danger of corrosion, lifts o"er the tan# structure should be a"oided, and thesteam inlet and outlet connections may be ta#en through the tan# side. The presence of any lift will result inwaterlogging of a proportion of the coil length, and possibly waterhammer, noise and lea#ing pipewor#.

Steam heating coils should generally ha"e a gradual fall from the inlet to the outlet to ensure thatcondensate runs toward the outlet and does not collect in the bottom of the coil.

@here a lift is una"oidable, it should be designed to include a seal arrangement at the bottom of the lift anda small bore dip pipe, as shown in Digure .87..

#ig. 2.10.2 Tank with a rising discharge pipe

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The seal arrangement allows a small amount of condensate to collect to act as a water seal, and pre"entsthe occurrence of steam loc#ing. @ithout this seal, steam can pass o"er any condensate collecting in thebottom of the pipe, and close the steam trap at the top of the riser.

The condensate le"el would then rise and form a temporary water seal, loc#ing the steam between thebottom of the riser and the steam trap. The steam trap remains closed until the loc#ed steam condenses,during which time the coil continues to waterlog.

@hen the loc#ed steam condenses and the steam trap opens, a slug of water is discharged up the riser. !ssoon as the water seal is bro#en, steam will enter the rising pipe and close the trap, while the bro#en columnof water falls bac# to lie at the bottom of the heating coil.

The small bore dip pipe will only allow a "ery small "olume of steam to become loc#ed in the riser. It enablesthe water column to be easily maintained without steam bubbling through it, ensuring there is a steady andcontinuous condensate flow to the outlet.

@hen the seal is ultimately bro#en, a smaller "olume of water will return to the heating coil than with anunrestricted large bore riser, but as the water seal arrangement re&uires a smaller "olume of condensate toform a water seal, it will immediately reform.

If the process in"ol"es articles being dipped into the li&uid, it may not be con"enient to install the coil at thebottom of the tan# it may be damaged by the ob$ects being immersed in the solution. !lso, during certain

processes, hea"y deposits will settle at the bottom of the tan# and can &uic#ly co"er the heating surface,inhibiting heat transfer.

Dor these reasons side hung coils are often used in the electroplating industry. In such cases serpentine orplatetype coils are arranged down the side of a tan#, as shown in Digure .87.0. These coils should alsoha"e a fall to the bottom with a water seal and a small bore dippipe. This arrangement has the ad"antagethat it is often easier to install, and also easier to remo"e for periodic cleaning if re&uired.

#ig. 2.10." Side hng coils

If articles are to be dipped into the tan#, it may not be possible to use any sort of agitator to induce forcedcon"ection and pre"ent temperature gradients occurring throughout the tan#. @hether bottom or side coilsare used, it is essential that they are arranged with ade&uate co"erage so that the heat is distributed e"enlythroughout the bul# of the li&uid.

The diameter of the coil should pro"ide sufficient length of coil for good distribution. ! short length of coil with

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a large diameter may not pro"ide ade&uate temperature distribution. owe"er a "ery long continuous lengthof coil may e'perience a temperature gradient due to the pressure drop from end to end, resulting in une"enheating of the li&uid.

&hilst the ne't two headings( )Si*ing the control +al+e, and )The condensate remo+al de+ice, areinclded in this -odle( the new reader shold refer to later locks and -odles in The Steam andCondensate Loopfor fll and comprehensi+e information( $efore attempting si*ing and selection of

eqipment.

6* and a 1F/ piston actuated "al"e )G"s> 8?.;* operating together will caterfor the startup load. @hen approaching the control temperature, the larger "al"e would be set to shut down,allowing the smaller "al"e to gi"e good control.

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The condensate remo"al de"iceThe selection and si+ing of the condensate remo"al de"ice will be "ery much influenced by the condensatebac# pressure. Dor the purpose of this e'ample, it is assumed the bac# pressure is atmospheric pressure.The de"ice should be si+ed so it is able to satisfy both of the following conditions:

8. Pass ?/7 #g-h of condensate with 8.8 bar g in the coil, i.e. the fullload condition.

. Pass the condensate load when steam pressure in the coil e&uals the condensate bac# pressure, i.e. thestall load condition.

If the steam trap is only si+ed on the first condition, it is possible that it may not pass the stall load )thecondition where the product approaches its re&uired temperature and the control "al"e modulates to reducesteam pressure*. The stall load may be considerable. @ith respect to nonflow type applications such astan#s, this may not be too serious from a thermal "iewpoint because the contents of the tan# will almost beat the re&uired temperature, and ha"e a huge reser"oir of heat.

!ny reduction in heat transfer at this part of the heating process may therefore ha"e little immediate effecton the tan# contents.

owe"er, condensate will bac# up into the coil and waterhammer will occur, along with its associatedsymptoms and mechanical stresses. Tan# coils in large circular tan#s tend to be of robust construction, and

are often able to withstand such stresses. Problems can howe"er occur in rectangular tan#s )which tend tobe smaller*, where "ibration in the coil will ha"e more of an effect on the tan# structure. ere, the energydissipated by the waterhammer causes "ibration, which can be detrimental to the life of the coil, the tan#,and the steam trap, as well as creating unpleasant noise.

@ith respect to flowtype applications such as plate heat e'changers, a failure to consider the stall conditionwill usually ha"e serious implications. This is mainly due to the small "olume in the heat e'changer.

Dor heat e'changers, any unwanted reduction in the heating surface area, such as that caused bycondensate bac#ing up into the steam space, can affect the flow of heat through the heating surface. Thiscan cause the control system to become erratic and unstable, and processes re&uiring stable or accuratecontrol can suffer with poor performance.

If heat e'changers are o"ersi+ed, sufficient heating surface may remain when condensate bac#s up into thesteam space, and reduction of thermal performance may not always occur. owe"er, with heat e'changersnot designed to cope with the effects of waterlogging, this can lead to corrosion of the heating surface,ine"itably reducing the ser"ice life of the e'changer. @aterlogging can, in some applications, be costly.

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#ig. 2.10./ con+entional acketed +essel

The heat transfer area )the "essel wall surface area*, can be calculated in the same manner as with a steamcoil, using (&uation ./.0 and the o"erall heat transfer coefficients pro"ided inTable .87.6.

!lthough steam $ac#ets may generally be less thermally efficient than submerged coils, due to radiationlosses to the surroundings, they do allow space for the "essels to be agitated so that heat transfer ispromoted. The 3 "alues listed in Table .87.6. are for moderate nonpro'imity agitation.