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Indirect heating of fluids is covered in this tutorialincluding layouts, control and drainage of coils andjackets, and heat transfer calculations.

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Steam Engineering Principles and Heat Transfer Heating w ith Coils and Jackets

Heating with Coils and Jackets

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

Submerged steam coils - A widely used form of heat transfer involves the installation inside a tankof a steam coil immersed in a process fluid.

Steam jackets - Steam circulates in the annular space between a jacket and the vessel walls, andheat is transferred through the wall of the vessel.

Submerged steam coilsThe use of tank coils is particularly common in marine applications where cargoes of crude oil, edible oils,tallow and molasses are heated in deep tanks. Many of these liquids are difficult to handle at ambienttemperatures due to their viscosity. Steam heated coils are used to raise the temperature of these liquids,lowering their viscosity so that they become easier to pump.

Tank coils are also extensively used in electroplating and metal treatment. Electroplating involves passingarticles through several process tanks so that metallic coatings can be deposited on to their surfaces. Oneof the first stages in this process is known as pickling, where materials such as steel and copper aretreated by dipping them in tanks of acid or caustic solution to remove any scale or oxide (e.g. rust) whichmay have formed.

Steam coil sizingHaving determined the energy required (in Tutorial 2.9), and with knowledge of the steampressure/temperature in the coil, the heat transfer surface may be determined using Equation 2.5.3:

Equation 2.5.3

The heat transfer area calculated is equivalent to the surface area of the coil, and will enable an appropriatesize and layout to be specified.

Determining the 'U' valueTo calculate the heat transfer area, a value for the overall heat transfer coefficient, U, must be chosen. Thiswill vary considerably with the thermal and transport properties of both fluids and a range of otherconditions.

On the product side of the coil a thermal boundary layer will exist in which there is a temperature gradientbetween the surface and the bulk fluid. If this temperature difference is relatively large, then the naturalconvective currents will be significant and the heat transfer coefficient will be high.

Assisted circulation (such as stirring) that will induce forced convection, will also result in highercoefficients. As convection is partially dependent on the bulk motion of the fluid, the viscosity (which varieswith temperature) also has an important bearing on the thermal boundary layer.

Additional variations can also occur on the steam side of the coil, especially with long lengths of pipe. Thecoil inlet may have a high steam velocity and may be relatively free from water. However, further along thelength of the coil the steam velocity may be lower, and the coil may be running partially full of water. In verylong coils, such as those sometimes found in seagoing tankers or in large bulk storage tanks, a significantpressure drop occurs along the length of the coil. To acheive the mean coil temperature, an average steampressure of approximately 75% of the inlet pressure may be used. In extreme cases the average pressureused may be as low as 40% of the inlet pressure.

Another variable is the coil material itself. The thermal conductivity of the coil material may vary considerably.

Steam EngineeringPrinciples and HeatTransfer

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What is Steam?

Superheated Steam

Steam Quality

Heat Transfer

Methods of Estimating Steam

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Measurement of Steam

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Thermal Rating

Energy Consumption of Tanks

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Heating w ith Coils and

Jackets

Heating Vats and Tanks by

Steam Injection

Steam Consumption of Pipes

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Steam Consumption of Heat

Exchangers

Steam Consumption of Plant

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Entropy - A Basic

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However, overall heat transfer is governed to a large extent by the heat resistant films, and the thermalconductivity of the coil material is not as significant as their combined effect. Table 2.10.1 provides typicaloverall heat transfer coefficients for various conditions of submerged steam coil application. 'U' values forsteam pressures between 2 bar g and 6 bar g should be found by interpolation of the data in the table.

The range of figures shown in Table 2.10.1 demonstrates the difficulty in providing definitive 'U' values.Customary figures at the higher end of the scale will apply to installations that are supplied with clean drysteam, small coils and good condensate drainage. The lower end is more applicable to poor quality steam,long coils and poor condensate drainage.

The recommended overall heat transfer coefficients will apply to typical conditions and installations. Theserecommended rates are empirically derived, and will generally ensure that a generous safety marginapplies to the coil sizing.

In the case of fluids other than water, the heat transfer coefficient will vary even more widely due to the wayin which viscosity varies with temperature. However, the values shown in Table 2.10.2 will serve as a guidefor some commonly encountered substances, while Table 2.10.3 gives typical surface areas of pipes permetre length.

Example 2.10.1Continuing from Example 2.9.1 determine:

Part 1. The average steam mass flowrate during start-up. (Mean heat load = 367 kW)

Part 2. The heat transfer area required.

Part 3. A recommended coil surface area.

Part 4. The maximum steam mass flowrate with the recommended heat transfer area.

Part 5. A recommendation for installation, including coil diameter and layout.

The following additional information has been provided:

Steam pressure onto the control valve = 2.6 bar g (3.6 bar a).

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A stainless steel steam coil provides heat.

Heat transfer coefficient from steam/coil/liquid, U = 650 W/m²°C

Part 1 Calculate the average steam mass flowrate during start-up

Steam pressure onto the control valve = 2.6 bar g (3.6 bar a)

Critical pressure drop (CPD) will occur across the control valve during start-up, therefore the minimumsteam pressure in the heating coil should be taken as 58% of upstream absolute pressure. An explanationof this is given in Block 5.

Part 2 Calculate the heat transfer area required.

Part 3 A recommendation for coil surface area

Because of the difficulties in providing accurate 'U' values, and to allow for future fouling of the heatexchange surface, it is usual to add 10% to the calculated heat transfer area.

Part 4 The maximum steam mass flowrate with the recommended heat transfer area

Maximum heat transfer (and hence steam demand) will occur when the temperature difference between thesteam and the process fluid is at its maximum, and should take into consideration the extra pipe areaallowed for fouling.

(a) Consider the maximum heating capacity of the coil (coil)

Using Equation 2.5.3: = UAΔT

(b) Steam flowrate to deliver 519 kW

Part 5 A recommendation for installation, including coil diameter and layout

(a) Determine coil diameter and length

From Table 2.10.3, a 100 mm pipe has a surface area of 0.358 m²/m run. This application will require:

It may be difficult to accommodate this length of large bore heating pipe to install in a 3 m × 3 m tank.

One solution would be to run a bank of parallel pipes between steam and condensate manifolds, set atdifferent heights to encourage condensate to run to the lower (condensate) manifold. The drain line mustfall from the bottom of the condensate manifold down to the steam trap (or pump-trap). See Figure 2.10.1for a suggested layout.

Fig. 2.10.1Possible layout of coils in a rectangular tank

Note the steam supply is situated at one end of its manifold, whilst the trap set is at the other end. This willhelp steam to flow and push condensate through the coils.

In the application, the steam and condensate headers would each be 2.8 m long. As the condensatemanifold is holding condensate, the heat from it will be small compared to the steam manifold and this canbe ignored in the calculation.

The steam manifold should be 100 mm diameter as determined by the previous velocity calculation. Thiswill provide a heating area of:

2.8 m x 0.358 m²/m = 1.0 m²

Consequently 7 m² - 1 m² = 6 m² of heat transfer area is still required, and must be provided by theconnecting pipes.

Arbitrarily selecting 32 mm pipe as a good compromise between robustness and workability:

The lengths of the connecting pipes are 2.5 m.

CHECK

It is necessary to confirm the steam velocity through the connecting tubes:

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

This leaves 86% of the 850 kg/h = 731 kg/h of steam which must pass through the 18 connecting pipes andalso into the lower (condensate) manifold.

Other steam coil layouts

The design and layout of the steam coil will depend on the process fluid being heated. When the processfluid to be heated is a corrosive solution, it is normally recommended that the coil inlet and outletconnections are taken over the lip of the tank, as it is not normally advisable to drill through the corrosionresistant linings of the tank side. This will ensure that there are no weak points in the tank lining, wherethere is a risk of leakage of corrosive liquids. In these cases the coil itself may also be made of corrosionresistant material such as lead covered steel or copper, or alloys such as titanium.

However, where there is no danger of corrosion, lifts over the tank structure should be avoided, and thesteam inlet and outlet connections may be taken through the tank side. The presence of any lift will result inwaterlogging of a proportion of the coil length, and possibly waterhammer, noise and leaking pipework.

Steam heating coils should generally have 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.

Where a lift is unavoidable, it should be designed to include a seal arrangement at the bottom of the lift anda small bore dip pipe, as shown in Figure 2.10.2.

Fig. 2.10.2Tank with a rising discharge pipe

The seal arrangement allows a small amount of condensate to collect to act as a water seal, and preventsthe occurrence of steam locking. Without this seal, steam can pass over any condensate collecting in thebottom of the pipe, and close the steam trap at the top of the riser.

The condensate level would then rise and form a temporary water seal, locking the steam between thebottom of the riser and the steam trap. The steam trap remains closed until the locked steam condenses,during which time the coil continues to waterlog.

When the locked steam condenses and the steam trap opens, a slug of water is discharged up the riser.As soon as the water seal is broken, steam will enter the rising pipe and close the trap, while the brokencolumn of water falls back to lie at the bottom of the heating coil.

The small bore dip pipe will only allow a very small volume of steam to become locked in the riser. Itenables the water column to be easily maintained without steam bubbling through it, ensuring there is asteady and continuous condensate flow to the outlet.

When the seal is ultimately broken, a smaller volume of water will return to the heating coil than with anunrestricted large bore riser, but as the water seal arrangement requires a smaller volume of condensateto form a water seal, it will immediately re-form.

If the process involves articles being dipped into the liquid, it may not be convenient to install the coil at thebottom of the tank - it may be damaged by the objects being immersed in the solution. Also, during certainprocesses, heavy deposits will settle at the bottom of the tank and can quickly cover the heating surface,inhibiting heat transfer.

For these reasons side hung coils are often used in the electroplating industry. In such cases serpentine orplate-type coils are arranged down the side of a tank, as shown in Figure 2.10.3. These coils should alsohave a fall to the bottom with a water seal and a small bore dip-pipe. This arrangement has the advantagethat it is often easier to install, and also easier to remove for periodic cleaning if required.

Fig. 2.10.3Side hung coils

If articles are to be dipped into the tank, it may not be possible to use any sort of agitator to induce forcedconvection and prevent temperature gradients occurring throughout the tank. Whether bottom or side coilsare used, it is essential that they are arranged with adequate coverage so that the heat is distributed evenlythroughout the bulk of the liquid.

The diameter of the coil should provide sufficient length of coil for good distribution. A short length of coilwith a large diameter may not provide adequate temperature distribution. However a very long continuouslength of coil may experience a temperature gradient due to the pressure drop from end to end, resulting inuneven heating of the liquid.

Whilst the next two headings, 'Sizing the control valve' and 'The condensate removal device' areincluded in this Tutorial, the new reader should refer to later Tutorials for full and comprehensiveinformation, before attempting sizing and selection of equipment.

Control valve arrangementThe control valve set may be either one or two valves in parallel. A single control valve, large enough to copewith the maximum flowrate encountered at start-up, may be unable to control flow accurately at theminimum expected flowrate. This could cause erratic temperature control. An alternative is to fit twotemperature control valves in parallel:

One valve (running valve) sized to control at the lower flowrate.

A second valve (starting valve) to pass the difference between the capacity of the first valve, and themaximum flowrate.

The starting valve would have a set-point slightly lower than the running valve, so it would close first, leavingthe running valve to control at low loads.

Sizing the control valveThe control valve set (either one valve or two valves in parallel).

The coil has been sized on mean heat transfer values. However, it may be better to size the control valve tosupply the maximum (start-up) load. With large coils in tanks, this will help to maintain a degree of steampressure throughout the length of the coil when the steam is turned on, helping to push condensatethrough the coil to the steam trapping device. If the control valve were sized on mean values, steampressure in the coil at start-up will tend to be lower and the coil may flood.

Using one valveContinuing with Example 2.10.1 the maximum steam load is 850 kg/h and the coil is designed to deliverthis at a pressure of 1.1 bar g. A steam valve sizing chart would show that a Kv of about 20 is required to

pass 850 kg/h of steam with a pressure of 2.6 bar g at the inlet of the control valve, and Critical PressureDrop (CPD) across the valve. (Tutorial 6.4 will show how the valve size can be determined by calculation).

A DN40 control valve with a larger Kvs of 25 would therefore need to be selected for the application.

If one valve is to be used, this valve must ensure the maximum heat load is catered for, while maintainingthe required steam pressure in the coil to assist the drainage of condensate from it at start-up. However, forreasons previously explained, two valves may be better.

The running load is 52 kW and with the coil running at 1.1 bar g, the running steam load:

The steam valve sizing chart shows a Kv of 2 is required to pass 85 kg/h with 3.6 bar upstream, operating at

critical pressure drop.

A DN15 KE type valve (Kvs = 4) and a DN25 piston actuated valve (Kvs = 18.6) operating together will cater for

the start-up load. When approaching the control temperature, the larger valve would be set to shut down,allowing the smaller valve to give good control.

The condensate removal deviceThe selection and sizing of the condensate removal device will be very much influenced by the condensatebackpressure. For the purpose of this example, it is assumed the backpressure is atmospheric pressure.The device should be sized so it is able to satisfy both of the following conditions:

Pass 850 kg/h of condensate with 1.1 bar g in the coil, i.e. the full-load condition.

Pass the condensate load when steam pressure in the coil equals the condensate backpressure,i.e. the stall load condition.

If the steam trap is only sized on the first condition, it is possible that it may not pass the stall load (thecondition where the product approaches its required temperature and the control valve modulates to reducesteam pressure). The stall load may be considerable. With respect to non-flow type applications such astanks, this may not be too serious from a thermal viewpoint because the contents of the tank will almost beat the required temperature, and have a huge reservoir of heat.

Any reduction in heat transfer at this part of the heating process may therefore have little immediate effect onthe tank contents.

However, condensate will back up into the coil and waterhammer will occur, along with its associatedsymptoms and mechanical stresses. Tank coils in large circular tanks tend to be of robust construction,and are often able to withstand such stresses. Problems can however occur in rectangular tanks (whichtend to be smaller), where vibration in the coil will have more of an effect on the tank structure. Here, theenergy dissipated by the waterhammer causes vibration, which can be detrimental to the life of the coil, thetank, and the steam trap, as well as creating unpleasant noise.

With respect to flow-type applications such as plate heat exchangers, a failure to consider the stall conditionwill usually have serious implications. This is mainly due to the small volume in the heat exchanger.

For heat exchangers, any unwanted reduction in the heating surface area, such as that caused bycondensate backing 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 requiring stable or accuratecontrol can suffer with poor performance.

If heat exchangers are oversized, sufficient heating surface may remain when condensate backs up into thesteam space, and reduction of thermal performance may not always occur. However, with heat exchangersnot designed to cope with the effects of waterlogging, this can lead to corrosion of the heating surface,inevitably reducing the service life of the exchanger. Waterlogging can, in some applications, be costly.Consider a waterlogging air heater frost coil. Cold air at 4°C flowing at 3 m/s can soon freeze condensatelocked in the coils, resulting in premature and unwarranted failure. Proper drainage of condensate isessential to maintain the service life of any heat exchanger and air heater.

Steam traps are devices which modulate to allow varying amounts of condensate to drain from applicationsunder varying conditions. Float traps are steam traps designed to modulate and release condensate closeto steam temperature, offering maximum plant performance, maximum plant life, and maximum return onplant investment.

When stall conditions occur, and a steam trap cannot be used, an automatic pump-trap or pump and trap incombination will ensure correct condensate drainage at all times, thus maximising the thermal capabilityand lifetime costs of the plant.

Steam jacketsThe most commonly used type of steam jacket consists simply of an outer cylinder surrounding the vessel,as shown in Figure 2.10.4. Steam circulates in the outer jacket, and condenses on the wall of the vessel.Jacketed vessels may also be lagged, or may contain an internal air space surrounding the jacket. This isto ensure that as little steam as possible condenses on the outer jacket wall, and that the heat istransferred inwards to the vessel.

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Fig. 2.10.4A conventional jacketed vessel

The heat transfer area (the vessel wall surface area), can be calculated in the same manner as with asteam coil, using Equation 2.5.3 and the overall heat transfer coefficients provided in Table 2.10.4.

Although steam jackets may generally be less thermally efficient than submerged coils, due to radiationlosses to the surroundings, they do allow space for the vessels to be agitated so that heat transfer ispromoted. The U values listed in Table 2.10.4. are for moderate non-proximity agitation.

Commonly the vessel walls are made from stainless steel or glass lined carbon steel. The glass lining willoffer an additional corrosion resistant layer. The size of the steam jacket space will depend on the size ofthe vessel, but typically the width may be between 50 mm and 300 mm.

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