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  • 1. Can superheated steam be used in process heat exchangers andother heating processes?Although not the ideal medium for transferring heat, superheated steam is sometimes used for process heating inmany steam plants around the world, especially in the HPIs (Hydrocarbon Processing Industries) which produceoils and petrochemicals. This is more likely to be because superheated steam is already available on site forpower generation, being the preferred energy source for turbines, rather than because it has any advantage oversaturated steam for heating purposes. To be clear on this point, in most cases, saturated steam should be usedfor heat transfer processes, even if it means desuperheating the steam to do so. HPIs often desuperheat steamto within about ten degrees of superheat. This small degree of superheat is removed readily in the first part of theheating surface. Greater amounts of superheat are more difficult, and often uneconomic to deal with and (forheating purposes) are best avoided.There are quite a few reasons why superheated steam is not as suitable for process heating as saturated steam:Superheated steam has to cool to saturation temperature before it can condense to release its enthalpy ofevaporation. The amount of heat given up by the superheated steam as it cools to saturation temperature isrelatively small in comparison to its enthalpy of evaporation.If the steam has only a few degrees of superheat, this small amount of heat is quickly given up before itcondenses. However, if the steam has a large degree of superheat, it may take a relatively long time to cool,during which time the steam is releasing very little energy.Unlike saturated steam, the temperature of superheated steam is not uniform. Superheated steam has to cool togive up heat, whilst saturated steam changes phase. This means that temperature gradients over the heattransfer surface may occur with superheated steam.In a heat exchanger, use of superheated steam can lead to the formation of a dry wall boiling zone, close to thetube sheet. This dry wall area can quickly become scaled or fouled, and the resulting high temperature of thetube wall may cause tube failure.This clearly shows that in heat transfer applications, steam with a large degree of superheat is of little usebecause it: Gives up little heat until it has cooled to saturation temperature. Creates temperature gradients over the heat transfer surface as it cools to saturation temperature. Provides lower rates of heat transfer whilst the steam is superheated. Requires larger heat transfer areas.So, superheated steam is not as effective as saturated steam for heat transfer applications. This may seemstrange, considering that the rate of heat transfer across a heating surface is directly proportional to thetemperature difference across it. If superheated steam has a higher temperature than saturated steam at thesame pressure, surely superheated steam should be able to impart more heat? The answer to this is no. Thiswill now be looked at in more detail.It is true that the temperature difference will have an effect on the rate of heat transfer across the heat transfersurface, as clearly shown by Equation 2.5.3. Equation 2.5.3Where: = Heat transferred per unit time (W) 2U = Overall thermal transmittance (heat transfer coefficient) (W/m °C) 2A = Heat transfer area (m )ΔT = Temperature difference between primary and secondary fluid (°C)Equation 2.5.3 also shows that heat transfer will depend on the overall heat transfer coefficient U, and the heattransfer area A.For any single application, the heat transfer area might be fixed. However, the same cannot be said of the Uvalue; and this is the major difference between saturated and superheated steam. The overall U value forsuperheated steam will vary throughout the process, but will always be much lower than that for saturated steam.It is difficult to predict U values for superheated steam, as these will depend upon many factors, but generally,the higher the degree of superheat, the lower the U value. 1
  • 2. 2Typically, for a horizontal steam coil surrounded with water, U values might be as low as 50 to 100 W/m )°C for 2superheated steam but 1 200 W/m )°C for saturated steam, as depicted in Figure 2.3.4. 2For steam to oil applications, the U values might be considerably less, perhaps as low as 20 W/m )°C for 2superheated steam and 150 W/m )°C for saturated steam. 2 2In a shell and tube heat exchanger, 100 W/m )°C for superheated steam and 500 W/m )°C for saturated steamcan be expected. These figures are typical; actual figures will vary due to other design and operationalconsiderations. Figure 2.3.4 Typical ‘U’ values for superheated and saturated steam coils in waterAlthough the temperature of superheated steam is always higher than saturated steam at the same pressure, itsability to transfer heat is therefore much lower. The overall effect is that superheated steam is much less effectiveat transferring heat than saturated steam at the same pressure. The next Section Fouling gives more detail.Not only is superheated steam less effective at transferring heat, it is very difficult to quantify using Equation2.5.3, = U A ΔT, as the temperature of the steam will fall as it gives up its heat while passing along the heatingsurface.Predicting the size of heat transfer surfaces utilising superheated steam is difficult and complex. In practice, thebasic data needed to perform such calculations is either not known or empirically obtained, putting their reliabilityand accuracy in doubt.Clearly, as superheated steam is less effective at transferring heat than saturated steam, then any heating areausing superheated steam would have to be larger than a saturated steam coil operating at the same pressure todeliver the same heat flowrate.If there is no choice but to use superheated steam, it is not possible to maintain steam in its superheated statethroughout the heating coil or heat exchanger, since as it gives up some of its heat content to the secondary fluid,it cools towards saturation temperature. The amount of heat above saturation is quite small compared with thelarge amount available as condensation occurs.The steam should reach saturation relatively soon in the process; this allows the steam to condense to producehigher heat transfer rates and result in a higher overall U value for the whole coil, see Figure 2.3.5.To help to enable this, superheated steam used for heat transfer purposes should not hold more than about 10°Cof superheat. Figure 2.3.5 Less superheat allows the steam to condense in the major part of the coil thus increasing the overall ‘U’ value approaching that of saturated steam. 2
  • 3. If this is so, it is relatively easy and practical to design a heat exchanger or a coil with a heating surface areabased upon saturated steam at the same pressure, by adding on a certain amount of surface area to allow for thesuperheat. Using this guideline, the first part of a coil will be used purely to reduce the temperature ofsuperheated steam to its saturation point. The rest of the coil will then be able to take advantage of the higherheat transfer ability of the saturated steam. The effect is that the overall U value may not be much less than ifsaturated steam were supplied to the coil.From practical experience, if the extra heating area needed for superheated steam is 1% per 2°C of superheat,the coil (or heat exchanger) will be large enough. This seems to work up to 10°C of superheat. It is notrecommended that superheated steam above 10°C of superheat be used for heating purposes due to theprobable disproportionate and uneconomic size of the heating surface, the propensity for fouling by dirt, and thepossibility of product spoilage by the high and uneven superheat temperatures.FoulingFouling is caused by deposits building up on the heat transfer surface adding a resistance to heat flow. Manyprocess liquids can deposit sludge or scale on heating surfaces, and will do so at a faster rate at highertemperatures. Further, superheated steam is a dry gas. Heat flowing from the steam to the metal wall must passthrough the static films adhering to the wall, which resist heat flow.By contrast, the condensation of saturated steam causes the movement of steam towards the wall, and therelease of large quantities of latent heat right at the condensing surface. The combination of these factors meansthat the overall heat transfer rates are much lower where superheated steam is present, even though thetemperature difference between the steam and the secondary fluid is higher.Example 2.3.3 Sizing a tube bundle for superheated steamSuperheated steam at 3 bar g with 10°C of superheat (154°C) is to be used as the primary heat source for a shelland tube process heat exchanger with a heating load of 250 kW, heating an oil based fluid from 80°C to 120°C(making the arithmetic mean secondary temperature (ΔT AM) 100°C). Estimate the area of primary steam coilrequired.(Arithmetic mean temperature differences are used to keep this calculation simple; in practice, logarithmic meantemperatures would be used for greater accuracy. Please refer to Tutorial 2.5 Heat Transfer for details onarithmetic and logarithmic mean temperature differences).First, consider the coil if it were heated by saturated steam at 3 bar g (144°C). 2The U value for saturated steam heating oil via a new carbon steel coil is taken to be 500 W/m °C. 3
  • 4. Other applications using superheated steamAll the above applies when steam is flowing through a relatively narrow passage, such as the tubes in a shell andtube heat exchanger or the plates in a plate heat exchanger.In some applications, perhaps a drying cylinder in a paper machine, superheated steam is admitted to a greatervolume, when its velocity plummets to very small values. Here, the steam near the wall of the cylinder quicklydrops in temperature to near saturation and condensation begins. The heat flow through the wall is then thesame as if the cylinder were supplied with saturated steam. Superheat is present only within the core in thesteam space and has no discernible effect on heat transfer rates.There are instances where the presence of superheat can actually reduce the performance of a process, wheresteam is being used as a process material.One such process might involve moisture being imparted to the product from the steam as it condenses, such as,the conditioning of animal feedstuff (meal) prior to pelletising. Here the moisture provided by the steam is anessential part of the process; superheated steam would over-dry the meal and make pelletising difficult.The effects of reducing steam pressureIn addition to the use of an additional heat exchanger (generally called a superheater), superheat can also beimparted to steam by allowing it to expand to a lower pressure as it passes through the orifice of a pressurereducing valve. This is termed a throttling process with the lower pressure steam having the same enthalpy (apartfrom a small amount lost to friction in passing through the valve) as the upstream high pressure steam. However,the temperature of the throttled steam will always be lower than that of the supply steam.The state of the throttled steam will depend upon: The pressure of the supply steam. The state of the supply steam. The pressure drop across the valve orifice.For supply steam below 30 bar g in the dry saturated state, any drop in pressure will produce superheated steamafter throttling. The degree of superheat will depend on the amount of pressure reduction.For supply steam above 30 bar g in the dry saturated state, the throttled steam might be superheated, drysaturated, or even wet, depending on the amount of pressure drop. For example, dry saturated steam at 60 bar gwould have to be reduced to approximately 10.5 bar g to produce dry saturated steam. Any less of a pressuredrop will produce wet steam, while any greater pressure drop would produce superheated steam.Equally, the state of the supply steam at any pressure will influence the state of the throttled steam. For example,wet steam at a pressure of 10 bar g and 0.95 dryness fraction would need to be reduced to 0.135 bar g toproduce dry saturated steam. Any less of a pressure drop would produce wet steam while any greater pressuredrop would superheat the throttled steam.The Mollier chart is a plot of the specific enthalpy of steam against its specific entropy (sg). Fig. 2.3.7 Enthalpy - entropy or Mollier chart for steam 4
  • 5. Figure 2.3.7 shows a simplified, small scale version of the Mollier chart. The Mollier chart displays many differentrelationships between enthalpy, entropy, temperature, pressure and dryness fraction. It may appear to be quitecomplicated, due to the number of lines: Constant enthalpy lines (horizontal). Constant entropy lines (vertical). The steam saturation curve across the centre of the chart divides it into a superheated steam region, and a wet steam region. At any point above the saturation curve the steam is superheated, and at any point below the saturation curve the steam is wet. The saturation curve itself represents the condition of dry saturated steam at various pressures. Constant pressure lines in both regions. Constant temperature lines in the superheat region. Constant dryness fraction (Χ) lines in the wet region.A perfect expansion, for example within a steam turbine or a steam engine, is a constant entropy process, andcan be represented on the chart by moving vertically downwards from a point representing the initial condition toa point representing the final condition.Filmwise condensationThe elimination of the condensate film, is not quite as simple. As the steam condenses to give up its enthalpy ofevaporation, droplets of water may form on the heat transfer surface. These may then merge together to form acontinuous film of condensate. The condensate film may be between 100 and 150 times more resistant to heattransfer than a steel heating surface, and 500 to 600 times more resistant than copper.Dropwise condensationIf the droplets of water on the heat transfer surface do not merge immediately and no continuous condensate filmis formed, dropwise condensation occurs. The heat transfer rates which can be achieved duringdropwisecondensation, are generally much higher than those achieved during filmwise condensation.As a larger proportion of the heat transfer surface is exposed during dropwise condensation, heat transfercoefficients may be up to ten times greater than those for filmwise condensation.In the design of heat exchangers where dropwise condensation is promoted, the thermal resistance it produces isoften negligible in comparison to other heat transfer barriers. However, maintaining the appropriate conditions fordropwise condensation have proved to be very difficult to achieve.If the surface is coated with a substance that inhibits wetting, it may be possible to maintain dropwisecondensation for a period of time. For this purpose, a range of surface coatings such as Silicones, PTFE and anassortment of waxes and fatty acids are sometimes applied to surfaces in a heat exchanger on whichcondensation is to be promoted. However, these coatings will gradually lose their effectiveness due to processessuch as oxidation or fouling, and film condensation will eventually predominate.As air is such a good insulator, it provides even more resistance to heat transfer. Air may be between 1 500 and3 000 times more resistant to heat flow than steel, and 8 000 to 16 000 more resistant than copper. This meansthat a film of air only 0.025 mm thick may resist as much heat transfer as a wall of copper 400 mm thick! Ofcourse all of these comparative relationships depend on the temperature profiles across each layer.Figure 2.5.4 illustrates the effect this combination of layers has on the heat transfer process. These barriers toheat transfer not only increase the thickness of the entire conductive layer, but also greatly reduce the meanthermal conductivity of the layer.The more resistant the layer to heat flow, the larger the temperature gradient is likely to be. This means that toachieve the same desired product temperature, the steam pressure may need to be significantly higher.The presence of air and water films on the heat transfer surfaces of either process or space heating applicationsis not unusual. It occurs in all steam heated process units to some degree.To achieve the desired product output and minimise the cost of process steam operations, a high heatingperformance may be maintained by reducing the thickness of the films on the condensing surface. In practice, airwill usually have the most significant effect on heat transfer efficiency, and its removal from the supply steam willincrease heating performance. 5
  • 6. Why return condensate and reuse it?Financial reasonsCondensate is a valuable resource and even the recovery of small quantities is often economically justifiable. Thedischarge from a single steam trap is often worth recovering.Un-recovered condensate must be replaced in the boiler house by cold make-up water with additional costs ofwater treatment and fuel to heat the water from a lower temperature.Water chargesAny condensate not returned needs to be replaced by make-up water, incurring further water charges from thelocal water supplier.Effluent restrictionsIn the UK for example, water above 43°C cannot be returned to the public sewer by law, because it is detrimentalto the environment and may damage earthenware pipes. Condensate above this temperature must be cooledbefore it is discharged, which may incur extra energy costs. Similar restrictions apply in most countries, andeffluent charges and fines may be imposed by water suppliers for non-compliance.Maximising boiler outputColder boiler feedwater will reduce the steaming rate of the boiler. The lower the feedwater temperature, themore heat, and thus fuel needed to heat the water, thereby leaving less heat to raise steam.Boiler feedwater qualityCondensate is distilled water, which contains almost no total dissolved solids (TDS). Boilers need to be blowndown to reduce their concentration of dissolved solids in the boiler water. Returning more condensate to thefeedtank reduces the need for blowdown and thus reduces the energy lost from the boiler.Summary of reasons for condensate recovery: Water charges are reduced. Effluent charges and possible cooling costs are reduced. Fuel costs are reduced. More steam can be produced from the boiler. Boiler blowdown is reduced - less energy is lost from the boiler. Chemical treatment of raw make-up water is reduced.Figure 14.1.5 compares the amount of energy in a kilogram of steam and condensate at the same pressure. Thepercentage of energy in condensate to that in steam can vary from 18% at 1 bar g to 30% at 14 bar g; clearly theliquid condensate is worth reclaiming. Fig. 14.1.5 Heat content of steam and condensate at the same pressures 6