Efficient Steam Generation & DistributionDr. Bipin Thapliyal Scientist Central Pulp & Paper Research Institute, Saharanpur
ContentsSteam and its properties Condensate and Flash Steam Efficient Steam Generation Steam Distribution
Why Steam is so popular as heat conveying media in industry?Highest specific heat and latent heat Highest heat transfer coefficient Easy to control and distribute Cheap and inert
Energy Energy can neither created nor destroyed. It can always be accounted for, and if it disappears at point. A then it reappears in equal amount at point B. The steam tables can be relied on always to provide information on the properties of steam. Heat Flow A temperature difference is necessary for heat to flow. Heat flows from highertemperature to lower-temperature. The rate of flow varies directly with temperature differences, and inversely with any resistances to this flow.! Fluid Flow Any fluid tends to flow from high-pressure to lower-pressure regions because of the effects of friction, The rate of flow varies directly with pressure differences and inversely with any resistances to this flow. Gravity acts downwards! the denser constituents in a mixture often tend to move to the bottom of a space, unless other forces acting on them oppose such motion.
Saturated Steam Super Heated Steam Condensate Flash Steam
What is steam ?Adding heat energy to water raises its temperature: some 419.04 KJ will raise 1 kg to 1000C, any further addition of heat evaporates the water. If 2257 KJ are added to each kg of water, then all the water becomes the dry gas, steam.
Specific Volume of Steam In case, if only part of this extra energy is added say, 90% - then 90% of the water evaporates and the other 10% remains liquid. The specific volume of steam at atmospheric pressure is 1.673 m3/ kg, so the mixture 1 kg steam containing 90% steam and 10% water would occupy a volume of (0.9x 1.673)+ (0.1x 0.001) = 1.5057 m3. This mixture would be described as steam with a dryness fraction of 0.9.
If the water is kept at a pressure above atmospheric, its temperature can be raised above 1000C before boiling begins. At 10 bar gauge, for example, boiling point is at about 184.10C. The extra energy needed to convert water at this pressure and temperature into steam (the enthalpy of evaporation) is now rather less at 2000.1 KJ/ kg, while the volume of 1 kg of pure steam is only 0.177 m3.
Heat & Mass Flow of Steam When steam at the saturation temperature contacts a surface at a lower temperature, and heat flows to the cooler surface, some of the steam condenses to supply the energy. The pressure and temperature of the steam remain constant after condensing, due to a sufficient supply of steam moving into the volume which had been occupied by the steam and is now condensed.
The condensate produced within the heat exchangers, as also within the steam lines, is initially at the saturation temperature and carries the same pressure. If it is discharged to a lower pressure, through a manual or automatic drain valve (steam trap) or even through a leak, it then contains more energy than water is able to hold at the lower pressure if it is to remain liquid.
If steam at the saturation temperature were to contact a surface at a higher temperature, as in some boilers, its temperature could be increased above the evaporation temperature and the steam would be described as superheated. Superheated steam is very desirable in turbines, where its use allows higher efficiencies to be reached, but it is much less satisfactory than saturated steam in heat exchangers. It behaves as a dry gas, giving up its heat content rather reluctantly as compared with saturated steam, which offers much higher heat transfer coefficients.
Condensate & Flash Steam The condensate often contains excess of energy. If the excess energy amounts to, say, 5% of the enthalpy of evaporation at the lower pressure, then 5% of the water would be evaporated. The steam released by this drop in pressure experienced by hightemperature water is usually called flash steam. Recovery and use of this low-pressure steam, released by flashing, is one of the easiest ways of improving the efficiency of steam-utilization systems.
Condensate & Flash Steam It is equally true that condensate, even if it has been released to atmospheric pressure, carries the same 419.04 KJ/kg of heat energy that any other water at the same temperature would hold. Condensate is a form of distilled water, requiring little chemical feed treatment of softening. And it already holds energy which may amount 15% of the energy which would have to be supplied to cold make-up feed water, even in relatively low-pressure systems.
Steam Generation: BoilerA boiler is an enclosed vessel that provides a means for combustion heat to be transferred into water until it becomes heated water or steam. The hot water or steam under pressure is then usable for transferring the heat to a process. When water is boiled into steam its volume increases about 1,600 times, producing a force that is almost as explosive as gunpowder. This causes the boiler to be extremely dangerous equipment that must be treated with utmost care.
Boiler Types and ClassificationsThere are virtually infinite numbers of boiler designs but generally they fit into one of two categories: Fire tube or fire in tube boilers;contain long steel tubes through which the hot gasses from a furnace pass and around which the water to be converted to steam circulates. Fire tube boilers, typically have a lower initial cost, are more fuel efficient and easier to operate, but they are limited generally to capacities of 25 tons/hr and pressures of 17.5 kg/cm2.
Boiler Types and ClassificationsWater tube or water in tube boilers are in which the the water passes through the tubes and the hot gasses passes outside the tubes.
These boilers can be of single- or multiple-drum type. These boilers can be built to any steam capacities and pressures, and have higher efficiencies than fire tube boilers.
Boiler Types and ClassificationsPackaged Boiler: The packaged boiler is so called because it comes as a complete package. Once delivered to site, it requires only the steam, water pipe work, fuel supply and electrical connections to be made for it to become Package boilers are generally of shell type with fire tube operational. design so as to achieve high heat transfer rates by both radiation and convection
Stoker Fired Boiler: Stokers are classified according to the method of feeding fuel to the furnace and by the type of grate. The main classifications are: 1. Chain-grate or traveling-grate stoker and 2. Spreader stoker
Chain-Grate or TravelingGrate Stoker Boiler
Spreader Stoker Boiler Spreader stokers utilize a combination of suspension burning and grate burning. The coal fines are burned in suspension; the larger particles fall to the grate, where they are burned in a thin, fastburning coal bed. This method of firing provides good flexibility to meet load fluctuations, since ignition is almost instantaneous when firing rate is increased. Hence, the spreader stoker is favored over other types of
Pulverized Fuel Boiler The coal is ground (pulverised) to a fine powder, so that less than 2% is +300 micro metre (m) and 70-75% is below 75 microns, for a bituminous coal. The pulverised coal is blown with part of the combustion air into the boiler plant through a series of burner nozzles. Secondary and tertiary air may also be added. Combustion takes place at temperatures from 1300-1700C, depending largely on coal grade. One of the most popular systems for firing pulverized coal is the tangential firing using four burners corner to corner to create a fireball at the center of the furnace
FBC Boiler Air & finely divided bed of solid particles (sand) are supported on a fine mesh. At a particular air velocity, a stage is reached when the individual particles are suspended in the air stream. Further, increase in velocity gives rise to bubble formation, vigorous turbulence and rapid mixing and the bed is said to be fluidized. The sand in a fluidized state is heated to the ignition temperature of the coal and the coal is injected continuously in to the bed. The coal burns rapidly, and the bed attains a uniform temperature due to effective mixing.
Advantages of Fluidised bed combustion over conventional firing systems Fuel flexibility, Reduced emission of noxious pollutants such as SOx and NOx, Compact boiler design and Higher combustion efficiency.
Efficient Steam GenerationThe various efficient steam generation opportunities in boiler system are related to; Combustion of fuel, Heat transfer, Avoidable losses, High auxiliary power consumption, Water quality and Blowdown.
Efficient Steam Generation
Examining the following factors can indicate if a boiler is being run to maximize its efficiency: 1. Stack Temperature Stack temperatures greater than 200C indicates potential for recovery of waste heat. It also indicates the scaling of heat transfer/recovery equipment and hence the urgency of taking an early shut down for water / flue side cleaning.
Efficient Steam Generation 2. Feed Water Preheating using Economiser The potential for energy saving depends on the type of boiler installed and the fuel used. For a typically older model shell boiler, with a flue gas exit temperature of 260 C, an economizer could be used to reduce it to 200 C, increasing the feed water temperature by 15 C. Increase in overall thermal efficiency would be in the order of 3%. For a modern 3-pass shell boiler firing natural gas with a flue gas exit temperature of 140 C a condensing economizer would reduce the exit temperature to 65 C increasing thermal efficiency by 5%.
Efficient Steam Generation 3. Combustion Air Preheat Combustion air preheating is an alternative to feedwater heating. In order to improve thermal efficiency by 1%, the combustion air temperature must be raised by 20 C. Most gas and oil burners used in a boiler plant are not designed for high air preheat temperatures. Modern burners can withstand much higher combustion air preheat, so it is possible to consider such units as heat exchangers in the exit flue as an alternative to an economizer, when either space or a high feed water return
Efficient Steam Generation 4. Incomplete Combustion In the case of oil and gas fired systems, CO or smoke with normal or high excess air indicates burner system problems. poor mixing of fuel and air at the burner. Poor oil fires can result from improper viscosity, worn tips, carbonization on tips and deterioration of diffusers or spinner plates.
Efficient Steam Generation 5. Excess Air Control Excess air is required in all practical cases to ensure complete combustion, to allow for the normal variations in combustion and to ensure satisfactory stack conditions for some fuels. The optimum excess air level for maximum boiler efficiency occurs when the sum of the losses due to incomplete combustion and loss due to heat in flue gases is minimum. This level varies with furnace design, type of burner, fuel and process variables. It can be determined by conducting tests with different air fuel ratios.
Relation Between CO2 and Excess Air for Fuel Oil
Relation Between Residual Oxygen and Excess Air
Efficient Steam Generation Various methods available to control the excess air are: Portable oxygen analysers and draft gauges - Excess air reduction up to 20% is feasible. Continuous oxygen analyzer with a local readout mounted draft gauge, by which the operator can adjust air flow. A further reduction of 1015% can be achieved. Damper control by continuous oxygen analyzer. This enables an operator to remotely control a number of firing systems simultaneously. The automatic fan speed control from O2 analyser feed, Its cost is really justified only for large systems.
Efficient Steam Generation 6. Radiation and Convection Heat Loss With modern boiler designs, this may represent only 1.5% on the gross calorific value at full rating, but will increase to around 6%, if the boiler operates at only 25 percent output. Repairing or augmenting insulation can reduce heat loss through boiler walls and piping. 7. Automatic Blowdown Control Uncontrolled continuous blowdown is very wasteful. Automatic blowdown controls can be installed that sense and respond to boiler water conductivity and pH. A 10% blow down in a 15 kg/cm2 boiler results in 3%
Efficient Steam Generation 8. Reduction of Scaling and Soot Losses Elevated stack temperatures may indicate excessive soot buildup or scaling on the water side. When the flue gas temperature rises about 20 C above the temperature for a newly cleaned boiler, it is time to remove the soot deposits. It is estimated that 3 mm of soot can cause an increase in fuel consumption by 2.5% due to increased flue gas temperatures. Periodic off-line cleaning of radiant furnace surfaces, boiler tube banks, economizers and air heaters may be
Efficient Steam Generation
9. Reduction of Boiler Steam Pressure This is an effective means of reducing fuel consumption, if permissible, by as much as 1 to 2%. Lower steam pressure gives a lower saturated steam temperature and without stack heat recovery, a similar reduction in the temperature of the flue gas temperature results. Pressure should be reduced in stages, and no more than a 20 percent reduction should be considered.
Efficient Steam Generation 10. Variable Speed Control for Fans, Blowers and Pumps In general, if the load characteristic of the boiler is variable, the possibility of replacing the dampers by a VSD should be evaluated. 11. Effect of Boiler Loading on Efficiency The maximum efficiency of the boiler does not occur at full load, but at about two-thirds of the full load. If the load on the boiler decreases further, efficiency also tends to decrease.
Efficient Steam Generation 12. Proper Boiler Scheduling It is usually more efficient, on the whole, to operate a fewer number of boilers at higher loads, than to operate a large number at low loads. 13. Boiler Replacement A change in a boiler can be financially attractive if the existing boiler is : old and inefficient not capable of firing cheaper substitution fuel over or under-sized for present requirements not designed for ideal loading conditions Replacement must be carefully studied.
Efficient Steam Utilization and Distribution
Energy in Fuel to Useful EnergyEnergy in Fuel Purchased Cost of Energy into Factory Heat Generation Distribution Cost of Energy To Process Utilisation in Production Process Final Utilisation Cost Heat to Product
Example of System LossThe typical steam system overall efficiency is about 35% as follows:25% 20% 5%
P R O C E S S15%
P R O35%
D U C T
Condensate Return System
Generation efficiency 80%
Distribution efficiency= 83% (including condensate return)
Utilisation efficiency 47%
Efficient Steam UtilisationAvoid steam leakages Provide dry steam for process Utilising steam at the lowest possible pressure Insulation of steam pipelines and hot process equipment Minimising barriers to heat transfer Condensate recovery Flash steam recovery Proper selection and maintenance of steam traps Proper sizing of steam and condensate piping Reducing the work to be done by steam
Avoiding Steam Leakages
Leaking Steam Pipe / ValveAudible Leak Visible Leak
Weak whistling Almost invisible steam jet
Weak hissing Visible steam jet
800 litre oil per year 800 litre oil per year
2,000 to 4,000 litre oil per year 2,000 to 4,000 litre oil per year
Provide dry steam for the processDisadvantages of wet steamLess heat content, Extended process time, Irregular heating, Barrier to heat transfer, Overloading of steam traps
Disadvantages of superheated steamPoor heat transfer coefficient, takes time to give up superheat by conduction
Benefits of dry steamHeat transfer is rapid and regular
Providing Dry Steam for ProcessUse Dry Saturated steam for processes Steam Separators to be fitted at point of steam use
Provide a little superheat to ensure dry saturated steam at the process end
Utilising steam at the lowest possible pressure2730.7 KJ/kg 2770.8 KJ/kg
Steam should always be generated and distributed at the highest possible pressure but utilised at the lowest practicable pressure
2.4 bar, 121.5oC
6.8 bar, 164.3oC
Optimal InsulationHeat loss, 89 mm black steel pipe, 90 oC
Uninsulated 320 W/m
50 mm insulation 29 W/m
100 mm insulation 19 W/m 320 - 29 = 291 W per m
50 mm insulation compared with an uninsulated pipe:
263 litre oil per year50 mm insulation compared with 100 mm insulation: 29 - 19 = 10 W per m
9 litre oil per year
...But dont Over-Insulate:There is always an optimum insulation level (1-3 years payback)
Direct Utilization of SteamDirect Steam use involves both Latent Heat and Sensible Heat Use temperature controller in Direct Use to avoid steam wastage
Minimising barriers to heat transferat e film all Sta g pro nant duc t pro duc t
Resistance to heat transfer of water is 60 70 times more than steel and 500 600 times than copper Resistance to heat transfer of Air is 1500 times more than steel and 19,000 times than copper
Sc a le
Sc a le
Effect of air and water filmSteam at 1 kg/cm2 Steam at 0.75 kg/.cm2:Air and water film reduced by 50 % ; Quicker process timefilm film all all tal w Sta g pro nant duc t pro duc t Sta g pro nant duc t pro duc t210OC
ilm Air f
e ns Co nd
Boiler Fuel Saving by Condensate ReturnSaving in percent if condensate is returned to the boiler instead of draining14 12 10 Percentage 8 saved 6 4 2 0 0 20 40 60 80 100 Condensate return temp. oC
100% returned 50 % returned
For every 6OC rise in boiler feed water temperature, there is a 1 % raise in boiler efficiency
Reducing the work to be done by steamHave shortest route of piping Remove moisture mechanically to the fullest before steam drying / avoid bone drying Optimise humidity of drier exhaust Explore process integration Use thermostatic controls Remove / blank redundant lines Productive use of machinery (Maximise equipment loading) Look for cheaper alternatives of doing the job (waste heat
boilers, thermic fluid heater etc)
Steam Piping : Featureswhile laying new pipes ,it is a compromise between aesthetic design and architects plans. Steam pipes should be laid by the shortest possible distance. Provision for proper draining of condensate. For example, a 100mm well lagged pipe of 30meter length carrying steam at 7 Kg/cm2 pressure can condense nearly 10 Kg. of water in the pipe in one hour unless it is removed from the pipe through traps. The pipes should run with a fall (slope)of not less than 12.5 mm in 3 meter in the direction of flow.
Large pockets in the pipes to enable water to collect Drain pockets should be provided at every 30 to 50 meters and at any low point in the pipe network. Expansion loops are required to take care of the expansion of pipes when they get heated up. Automatic air vents should be fixed at the dead end of steam mains, which will allow removal of air, which will tend to accumulate.
Steam Pipe Sizing and Design1. Pipe SizingProper sizing of steam pipelines help in minimizing pressure drop. The velocities for various types of steam are:Superheated Saturated Wet or Exhaust 50-70 m/sec 30-40 m/sec 20-30 m/sec
The steam piping should be sized, based on permissible velocity and the available pressure drop in the line. A higher pipe size will reduce the pressure drop and thus the energy cost. However, higher pipe size will increase the initial installation cost.
By use of smaller pipe size, even though the installation cost can be reduced, the energy cost will increase due to higher-pressure drop. Pressure drop change is inversely proportional to the 5th power of diameter change. Hence, care should be taken in selecting the optimum pipe size.
2) Pipe Redundancy 3) Drain PointsThese points help in removing water in the pipes due to condensation of steam. The presence of water causes water hammering. A steam trap must be provided at the drain points to avoid leakage of steam.
Steam average velocity (in m/s)Nominal pipe size (in mm) Below 50 50 to 150 200 & above
Saturated steam at sub-atmospheric pressure
10 - 15
15 - 20
Saturated steam at 0-1 kg/cm2.(g)
15 - 20
17 - 30
20 - 30
Saturated steam at 1.1 - 7 kg/cm2.(g)
15 - 22
20 - 33
25 - 43
Saturated steam over 7 kg/cm2.(g)
15 - 25
20 - 35
30 - 50
Superheated steam at 0 - 7 kg/cm2.(g)
20 - 30
25 - 40
30 - 50
Superheated steam at 7.1-35 kg/cm2.(g)
20 - 33
28 - 43
35 - 55
Superheated steam at 35.1 - 70 Kg/cm2.(g)
22 - 33
30 - 50
40 - 61
Superheated steam over 70 kg/cm2.(g)
22 - 35
35 - 61
50 - 76
Steam Traps?A steam trap is a valve device that discharges condensate and air from the line or piece of equipment without discharging the steam. The purpose of installing the steam traps is to obtain fast heating of the product and equipment by keeping the steam lines and equipment free of condensate, air and non-condensable gases. FunctionsTo discharge condensate as soon as it is formed Not to allow steam to escape. To be capable of discharging air and other incondensable gases
Types of Steam TrapsG roup M e c h a n ic a l tr a p P r in c ip le D if f e r e n c e in d e n s ity b e tw e e n s te a m and c o n d e n s a te . S u b -g r o u p B u c k e t ty p e O pen bucket I n v e r te d b u c k e t, w ith le v e r , w ith o u t le v e r F lo a t ty p e F lo a t w ith le v e r F r e e f lo a t D is c ty p e O r if ic e ty p e
D if f e r e n c e in th e r m o d y n a m ic p r o p e r tie s b e tw e e n s te a m a n d c o n d e n s a te T h e r m o s ta tic D if f e r e n c e in te m p e r a tu r e B im e ta llic ty p e m e ta l tr a p b e tw e e n s te a m a n d e x p a n s io n ty p e . c o n d e n s a te To discharge condensate as soon as it is formed Not to allow steam to escape. To be capable of discharging air and other incondensable gases
T h e r m o d y n a m ic tr a p
Flash SteamFlash steam available in % S1 - Sensible heat of high pressure condensate S2 - Sensible heat of steam at lower pressure (at which it is flashed) L2 - Latent heat of flash steam at lower pressure
S1 - S2 L2
Make a steam balance24 TPH Boiler Steam PRDV 12 Bar Distribution Header 8 bar 7 TPH VAPOR ABSORPTION REFRIGERATION CONTINUOUS STERILISER
4 TPH DG Set WHR Steam 2 TPH 8 Bar PRDV 3 Bar
Bleed 4 TPH Media Sterilisation Germinator 4 TPH Sterile vessels Pre fermentor 0.5 TPH
PILOT PLANT MICRO BIOLOGY LAB EXTRACTION SOLVENT RECOVERY FUEL OIL TANK FARM
Conduct Steam auditSpecific Steam Consumption300 Production (Tonnes) 250 200 150 100 50 0Sep'96 Oct'96 Nov'96 Dec'96 Jan'97 Feb'97 Mar'97 Apr'97 May'97 Jun'97 Jul'97 Aug'97
40 35 30 25 20 15 10 5 0 MonthProduction (Total PE) Steam (T) /Ton of PE
KL / Tonne of PE
ConclusionAt your plant; Ensure proper sizing of steam lines Select right type of traps Test and identify malfunctioning traps Quantify steam leakages Determine heat loss from leakages Quantify flash steam and its recovery Identify energy saving opportunities in steam distribution and utilization systems