INTRODUCTION AND LITERATURE REVIEW1 1.1 ntroduction of Condenser:IThe Work done in any heat engine is directly proportional to the temp. Range of the working fluid in the engine, OR For greater amount of expansion of working fluid, greater will be the work done. In case of steam engines, the lower limit of expansion (or exhaust) may be decreased below the atmospheric pressure. This is done by exhausting the steam into a closed vessel called condenser. In condenser exhaust steam is condensed & vacuum is maintained in it. Steam condenser is an appliance in which the steam exhausted from a steam engine is condensed. Condenser is installed such that the exhaust steam from a Steam Engine is delivered into it. In the condenser, the heat is removed from the exhausted steam in form of latent heat by means of cooling water which absorbs this latent heat. The exhaust steam after losing its latent heat changes its state into water, which is termed as Condensate and this process, is called as condensation. In effect a Condenser is a heat exchanger wherein the exhaust steam of a Steam Engine is condensed either in direct or indirect contact with cooling water through a heat transfer medium separating them. In short the purpose of the condenser in a vapor compression cycle is to accept the hot, highpressure gas from the compressor and cool it to remove first the superheat and then the latent heat, so that the refrigerant will condense back to a liquid. In addition, the liquid is usually slightly sub cooled. In nearly all cases, the cooling medium will be air or water.1.2 Condenser in power plant: When the steam has completed its work in the turbine and before it can be returned to the boiler, it is necessary to change it back into water.Figure 1.2.1: Inner view of steam condenser used in power plant This is the duty the condenser must perform as efficiently as possible and, for this reason, it2 is the largest and most important of the heat exchangers in a power station. The heat in the exhaust steam, which can no longer be converted into mechanical energy, must be transferred from the steam to the cooling water. Steam condenser is a closed space into which steam exits the turbine and is forced to give up its latent heat of vaporization. It is a necessary component of a steam power plant because of two reasons. It converts dead steam into live feed water. It lowers the cost of supply of cleaning and treating of working fluid. It is far easier to pump a liquid than a steam. It increases the efficiency of the cycle by allowing the plant to operate on largest possible temperature difference between source and sink. The steams latent heat of condensation is passed to the water flowing through the tubes of condenser. After steam condenses, the saturated water continues to transfer heat to cooling water as it falls to the bottom of the condenser called, hotwell.This is called sub cooling and certain amount is desirable. The difference between saturation temperature corresponding to condenser vacuum and temperature of condensate in hot well is called condensate depression.Figure 1.2.2: Condenser after manufacturingFig 1.2.1 shows the Inner view of steam condenser used in power plant. It is shown the path of steam comes from the turbine & shown the process how the steam converts in to the water. Fig 1.2.2 shows Condenser after manufacturing. Fig 1.2.3 shows the line diagram of condenser.3 Figure 1.2.3: Line diagram of condenser used in power plant1.3 Functions of a Condenser:By designing the turbine to exhaust into a condenser which maintains a pressure lower than atmospheric there are three important advantages to be gained. Saving in steam: o There is a big reduction in the amount of steam required to generate each unit of electricity by using a condenser. In a turbine without a condenser the lowest pressure to which the steam can be expanded is that of the atmosphere. It can be said that in this case the back pressure against which the steam is exhausted is atmospheric pressure.Atmospheric pressure is equivalent to the pressure which would support a column of mercury approximately 760 mm high. o If the last stages of the turbine were under vacuum, and the back pressure reduced by a condenser to 68 mbar, then the steam would be able to continue its expansion down to 68 mbar. During this expansion each pound of steam is capable of doing a great deal more work. For example, in a 62 bar turbine with a back pressure 51mbar, the steam does nearly 30% of its work as it expands below atmospheric pressure. Thus the use of a condenser brings considerable saving. Conservation of pure feed water: o Very large quantities of steam pass though a turbine, for example, a 660 MW machine on full load uses some 532 kg/sec. It would, of course, be not only very wasteful but almost impracticable to allow this vast amount of steam to be exhausted to atmosphere. By using a condenser the exhaust steam is converted back to hot water which is removed from the condenser for continuous use in the power station heat cycle. This water is known as condensate. It being free from impurities and non-condensable gases, does not produce4 corrosive action and also it being hot (@ 40-50 C), saves a considerable amount of fuel. Thus the overall efficiency of the plant is increased. De aeration of make-up water: o Due to leakage, blowing down, steam soot blowing, etc., some of the feed water in the boiler/turbine system is lost, and must be made-up. o Make-up water is usually supplied from Reserve Feed Water (RFW) tanks, where the air in contact with the water surface introduces oxygen into the RFW. Dissolved oxygen in feed water must be kept to the lowest practicable minimum, since oxygen causes corrosion of tubes and turbine internals. o The most convenient method of removing oxygen form the make-up water is to admit this water to the condenser, where due to the very low pressure and corresponding low boiling point, the water flashes off to steam, and dissolved oxygen separates off to be removed with other air and gases by the air-ejectors. 1.3.1 Based on above discussion here are the some advantages of condenser in power plant: o Improved work done & efficiency due to low pressure of condenser. o Recovery of the condensate to be fed to the boiler as a high quality feed water for reuse. o Reduce steam consumption for the same power output due to increased work done. o Economy in water softening plant as only make up water is to be treated instead of full feed water. o Reduced thermal stresses due to high temp. of feed water entering to boiler.1.4 About condensing plant:Figure1.4.1: Main elements of condensing unit5 1.4.1 Elements of a condensing plant:The auxiliaries, which are essentially required for the proper functioning of a condensing plant, are known as elements of a condensing plant. Figure below shows Condensing plant lay-out. The main elements of a condenser are: Condenser where steam is condensed, Hot well in which the condensate is collected. Pump for circulating Cooling Water,supply of Circulating Cooling Water, Air pump (or ejector) to remove air and non-condensing gases from the condenserFigure 1.4.2: Elements of a condensing plantCondensate Pump (extraction pump) to remove Condensate from the condenser, Feed Pump for supplying feed water from De aerator to the boiler. 1.4.2 Features of good condensing plant: The desirable features of a good condenser are: Minimum quantity of circulating cooling water. Minimum cooling surface area per kW capacity. Minimum auxiliary power. Minimum steam condensed per M2 of surface area As the cooling water temperature entering the condenser increases vacuum decreases. The following figure shows this:6 Figure 1.4.3: Features of a good condenserHeat to be removed: The heat to be removed in the condenser is shown in the ph diagram and, apart from comparatively small heat losses and gains through the circuit will be Heat taken in by evaporator heat of compression. This latter, again ignoring small heat gains and losses will be the power input to the compressor, giving Evaporator load compressor input power condenser load. Condenser load is stated as the rate of heat rejection. Some manufacturers give ratings in terms of the evaporator load, together with a de-rating factor, which depends on the evaporating and condensing temperatures. Evaporator load factor condenser load the provision of a separate oil cooler will reduce condenser load by the amount of heat lost to the oil and removed in the oil cooler. This is of special note with oil-injected screw compressors, where a high proportion of the compressor energy is taken away in the oil. This proportion varies with the exact method of oil cooling, and figures should be obtained from the compressor manufacturer for a particular application.Here in the figure the condenser load p-h diagram is shown. From the figure it is easily understand.7 Figure 1.4.4: Condenser load p h diagram1.5 Operation of condenser:Before starting up, the system should be checked for correct piping and proper control and safety devices. The following procedure is generally applicable to all types of condensers: Start the moderate temperature fluid and operate vents to remove trapped air. Gradually start the extreme temperature fluid, allowing the exchanger to adjust its temperature slowly. Operate the vents. When shutting down, stop the extreme temperature fluid first, and then operate the vents. Avoid thermal shock due to rapidly altering the flow of either stream, unless the unit is specifically designed to withstand this type of service. Be sure all condensate is drained from the steam units before admitting steam to avoid water hammer. When the exchanger has reached operating conditions, check all bolted connections for tightness. Record operating pressures and temperature for future reference. Loss of capacity and increase of pressure drop will indicated the progress of fouling.8 Internal leaks will be indicated by contamination of the low pressure stream. External leaks can usually be seen or heard. To locate leaks, follow the procedure for testing a repaired exchanger.1.6 Condenser maintenance:o As with any mechanical equipment, condensers should never be located where they are difficult of access, since there will then be less chance of routine maintenance being carried out. Periodic maintenance of a condenser is limited to attention to the moving parts fans, motors, belts, pumps and cleaning of water filters, if fitted. o The overall performance will be monitored from the plant running log and the heat exchange surfaces must be kept clean for maximum efficiency meaning the lowest head pressure and lowest power. o Air-cooled surfaces may be cleaned by brushing off the accumulation of dust and fluff where the air enters the coil, by the combination of a high-pressure air hose and a vacuum cleaner, or, with the obvious precautions, by a water hose. o Scale within the tubes of a straight double pipe or shell-and-tube condenser can be mechanically removed with suitable wire brushes or high-pressure water lances, once the end covers have been removed. Tubes which cannot be dealt with in this way must be chemically cleaned. o It will be appreciated that, where air and water are present, as in a water cooling tower or evaporative condenser, the apparatus will act as an air washer, removing much of the dust from the air passing through it. o Such dirt may be caught in a fine water filter, but is more commonly allowed to settle into the bottom of the tank and must be flushed out once or twice a year, depending on the severity of local contamination. o Where heavy contamination is expected, it is good practice to provide a deeper tank than usual, the pump suction coming out well clear of the bottom, and tanks 3 m deep are in use. Where plant security is vital, the tank is divided into two parts, which may be cleaned alternately.1.7 Repairs of condenser:Cleaning: o The method of cleaning will be indicated by the composition of the dirt or scale, its location and the type of exchanger. Each cleaning job must be considered individually with the help of cleaning tool manufactures and chemical cleaning contractors. o Generally, the tube side of exchangers can be cleaned mechanically, except for u-tubes where the small radius inside bends is usually not cleanable. Removable bundles with tubes9 on square pitch can be cleaned mechanically on the shell side. Removable bundles with triangular pitch can sometimes be cleaned by jetting but generally these and non-removable bundles must be cleaned chemically on the shell side. o Mechanical cleaning tools must be applied carefully to avoid cutting or otherwise damaging the tubes. Sharp, hard cleaning tools should be avoided. Chemical cleaning solutions must be used with care and completely flushed out to avoid corrosion damage. Never use cleaning solutions containing hydrochloric acid on galvanized exchangers. Tube Bundle Removal: o Before attempting to dismantle a heat exchanger, be sure internal pressure is relieved. Remote valves should be locked closed. o Remove channels or cast iron heads. Small bundles can usually be pulled out manually after starting with jackscrews or a pry. When using a pry, be careful not to damage gasket faces. o Large bundles may be pulled with a chain fall. o On single tube sheet exchangers, the bundle may be pried out using a strong-back making sure that the strong back includes enough tube sheet bolt holes to prevent bending of the tube sheet. o Use wide slings or cradles when handling the bundle to prevent damage to individual tubes. Tube Replacement: o Where a small portion of the bundle is concerned, it may prove more practical to plug defective tubes than to replace them. The following procedure is generally acceptable for average exchangers of ordinary materials. o For larger units and stronger materials, better tooling will probably be justified and may be necessary. In these cases, follow the recommendations of the tool manufacture. o Face off the tube ends flush with the tube sheet with a cut-off tool. o Loosen one end of the tube with a knock-out tool, and then drive the tube out from the other end. Some judgment is required here and several tries may be necessary before the tube can be driven out. If an inner tube of a U-tube bundle is to be removed it will be necessary to remove some outer tubes to gain access to the faulty one. o Clean the hole of all tube material and dirt. o Insert the new tube so that it projects 1 / 16 inch beyond the face of the tube sheet and roll. The rolling is a skilled job and must be done carefully. Lubricate the roller well and keep it clean. Do not over expand the tubes. If the maintenance department is not experienced in tube rolling, it will probably be more satisfactory to return the bundle to the manufacturer for repairs.10 Reassembly and Testing: o Clean all gasket faces and use new gaskets of the same material, thickness and dimensions as originally supplied. Pull up bolts only enough to produce a tight seal, alternating between diametrically opposed bolts to keep flange faces parallel. Do not overstress bolts or flanges. o When replacing bolts, be sure to use the same alloy and grade as originally supplied. Heat exchangers are usually tested for tightness with water. Do not pressurize an exchanger with gas until it has been tested up to the gas pressure with a liquid. Do not exceed the test pressure stamped on the nameplate of the exchanger. o To test tube joints, fill and pressurize the shell side with the channels removed to expose the tube ends. Have the shell side pressure held at the test pressure stamped for tubes and hold for several minutes. Leaking tube joints can be rolled lightly. Do not over-roll. o Replace the channels and pressurize both shell and tube side to test flange joints. o Be careful to protect instruments and fixtures which are not designed for the test pressure. Routine Inspection: Exchangers should be painted and inspected periodically for corrosion. Check for corrosion, particularly around connections and under insulation. Temperature Shocks: Exchangers normally should not be subjected to abrupt temperature fluctuations. Hot fluid must not be suddenly introduced when the unit is cold, nor cold fluid suddenly introduced when the unit is hot. Bolted Joints: Heat exchangers are pressure tested before leaving the manufacturer's shop in accordance with ASME Code requirements. However, normal relaxing of the gasketed joints may occur in the interval between testing in the manufacturer's shop and installation at the jobsite. Therefore, all external bolted joints may require retightening after installation and, if necessary, after the exchanger has reached operating temperature.1.8 Condenser fittings:o The inlet pipe bringing high-pressure gas from the compressor must enter at the top of the condenser, and adjacent piping should slope in the direction of flow so that oil droplets and any liquid refrigerant which may form will continue in the right direction and not back to the compressor. o The outlet pipe must always be from the lowest point, but may have a short internal up stand so that any dirt such as pipe scale or metal swarf will be trapped and not taken around the circuit. Condensers for ammonia systems may have an oil trap, usually in the form of a drain pot, and the liquid outlet will be above this.11 o Water connections to a shell-and-tube condenser must always be arranged so that the end covers can easily be removed for inspection, cleaning, and repair of the tubes. o Heavy end covers require the use of lifting tackle, and supports above the lifting points should be provided on installation to facilitate this work. o Condensers contain pressurized refrigerant and where they exceed certain volumes they will be subject to the requirements of the Pressure Vessel Directive (PED) and EN378. Manufacturers will be aware of these requirements, and proprietary products will be correctly equipped.1.9 Common Condenser Problems:Figure 1.9.1: Different parts of condenserSince 1994, Intek has been the leading source of new information on condenser dynamics, which has been made possible with the RheoVac instrument. The RheoVac instrument measures the mixture of water vapor and no condensable gases in the vent line. With this information and other plant measurable, Intek developed a Comprehensive Condenser Model and Theory (CCMT) based on physical principles. The CCMT has been validated under a variety of different operating conditions, and has been used to identify deficiencies in condenser design and problems with condenser operations and maintenance. Intek has established itself as a leader in solving condenser related problems and improving condenser performance. Some of them are given below; High Condenser Pressure Corrosion12 Dissolved Gases (Oxygen, Carbon Dioxide, Ammonia, etc.) Low Cleanliness Factor Low Heat Transfer Coefficient Air In-Leakage Tube Fouling Low Pump/Exhaust Capacity Other Efficiency/Maintenance/Design/Operations Concerns High Condenser Pressure:1.9.1High condenser back pressure is the most obvious plant measurable that results in lost revenue or excess operating costs. Simply, back pressure is directly related to the power output from the turbines and excess back pressure means reduced efficiency and dollars wasted. How excess back pressure affects the bottom line: Revenue and profit loss can be significant. As shown in the charts below, even a small excess back pressure will have a dramatic impact. These results are based on a PEPSE analysis of a 525MW generating unit which shows that each 0.1HgA rise in back pressure results in an increase of 0.17% in heat rate. In summary, a 0.3 HgA of excess back pressure, on a base loaded plant condenser, correlates to a loss of 2.68MW of power and nearly $770,000/year lost revenue. Corrosion/Dissolved Gases: Dissolved Oxygen (DO) and other gases are a major cause of corrosion in the steam cycle. Corrosion leads to forced outages and increased maintenance costs. A common misconception is that high DO is concurrent with high air in-leak; this is not always the case. High DO and condensate ammonia concurrent with low air in-leakage is an indication that the condenser configuration may be inadequate. Low Cleanliness Factor/Low Heat Transfer Coefficient: Cleanliness Factor (CF) is calculated by measuring the actual heat transfer coefficient as a percent of the associated HEI specified design heat transfer coefficient. Low CF measurement could be the result of one or many problems occurring in a plant, and indicates a lower than desired power generation efficiency; thus, dollars wasted. Air In-Leakage: Multiple pathways for air to leak into the steam path are inherent to the sub-atmospheric side of steam turbine power plants. Air in the steam path, along with deficiencies in condenser configuration, are major causes for a number of plant related problems such as excess back pressure, dissolved oxygen, corrosion, and low cleanliness factor. Quantifying this air in-leakage is essential for maintaining plant operations.13 RheoVac instruments are the solution to measuring air in-leakage. The RheoVac instrument succeeds where all other air flow monitors fail, because it is the only instrument that is based on the direct measurement of vent line gas parameters to accurately calculate air in-leakage. Additionally, RheoVac instruments provide plant operators with other essential data to respond to common plant maintenance issues, such as monitoring exhaust pump capacity, real-time verification of leak repairs, vacuum quality, and tube fouling. 1.9.2 Tube Fouling:Tube fouling occurs when biologic growth or material deposits obstruct the cooling circulating water flow through condenser tubes. Tube fouling manifests itself in many plant measurable. Without direct measurement of individual tube flow rates, however, other problems such as poor condenser configuration can lead to false presumption of tube fouling. Intek offers instrumentation for monitoring individual tube fouling.Figure1.9.2: Performance Loss Due to Scaling & FoulingFouling factors are best determined from experience with similar units in the same or similar service. When such information is not available, recourse may be had to publish data. The most comprehensive tabulation of fouling factors is the one developed by TEMA, which is available in Refs. The values given below in Table are representative of the data available in the public domain. It should be realized that there are very significant uncertainties associated with all such data. Fouling is a complex process that can be influenced by many variables that are not specifically accounted for in these tabulations. Fouling can occur by a number of mechanisms operating either alone or in combination. These include:14 1.9.3Corrosion:Corrosion products such as rust can gradually build up on tube walls, resulting in reduced heat transmission and eventual tube failure. This type of fouling can be minimized or eliminated by the proper choice of corrosion-resistant materials of construction in the design process. Why does corrosion occur? o The tubes, the tube sheets and the water boxes are all made up of materials having different compositions. These materials are always in contact with circulating water. The circulating water, depending on its chemical composition will act as an electrolyte between the tubes and water boxes. This will give rise to electrolytic corrosion which will start from more anodic materials first. The condenser tubes being the lowest in series of anodic materials will be the first to be effected. This makes the condenser tube ends to get eaten away first. o Sea water based condensers, in particular when sea water has added chemicals pollutants, has the worst corrosion characteristics. River water with pollutants also is not desirable for condenser cooling water. o However due to large quantity of water flow requirement for large condensers, the corrosive effect of sea water or river water has to be tolerated and remedial methods have to be adopted in practice.The concentration of un dissolved gases is high over air zone tubes. Therefore these tubes are exposed to higher corrosion rates. Sometimes these tubes are affected by stress corrosion cracking, if originally stress is not fully relieved during manufacture. o To overcome these effects of corrosion some manufacturers provide higher corrosive resistant tubes in this area. Effects of corrosion on condenser As the tube ends get corroded there is the possibility of cooling water leakage to the steam side contaminating the condensed steam or condensate, which is harmful to steam generators. The other parts of water boxes may also get affected in the long run requiring repairs or replacements involving long duration shut downs. Crystallization Crystallization typically occurs with cooling water streams containing dissolved sulfates and carbonates. Since the solubility of these salts decreases with increasing temperature, they tend to precipitate on heat-transfer surfaces when the water is heated, forming scale. This type of fouling can be minimized by restricting the outlet water temperature to a maximum of 110-125F. Decomposition Some organic compounds may decompose when they are heated or come in contact with hot surface, forming carbonaceous deposits such as coke and tar. In cracking furnaces, partial15 decomposition of the hydrocarbon feedstock is the objective and coke formation is an undesired but unavoidable result. Polymerization Polymerization reactions can be initiated when certain unsaturated organic compounds are heated or come in contact with a hot metal tube wall. The resulting reaction products can form a very tough plastic-like layer that can be extremely difficult to remove from heat-transfer surfaces. Sedimentation Sedimentation fouling results from the deposition of suspended solids entrained in many process streams such as cooling water and flue gases. High fluid velocities tend to minimize the accumulation of deposits on heat-transfer surfaces. 1.9.4oBiological activity:Biological fouling is most commonly caused by micro-organisms, although macroscopic marine organisms can sometimes cause problems as well. Cooling water and some other process streams may contain algae or bacteria that can attach and grow on heat-transfer surfaces, forming slimes that are very poor heat conductors. Metabolic products of these organisms can also cause corrosion of metal surfaces. Biocides and copper-nickel alloy tubing can be used to inhibit the growth of micro-organisms and mitigate this type of fouling.o It can be seen from above Table that the range of values of fouling factors spans more than an order of magnitude. For very clean streams, values of 0.0005 h*ft*f/BTU. Or less are appropriate, whereas very dirty streams require values of 0.005-0.01 h*ft*f/BTU. However, values in the range 0.001- 0.003 h*ft*f/BTU are appropriate for the majority of cases.Figure 1.9.3: The fouling effectsA heat exchanger in a steam power station contaminated with macro fouling. Fouling occurs16 when a fluid goes through the heat exchanger, and the impurities in the fluid precipitate onto the surface of the tubes. Low Pump/Exhaust Capacity Exhaust pump capacity must be maintained to ensure proper air removal from the condenser. Insufficient air removal can lead to increased back pressure, high dissolved oxygen, and low cleanliness factor. RheoVac instruments provide an accurate and real time measurement of pump capacity. Other Efficiency/Maintenance/Design Concerns For decades, Intek has solved flow and engineering problems for many customers. Intecs success is based on a fundamental understanding of physics and engineering backed by a team of knowledgeable and experienced science and engineering professionals. Variation of Steam Partial Pressure & Saturation TemperatureFigure 1.9.4: Variation of Steam Partial Pressure & Saturation TemperatureCondensate Depression The temperature of condensate is always a few degrees lower than the coincident condensing steam temperature. Sub cooling of condensate is undesirable on two accounts It lowers the thermodynamic efficiency of the power cycle.17 It enhances the propensity of the condensate to reabsorb non-condensable. Power Loss Due to Excess Back Pressure:Figure 1.9.5: Power Loss Due to Excess Back Pressure18 SURFACE CONDENSER HISTORY19 2.1 Classification of condenser:Broadly, condensers are classified in to two categories. Direct contact type, where the cooling water and steam directly meet and come out as a single stream. o They are classified in to three categories. Spray condenser Barometric condenser Jet condenser Surface condensers (indirect contact type) where there is no mixing of cooling water and steam. It is shell and tube type heat exchanger. The heat released upon condensation is transferred to circulating cooling water through the walls of the tube. o They are classified in to five categories. Down flow surface condensers. Central flow surface condensers. Inverted type surface condensers. Regenerative surface condensers. Evaporative condensers Our main intension is to design the surface condenser so directly we go to the surface condenser description. 2.2 Surface condensers: It is also known as Non-mixing type of condenser as in this type of condenser the exhaust steam and the cooling water do not come in direct contact with each other. Surface condenser is a shell & tubes type of heat exchanger where normally cooling water is on the tube side & exhaust steam is on the shell side. It is generally used where large quantity of inferior quality water is available & condensate formed from exhaust steam is to be recalculated to be used in boiler. This results in considerable saving in make-up water to boiler. If we think thermodynamically, in surface condenser, the latent heat of exhaust steam is being removed there by converting it into hot condensate. The surface condenser requires three (or two) pumps -- one for circulating cooling water one for extracting the condenser & the third one is for removing air from condenser. In case of third pump, we can use steam ejector. The surface condensers may be classified according to No. of water passes: single or multi pass. Direction of condensate flow and tube and tube arrangement.20 In all surface condensers the cooling water is passed through the tubes & steam surrounds the tube. the volume occupied by these tubes in the condenser shell is hardly 10% of the total shell volume due to very large volume of exhaust steam. A good surface condenser should have a low-pressure drop, maximum effective surface arrangement and should be economical in first cost. In modern condensers, a steam bypass line is provided along the side of the shell to pass steam to the hot well for reheating the condensate; this also helps for de aeration of condensate before use in boiler. The rise in temperature of the cooling water passing through the condenser is maximum 10 degree C therefore, quantity of water & surface are condenser causing the heat flow required are large for calculation purpose, we can write, Heat given away by steam = Heat gain by cooling water Following are description for the types of surface condensers: 2.2.1 Down flow type: Exhaust steam is admitted to the top of the condenser which is tube-and-shell type cross flow heat exchanger. Cooling water flows through the tubes and extracts heat from the steam which is on the shell-side.Figure 2.2.1: down flow type surface condenser.After having condensed on the surface of the water tubes, steam is converted into condensate which is discharged from the condenser bottom. 2.2.2 Central flow type surface condenser: It is also a shell-and-tube type cross flow heat exchanger, at the centre of which is located the suction of an air extraction pump so that the entire steam moves radially inward and21 Figure 2.2.2: Central flow type surface condensercomes in better contact with the outer surface of the best of tubes through which the cooling water flows. The steam condensate is extracted from the bottom by the condensate-extraction pump. 2.2.3 Inverted type surface condenser: Steam is admitted at the bottom and flows upward in cross-flow with the cooling water flowing in the tubes. The air extraction pump draws its suction from the top of the condenser, maintaining a steady upward current of steam which after having been condensed on the outer surface of the water tubes is removed by the condensate extraction pump. 2.2.4 Evaporative condenser: Exhaust steam is condensed inside the finned tubes as cooling water rains down from the to through the nozzles. Apart of the cooling water in contact with the tube surface evaporatesFigure 2.2.3: Evaporative condenserby drawing enthalpy from the steam which upon losing its latent heat condenses and discharges out as condensate.22 DESIGN CONCEPT23 3.1 Design aspects of surface condenser: The calculation of heat transfer for determining the tubes and total surface area required by a surface condenser are rather complex. They required the knowledge of the total heat load on the condenser, the heat transfer mechanism and co efficient in various parts of condenser. When the condenser is new, the outside surfaces are usually clean but quickly developed an oily film that changes condensation from drop wise to film wise condensation. Thus the shell side heat transfer co efficient is conservatively based on the lower film wise condensation mechanism. The shell side heat transfer co efficient depends upon the difference between the steam saturation temperature and tube well temperature, the relative position of the tubes steam velocity and turbulence, the extant of non condensable, and the existence of superheat steam if any. The circulating water side heat transfer co efficient depends upon its velocity, temperature and cleanliness of the inside surface. Owing to the large number of variables with many uncertainties, manufacturers have usually based their design in general proposed by the Heat Exchanger Institutes Standards for Steam Surface Condenser. The method is based on the usual heat transfer equation for the heat exchanger as given below-Q = U o Ao TmWhere,Q = heat load on condenser (W)Uo = over all condenser heat transfer co efficient based on outside tube area ( W Ao = total outside tube surface area ( m 2 ) Tm = log mean temperature difference in condenser ( oC )m2 K)Tm is expressed by below given equation,Tm =Ti Te T ln i Te Ti , Te is define below,Where,Ti =difference between saturation stem temperature & inlet circulating water ( oC )Te = difference between saturation stem temperature & outlet circulating water ( oC )24 3.1.1 Heat Exchanger Institutes (HEI) method: The overall heat transfer co efficient ( Uo ) is expressed empirically byU o = C1C2C3C4 CwWhere,Cw = circulating cold water velocity in tubes inlet ( m ) sC1 = dimensionless factor depending upon the tube outer diameterC 2 = dimensionless correction factor for circulating water inlet temperature C3 = dimensionless correction factor for tube material and gauge C 4 = values of these factors are given in tableFigure 3.1.1: Temperature distribution in a condenser25 Table 3.1: Standard factors ( C1C2C3C4 ) valuesTube outer 19 diameter (mm)2225 .4CwWater temperature ( oC )2777270525821.66 4.44 7.22 1012.77 15.55 21.11 26.66 32.22 37.7 - - -C2Tube material0.57 0.64 0.72 0.79 0.86 304 Stainless steel Admiralty arsenic copper0.921.001.041.081.10 - - 70-30 -Aluminum brass, Aluminum muntz metalbronze 90-10 Cu-Ni Cu-NiC3 C3 C3C40.58 0.56 0.54 0.851.00 0.98 0.960.96 0.94 0.910.90 0.87 0.890.83 0.80 0.76-for clean tubes, less for algae covered or sludge tubes3.1.2 Conventional method: In conventional method, the usual head transfer equations are used to calculate Uo . For film wise condensation the average heat transfer coefficient on steam side for a horizontal tube is given by Nusselt;( hav )ssWhere, k 3 2 gh fg 4 f f = ho = 0.725 N f d o 1N =no of horizontal tubes in vertical tier = Tsat Twallh fg = latent heat of condensation of steam f = viscosity of condensate (fluid)26 f = density of condensate (fluid)k f = thermal conductivity of condensate (fluid)do = outside tube diameterIt should be noted that Nusselts equation for ho gives a conservative value for the condensing film coefficient for heat transfer. However this value also be influenced by many factors such as super heat, vapor velocity, turbulence and the inside released gases and air leaked. The inside heat transfer co efficient on the water side is given by Mc Adams equationNu =Where,hi di = 0.023Re0.8 Pr 0.4 kRe =Reynolds number due to flow of circulating water through tubes=Cd iPr = Prandtle number=Cp kThe overall heat transfer co efficient for a condenser is given byx 1 1 1 1 = + + wall + U o Ao hi Ai hscale Ai kwall Aim ho AoWhere,hscale = heat transfer co efficient of scale formed kwall = thermal conductivity of wall Aim = mean inside area including scale formed xscale = wall thicknessFor simplicity, the tube wall resistance due to thin tube and good thermal conductivity may be neglected. Hence,1 1 1 1 = + + U o hi hscale ho27 It is to be noted that ho is much larger then0.8 hi Cwhiand U o mainly depends on water velocity as1 1 = A + B 0.8 Uo CwWhere,A=1 1 1 B= + & k f r0.4 ho hscale 0.023 0.2 0.8 di v foThe rate of heat transfer from the condensing vapor to the cooling water is expressed asQ = m ( hsteam hcondensate ) so= m C pc Tc2 Tc1 c = U o Ao TmWhere,o()ms = mass flow rate of steam entering to condenseromc = mass flow rate of coolantTc1 & Tc2 = inlet and outlet temperatureGenerally Tc2 Tc1o() is around10 C , Tciis around 11 to 17oC and Te =TTD should notbe less than 3 C . In practice, due to losses in condenser, the under cooling of the condensate(Tsatu Tcondensate ) is around 4 oCoNow the mass flow rate of coolant, i.e. water (mc ) is given bymc =oms ( hsteam hcondensate ) c pc Tc Tc228 o(1)The outside surface area Ao is thus calculated from above equationsAo =ms ( hosteam hcondensate )U o Tmn d ol n 2 mc = d i wC w 4 Where,w = density of water ( 103 Kgm3)Cw = velocity of water which varies from 1.8 to 2.5 m sTherefore, the length and number of tubes can be calculated from above equations. Generally tube length and diameter are selected so the estimation is made for number of tubes. In the case of large power plant, the number of tubes may be high as 50000 or even more.The pressure drop in the condenser is composed of the pressure drop in the water box and the friction pressure drop in the tubes. The pressure drop in terms of head is Thermal Processes Occurring in Condensers The condenser never receives pure seam from the turbine. A mixture of steam and non-condensable gases (Air-steam mixture) enters the condenser. The ratio of the quantity of gas that enters the condenser to the quantity of steam is called the P = gHrelative air content.=m air mc , sThe value of e depends on type, capacity, load and design dimensions of the condenser plant.Variation of Steam-air Mixture Parameters:mc , s + m airpc = psteam + pairpc psteamTsatFigure 3.1.2: Variation of Steam-air Mixture Parameters29 Using Dalons Law:pc = ps + pa Gas laws:pa v a = m a RaTa & ps v s = m s RsTs Volumes and temperatures are same: pa Ra m a = ps Rs m s = 0.622 ps =pc 1 + 0.622At the entry to condenser the relative content of air is very low and partial pressure of steam is almost equal to condenser pressure. As air-steam mixture moves in the condenser, steam is condensed and the relative content of air increases. Accordingly, the partial pressure of steam drops down. The pressure in the bottom portion of condenser is lower than that of the top portion. The pressure drop from inlet to exit of condenser is called steam exhaust resistance of a condenser.pc = pc pce The partial pressure of air at the bottom of the condenser cannot be neglected. The temperature of steam is a function of condenser pressure. This is due to increase in relative content of air in the mixture. The pressure also decreases due to resistance to flow of steam.30 pc psteamTsat ' TcpseTspe aTc = TseFigure 3.1.3: Variation of Steam-air Mixture ParametersCooling Water Outlet Temperature Calculation: The outlet temperature for the fluid flowing through the tube isTcw,out =& msteam ( h ) + Tcw,in & mcwc p ,cwThe surface area of the heat exchanger for the fouled condition is:Condensate Loading& & Qtransfer = AsurfaceU f F [ LMTD ] = mhThis can be used to calculate a Reynolds numberMass flor of condensate Perimeter & m = condensate for vertical tubes. d0 & m = condensate for horizontal tubes. Ltube=Recondensation = 4 filmFlow is considered laminar if this Reynolds number is less than 1800. The driving force for condensation is the temperature difference between the cold wall surface and the bulk temperature of the saturated vaporTdriving = Tsat Twall Tvapour Tsurface31 The viscosity and most other properties used in the condensing correlations are evaluated at the film temperature, a weighted mean of the cold surface (wall) temperature and the (hot) vapor saturation temperatureT film = Tsat Wall Temperatures 3T 3 (Tsaturation Twall ) = Tsat driving 4 4It is often necessary to calculate the wall temperature by an iterative approach. The summarized procedure is: Assume a film temperature, TfEvaluate the fluid properties (viscosity, density, etc.) at this temperature Use the properties to calculate a condensing heat transfer coefficient (using the correlations to be presented) Calculate the wall temperature. The relationship will typically be something likeTwall 1 UA = Tsat (Tsat Tcoolant ) 1h A o oUse the wall temperature to calculate a film temperature Compare the calculated film temperature to that from the initial step. If not equal, reevaluate the properties and repeat. Laminar Flow outside Vertical Tubes If condensation is occurring on the outside surface of vertical tubes, with a condensate loading such that the condensate Reynolds Number is less than 1800, the recommended correlation is: k 3 f ( f v ) g 3 f hcond 2 f Since the vapor density is usually much smaller than that of the condensate film, some11.47 = 3 Re condensationauthors neglect it and use the film density squared in the denominator. The presence of ripples (slight turbulence) improves heat transfer, so some authors advocate increasing the value of the coefficient by about 20%. Another form of writing h is:hcond k 3 f ( f v ) g f = 0.925 f 13This may also be compensated for rippling (0.925*1.2=1.11). Turbulent Flow outside Vertical Tubes:32 When the condensate Reynolds Number is greater than 1800, the recommended correlation is:hcond k 3 f ( f v ) g 0.4 f = 0.0076 Re 2 f 13Laminar Flow outside Horizontal Tubes: When vapor condenses on the surface of horizontal tubes, the flow is almost always laminar. The flow path is too short for turbulence to develop. Again, there are two forms of the same relationship:hcond1.51 = 3 Re condensation k 3 f ( f v ) g f 2 f 113 k 3 f ( f v ) gh fg 4 f hcond = 0.725 f Tdriving d 0 The constant in the second form varies from 0.725 to 0.729. The rippling condition (add 20%) is suggested for condensate Reynolds Numbers greater than 40. Condenser tubes are typically arranged in banks, so that the condensate which falls off one tube will typically fall onto a tube below. The bottom tubes in a stack thus have thicker liquid films and consequently poorer heat transfer. The correlation is adjusted by a factor for the number of tubes, becoming for the nth tube in the stack.hcond k 3 f ( f v ) gh fg f = 0.725 N f Tdriving d 0 14=htop4NSplashing of the falling fluid further reduces heat transfer, so some authors recommend a different adjustment. Performance of Condensers: The following needs to be computed: Condenser heat load = Q x T x Cp33 Table 3.2: Parameter Details for Performance of CondensersParameter Q T CpDetails Water flow rate Average CW temperature rise Specific heatUnit Kg/hoCokcal/kgCCalculated condenser vacuum =Atmospheric pressure Condenser back-pressure Deviation in condenser vacuum =Expected condenser vacuum - Measured condenser vacuu Condenser TTD = Saturation temperature Cooling water outlet temperatureCondenser Effectiveness =Rise incooling water temperature Saturation temperature Cooling water inlet temperatureCondenser heat duty in kcal/h =Heat added by main steam + heat added by reheater + heat added by SH attemperation + heat added by RH attemperation + heat added by BFP - 860 x (Pgen + Pgen losses + heat loss due to radiation)Condenser tube velocity (m/s) =*106 hr 3600 x tube area ( mm 2 ) * ( no. of tubes per pass no. of tubes plugged per pass3 Cooling water flow rate m())Determination of actual LMTDLMTD =Tout Tin T T ln sat in Tsat ToutLMTD expected = LMTD test xftx fw xfqft =Correction for cooling water inlet temperature SaturationTemperature during test LMTD during test ft = SaturationTemperature design LMTD design 0.25f w =correction for water flow rate34 Tube velocityduring test fw = Tube velocitydesign 0.50f q =correction for cooling water heat load Condenser designduty fq = Condenser dutyduring test Observations during Condenser Energy Audit Tubes in operation Vs total installed. Cleaning system operation. Filtering system for cooling water. Regular monitoring system for performance. Comparison of LMTD, TTD, heat load, condenser vacuum, flow, temperatures, pressures. with design / PG test- arriving the factors causing deviation. Modifications carried out in the recent past. Cooling water flow. Pressure drop on water side and choking. Affect of present performance of cooling tower. Accurate metering of vacuum. Absolute back pressure deviation from expected value.35 PROBLEM FORMULATION36 4.1 Data for designing the surface condenser for 60 MW capacity power plant:Table 4.1: Input design data for condenserSteam inlet temp. Steam outlet temp. Pressure within condenser Mass flow rate of steam Mass flow rate of water Cooling water inlet temp. No. of pass Inner and outer dia. Of tube Velocity of water180 C to 30C 55C to 90C 0.1 bar 30 tone 165.5 tone 25C to 40C 4 as well as 2 10 mm & 12 mm 1 m/sFrom above given data we have to design the surface condenser for steam power plant and we have to find; Out let temperature of cooling water Numbers of tubes Tube length Condenser area Over all heat transfer co efficient Mass flow rate of condensation Condenser efficiency37 NOMENCLATURE:2 A : Area, mC p : Specific heat,Kj Kg KDi : Inner diameter, mDo : Outer diameter, mK : Thermal conductivity,L : Length, mWmKM : Mass flow rate, p : Pressure, barKg sQ : Heat transfer rate per unit time, wattT : Temperature, oCU : Over all heat transfer coefficient, W 2 m K LMTD : log mine temperature, oC W H i : Heat transfer rate at inner surface, m 2 KH o : Heat transfer rate at outer surface, m 2 KN : No. of tubesWTw1 : Inlet temperature of cooling water, oCTw2 : Outlet temperature of cooling water, oCTs1 : Inlet temperature of steam, oC Ts2 : Outlet temperature of steam oCThese nomenclatures are used when we designing the steam condenser which we are used in power plant.,38 DESIGN OF CONDENSER39 5.1 Steps taken under consideration when designing the condenser:Effective steam condenser design of necessity must take many performance variables into consideration. Some of them are given below:The configuration of the exhaust flow pattern entering the condenser. The moisture content of incoming steam. The design of the transition piece to achieve the most uniform distribution of incoming steam over the tube bundle. Shell design parameters that provide maximum supports to the tube bundle without impacting pressure drop. De-aeration and reheat requirements for the steam condensate. Tube sheet design consideration that will enhance the steam flow over and through the tube bundle to achieve maximum performance efficiencies. Plus many more design variables that impact condenser performance.5.2 Design calculation for condenser:Mass of steam:-ms = 30 tone 30 103 3600 = 8.33 Kg =hrssHeat lost by steam = Heat gain by water ms C ps Ts1 Ts2 = mwC pw Tw1 Tw2()()( )30, 000 2.1 (150 40 ) = 46 4.186 Tw2 30 3600 (Answer)Tw2 = 39.99 oCRate of heat transfer is given by; Q = mwC pw Tw2 Tw1()= 46 4.186 ( 39.99 30 ) Q = 1925560 W40 Mass flow rate of water; mw = 46 =4di2 V N4( 0.010 )21000 1 N N = 585.98No of passes = 4 No of tubes = 585.98 4= 2345 (Answer)Figure 5.1.1: Tubing process41 Property of water at 35o CKg m3 Kj C p = 4.176 KgK = 993.95 = 262.73*102 Kg / hr mv = 0.752*106 m 2 = 5.43*104 m 2 / hrk = 62.5343 102Pr = 4.86Now from all these data we calculate Rey. & Prant. Nos. Reynolds number: R e = Vd= 993.95 0.752 106 0.010 262.73 102= 284.49Prenatal number:Pr =C pk 262.73 4.176 = 62.53 102 = 17.53Nusset number:Nu = 0.023Re0.8 Pr0.4= 0.023 (284.49)0.8 (17.53)0.4 = 6.64d hi Nu = k h 0.010 6.64 = i 62.53 1022Heat transfer co efficient of inside boundary of tube:-And also,42