PERFORMANCE ANALYSIS OF BOILER AND COOLING TOWER IN SAKTHI SUGARS (JOTHI NAGAR) AND IMPROVEMENT OF THEIR EFFICIENCIES
ABSTRACT
In a broad sense energy efficiency means economizing on the use energy without adversely affecting economic growth and development. It includes improving the efficiency of energy generation, transmission, distribution and efficient end use of energy. Cogeneration in sugar industries is not a new concept. It has been practiced by the sugar industries for a long time. Previously the sugar industries utilised the bagasse to generate electricity in low pressure boilers .The steam generated from those low pressure boilers was sufficient to meet their process, heat demands and their captive power demands. The additional electrical energy, if needed, was drawn from the POWER GRID. But now the word COGENERATION refers to process industries only if they generate exportable surplus power The main aim of this project is to carry out the performance analysis of the boiler and the cooling tower of the cogeneration plant, which helps in both conservation of energy and cost and thereby used o improve the efficiency of the unit. The performance analysis the boiler and cooling tower in the cogeneration plant is taken for study with recorded statistical data. As per performance calculation, the energy intensive areas are illuminated adding the above data and necessary solutions are suggested to diminish the losses.
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INTRODUCTION ABOUT THE PLANT SAKTHI SUGARS LIMITED, JOTHI NAGAR UNIT ΙΙ Sakthi sugars limited, Jothi nagar is situated in Sivagangai district of Tamilnadu (state). Its present capacity is 3000 TCD. General details: Installed crushing capacity Steam Generation rate Present crushing capacity
4000TCD 135TPH 3500TCD
Power Generation Details: During season Plant Power requirement Export to the grid During off season Plant power requirement Power export Other Details: Bagasse percent on cane Steam percent on cane Sugar recovery Fibre percent Moisture percent GCV value of Bagasse
30% 43% 10% 14% 54 2272kcal/kg
Co-generation in Sakthi sugars limited
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The cogeneration plant in Sakthi sugars limited produces a maximum capacity of 35.1 MW of power generation and 135 tons of steam generation for their sugar plant. Steams are extracted at a pressure of 20 ata, 9 ata and at a pressure of 3 ata for process heating application. The boiler is designed to handle biomass fuel as well as imported coal. The boiler is designed for moderate calorific value fuels. It is a high pressure boiler. It is a water tube boiler with travelling grates spreader stoker having economizer, air preheater, electrostatic precipitator, and an ash handling system. The steam generated is used to generate the power at first with high temperature and pressure and then for process in sugar plant with low temperature. It is called Topping cycle. The outlet of the boiler steam of 102 kg/cm² pressure and 545°C temperatures enters into the turbine having 24 stages of blades of 35.1 MW rotor with TPH flow It is triple extraction cum condensing turbo generator. The first extraction shall be uncontrolled bleeding of 12 TPH with 20 ata, second extraction shall be controlled bleeding 23 TPH with 9 ata and the third extraction shall be controlled bleeding 70TPH with 3 ata. The exhaust of 135 TPH in vacuum is entered into the condenser where cooling water cools it to condensate at saturated state. The condensate is pumped to boiler again through de-aerator, and economizer. To evacuate non condensable gas and air in exhaust steam, steam jet erector is used and it also maintain a vacuum inside the condenser. Boiler is used with the De-mineralised water treated by ion- exchange process in DM water treatment plant of data 30 TPH. This is meant for the make up of the system losses in the process and leakage of steam valves etc.
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To cool the exhaust steam, we have a cooling tower of capacity of m³/hr circulating water with the cooling range of 6 ºC and approach of 8 ºC The plant has fuel handling equipment such as belt conveyor, crusher, vibrating screen bunkers, rotary feeders to handle about TPH of coal. The flue gas passes through bank tubes inside the furnace and economizer, air preheater outside the furnace causing temperature drop from ºCºC and it escapes to the atmosphere from the chimney through the electrostatic precipitator. The treated boiler water has hardness with less TDS, silica, chloride, and ph of .The raw water has a hardness of The plant has an Alternator of Nayang mfg ltd make of 35.1MW, 3 phases, 50HZ, 1500 RPM coupled with reduction gear bore (5758RPM to 1500RPM)
INTRODUCTION TO CO-GENERATION
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Co-generation is the simultaneous generation of heat and power from the same fuel source, both of which are used. India is currently one among the largest producers of sugar in the world. It produces % of world’s sugar production. In India about million tonnes of sugarcane is cultivated per annum. There are about sugar mills in India. The crushing of these sugar mills ranges from 1250 TCD to 10000TCD. Of all the Co-generation options in India, sugar mills hold a specific advantage compared to other process industries. The sugar mills use bagasse, a residue in sugar manufacturing as also a fuel. The fuel bagasse is regenerative. The potential of power generation possible through Co-generation from Indian sugar mills is about 3500MW. In early 1930s when sugar factories were introduced in India, most of the working units were operated by steam engines of low steam pressure and temperature. But after the development in technology and considering the economy in sugar industries, high pressure and high temperature boilers were introduced and it made the low pressure and temperature steam engines obsolete. During the last decade or so, concept of electrically operated units came into existence and were introduced because of higher efficiency and lower maintenance cost. After implementation of steam economy, measures at various stages, the requirement of steam reduced to great extent from 65% to 45% on cane, which resulted in large bagasse savings. However sale of bagasse is not very lucrative and its price fluctuates widely from year to year. In recent past, to meet increasing power demand and with a view to efficiently utilise renewable energy sources into power generation. Government of India is encouraging sugar industries to supply surplus power to GRID and having realised the scope of value additions by Co-generation, many sugar industries are looking at this concept seriously.
Co-generation is likely to be most attractive under the following circumstance:
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• The demand for both steam and power is balanced • A single plant or group of plants has sufficient demands for steam and power to permit economies of scale to be achieved • Peaks and troughs in demands can be managed or, in case of electricity, adequate backup supplies can be obtained from utility company Co-generation: Co-generation is a simultaneous generation of heat and power, from the same fuel source, both of which are used. Classification of Co-generation system: There are two main types of Co-generation concepts: • Topping cycle plants • Bottoming cycle plants Topping cycle •
Topping cycle plant generates electricity or mechanical power first. There are mainly four types of Topping cycles.
Types of Topping cycles:
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Bottoming cycle • Bottoming cycle plant generates heat first • These plants are much less common than topping cycle plants In sugar industries they normally follow the Topping cycle concept. The steam generated is used to generate the power at first with high temperature and pressure and then for process in sugar plant with low temperature.
Prime movers for Co-Generation
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• Steam Turbine Steam turbines (Figure 1) are the most commonly employed prime movers for cogeneration applications. In the steam turbine, the incoming high pressure steam is expanded to a lower pressure level, converting the thermal energy of high pressure steam to kinetic energy through nozzles and then to mechanical power through rotating blades.
Figure 1 1. Back Pressure turbine In this type steam enters the turbine chamber at High Pressure and expands to Low or Medium Pressure. Enthalpy difference is used for generating power / work. Depending on the pressure (or temperature) levels at which process steam is required, backpressure steam turbines can have different configurations as shown in Figure 2.
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Figure 2 In extraction and double extraction backpressure turbines, some amount of steam is extracted from the turbine after being expanded to a certain pressure level. The extracted steam meets the heat demands at pressure levels higher than the exhaust pressure of the steam turbine. The efficiency of a backpressure steam turbine cogeneration system is the highest. In cases where 100 per cent backpressure exhaust steam is used, the only inefficiencies are gear drive and electric generator losses, and the inefficiency of steam generation. Therefore, with an efficient boiler, the overall thermal efficiency of the system could reach as much as 90 per cent. 1. Extraction Condensing turbine In this type, steam entering at High / Medium Pressure is extracted at an intermediate pressure in the turbine for process use while the remaining steam continues to expand and condenses in a surface condenser and work is done till it reaches the Condensing pressure.(vacuum). In Extraction cum Condensing steam turbine as shown in Figure 3, high Pressure steam enters the turbine and passes
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out from the turbine chamber in stages. In a two stage extraction cum condensing turbine MP steam and LP steam pass out to meet the process needs. Balance quantity condenses in the surface condenser. The Energy difference is used for generating Power. This configuration meets the heatpower requirement of the process.
Figure 3 The extraction condensing turbines have higher power to heat ratio in comparison with backpressure turbines. Although condensing systems need more auxiliary equipment such as the condenser and cooling towers, better matching of electrical power and heat demand can be obtained where electricity demand is much higher than the steam demand and the load patterns are highly fluctuating. The overall thermal efficiency of an extraction condensing turbine cogeneration system is lower than that of back pressure turbine system, basically because the exhaust heat cannot be utilized (it is normally lost in the cooling water circuit). However, extraction condensing cogeneration systems have higher electricity generation efficiencies Advantages of co-generation in sugar mills:
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• The sugar factory does not require any costly fuel. Bagasse, a residue from sugar mills can be used as fuel. Ultimate analysis Carbon Hydrogen Oxygen Ash Sulphur Moisture
Proximate analysis
-23.5% -3.25% -21.75% -1.5% -traces -50±2%
Fixed carbon Volatile matter Moisture Ash
-9.95% -24.26% -54±2% -1.5%
Gross calorific value of Bagasse (@ 50% moisture)-2272kcal/kg Gross calorific value of Coal (@ 25% moisture) -5100 kcal/kg • Fuel is regenerative • Additional source of income through exportable power to the grid, gives the sugar factories a competitive edge • Net contribution to green house effect is zero. Very little fly ash, and no sulphur emission • Minimum T&D losses • Saves the expenditure on safe storage and disposal of bagasse • Improved profitability of operation • Low gestation period • Well established technology • Power can also be generated in off season with saved bagasse or any other supplementary fuel like coal Technology used in SAKTHI SUGARS PVT LTD. (Co-generation) High pressure boilers are used in the production of power in the co-generation plant. The technology involved in the co-generation plant includes: •
Bagasse Silo: An effective silo system for the storage of bagasse of the requirement 10-15 minutes for boiler is stored for effective combustion control. For the system bagasse should be well prepared and the moisture should not exceed 50% to avoid choking of the system.
•
Travelling grate is successful for bagasse
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•
Bagasse Handling: During crushing season surplus bagasse is taken to storage yard and this saved bagasse could be used for cleaning day or off season in co-operation with the co-generation plant. But as unit size is larger, back feeding of larger quantum of bagasse may be nonuniform by bulldozers or pushers resulting in overloading and damage of conveyors.
•
Milling system: To run the co-generation plant effectively better milling such that well preparation of bagasse and controlling of moisture by a value of around 50% is essential otherwise problem in combustion control and handling of bagasse will be faced and also by increase of 1% moisture in bagasse, boiler efficiency reduces by 0.6%. In addition optimum imbibitions should be adopted for steam economy in boiling house and also all drives should be changed to electric or hydraulic systems from steam turbine. By changing these drives exportable power could be increased by about 1MW for 2500TCD plant.
•
ESP system: Operation of Electrostatic precipitator for bagasse firing has been proved to be successful. It reduces the emission levels as low as 50milligram per normal cubic metre, But the large unburnt in the ash may create some hazardous situations.
Feed Water Quality Management: This is essential for High pressure boiler and smooth running of turbine. Following are the points of attention: 1. Exhaust condensate may be used however sincere and disciplined monitoring to keep the condensate free from contamination, for this plant should ensure that piping and equipment using those steams and the condensate are maintained properly for no contamination of this condensate. 2. If contamination is suspected better to use de-mineralised water. 3. As quality of vapour condensate varies, generally ph is low, the TDS and silica are high and there could be traces ammonia and organic compounds. This condensate can be discarded, but its heat energy should be recovered. Then the requirement of boiler water increases. A system of online monitoring and batching to ensure that good quality of vapour condensate is taken has been devised, and also it was reported in “in power magazine” that the sugar contamination could be treated with activated carbon and hydrogen per oxide to eliminate contamination. •
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By using these techniques vapour condensate may be used in some extent to save the requirement of DM water. 4. Resin and regeneration in DM plant should be checked regularly. • Heat to power ratio: Most important technical parameter influencing the selection of the type of co-generation system It is defined as the “ratio of thermal energy to electricity required by the energy consuming facility”. It can be expressed in different units such as Btu/kWh, kcal/kWh, lb/hr/kW.
• Control and Instrumentation: Differential control system is preferred for fast response. The operations at various units of the power plant can be easily monitored with help of this DCS. Thus this reduces the involvement of human interface. • Condensing and Cooling sections: This is another area which consumes about 30% of the energy consumption in plant. It is essential to revamp the existing multijet condenser with barometric condensed .By adopting suitable energy conservation measures it is possible to bring down the energy consumption in this area by 50% of the present level. Vacuum to be maintained in the condenser should be selected optimally.
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ENERGY PERFORMANCE ASSESMENT OF BOILERS The performance parameters of boiler, like efficiency and evaporation ratio reduces with time due to poor combustion, heat transfer surface fouling and poor operation and maintenance. Even for a new boiler, reasons such as deteriorating fuel quality, water quality etc. can result in poor boiler performance. Boiler efficiency tests help us to find out the deviation of boiler efficiency from the best efficiency and target problem area for corrective action. Boiler Efficiency Thermal efficiency of boiler is defined as the percentage of heat input that is effectively utilised to generate steam. There are two methods of assessing boiler efficiency. 1) The Direct Method: Where the energy gain of the working fluid (water and steam) is compared with the energy content of the boiler fuel. 2) The Indirect Method: Where the efficiency is the difference between the losses and the energy input. a. Direct Method This is also known as ‘input-output method’ due to the fact that it needs only the useful output (steam) and the heat input (i.e. fuel) for evaluating the efficiency. This efficiency can be evaluated using the formula
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Heat output Boiler efficiency= ————— Heat input Parameters to be monitored for the calculation of boiler efficiency by direct method are: • Quantity of steam generated per hour (Q) in kg/hr. • Quantity of fuel used per hour (q) in kg/hr. • The working pressure (in kg/cm2 (g)) and superheat temperature (°C), if any • The temperature of feed water (°C) • Type of fuel and gross calorific value of the fuel (GCV) in kcal/kg of fuel
Boiler Efficiency=
Q × (H — h) × 100 ————— q×GCV
Q =quantity of steam generated per hour (kg/hr) q =quantity of fuel used per hour (kg/hr) GCV =gross calorific Value of fuel (kcal/kg) H =Enthalpy of saturated steam in kcal/kg of steam h = Enthalpy of feed water in kcal/kg of water
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Advantages of direct method: • Plant people can evaluate quickly the efficiency of boilers • Requires few parameters for computation • Needs few instruments for monitoring Disadvantages of direct method: • Does not give clues to the operator as to why efficiency of system is lower • Does not calculate various losses accountable for various efficiency levels
Indirect Method Description There are reference standards for Boiler Testing at Site using indirect method namely British Standard, BS 845: 1987 and USA Standard is ASME PTC-4-1 Power Test Code Steam Generating Units’. Indirect method is also called as heat loss method. The efficiency can be arrived at, by subtracting the heat loss fractions from 100. The standards do not include blow down loss in the efficiency determination process. A detailed procedure for calculating boiler efficiency by indirect method is given below. However, it may be noted that the practicing energy mangers in industries prefer simpler calculation procedures.
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The principle losses that occur in a boiler are: 1. 2. 3. 4.
Loss of heat due to dry flue gases Loss of heat due to moisture in fuel Loss of heat due to moisture in air Loss of heat due to combustion of hydrogen 5. Loss of heat due to surface radiation and convection heat loss 6. Unburnt losses in fly ash 7. Unburnt losses in bottom ash
BOILER EFFICIENCY (1+2+3+4+5+6+7)
(indirect
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method)
=
100
–
In the above, loss due to moisture in fuel and the loss due to combustion of hydrogen are dependent on the fuel, and cannot be controlled by design. The data required for calculation of boiler efficiency using indirect method are: • Ultimate analysis of fuel (H2, O2, S, C, moisture content, ash content) • Percentage of Oxygen or CO2 in the flue gas • Flue gas temperature in °C (Tf) • Ambient temperature in °C (Ta) & humidity of air in kg/kg of dry air • GCV of fuel in kCal/kg • Percentage combustible in ash (in case of solid fuels) • GCV of ash in kCal/kg (in case of solid fuels) Normally instruments such as flue gas analyzer, temperature indicators are used to acquire the above mentioned datas. Now a day’s differential control system is used in power plants which make it relatively easier to acquire the datas
Boiler Efficiency by Indirect Procedure and formula
Method:
Calculation
In order to calculate the boiler efficiency by indirect method, all the losses that occur in the boiler must be established. These losses are conveniently related to the amount of fuel burnt. In this way it is easy to compare the performance of various boilers with different ratings.
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However it is suggested to get an ultimate analysis of the fuel fired periodically from a reputed laboratory. Theoretical (stoichiometric) air fuel ratio and excess air supplied are to be determined first for computing the boiler losses. The formula is given below for the same.
The various losses associated with the operation of a boiler are discussed below with required formula. 1. Heat loss due to dry flue gas This is the greatest boiler loss and can be calculated with the following formula
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2. Heat loss due to moisture present in fuel Moisture entering the boiler with the fuel leaves as a superheated vapour. This moisture loss is made up of the sensible heat to bring the moisture to boiling point, the latent heat of evaporation of the moisture, and the superheat required bringing this steam to the temperature of the exhaust gas. This loss can be calculated with the following formula
3. Heat loss due to moisture present in air
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Vapour in the form of humidity in the incoming air, is superheated as it passes through the boiler. Since this heat passes up the stack, it must be included as a boiler loss. To relate this loss to the mass of coal burned, the moisture content of the combustion air and the amount of air supplied per unit mass of coal burned must be known. The mass of vapour that air contains can be obtained from psychometric charts and typical values are included below:
4. Heat loss due to evaporation of water formed due to H2 in fuel (%) The combustion of hydrogen causes a heat loss because the product of combustion is water. This water is converted to steam and this carries away heat in the form of its latent heat.
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5. Heat loss due to radiation and convection: The other heat losses from a boiler consist of the loss of heat by radiation and convection from the boiler casting into the surrounding boiler house. Normally surface loss and other unaccounted losses is assumed based on the type and size Of the boiler as given below For industrial fire tube / packaged boiler = 1.5 to 2.5% For industrial water tube boiler = 2 to 3% For power station boiler = 0.4 to 1% However it can be calculated if the surface area of boiler and its surface temperature are known as given below:
Heat loss due to unburned carbon in fly ash and bottom ash: Small amounts of carbon will be left in the ash and this constitutes a loss of potential heat in the fuel. To assess these heat losses, samples of ash must be analyzed for carbon
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content. The quantity of ash produced per unit of fuel must also be known 6. Heat loss due to unburnt in fly ash (%):
L6 = (Total ash collected per kg of fuel burnt × GCV of fly ash) × 100 ———————————————— GCV of fuel 7. Heat loss due to unburnt in bottom ash (%): L7 = (Total ash collected per kg of fuel burnt × GCV of bottom ash) × 100 ———————————————— GCV of fuel
Performance Analysis of Boiler by Indirect Method Here in Sakthi sugars pvt ltd. both imported coal and Bagasse is used as a fuel. This methodology is followed for uninterrupted power supply to the Power Grid even during off season. So the Boiler Efficiency is determined by taking the average of Coal fired boiler and Bagasse fired boiler. Fuel analysis for Bagasse: Ultimate analysis (wt
%):
Carbon
23.5%
Hydrogen
3.25%
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Oxygen
21.75%
Ash
1.5%
Moisture
50%
Fuel Analysis of Coal: Ultimate analysis (wt %): Carbon
57.17%
Hydrogen
5.24%
Oxygen
7.08%
Nitrogen
0.49%
Sulphur
1.04%
Ash
5.62%
Moisture
25%
Boiler Efficiency calculation of Bagasse fired boiler Theoretical air requirement = [(11. 6×C) + {34.8 × (H2 - O2/8)} + (4.35 × S)] / 100 = [(11.6×23.5) + {34.8× (3.25-21.75/8)} + (4.35×0)]/100 = 2.91 kg/kg Excess air supplied, EA % = [O2% / (21 – O2%)] × 100
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= [21.75 / (21 -20.75)] × 100 =44.67 kg/kg
Actual mass of air supplied/kg of fuel (AAS) = {1+ EA/100} ×Theoretical air = {1+ (44.67/100)} ×2.91 =4.21 kg/kg 1. Heat loss due to dry flue gas (%)
m×cp×(Tf –Ta)
L1 = ————————— GCV of fuel = (4.21×.23× (145-27)) ×100 / 2100 =5.44%
2. Heat loss due to H2 in fuel (%)
{9×H2× {584+Cp (Tf-Ta)} ×100 L2 = —————————— GCV of fuel = [9×.0325× {584+0.45(145-27)}] ×100/2100 = 8.87% 3. Heat loss due to moisture in fuel (%)
M× {584+ Cp (Tf-Ta)} L3
=
×100
—————————— GCV of fuel
= [0.5× {584+0.45(145 -27)}] ×100/2100
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=15.16%
4. Heat loss due to moisture in air (%)
AAS × humidity×Cp× (Tf –Ta) L4
=
×100
—————————— GCV of fuel
= {3.91×0.018×0.45× (145 – 27)} ×100 / 2100 = 0.177% 5. Heat loss due to unaccounted loss & radiation loss (%)
L5 = 0.548× [{Ts/55.55}] +1.957× (Ts –Ta) ×√Vm The loss due to unaccounted loss and radiation are assumed according to the manufacturer’s margin. L5 =5% 6. Heat loss due to unburnt in fly ash (%)
Total ash collected/Fuel consumed × GCV of fly ash ×100 L6 =
————————————————————— GCV of fuel 7.542/429 × 231 ×100
=
——————————— 2100
= 0.193%
7. Heat loss due to unburnt in bottom ash (%)
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Total ash collected/Fuel consumed ×GCV of bottomash×100 L7 =
————————————————————— GCV of fuel
0.838/429×231 × 100 =
————————— 2100
=0.024%
Boiler efficiency by (L1+L2+L3+L4+L5+L6+L7)
indirect
method=100
–
=100-(5.44+8.87+15.16+0.177+5+0.1 93+0.024) =65.07%
Boiler Efficiency calculation of Coal fired boiler Theoretical air requirement= [(11. 6×C) + {34.8 × (H2 - O2/8)} + (4.35 × S)] / 100
= [(11.6×57.17) + {34.8× (5.24-7.08/8)} + (4.35×.49)]/100 = 8.16 kg/kg
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Excess air supplied, EA % = [O2% / (21 – O2%)] × 100 = [7.08 / (21 -7.08)] × 100 =50.86 kg/kg
Actual mass of air supplied/kg of fuel (AAS) = {1+ EA/100} ×Theoretical air = {1+ (50.86/100)} ×8.16 =12.322 kg/kg Mass of Dry flue gas in kg/kg = 13.322 kg/kg
1. Heat loss due to dry flue gas(%)
m×c p×(Tf –Ta)
× 100 L1 = ————————— GCV of fuel = (13.322×.23× (145-27)) ×100 / 5100 =6.557%
2. Heat loss due to H2 in fuel (%)
9×H2× {584+Cp (Tf-Ta)} ×100 L2 = —————————— GCV of fuel
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= [9×0.0524× {584+0.45(145-27)}] ×100/5100 = 5.891%
3. Heat loss due to moisture in fuel (%)
M× {584+ Cp (Tf-Ta)} L3
=
×100
—————————— GCV of fuel
= [0.25× {584+0.45(145 -27)}] ×100/5100 =3.123%
4. Heat loss due to moisture in air (%)
AAS × humidity×Cp× (Tf –Ta) L4 =
×100
—————————— GCV of fuel
= {12.322×0.018×0.45× (145 – 27)} ×100 / 5100 = 0.23%
5. Heat loss due to unaccounted loss & radiation loss (%)
L5 = 0.548× [{Ts/55.55}] +1.957× (Ts –Ta) ×√Vm The loss due to unaccounted loss and radiation are assumed according to the manufacturer’s margin. L5 =5%
6. Heat loss due to unburnt in fly ash (%)
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Total ash collected/Fuel consumed × GCV of fly ash ×100 L 6=
————————————————————— GCV of fuel
22.348/358 × 561 ×100 =
——————————— 5100
= 0.686%
7. Heat loss due o unburnt in bottom ash (%)
Total ash collected/Fuel consumed ×GCV of bottomash×100 L7 =
————————————————————— GCV of fuel
8.532/358×627.3 × 100 =
————————— 5100
=0.2931%
Boiler efficiency by (L1+L2+L3+L4+L5+L6+L7)
indirect
= 100 – (6.557+3.123+5.891+0.23+ +5+0.686+0.293)
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method=100
–
=78.22%
Boiler efficiency for a Bagasse/coal fired boiler Input / output parameter
Bagas se
Heat input in fuel
Coal
= 100
100
1. Dry flue gas loss
= 5.44
2.Loss due to moisture in fuel
= 15.16
3.Loss due to moisture in air
= 0.177
6.55 7 2.12 3 0.23
4.Loss due to hydrogen in fuel
= 8.87
5.89
Various heat losses in boiler
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5.Loss due to radiation and = 5 convection 6.Loss due to unburnt in fly ash = 0.193 7.Loss due to unburnt in bottom = 0.024 ash Total losses = 34.864 Boiler efficiency
5 0.68 9 0.28 3 21.7 7
= 65.13 78.2 2
The Overall Boiler efficiency was found to be 71.6%
DRYING OF FUEL Apart from the economiser and the air heater, there is a third means of improving combustion in the furnaces, that is to dry the bagasse leaving the mills before sending it to the boilers. This procedure is much less used at present, but has every prospect of being employed more and more in the future. Calculations for Moisture removal from fuel.
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To consider how drying can be realised, we assume typical or frequent conditions. Suppose: Fibre in cane: • / = 0.15 Ambient air: t = 30 °C Excess air to the furnace: m = 1.4 Bagasse at 50% moisture per Bo= 300 kg Moisture in bagasse leaving the mills: w = 0.5 Ash % in bagasse: c =2.5% Temperature of gas admitted to the outlet from the dryer: T = 70 °C Temperature of gases leaving the boiler tubes: To = 145°C Water to be evaporated. To determine the quantity X of water to be evaporated per kg of bagasse/coal to reduce it to any moisture content w, starting from the default moisture content, this quantity will be such that w= (0.5 – x) / (1 –x)
hence: x= (0.5 – w)/ (1-w) kg/kg of fuel
Weight of gas. The weight of gas leaving the boiler has the value: P= 5.76 (1 - w) 1.4 + 1 = 9.06 -8.06 w kg/kg of fuel. Hence the total weight of gases: BP = (0.5 + w) (9.06 – 8.06 w) = 4.53 +5.03 w -8.06w² kg/ kg of fuel. Heat required. The fuel must be heated to the final temperature at which it leaves the dryer. The specific heat of fibre has a value: cf=0.36 The heat required to heat the fuel is thus: Hr= m × w×cp× (Ti-To) kcal/kg We require heating the water contained in the fuel and evaporate it. The total heat of this vapour at 70C is 637.5. We thus have: (637.5 -30) = 607.5(0.5 – w)/ (1-w) = (303.75 – 607.5w)/ (1-w), kcal/kg of fuel The total heat necessary is thus: Ch.N= Hr + {(303.75- 607.5w)/ (1-w)} kcal/kg of fuel. The heat available (Ch.D) has the value, assuming an efficiency of 0.85:
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Ch.D= B.P (145- 70) × 0.285×0.85=18.168×B.P Or Ch.D= (4.53 +5.03 w -8.06w²) ×18.168 kcal/kg of fuel Or Ch.D= 82.30+91.38w -146.434 w²
kcal/kg of fuel
There is no advantage in drying the fuel below 10% moisture, as it would be liable to increase to 10% by absorption of atmospheric humidity. The volume of the dryer is approximately12-15m³/t/h of water to be evaporated. The drying of bagasse is rather expensive, but it offers the great advantage of being the procedure which permits the flue gases to be brought to the lowest temperature, and thus giving the equipment the greatest recovery of heat from the flue gases. The only limit being imposed by the necessity to avoid cooling to the dew point of 60 — 69°C. Another advantage of the dryer is that the bagasse introduced to the furnace is heated, which facilitates combustion and rapid ignition. This is not taken into account in the calculations, but does contribute to improved combustion. On the other hand, dry bagasse is readily ignited and precautions should be taken in the dryer and in the transfer of bagasse to the boilers. These consist especially in placing a steam valve at the entry to the dryer so as to extinguish readily any fire which may break out in the dryer. The velocity of gas in the dryer should not exceed 2.5m/s to avoid entraining fine particles. It is generally kept between 2 and 2.5 m/s. The main disadvantage of bagasse drying is its cost, which may attain three times that of economisers or air heaters. For this reason it is not often practised; but it is the solution which should be considered when it is desired to conserve bagasse for slack-season use and in particular for conversion to pellet. However, for combustion of bagasse in the spreader-stoker furnaces used at present, drying is generally not taken below 30% in the dried bagasse, since a drier fuel would involve risk of higher combustion temperatures which could cause deposit of fused ash on the boiler tubes.
BAGASSE DRYING OPTIONS AVAILABLE 1. Indirect contact dryers 2. Direct contact dryers – Stationary/Rotary
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In stationary type dryers two arrangements are available 1. Parallel arrangement 2. Sequence arrangement COAL DRYING OPTIONS AVAILABLE 1. Vibratory Fluid Bed Dryer Parameters considered for selection of dryers a. Heat transfer rate b. Retention time c. Uniformity of drying d. Number of installations e. Operating results f. Availability of designs and drawings It was observed that direct type dryers were more efficient in removing the moisture. After technical evaluation and based on site conditions, it was decided to prefer direct contact type stationary dryer with sequence arrangement. There were about
Bagasse Dryer : Dryer Installation The dryer is installed between the boiler ID fan and the chimney. The wet bagasse from the mill is drawn from the bagasse elevator and isn’t transported to the dryer by belt conveyor. The dried bagasse is transported back to the main bagasse carrier to boiler by belt conveyor. The flue gas to the drier is taken from the existing ducting that connects the outlet of the boiler to the chimney through the drier fan. The fuel feed to the drier is controlled through the rotary valve.
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The drier fan moves the flue gases and fuel mix through out the dryer tube, where the temperature of flue gas are separated in cyclones separators. The dried fuel is discharged on belt conveyor at the bottom of the cyclone, and the flue gases are discharged through atmosphere.
Bagasse dryer-Arrangement
Dryer operation It is to be understood that main aim of bagasse drying action, according to the design is to get flue gas temperature at 75deg C minimum rather than any bagasse moisture content Dryer operations are controlled through a control loop which regulates the quantity of bagasse elevator. The quantity of flue gas is controlled by regulating the rpm of dryer fan in order to keep the flue gas temperature close to 75 deg C The bagasse dryer can be put in and out of operation without disturbing the boiler and does not require any extra manpower for operation. Also the
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dryer does not require any extra manpower for operation, the dryer does not require any special maintenance.
BAGASSE DRYER - SCHEMATIC ARRANGEMENT
Operational safety Utmost care is required to prevent jamming of dryer tube due to fly ash and bagasse. Though the process time in the dryer is only a few seconds, care should be taken to prevent fire in the dryer. The fire can be due to succession of events-e.g. there is a bagasse jam at dryer outlet and process consiousness is interrupted. Some quantity of bagasse remains inside the dryer- mostly in the cyclone and is exposed to larger than normal residence or contact time with the flue gases or hot metal parts. As a result moisture content of bagasse is further reduced. Under these conditions bagasse is further reduced. Under
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these conditions bagasse is susceptible to catch fire. Therefore the flow of bagasse in and out of dryer should be monitored continuously during operations Calculations for moisture removal from bagasse: Water to be evaporated. w= (0.5 – x) / (1 –x)
hence: x= (0.5 – w)/ (1-w) kg/kg of fuel
We are reducing the moisture content of Bagasse to 45% from 50% So, w=0.45 x= (0.5- 0.45)/ (1-0.45) x=0.09 kg/kg of bagasse Weight of gas. The weight of gas leaving the boiler has the value: P= 5.76 (1 - w) 1.4 + 1 = 9.06 -8.06 w kg/kg of fuel. P=5.76(1- 0.45)1.4+1= 9.06-(8.06×0.45) kg/kg of bagasse P= 5.435 kg/kg of bagasse. Hence the total weight of gases: BP = (0.5 + w) (9.06 – 8.06 w) = 4.53 +5.03 w -8.06w² kg/ kg of fuel. BP = (0.5 + 0.45) (9.06 – 8.06×0.45) = 4.53 +5.03 w -8.06w² kg/ kg of fuel. BP = 5.16 kg/kg of bagasse Heat required. The fuel must be heated to the final temperature at which it leaves the dryer. The specific heat of fibre has a value: cf=0.36 The heat required to heat the fuel is thus: Hr= m × w×cf× (Ti-To) kcal/kg Hr= 1 × 0.5×0.36× (70-30) kcal/kg
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Hr= 7.2 kcal/kg We require heating the water contained in the fuel and evaporate it. The total heat of this vapour at 70°C is 637.5. We thus have: (637.5 -30) = 607.5(0.5 – w)/ (1-w) = (303.75 – 607.5w)/ (1-w), kcal/kg of fuel The total heat of this vapour: = [303.75- (607.5×0.45)]/ (1-0.45) = 55.22 kcal/kg The total heat necessary is thus: Ch.N= Hr + {(303.75- 607.5w)/ (1-w)} kcal/kg of fuel. Ch.N= 7.2 + 55.22 = 62.42 kcal/kg of bagasse. Ch.N= 62.42 kcal/kg The heat available (Ch.D) has the value, assuming an efficiency of 0.85: Ch.D= B.P (145- 70) × 0.285×0.85=18.168×B.P Or Ch.D= (4.53 +5.03 w -8.06w²) ×18.168 kcal/kg of fuel Or Ch.D= 82.30+91.38w -146.434 w²
kcal/kg of fuel
̥ ° ̥ The heat available Ch.D = [82.3+ (91.38×0.45)-(146.434×0.45²)] kcal/kg of bagasse Ch.D = 93.76 kcal/kg of bagasse Excess heat available=Ch.D-Ch.N=93.76-62.42=31.34 kcal/kg of bagasse Therefore, The heat available is more than the heat required. So the moisture content in bagasse can reduced.
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COAL DRYER: The vibratory fluid bed dryer has several advantages compared to the conventional rotary dryers used for such applications.
These dryers are suitable for handling different types of coals such as bituminous, sub bituminous, anthracite, lignite etc, THE PROCESS The hot gases required for drying The exhaust gas from the dryer is are generated in the hot gas passed through a bag filter for generator by firing fuel such as removing the fines carried over before releasing it to the atmosphere. It is oil, natural gas, coal or any waste also possible to re-circulate part of the gas available in the plant. The hot flue gas from bag filter outlet to dryer. gas is introduced in the Fluid Bed This minimizes chances of explosion Dryer at the bottom of vibrating by reducing oxygen content in flue bed through a perforated plate. gas and also provides heat recovery. The coal is conveyed from feed end to discharge end by vibrations and comes in contact with hot gas resulting in drying the coal. The combined action of vibrations and fluidization ensures uniform drying and product moisture.
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VIBRATING FLUID BED DRYER-ONCES THOURGH (Schematic Diagram)
VIBRATING FLUID RECIRCULATION
BED
DRYER-WITH
FLUE
GAS
(Schematic Diagram) GENERAL SPECIFICATIONS OF COAL DRYING SYSTEM The specifications vary from case to case. The drying systems are tailor made for specific applications. However broad specifications are as under: Capacity Inlet Moisture Outlet Moisture Particle Size Drying Temperature
Up to 150 Tons/hr Up to 40% 4 to 5 % 0.5mm to 75mm 300 to 400ºC
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SPECIAL FEATURES & ADVANTAGES Completely dust proof construction with explosion vents. Minimum capital expenditure. More efficient compared to rotary dryers. Minimum fines generation compared to rotary dryers. Uniform drying and product moisture. Compact and sturdy design. Easy to install, operate and maintain.
Calculations for moisture removal from coal: Water to be evaporated. w= (0.25 – x) / (1 –x)
hence: x= (0.25 – w)/ (1-w) kg/kg of fuel
We are reducing the moisture content of Coal to 20% from 25% So, w=0.20 x= (0.25- 0.20)/ (1-0.20)
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x=0.0625 kg/kg of coal Weight of gas. The weight of gas leaving the boiler has the value: P= 5.76 (1 - w) 1.4 + 1 = 9.06 -8.06 w kg/kg of fuel. P=5.76(1- 0.20)1.4+1= 9.06-(8.06×0.20) kg/kg of coal P= 7.44 kg/kg of coal. Hence the total weight of gases: BP = (0.25 + w) (9.06 – 8.06 w) = 2.265 +7.045 w -8.06w² kg/ kg of fuel. BP = (0.25 + 0.20) (9.06 – 8.06×0.20) = 2.265 +7.04 w -8.06w² kg/ kg of fuel. BP = 3.35 kg/kg of coal Heat required. The fuel must be heated to the final temperature at which it leaves the dryer. The specific heat of coal has a value: cf=0.329 The heat required to heat the fuel is thus: Hr= m × w×cf× (Ti-To) kcal/kg Hr= 1 × 0.25×0.329× (70-30) kcal/kg Hr= 3.29 kcal/kg We require heating the water contained in the fuel and evaporate it. The total heat of this vapour at 70°C is 637.5. We thus have: (637.5 -30) = 607.5(0.25– w)/ (1-w) = (151.81 – 607.5w)/ (1-w), kcal/kg of fuel The total heat of this vapour: = [151.81- (607.5×0.20)]/ (1-0.20) = 37.88 kcal/kg The total heat necessary is thus: Ch.N= Hr + {(151.81- 607.5w)/ (1-w)} kcal/kg of fuel.
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Ch.N= 3.29 + 37.88 = 41.17 kcal/kg of coal. Ch.N= 41.17 kcal/kg The heat available (Ch.D) has the value, assuming an efficiency of 0.85: Ch.D= B.P (145- 70) × 0.285×0.85=18.168×B.P Or Ch.D= (2.265 +7.04 w -8.06w²) ×18.168 kcal/kg of fuel Or Ch.D= 41.15+ 127.9w – 146.43 w²
kcal/kg of fuel
̥ ° ̥ The heat available Ch.D = [41.15+ (127.9×0.20)-(146.434×0.20²)] kcal/kg of coal Ch.D = 60.8 kcal/kg of coal Excess heat available=Ch.D-Ch.N=60.8-41.17=19.63 kcal/kg of coal. The calculation shows that the moisture content in coal can be reduced by 5%. So the installation of dryer would be successful.
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Efficiency calculations of boiler after reduced moisture content and flue gas outlet temperature. Boiler calculation of bagasse reduction in moisture content:
fired
boiler
after
The moisture content in the bagasse is reduced by 5% after passing through the dryer. So now the moisture content of bagasse, m=45%. Moreover the temperature of the flue gas released into the atmosphere is reduced considerably, the new flue gas stack temperature Tf=70°C 1. Heat loss due to dry flue gas (%)
m×cp×(Tf –Ta)
L1 = ————————— GCV of fuel = (4.21×.23× (70-27)) ×100 / 2100 =1.98%
2. Heat loss due to H2 in fuel (%)
{9×H2× {584+Cp (Tf-Ta)} ×100 L2 = —————————— GCV of fuel = [9×.0325× {584+0.45(145-27)}] ×100/2100 = 8.38%
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3. Heat loss due to moisture in fuel (%) M× {584+ C p (Tf-Ta)} ×100 L3
=
—————————— GCV of fuel
= [0.45× {584+0.45(70-27)}] ×100/2100 =12.92%
4. Heat loss due to moisture in air (%) AAS × humidity×Cp× (Tf –Ta) ×100 L4
=
—————————— GCV of fuel = {3.91×0.018×0.45× (70–
27)} ×100/2100 = 0.064%
5. Heat loss due to unaccounted loss & radiation loss (%) L 5 =0.548× [{Ts/55.55}] +1.957× (Ts –Ta) ×√Vm The loss due to unaccounted loss and radiation are assumed according to the manufacturer’s margin. L5 =5%
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6. Heat loss due to unburnt in fly ash (%) Total ash collected/Fuel consumed × GCV of fly ash×100 L 6=
————————————————————— GCV of fuel 7.542/429 × 231 ×100
=
——————————— 2100
= 0.193%
7. Heat loss due to unburnt in bottom ash (%) Total ash collected/Fuel consumed×GCV bottom ash×100 L7 =
————————————————————— GCV of fuel
0.838/429×231 × 100 =
————————— 2100
=0.024%
Boiler efficiency by (L1+L2+L3+L4+L5+L6+L7)
indirect
method=100
–
=100(1.98+8.38+12.92+0.064
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+0.024+5+0.19 3) =73.41%
Boiler efficiency of coal fired boiler after reduction in moisture content: The moisture content in the coal is reduced by 5%.So the new moisture content is 20%. The losses for the new moisture content and the reduced stack temperature.
1. Heat loss due to dry flue gas(%)
m×cp×(Tf –Ta)
×
100 L1 = ————————— GCV of fuel = (13.322×.23× (7027)) ×100/5100 =2.58%
2. Heat loss due to H2 in fuel (%)
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9×H 2× {584+Cp (Tf-Ta)} ×100 L2 = —————————— GCV of fuel = [9×0.0524× {584+0.45(7027)}] ×100/5100 = 5.57%
3. Heat loss due to moisture in fuel (%) M× {584+ C p (Tf-Ta)} ×100 L3
=
—————————— GCV of fuel
= [0.20× {584+0.45(70 -27)}] ×100/5100 =2.36%
4. Heat loss due to moisture in air (%) AAS × humidity×C p× (Tf –Ta) ×100 L4 =
—————————— GCV of fuel
= {12.322×0.018×0.45× (70 – 27)} ×100 / 5100 = 0.084% 5. Heat loss due to unaccounted loss & radiation loss (%)
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L 5 = 0.548× [{Ts/55.55}] +1.957× (Ts –Ta) ×√Vm The loss due to unaccounted loss and radiation are assumed according to the manufacturer’s margin. L5 =5% 6. Heat loss due to unburnt in fly ash (%) Total ash collected/Fuel consumed × GCV of fly ash ×100 L 6=
————————————————————— GCV of fuel 22.348/358 × 561 ×100
=
——————————— 5100
= 0.686% 7. Heat loss due o unburnt in bottom ash (%)
Total ash collected/Fuel consumed ×GCV of bottomash×100 L7 =
————————————————————— GCV of fuel
8.532/358×627.3 × 100 =
————————— 5100
=0.2931%
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Boiler efficiency by (L1+L2+L3+L4+L5+L6+L7)
indirect
method=100
–
= 100 – (2.583+2.36+5.57+0.084+ +5+0.686+0.2 93) =83.42%
Boiler efficiency for a Bagasse/coal fired boiler Input / output parameter
Bagas Coal se
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Heat input in fuel
= 100
100
1. Dry flue gas loss
= 1.98
2.58
2.Loss due to moisture in fuel
= 12.92
2.36
3.Loss due to moisture in air
= 0.064
4.Loss due to hydrogen in fuel
= 8.38
0.08 4 5.57
Various heat losses in boiler
5.Loss due to radiation and = 5 convection 6.Loss due to unburnt in fly ash = 0.193 7.Loss due to unburnt in bottom = 0.024 ash Total losses = 28.59 Boiler efficiency
5 0.68 6 0.29 3 17.5 8
= 71.41 82.4 2
Thus the overall efficiency of boiler = 77.41%
Therefore, The new Boiler efficiency = 77.41% Thus there is an increase in boiler efficiency by 5.8%, which constitutes of huge economic value. Therefore the method of utilising the flue gas to dry the fuel for reduction of moisture content proved to be possible, and if installed can yield a huge profit to the firm and also indirectly reduces the global warming . COOLING TOWER
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Introduction Cooling towers are a very important part of many chemical plants. The primary task of a cooling tower is to reject heat into the atmosphere. They represent a relatively inexpensive and dependable means of removing lowgrade heat from cooling water. The make-up water source is used to replenish water lost to evaporation. Hot water from heat exchangers is sent to the cooling tower. The water exits the cooling tower and is sent back to the exchangers or to other units for further cooling.
Cooling Tower Types
Cooling towers fall into two main categories: Natural draft and Mechanical draft. Natural draft towers use very large concrete chimneys to introduce air through the media. Due to the large size of these towers, they are generally used for water flow rates above 45,000 m3/hr. These types of towers are used only by utility power stations. Mechanical draft towers utilize large fans to force or suck air through circulated water. The water falls downward over fill surfaces, which help increase the contact time between the water and the air - this helps maximise heat transfer between the two. Cooling rates of Mechanical draft towers depend upon their fan diameter and speed of operation.
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Mechanical draft towers Mechanical draft towers are available in the following airflow arrangements: 1. Counter flows induced draft. 2. Counter flow forced draft. 3. Cross flow induced draft.
Figure 1 In the counter flow induced draft design, hot water enters at the top, while the air is introduced at the bottom and exits at the top. Both forced and induced draft fans are used.
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In cross flow induced draft towers, the water enters at the top and passes over the fill. The air, however, is introduced at the side either on one side (single-flow tower) or opposite sides (double-flow tower). An induced draft fan draws the air across the wetted fill and expels it through the top of the structure. The Figure1 illustrates various cooling tower types. Mechanical draft towers are available in a large range of capacities. Normal capacities range from approximately 10 tons, 2.5 m3/hr flows to several thousand tons and m3/hr. Towers can be factory built or field erected – for example concrete towers are only field erected. Many towers are constructed so that they can be grouped together to achieve the desired capacity. Thus, many cooling towers are assemblies of two or more individual cooling towers or “cells.” The number of cells they have, e.g., an eight-cell tower, often refers to such towers. Multiple-cell towers can be lineal, square, or round depending upon the shape of the individual cells and whether the air inlets are located on the sides or bottoms of the cells.
Components of Cooling Tower The basic components of an evaporative tower are: Frame and casing, fill, cold water basin, drift eliminators, air inlet, louvers, nozzles and fans. Frame and casing: Most towers have structural frames that support the exterior enclosures (casings), motors, fans, and other components. With some smaller designs, such as some glass fibre units, the casing may essentially be the frame. Fill: Most towers employ fills (made of plastic or wood) to facilitate heat transfer by maximising water and air contact. Fill can either be splash or film type. With splash fill, water falls over successive layers of horizontal splash bars, continuously breaking into smaller droplets, while also wetting the fill surface. Plastic splash fill promotes better heat transfer than the wood splash fill. Film fill consists of thin, closely spaced plastic surfaces over which the water spreads, forming a thin film in contact with the air. These surfaces
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may be flat, corrugated, honeycombed, or other patterns. The film type of fill is the more efficient and provides same heat transfer in a smaller volume than the splash fill. Cold water basin: The cold water basin located at or near the bottom of the tower, receives the cooled water that flows down through the tower and fills. The basin usually has a sump or low point for the cold water discharge connection. In many tower designs, the cold water basin is beneath the entire fill. Some forced draft counter flow design, however, the water at the bottom of the fill is handled to a perimeter trough that functions as the cold water basin. Propeller fans are mounted beneath the fill to blow the air up through the tower. With this design, the tower is mounted on legs, providing easy access to the fans and their motors. Drift eliminators: These capture water droplets entrapped in the air stream that otherwise would be lost to the atmosphere. Air inlet: This is the point of entry for the air entering a tower. The inlet may take up an entire side of a tower—cross flow design— or be located low on the side or the bottom of counter flow designs. Louvers: Generally, cross-flow towers have inlet louvers. The purpose of louvers is to equalize air flow into the fill and retain the water within the tower. Many counter flow tower designs do not require louvers. Nozzles: These provide the water sprays to wet the fill. Uniform water distribution at the top of the fill is essential to achieve proper wetting of the entire fill surface. Nozzles can either be fixed in place and have either round or square spray patterns or can be part of a rotating assembly as found in some circular cross-section towers. Fans: Both axial (propeller type) and centrifugal fans are used in towers. Generally, propeller fans are used in induced draft towers and both propeller and centrifugal fans are found in forced draft towers. Depending upon their size, propeller fans can either be fixed or variable pitch. A fan having non-automatic adjustable pitch blades permits the same fan to be used over a wide range of kW with the fan adjusted to deliver the desired air
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flow at the lowest power consumption. Automatic variable pitch blades can vary air flow in response to changing load conditions. Tower Materials Today, tower manufacturers fabricate towers and tower components from a variety of materials. Often several materials are used to enhance corrosion resistance, reduce maintenance, and promote reliability and long service life. Galvanized steel, various grades of stainless steel, glass fibre, and concrete are widely used in tower construction as well as aluminium and various types of plastics for some components. Larger towers sometimes are made of concrete. Many towers— casings and basins—are constructed of galvanized steel or, where a corrosive atmosphere is a problem, stainless steel. Sometimes a galvanized tower has a stainless steel basin. Glass fibre is also widely used for cooling tower casings and basins, giving long life and protection from the harmful effects of many chemicals
Cooling Tower Performance The important parameters, from the point of determining the performance of cooling towers, are:
57
i) “Range” is the difference between the cooling tower water inlet and outlet temperature. ii) “Approach” is the difference between the cooling tower outlet cold water temperature and ambient wet bulb temperature. Although, both range and approach should be monitored, the `Approach’ is a better indicator of cooling tower performance. (See Figure 7.3). iii) Cooling tower effectiveness (in percentage) is the ratio of range, to the ideal range, i.e., difference between cooling water inlet temperature and ambient wet bulb temperature, or in other words it is = [Range / (Range + Approach)]. iv) Cooling capacity is the heat rejected in kcal/hr or TR, given as product of mass flow rate of water, specific heat and temperature difference. v) Evaporation loss is the water quantity evaporated for cooling duty and, theoretically, for every 10, 00,000 kCal heat rejected, evaporation quantity works out to 1.8 m3. An empirical relation used often is:
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Evaporation Loss (m3/hr) = Circulation water flow rate × Range ————————————–——— 675 Range = Temp. difference between inlet and outlet water. vi) Cycles of concentration (C.O.C) are the ratio of dissolved solids in circulating water to the dissolved solids in make up water. vii) Blow down losses depend upon cycles of concentration and the evaporation losses and are given by relation: Blow Down = Evaporation Loss / (C.O.C. – 1)
COOLING TOWER PERFORMANCE ANALYSIS Cooling tower details
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Cooling tower Type
Induced draft flow Capacity per cell 2820 m3/hr Number of cells 3 Number of cells in operation 3 Height of the cooling tower 5m From the ground level air opening height 11 m Temperature of hot water entering the tower 37°C Temperature of cold water leaving the tower 31°C Dry bulb temperature of air 26°C Wet bulb temperature of air 23°C Temperature of air leaving the cooling tower 29°C TDS of make up water 450ppm TDS of circulating water 1800ppm Circulating water flow rate 3600m³/hr
counter
Cooling Tower calculation Cooling tower Range = (Temperature of Hot water — Temperature of Cold water) = [37°C -31°C] =
6°C
Cooling Tower Approach= (Temperature of cold water— Wet bulb temperature) = [31C – 23C] = 8C Evaporation loss =
Circulation water flow rate × Range ————————————–——— 675
3600 × 6 = ————
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675 =32m³/hr Cycles of concentration = (TDS of circulating water/TDS of make up water) =1800/450 =4 Blow down requirement= {Evaporation loss (m³/hr) ÷ (C.O.C -1)} =32/ (4-1) =10.66m³/hr Make up water requirement= Evaporation loss + Blow down requirement =32+10.66 =42.66m³/hr Cooling tower effectiveness=Range/ (Range + Approach) ×100 = (6 / 14) × 100 = 42.85 %
Energy Saving Opportunities in Cooling Towers • Follow manufacturer’s recommended clearances around cooling towers and relocate or modify structures that interfere with the air intake or exhaust. • Optimise cooling tower fan blade angle on a seasonal and/or load basis. •
Correct excessive and/or uneven fan blade tip clearance and poor fan balance.
•
On old counter-flow cooling towers, replace old spray type nozzles with new square spray ABS practically non-clogging nozzles.
• Replace splash bars with self-extinguishing PVC cellular film fill.
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• Install new nozzles to obtain a more uniform water pattern • Periodically clean plugged cooling tower distribution nozzles. • The water efficiency of the cooling tower can be improved by a certain method
Increasing Cooling Tower Water Efficiency This project aims to improve overall water efficiency of cooling towers by Examining applicable systems that can recover vapour. A vapour recovery system consisting of a circular fibre filter on top of cooling towers absorbs and condenses water vapour coming out of the cooling towers. A feasibility experiment showed that approximately 10 percent of evaporated water could be recovered using this method. Further research should be carried out to study the type of the fibre filter that should be installed to optimize water vapour absorption without obstructing airflow. Vapour Recovery Experiment In order to estimate how much vapour can be absorbed by different filters and parameters, a simple experiment was designed using an ultrasonic humidifier (Fig. 3) as a cooling tower prototype. Supply rate of vapour from the humidifier was calculated by dividing the difference of water remaining in the tank of the humidifier by amount of time passed.
Table 2 Amount of water emitted by the ultrasonic humidifier over different periods of time Table 2 shows that 1.9 to 2.2 g of vapour was emitted by the humidifier per minute. This variation, with changing temperature and humidity levels in the Laboratory, the source of error is estimated to be around 15 per cent.
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Fibre filter (Fig. 4) is installed on top of a humidifier and the amount of time passed and according amount water (in grams) absorbed by each filter is measured. For fibre filter pad, variations in thickness and area were imposed.
Figure 3
Figure 4
Conclusion
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Here the dryer is used to reduce the moisture content of the fuel. Thus the efficiency of the boiler increases by 5 percentage. Hence application of dryer is preferred
The flue gas outlet temperature is considerably reduced, which helps in reducing the effect global warming.
Since the moisture content of the fuel is reduced, the fuel feeding rate can be reduced to a very small amount.
Cooling tower losses can be minimized by implementing vapour recovery set-up in the cooling tower, which increases the water efficiency in the cooling tower.
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BIBLIOGRAPHY
Hugot book on canes
- Hugot
Bureau of energy efficiency books
Energy efficiency management website Downloaded materials from website
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