ENERGY SAVING IN TEXTILE PROCESSING Nandish Mehta (Technical Director) Harish Enterprise Pvt. Ltd. Introduction: Cost of energy in Textile processing is a major factor, next only to the chemical cost and hence every processing unit strives to adopt ways and means best known to them to minimise the energy cost. An attempt is made here to provide practical tips and a ready reckoner for making investment decisions related to energy saving. 1.0 Basic Fuels Energy is consumed in processing units either in the form of fuel or in the form of electric power. Several fuels can be used like, furnace oil, cost, husk, natural gas or compressed natural gas stored in bullets. Electric power can be bought directly from electricity Board or self generated through DG Sets or turbines. Table A below compares the cost per kilo calorie of each fuel. [Figures are based on actual situation in Tarapur, Surat and Mumbai]. Table A Fuel Unit Kcal/Unit Boiler Eff. Price/Unit (Rs.) Cost per 1 Lac kilo Cal (Rs.) Furnace Oil Lit 9300 85% 15.00 189.75 Coal Kg 4500 70% 2.20 69.84 Natural Gas M 3 8600 90% 8.00 103.36 Electric power kwh 860 100% 5.00 581.40 Coal is undoubtedly the cheapest fuel available for steam boiler or a thermic fluid boiler even after taking into account extra electric power consumed for larger sized ID fan. As for maintaining precise temperature on a Coal fired thermic fluid boiler, it poses no problem with modern fluidised bed boiler with mechanized coal handling and coal charging equipment available. In Mumbai region where coal is not being permitted, the obvious choice is Natural Gas over furnace oil. In Surat region all the three fuels are available. Coal is the obvious choice for steam Boiler. However there are heated arguments about using a Coal fired thermic boiler or using natural gas directly for stenter and dryers. It would be worth while analyzing the economics of Coal fired thermic fluid boiler Vs. Direct Gas firing in Stenter and Dryers.
2.0 Coal fired Thermic fluid Boiler Vs Direct Gas firing in Stenter / Dryer: Energy Consumed in Stenter: For all practical purpose the average drying capacity in a stenter is 100 kg of water/hr/chamber i. e. 100,000 K. Cal/Hr/Chamber. (This figure is 80 kg of water/hr/chamber and 80,000 kcal/hr/chamber respectively for fine fabric like sari being processed in Surat). Let us consider 2 stenters each of 5-chambers, which is generally the case in most processing units either both using direct firing systems or both catered by a 1 million kcal/hr Coal fired thermic fluid boiler. Case I Direct fired system using natural gas Natural Gas Consumption = 100,000 K.Cal/Hr/Chamber x chambers 8600 x 0.95 (eff) = 122.40 M 3 /Hr. Cost of Natural Gas = Rs. 8 x 122.40 = Rs. 979.20 /hr. Electric power Consumed is = 120 Amps/m/c x 2 m/c x 0.415 x 3 = 172.3 kwh. Cost of power = 172,3 x 5 = Rs. 861. 54/h. Total cost of energy = Rs. 1840.74/h. Case II Coal fired Thermic Fuel system with Oil radiators in stenter Coal Consumption = 100,000 K.Cal/hr/Chamber x 10 Chambers 4500 x 0.70 (eff) x 0.98 (pipe line loss) = 323.93 kgs coal/hr. Cost of Coal Consumed =329.39 x 2.2 = Rs. 712.66 /hr Power consumption in this system is sum total of power consumed by Thermic Fluid Boiler and power consumed by Stenters. Power consumed by Thermic Fluid Boiler is ID fan Circulation Pump Total = 20 HP = 20 HP = 40 HP i.e.30 kwh i.e. Rs 150/ hr. Power consumed by Stenter: = 140 amp/m/c x 2 m/c x 0.415 x 3 = 201.026 kwh. Cost of power in Stenter = 201.026 x 5 = Rs. 1005. 13 /hr Total Energy Cost = Rs. 712.66 + Rs. 150.00 + 1005.13 = Rs. 1867.79 /hr. (Power consumed by stenter with Thermic Fluid system is higher compared to direct gas fired system due to Oil radiators offering resistance in Blower Air Circuit. Ideally the Blowers have to be larger in case of Oil radiators in comparison to direct gas fired system for identical drying capacity.).
Direct Gas fired stenters work out marginally better compared to thermic fluid system for the parameters assumed in above calculation. However, of late imported coal having calorific value of 6000 k cal/kg is available in the market in the same price range. This will change the equation in favour of coal-fired system. Reliability of gas supply too is in question an many units in Surat are experiencing frequent stoppage of gas supplies. 3.0 Means of Energy Saving on Dryer & Stenter The most energy consuming equipments. Drying of Wet fabric consumes the highest energy compared to any other process in textile processing. The known means of extracting water from fabric and the resultant remained water (expression in % is shown in Table B below. Table B Type of Equipment Mangle Mangle Mangle Centrifuge (Ebonite/Rubber) (Rubber/ (Ebonite + + (Hydro Rubber) Roberto Roll) Extractor) Fabric Condition Open width Open width Open width Rope Operation Continuous Continuous Continuous Batch Expression for 100 % Polyester 65% 55% 25% 25% 100% Cotton 70% 60% 55% 50% 65% Poly 35% Viscose 60% 52% 40% 35% Electric Power Consumed 1.25 kwh 1.45 kwh 1.45 kwh 6 kwh For 1000 mets 65/35 suiting 300 gm/i.m. (300 kgs/1000 mts) at 40m/min speed. Cost of Electric power/1000 mts Rs. 6.25 Rs. 7.25 Rs. 7.25 Rs.30.00 Let us examine the cost of energy on a Cylinder Drying Range for all the above cases. Thermal Energy 300kgx0.6. 900kcal 300kg/0.25x 900 300kgx0.40x 900 300x0.35x 900 required for 1000 mtrs (300kg) fabric = 16,2000 kcal =140,400 kcal = 108,000 kcal =94,500kcal Equivalent coal = 51.4 kg = 44.57 kg = 34.28 kg =30.00 kg consumption Cost of coal/1000 mtr =Rs.113.05 =Rs. 98.05 =Rs.75.43 =Rs.66.00 Total Power & energy = Rs.119.33 =Rs. 105.30 =Rs. 82.68 =Rs.96.00 cost/1000 mtr General tendency of buyers is to pay least attention to a Mangle while buying Cylinder Drying Range. From the table above, one can realise that a good mangle (10 Ton capacity) compared to ordinary (2T) will pay back in less than a years time. It is also worth while to have a hydroextractor if the previous process is in rope form.
Drying on Stenter vs. Drying on Cylinder Drying Range Drying on stenter not only needs thermic energy for heating but also needs electrical energy for blowing hot air on the fabric. This makes the stenter drying the costliest process, Table C below shows comparison between Cylinder Drying and Stenter considering coal as the fuel for Boiler/Thermic boiler together with Electric Power. Table C Cylinder Dryer (16 cyl) Stenter (5-Chamber) Drying speed of 65% poly 35% viscose 40 m/min 40 m/min Suiting with 52% mangle expression (300 gm/lmt) using a Rubber+Rubber Mangle Production in kgs/hr 2400 mts 2400 mts. Production in kgs/hr 720-kgs/hr 720 kgs/hr. Water to be evaporated 0.52 x 720 = 374.40 kgs/h 374.40 kgs/h Coal Consumption 374.40 x 900 374.40 x 1000 4500 x 0.7 4500 x 0.7 = 106. 97 kg =118.86 kg Cost of /hr = Rs. 235.33 Rs.261.49 Cost of power /hr = 5 kwh x Rs. 5 = Rs. 25.00 100 kwh x 5 = Rs.500 Total Energy cost/hr = Rs. 260.33 Rs. 761.49 It is obvious form the above cost analysis that it is atrocious to use stenter or any convection dryer for simply drying the fabric. Stenter however is an essential equipment for heat setting and equalising the width. Most processors also use Stenter for final finishing of fabric. Ideally even for finishing the fabric one should pad-dry on Mangle + Drying Range and then only bring on stenter for curing/width equalising and controlling the shrinkage. The cost of this process will work out cheaper as the stenter speed can be increased to 50 m/min for the fabric mentioned in the above example. The exact cost comparison is given in table D below.
Table D (Energy Cost of Finishing Process on 65/35 300-gm/lm suiting). Mangle + Drying Range + stenter Mangle + Stenter Drying speed on Cylinder 40 m/min NA Dryer Cost of Energy + Power (from Table-C) Rs. 260.33 /hr. or Rs.260.33 / 2400 mts Speed on Stenter 50 m/min 33 m/min. Energy required in one hr. 15000 Kcal/ch x 5 35 x 60 x 0.3 x 100 =75000 K.Cal =630,000 K.Cal. Energy required for 2400 mts. =75000 x 2400 = 630,000 x 2400 ------- ------- 50 x 60 35 x 60 = 60,000 K.Cal =7,75,384 K Cal. Coal Consumption = 60,000 = 775384 --------- = 19.04 kg ------ = 246.15 kg 4500 x 0.7 4500 x 0.7 Cost of coal / 2400 mts. = Rs. 41.89 =Rs. 541.54 Power consumption for 2400 mts. = 100 Kw x 2400 100 Kw x 2400 ------- -------- 50 x 60 35 x 60 = 80 Kwh. =114.29 Kwh. Cost of Power for 2400 mts. = Rs. 400.00 =Rs.571.45 Total Energy Cost for 2400 mts. = Rs. 441.89 =Rs. 1112.99. The cost saving is unbelievable and technicians must try to establish finishing process on Cylinder Drying Range. On cylinder dryer the fabric temperature never reaches above 130 o C which is far below the required temperature of 170 o C for polyester to be affected in any way. It is therefore only a myth that polyester blended fabric will get harsh and shiny when dried on cylinder drying range. Below are some practical measures one may take to minimise energy consumption on Cylinder Drying Range and Stenter. Cylinder Drying Range Precautions on a Cylinder Drying Range to run it efficiently are: No steam leakages in the pipe line. Good functional steam traps properly maintained, preferably independent steam trap for each cylinder. Recovery of condensate for Boiler feed. Proper tension in the fabric so as to have intimate contact between fabric and cylinder. Uneven tension will form creases as well as reduce drying efficiency. Maintain clean surface of cylinders teflon coated cylinders are recommended.
Stenter: Precautions on a Stenter to run it efficiently are: Sealing of chambers with proper asbestos rope packing and tightening top, bottom and door panels is a must. Leakages of fresh air into the machine through circulating Blower fans can increase the energy consumption by more than 30%. Exhaust Blower and exhaust damper control is meaningful only when the chamber is properly sealed, otherwise the leakage into the chambers will be much more than the exhaust capacity. This is the main reason for fumes leaking out from feeding and delivery openings of the chambers. If chamber sealing is proper exhaust Blower of 5 HP capacity for a 5 Chamber Stenter is more than sufficient to keep fumes leaking out. Controlling of Damper or Exhaust Blower RPM will than be useful for energy saving. During finishing process on Stenter, technicians prefer to keep actual speed about 10% lower than what it should be. This is mainly to keep a safe margin due to over shooting and under shooting of chamber temperature of as much as 10 O C. The main cause of temperature fluctuation is the poor quality of control valves used. Here the investment in good quality motorised valve and proportionate temperature controller will pay back in just few months due to higher production. Technicians also tend to run stenter at safe speed during finishing process, as it is impractical to adjust the speed with change in fabric colour and weave in the same batch. It would be worthwhile to install a device which will measure and control fabric temperature in the beginning of last chamber. If the fabric temperature is more than the desired temperature, the controller will switch the blower fans to lower speed in the last chamber, there by saving substantial power. (Rs 50/hr). Alternately, machine speed can be automatically increased. (This is generally not acceptable to technicians as they are worried about the finish). 4.0 Role of Process Instruments in Energy Savings Various electronic Instruments are available today for process controls. It would be worthwhile to study their real worth for energy saving. Invertors (Variable Frequency Drives) - Invertors for Stenter In a stenter Invertors can be used for the Main Drive, Mangle Drive, Over feed system and Blower Motors and Exhaust Blower Motors. Of the total power consumed of 140 Amps in a 5 Chamber Stenter, 110 Amps is taken up by Blower Motors and the remaining drives consume 30 Amps. Of this, Main Drive, Mangle Drive and overfeed Drive together do not consume more than 20 Amps. By using Invertors one can at the most save 10% power compared to D.C.Drives. i.e. Rs. 7 per hour. The additional investment is not worth. As regards to Blower Motors, most stenters have dual speed motors, thereby operator can choose required Blower speed. Practically speaking with Invertor for Blower Motors it is rather difficult for operator to select the precise Blower speed for different fabric and hence the investment is not worth. [However Invertors for Drives are recommended for maintenance free working].
Invertors for jet / Soft flow / Dyeing machines Most of the Jet Dyeing / Soft flow machines have pumps of larger capacity than actual requirement (suitable upto 500 gm/linear meter fabric). In practical use, nozzle flow is throttled by 50%. Here it will be worthwhile to use Invertor for the main pump and remove pressure control valve for the Nozzle. The savings could be upto 30% i.e. Rs. 15 /hr. Invertors for Hydraulic Jiggers For the large size (960 kgs capacity) the main pump motor consumes 15 HP throughout the operation. In actual practice though the machine is designed to operate at 150-m/min speed, it is usually run at 100-120 m/min speed. It would be worth while to use an invertor by replacing flow control valve. Savings could be upto 30% i.e. Rs. 15/hr. Moisture Control in Exhaust for Stenter / Dryers This instrument is very expensive in the first. Secondly, as explained earlier the exhaust blower speed or control of exhaust damper is meaningful provided that there are no leakages of fresh air into the machine. Further the belief that with exhaust closed drying speeds goes down is only a myth. In actual practice there is no difference in drying speeds with or without exhaust. Moisture control on Fabric on Stenters / Dryers This instrument is useful only for 100% cotton /viscose/wool fabric and its use is recommended, as it is affordable. With the desired moisture in the dryed fabric either the machine speed can be automatically varcid to get optimum speeds or the blower of the last chamber can be switched to high or low to save energy. Full Computerisation of Stenter Full computerrisation of Stenter provides several features as enumerated below. Accurate Temperature Control in each chamber. Measurement of fabric temperature and control of machine speed or high/ Low Blower speed in relation to exit fabric temperature. Measurement of total power consumed. Measuring of Fabric moisture and control of machine speed or High/Low of Blower speed in relation to fabric moisture. Production data logging like machine speed, temperature, Blower speed selection etc. These parameters can be stored in memory, which can be repeated when similar fabric is to be proccesed. All machine parameters are automatically set as per the programme. Exhaust Blower speed control to minimise loss of energy. Help for diagnosing machine fault during breakdown. Investment in computerising stenter is worth while as it eliminates human negligence and ensures optimum utilisation of machine with least energy consumption. 5.0 Conclusion: Energy saving in textile processing largely depends on the judicious investment in plant and Machinery, choosing the appropriate fuel depending upon the location of the plant. Computerisation of machines to eliminate human negligence and obtain real production and energy figures without manipulation to take corrective measures. Reference book: BTRA & FAITMA Seminar on Conservation of Utilities in Indian Textile Industry November 26, 2002