The rule of nature says that energy can neither be created nor destroyed. But theres no rule that says that energy cannot be wasted. We never realize the amount of energy we waste. Using two tubelights in a room where even one can do the need full is wastage of electric energy. Keeping the water tap on throughout the period of bathing is wastage of water energy. Man, since ages, has wasted energy and has never even realized it. Today the energy of air is being conserved, through the process of Compressed Air Energy Storage (CAES), to be used in the future. CAES refers to the compression of air during periods of low energy demand, for use in meeting periods of higher demand. Typically, compression is done with an electrically powered turbo-compressor; and expansion is done with a natural-gas powered expander (heater) which drives a combustion turbine.
Appropriate care needs to be taken for efficient utilisation of the compressed air, operation and maintenance of the compressors, dryers and related equipment. This will avoid unnecessary wastage of the compressed air, and electrical energy. Companies utilising compressed air systems need to implement just a few simple steps for efficient operation to help in considerable saving of energy. In a compressed air system, about 20 per cent losses are due to inefficient piping, 10-30 per cent can be branded as leakage losses, and about 35 per cent are conversion losses. Certain steps need to be taken to avoid wastage of energy, as listed below :
- Operate the compressors at the optimum required pressure. If the requirement is 6 kg in the shop, operate the compressor at 6.5 kg pressure in the compressor room. Table 1 shows the energy loss by operating at higher pressure
- Ensure that the line pressure drops are less than 0.5 kg at the farthest end compared to the compressor room
- Take better care of the inter-coolers and after-coolers, use soft water to avoid scale formation, and magnets to reduce scale
- Improperly maintained inter coolers can result in unnecessary wastage of energy. Use proper instrumentation to measure temperature, and pressure
- Establish ideal conditions of temperature, and cleanliness in the compressor room
- Match the supply, and demand of the compressed air
- Use variable frequency drives to match the demand for compressors higher than 50 hp
- Avoid leakages in the compressed air system
- Regularly (weekly) conduct a leak test, and control the leaks less than 5 per cent produce to carry a leak test. When none of the equipment are working
- Percentage of leak = load time/load time + unload time x 100
- In many industries, it is observed that there is a 20 to 25 per cent leak. Normally the leaks will be at joints/valves and hoses. An ultrasonic leak detector also can be used to detect leaks
- Use energy efficient nozzles for cleaning application. These are specially designed nozzles, which will draw outside air and increase the pressure, four to five times
- Use a number of pressure gauges in the shop floor to monitor the line pressure (digital type for better visibility)
- Provide receivers at the point of higher consumption of the air
- Use PVC pipes for the air transfer, which results in less friction, and hence scale formation does not take place
- Use energy efficient dryers as refrigerant and heat less type and monitor regularly
- Use thermostatic controls for the cooling tower fan motors
- Use FRP blades for the CT fans
- Use energy efficient pumps for the cooling water circulation, and maintaining optimum pressures
- Provide interlocks for the water pressure and compressor motor, and cooling water circulation pumps
- Use alternate methods of cleaning. Instead of using compressed air/vacuum cleaners use low loss nozzles
- Use flat belt drives for the compressor, and motor. Take proper care about alignment
- Separate the high and low-pressure lines, and provide separate compressors
- Use auto drain valves to remove the moisture, instead of timer controlled valves
- Pneumatic tools consume 10 times more energy compared to electrical tools. Unless it is compulsory do not use pneumatic tools
- Pneumatic instrumentation should be replaced by electronic instrumentation (conveying systems also)
- Proper lubrication must be used to reduce the friction and energy, in compressed air lines, in FRL units
- It is recommended to use two stage and multistage compressors instead of single stage compressors
- Use regulators, and reduce the pressure wherever lower pressures are needed, in the system
- Close individual valves or solenoid valves near the equipment when equipment is not in use
- Use proper instrumentation as pressure and temperature gauges in the compressed air system, and differential pressure gauges in the section filters
- Use lower pressures in the return lines, of the cylinders
Case Study 1
Proposal Use blower air for agitation instead of compressed air in the effluent treatment plant and save energy.
Present system Presently compressed air at 6 kg pressure is released in the effluent treatment plant for mixing. Producing compressed air at 6 kg and releasing in the effluent treatment plant is not the correct practice. The above operation can be replaced by use of a blower. There will be considerable advantage by use of blower air.
Benefit Rs 3.46 lakh/year
Investment For blower and pippins Rs 75,000
Payback Three months
Present power consumption by compressor motor = 14.5 kW
Expected power consumption by blower motor = 2 kW
Benefit 12.5 kW
Annual Saving = 12.5 x 24 x 330 x 3.50 = Rs 3,46,500
Investment - for blower and pipe lines = Rs 75,000
Case study 2
Descale the inter coolers and after coolers as per schedule, and maintain the inlet temperature to the second stage at ambient temperature.
Three reciprocating screws and one screw compressor is used to supply air to the various equipments in the factory. At a time two reciprocating compressors and one screw compressor are in operation. It is observed that the incoming air temperature of the reciprocating compressors is 42 0C, and the outlet surface air temperature at the intercooler is 54 0C. The suction temperature of stages I & II is higher. This results in higher power consumption by motor.
Proposal : Draw outside cool air and direct it towards the suction of reciprocating compressors. Descale and maintain the inter coolers efficiently.
Benefit Rs 3.13 lakh
Investment For the ducting of suction air Rs 0.24 lakh. For a new spare inter cooler and cleaning
Payback Less than one month
Advantage by reducing the in coming air to 36 0C in compressor 3 = 43 0C - 36 0C = 7 0C
For 4 0C, saving is 1 per cent
For 7 0C = 1.75 per cent
Expected saving per day in compressor 3 = 99.4 x 0.0175 x 24 = 41.74 kWh
Saving due to cleaning of the inter coolers
Present temperature = 56 0C
Expected temperature = 42 0C
Power lost due to the difference = 14 0C.
Total power lost in compressors 3 and 2 = 3.5 % + 1.75 % & 3.5 % + 1.25 %
= 5.25 % & 4.75 %
Saving in compressor 3 per day = 0.052 x 99.4 x 24 = 124 kWh
Saving in compressor 2 per day = 0.0475 x 92.8 x 24 = 105.792 kWh
Total = 229.79kWh/day.
Annual Saving = 78129.28 kWh
In Rupees = Rs. 3,13,298.00
To make ducts from exiting blowers to the compressor suction filter - 3 no. Approximate distance is 30 feet - 1 x 2 feet cross section 2 points are at each compressor. The cost is Rs 24,000
Cost for the spare inter cooler not considered the cost since it is a maintenance requirement.