Introduction: Compressed air (CA) is an inevitably essential utility of the cement plant from quarry operation to cement packing, however unfortunately in majority of the plants compressed air is taken as an out of sight and out of mind utility and considered as somewhat free of cost utility until plant starts to suffer due to drop in pressure or supply inadequacy, moreover in most of the plants an easy go solution for this problem is considered only to increase pressure set for compressor or simply addition of one more compressor.

In reality compressed air is an expensive form of energy, and is likely consuming a significant slice of your energy dolor. For an air cooled compressor over first ten years of life time with two shift operation, the operation cost (Electricity and Maintenance) will be equal to about 88% of the total life time cost, the remaining 12% will account for installation and original equipment cost. Moreover only 10% of the electric energy input is converted to useful mechanical work and 90% is lost in the compressed air system in the form of heat, use in system components and inefficiencies of end user. Although the heat of compression can be recovered from compressed air and can be potentially used for other processes, water heating etc. However this practice is not so common in cement plants so far. Therefore in general an efficient compressed air system is practical solution to achieve reliability and efficiency. Moreover avoiding miss use and chronic  leakages is the only most appropriate way to control the cost of this utility.In general it is worth to question if compressed air is the most appropriate power source for the end user application. This article will briefly explain the strategies for optimization of compressed air system as a whole.

Generally in cement plant compressed air is used for the following application.

• Wagon drills in quarry.
• De-dusting Bag filter cleaning.
• Air blasters and air cleaning lances for material movement in hoppers and pyro processing.
• Operation and control of valves.
• Instrument air for movement of livers and probes.
• For atomization of fuels and water spray in cement mills.
• Pinion-girth gear grease lubrication

Optimization of Compressed Air System.

Optimization of compressed air system needs to take care of both the sides equally, the supply side and the demand side and to know well how these two sides interact with each other. The supply side consists of compressors (fixed rpm or variable rpm), air treatment equipment, primary storage and control system. While as the demand side consists of distribution piping, secondary storage, online treatment equipment and end user. Various optimization strategies are discussed below.

Location of compressor: As a breathing machine compressor efficiency improves with cool, clean and dry air intake.This can be achieved by extending air intake to outside of the compressor room, at the same time we have to be careful that the intake area is not a damp place like near to cooling tower where air humidity is comparatively higher and the intake extension duct diameter, length and the number of bends has to be carefully chosen in order to keep pressure drop across the extended intake duct minimum possible. As a thumb rule “for every 10 degree centigrade rise in temperature of intake air will increases energy consumption of compressor by 2.5% for the same output”.

Intake air filter: Intake air filter is an essential component of compressor to ensure protection from the dust entering compressor, causing sticking on valves, scoring of cylinder, screw and excessive wear. The dust separation capacity of an efficient air filter should be high. Moreover Low pressure drop across the filter and robust design to avoid frequent cleaning and replacement are required. Manometers or differential pressure gauges across the filter may be provided for monitoring pressure drop so as to plan filter cleaning schedule. Due to chocking of filters by dust pressure drop increases, therefore regular cleaning is mandatory, it must be kept in mind always that “for every 250 mmwc of pressure drop increase across at the suction path due to chocked filters etc. the compressor power consumption increases by about 2% for the same output”.

Inter cooler ,After cooler, Dryer and drainage: Very often in cement plants single stage screw compressors are used for compressed air supply at the  pressure of 7-10 bar. So the impact of inter cooler is not included in this article.The function of after cooler is to remove moisture in air by reducing the temperature. And the residual traces of moisture are removed using air dryers.Moisture drain traps are used to remove moisture in the compressed air manually or automatically. Efficient operation of after cooler, dryer and moisture traps are essentially important to ensure moisture free compressed air supply to instruments, tools located upstream without the need of  an additional dryers, drain points which in turn consumes significant energy and introduces  pressure drop in air distribution system.

Reduction of delivery pressure:This is a best way to achieve an energy optimization. Care should be taken while exploring this option. Pressure loss in the distribution network has to be taken into account while reducing delivery pressure. There is no benefit what so ever to operate compressor at pressure higher than required, and the set maximum pressure is determined by the high pressure demand end user.

“Reduction in delivery pressure by 1 bar in compressor would decrease the power consumption by 6-10 %”.

Compressor modulation by Optimum Pressure Settings: Very often in an industry, compressed air is served to different sections from centralized or decentralized compressor rooms consisting of different types, different capacities and even different make compressors supplying air to a common distribution network. In such situations, proper selection of a right combination of compressors and optimum modulation of various compressors can save significant electric energy and consequently reduce the cost of compressed air generation.

It is quite important to give consideration to the compressor to be modulated and the control to be selected to achieve optimal energy consumption. For similar compressors in group only one should be modulated to handle load variation and all others must be operated at full load. For different types of compressors in a group the one with minimum part load may be modulated. For different capacity compressor in a group, smallest one should be set to modulate. In general, the compressor with lower part load power consumption should be modulated.

Various modulation controls strategies are just mentioned here in the order of decreasing energy efficient modulation.

• On-Off Control (For small compressors and big storage for compressed air).
• Variable speed control.
• Load/Unload control
• Variable displacement.
• Inlet vane control.

All parameters are to be considered at the same time to select the one or combination of above mentioned control strategies. It is worth to mention here that sufficient compressed air storage (10 US Gal per CFM) will ensure energy saving benefits of modulation control as discussed above.

Minimum pressure drop in Distribution lines:

Inadequate pipe size increases pressure very steeply, therefore it is recommended to consider large diameter to keep pressure drop minimum.Thisfact can be well understood by knowing that “for a flow rate of 170m³/h(100 CFM) if the pipe nominal bore is increased from 40 mm to 80 mm, the pressure drop per 100 m length will reduce from 1.8 bar to 0.04 bar. Reduce number of fittings in pipe line especially globe valves, u-bends. Prefer a loop pipe which will not only increase pipe area to reduce pressure drop, but as well ensure better pressure distributions for farthest end users.Consider decentralized small compressor rooms in various plant sections instead of having one centralized compressor room to keep the distribution losses low. The acceptable pressure drop in industrial practice is 0.3bar in mains header at the farthest point of use.

Segregation of low and high pressure applications: For low pressure end users compressed air should be preferably generated separately instead of reducing the pressure by reduction/regulator valves, which invariably wastes energy.

Avoid miss use of compressed air: Discourage compressed air use for the applications where actually a moving air will serve the purpose like floor cleaning, cooling instrument and bearings, body cleaning. Wherever safety provision allows use electric operated tools instead of pneumatic tools.

Avoid Compressed air leakages: Here is the biggest and best opportunity to save energy in compressed air system.

Due to poor maintenance and sometimes due to improper installation leaks occur at air storage/receivers, relief valves, pipe and hose joints, shut off valves, quick release couplings, tools and equipment’s. In most of the industries 15-30 % of energy can be saved by properly arresting of air leakages in compressed air system. Compressed air at 7bar pressure discharges at the rate of 32.5m³/minute through an orifice of 5mm.

A simple method is included here to find the percentage of compressed air leakage.

Shut off all compressed air end users, Run the compressor  to fill  the system to set specified pressure P2 (unloading  operation) Now leave it till pressure decreases to P1 (loading pressure) pressure, Note the sub-subsequent time taken for 'load' and 'unload' cycles of the compressors. Take 5-10 cycles for accuracy.

Let loading time be “T”minutes and unloading time be “t” minutes.

Then

 % leakage= [T/ (T+t)] x 100

Or system leakage quantity (m³/min) 

Q =% leakage x Q    m³/min.

Where Q is compressor capacity in m³/min

Conclusion: Compressed air as an energy of choice needs utmost optimization in generation and utilization. After installation of an energy efficient compressor(s) with proper auxiliary equipment’s like filter, dryers, etc. in a well-designed distribution network of minimum pressure drop with sufficient storage capacity receivers,it is the routine maintenance (such as fixing of leakages, replacing clogged filters, reducing air pressure) to ensure optimal operation cost. Even higher saving can be achieved by separating low & high pressure applications and adopting better control strategy for compressor modulation.

 

Bibliography

1. Ivor F. da Cunha P.Eng of Leap Frog Energy Technologies Inc.

For the CEA Technologies Inc. “COMPRESSED AIR Energy Efficiency Reference Guide”

2. Book-3 Bureau of energy efficiency, India

For National Certificate Examination for Energy Managers and Energy Auditors “Energy Efficiency in Electrical Utilities”.

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