Compressed air required in the mills is at pressures ranging from 4 – 8 kg/cm2. Although, atmospheric air is free, its compression to the required higher pressure by air compressors costs money as the compressors are operated by electric motors. With the rise in power cost, it has become necessary to optimise compressed air consumption so that compressor usage is curtailed thereby saving electrical power.
To optimise the compressed air consumption, it is necessary to monitor its consumption and compare it with some basic value or ‘Norm’. The only norms or recommended values for compressed air consumption available till date, are those provided by some of the machinery manufacturers. No data was available on compressed air consumption on actual basis for various machines in operation in the mills. The aim of the present study was to measure/ estimate compressed air consumption for various machines and to correlate it with machine production so that some sort of initial set of ‘Norms’ or practical values for compressed air consumption could be made available.
Compressed Air System
Reciprocating type of compressors is normally in use in the textile industry. However, of late, screw compressors are being preferred, as these are more energy efficient as compared to reciprocating compressors. For very high capacity requirements, centrifugal compressors are used.
The compressor capacity is stated in terms of CEM i.e. cubic feet per minute of air (being compressed or free air delivery). Capacity range of air compressors normally used in the industry is as follows:
v Reciprocating Compressor, Single Stage : Upto 50 CFM
v Reciprocating Compressor, Two Stage : 50 – 600 CFM
v Screw Compressors : 150 – 2500 CFM
v Centrifugal Compressors : 2000 – 4000 CFM
In the mill, previously large number of small capacity compressors was in vogue. Now-a-days, centralised compressor(s) of higher capacity are preferred. This is because higher capacity compressors are more energy efficient and also operation and maintenance of less number of compressors is easier.
The compressed air generated by compressor is stored in the air-receiver of sufficient capacity to smoothen fluctuations in the supply of compressed air vis-à-vis demand from individual consumption points (machines). The air receiver also helps in cooling of air and thus facilitates removal of much of the moisture in the air, which is harmful for the pneumatic system. For large users/bigger compressors, air dryer units are provided. Air dryer units of two types are normally employed in the mills. One, the heatless dryer and the other is refrigerated air dryer, which is preferred of late. Although, the refrigerated air dryer has higher initial cost, it has less operational air is lot in purging thereby wasting costly electrical energy. The air dryer helps in making the compressed air almost 100% dry which is essential for some machines such as air-jet looms.
As regards air distribution network, it should be ensured that pressure drop should not be more than 0.5 kg/cm2 in the longest line. The pipe sizes are to be decided based on this consideration. Also, the pipelines should be of minimum length with less bends and fittings. This will ensure minimum wastage of electrical power.
For proper and trouble-free operation of pneumatic systems provided for the machines, it is essential to ensure filtration, pressure regulation and lubrication of the compressed air. Providing filters in the pipeline after the air receiver ensures that filtration. These can remove dust particles upto 5 micron size. Pressure regulation i.e. monitoring the desired air pressure for the machines used is achieved by the air pressure regulator. For reducing friction, wear and tear and preventing corrosion, proper lubrication of compressed air is necessary. This is achieved by the ‘oil-fog’ airline lubricator.
Compressed Air Consumption Study – Methodology
Preliminary visits were made to the mills for shopfloor observations and for discussions with the concerned technical personnel to decide the course of action to be followed in execution of this project. It was decided to use “Rotameter” type Air Flow Monitor (which was available with BTRA) so that pattern of flow rate in CFM (Cubic Feet per Minute) of compressed air required for different machines could be established. As the Air Flow Monitor is required to be fitted online, certain arrangements/ assembly of Air Flow Monitor was decided. Accordingly, valves and pipefittings were procured and an Air Flow measurement assembly was prepared (Fig. 1).
Procedure Adopted for Measurements
After discussion with mills technicians as well as based on shopfloor observations, suitable location for fixing the instrument was selected. Necessary care was taken while selection of site viz.,
➢ Minimum replacement of pipeline.
➢ Near to the stop valve and union joint.
➢ Easy to plug and remove the flexible/removable assembly of Air Flow Monitor. ➢ Suitable height to note the Air Flow Monitor reading.
As most of the machines are operated on continuous basis, all the preliminary preparations such as providing distance pipe pieces, etc., are required to be done in advance and during the recess/ stoppages; the instrument assembly (fixed part) is required to be fitted. While commencing studies, instrument part (flexible/ removable assembly) is plugged in fixed part, which is provided in the pipeline. The stop valve of fixed assembly is then closed so that air is passing through the instrument.
