An insight into the challenges in compressor technology | The compressor technology today has reached a fairly matured stage. Various types of compressors have found their own typical place in the market. This depends on the application domain. Reciprocating compressors have found their place for high pressure applications. The pressures would range from 20 to 500 bar. These are both oil lubricated and oil free applications. Air separation industry, cylinder filling are some of the typical applications for this. It is also very common to find the low capacity oil free reciprocating compressors for 100 psi working pressures. The reason here is the expensive manufacturing of oil free screw compressors in smaller capacity range.
Reciprocating compressors also have a very strong domain of gas compression (like CNG) and other process gases. This is because of the versatility of reciprocating machines and the comparative ease of changing the material to suit the application. The inter-cooling material can also be chosen with relative ease as the coolers are eternal fitments on the compressor. Multi staging of recip machines is achieved with ease either by increasing the number of throws or by creating tandem fitments of cylinders.
Oil injected screw compressors have been a great success story over the years and they cover a very wide range of applications. Small and medium industrial air requirements are all taken care of by oil injected screw compressors. Compactness, ease of operation, low maintenance requirements are strong advantages of this technology. The skill sets required to operate and maintain these types of machines is of minimal level and is limited to change of filters, separators and oil.
Oil injected compressors with downstream filtration actually handle a large section of requirements of oil free compressors as the outlet air quality is practically oil free. There are some varieties of water injected screw compressors, as a cost effective solution to conventional oil free screw compressors. However, the success of this technology is only limited for reasons associated with effective water removal and the life of non metallic rotors. Water lubricated bearings is another area of concern.
Energy saving on the screw compressors has evolved over many years by improvement of profiles. The profiles have moved from A to D to N and G. There could be some more derivatives of the profiles. The lobe combination has also been experimented with the most popular being 4 x 6 and 5 x 7. All the design efforts have been focused on minimising the blow hole area so as to control the back flow of compressed air/gas/refrigerant.
Manufacturing methods like grinding have also added to the consistency and efficiency because of the achievement of closer clearances. Popular method of energy saving on screw compressors now is the adaptation of variable frequency drives. This is greatly possible because screw compressors are positive displacement machines with practically linear curve for speed to volume.
Oil free screw compressors find their place in the market in the domain of medium capacity and medium pressure applications. As a technology, this is fairly expensive to manufacture and also expensive to maintain over a longer period of typically seven years or so. However the technology is still popular as the size of recip machines in this area is very large compared to oil free screw compressors.
Oil free screw compressors are also quieter in operation as they typically come enclosed in canopy and are ready to use. They don’t need any preparations like foundation etc. When the demands are of higher magnitude, the choice is for non positive displacement centrifugal machines or the axial flow machines. These are typically multi stage machines even for moderate pressures as the ability to build pressure per stage is limited.
Of course the reduction in the specific power is a perennial challenge that all designers face forever. The compressor technology is not different to this concept. There are various measures being taken to reduce the required input energy. One very common driver in this area is to use higher efficiency electrical motors. Many experiments have been launched commercially using the permanent magnet motors to improve on the efficiency.
The gas compressors are invariably coupled to gas engines to encash on the low cost and readily available fuel. A typical compressor installation will have about 70% of the lifetime cost from the energy consumption. Also the receip machines are required to spend much frequently on the maintenance of valves, rings, gland packing etc. on process and critical installations. There are various instrumentations applied to actually detect the symptoms of the necessity of maintenance.
The challenge is really to expand on the life of the consumables. Lot of research has gone in the materials of piston rings and many non metallic varieties have come up claiming almost double the life than conventional materials. Valves of non metallic constructions are available but are currently far too expensive for common use. A lot of research is still required to make this available as commercially viable solutions.
Increasing the speed of compressor is also one area which is being tried. However, the governing standards do not allow crossing certain limits. Also, as the valve operations on higher speeds become very critical, these units are successful as booster compressors.
The maintenance intervals on screw compressors are much less as compared to recip and are comparatively less laborious. This is the prime reason for the preference on screw compressors. The day when some innovation comes up for oil free compressors, to be commercially at par with oil lubricated compressors, the entire market will shift to that technology. Nobody really prefers to add oil and then find technologies to remove it. However, currently the oil removal systems enjoy a large market volume.
Centrifugal compressors are more efficient compared to the screw compressors. The challenge is to make them available at competitive prices. The range of centrifugal machines today would generally start from 2000cfm and above. Substantial design efforts are being put to come up with machines in the range of 500 to 2000cfm. Once the scale of economics is achieved on this, it is certainly going to be the market changer.
The centrifugal machines practically don’t need to be touched for years together and since they are better in terms of energy efficiency, the power saving will be a definite advantage. The centrifugal compressor technology is having a challenge that the turn down available on the compressor has to be larger. Current trends are generally 30% of turndown. Any advancement in this without sacrificing efficiencies would expand the gains further.
Many times when substantial flow variations are required to be addressed, the operating point efficiency is compromised to get much larger turn downs. This is also a larger challenge on the turbochargers for variable speed engines like automobiles. The flatter pressure gain curve will help in maintaining the engine power characteristics at all speeds. Energy savings on the compressor applications is also widely being helped by the electronic control systems which are available at very reasonable pricing in today’s world. We can have the data logs also created to analyse the compressor behaviour over a period.
The modern day control systems actually allow us to perform a very tight control on the operating pressures which means the pressure available at the point of use can be just adequate. Every one bar drop in pressure would end up saving about 6% energy. Hence, even if one is able to set the compressor at half a bar lesser value, the savings over a period will be huge. Control logics can also allow the weekly/daily programming as per the anticipated air demands.
Shridhar Naik
Associate Vice President
Kirloskar Pneumatic Co Ltd