Compressor selection – what is the right compressor for each application?
There are several compressed air compression techniques and each technique has its own advantages over the others. Compression techniques include e.g. screw, piston, turbo and lamella technique. Different techniques also work in different pressure ranges, but here I will go through the most suitable techniques for the most commonly used pressure range 6 – 10 bar.
Benefits of oil-lubricated compressors:
• It is easy to recover heat from an oil-lubricated screw compressor via either oil or water cooling.
• Compressors with a frequency converter have a good control range and efficiency remains good throughout the output range.
• Up to 13 bar pressure can be applied with one screw element without intercooling.
Compressor oil thus works in many ways for the benefit of the compressor.
Disadvantages of oil-lubricated compressors:
• If purity of the compressed air is a critical factor, compressor oil is the biggest disadvantage. It is true that compressed air can be treated as virtually oil-free, but when using an oil-lubricated compressor, there is always a risk of oil entering the compressed air network. For example, a 75 kW compressor contains about 35 liters of oil.
• The oil is kept in the compressor by an oil separator. The oil separator is a wearing part that is changed regularly during scheduled maintenance. However, there is always a risk that the part in question is defective already new, and it fails when the compressor is running. In this case, all 35 liters of oil are compressed (with e.g. 8 bar pressure) into the compressed air network. Compressed air filters do not withstand such an oil load, so in practice the oil comes out of the last usage point of the pipeline network.
• In the food industry, the aim is to reduce this risk with food-grade oils. Food-grade oils are not harmful to health, but in such large quantities, the end product is certainly unusable, and the compressed air piping has to be washed.
The most common type of oil lubricated compressor is the screw compressor, which is a good “basic machine” for almost any type of use. The advantages of an oil lubricated screw compressor are e.g. long service life, good efficiency, low noise level and inexpensive price.
The advantages of a rotary vane include e.g. very long service life. The service life of a rotary vane is even longer than that of a screw compressor, and if the compressor “cuts out”, it is still possible to cope with a relatively inexpensive lamella replacement compared to replacing a screw compressor element.
Piston compressors are no longer seen frequently in the industry. They have the advantage of a very inexpensive price, but on the other hand piston compressor is generally not suitable for continuous use. In addition, maintenance is expensive in relation to its price. On the other hand, when aiming for high pressure (> 15 bar), a piston compressor is almost the only option.
Oil-free techniques include e.g. scroll compressors, water-lubricated and two-stage screw compressors, and turbo compressors. Oil free piston compressors also exist, but their use is very limited.
Most often, oil-free compressors are expensive, noisy, large, heavy, and “not so user-friendly” compared to oil-lubricated compressors. The biggest advantage of oil-free compressors is their oil-free nature.
When visiting a compressor room with oil-free compressors, the room is generally clean, odorless and tasteless. Oil-free compressors are generally cleaner. This is an advantage e.g. when the compressor is in the work area. In the case of oil-lubricated compressors, it is very common to see that there has been or still is oil, oily rags, etc. on the floor.
Advantages of oil-free compressors:
• Oil-free compressor does not increase the oil load of compressed air.
• The risk of oil entering the end-product or the compressed air network is completely eliminated.
Disadvantages of oil-free compressors:
• Oil-free compressors are often about three times as expensive as oil-lubricated compressors of the same size range.
• On the other hand, expensive analysis or production equipment that cannot withstand oil at all, are often connected to the air network. In this case, the price of the compressor, e.g. 10,000 euro vs. 30,000 euro, does not play a big role if the expensive equipment remain undamaged.
Scroll compressors generally consist of one or more 2.5 kW, 5 kW or 7.5 kW blocks. The advantages of these compressors are low price, good reliability, low noise level and low maintenance cost.
Disadvantages include poor efficiency and short service life of the blocks. On the other hand, in small and occasional use, the scroll compressor is an easy choice for compressor needs below 30 kW.
If a 4-block 30 kW compressor is dimensioned for use when normal operation requires about 15 kW, there are practically four compressors inside one compressor housing. Normally, two make compressed air and two compressors are in reserve. If one of the blocks breaks down, there is still one in reserve and two will make compressed air.
Scheduled maintenance of these devices is low compared to other techniques. With scroll compressors, the block is rather run until the end of its lifetime, and then replaced with a new one. The price of a new scroll block is most often a fraction of the price of a new screw element.
Water-lubricated screw compressor
When the operating demand is over 30 kW, the price of electricity starts to play a bigger role and the lower electricity consumption of screw technique overcomes the benefits of scroll technique. As a rule, the next step is water-lubricated screw compressors.
A water-lubricated screw compressor is basically similar to an oil-lubricated compressor, but water is used as the lubricant instead of oil. The advantages of these compressors are good control range, low noise level and good efficiency. The service life is average.
Price level of these compressors is between a scroll compressor and a two-stage screw compressor. As a rule, a water-lubricated compressor is a good choice if the compressed air demand is in the range of 30..75 kW.
Two-stage oil-free screw compressor
For larger needs >55 kW, an oil-free two-stage screw compressor is likely the right choice. Compressors are often costly, loud, and adjustment ranges generally worse. On the other hand, these compressors are long-lasting and reliable. Depending on the technology, the efficiencies can be very good, even better than oil-lubricated screw compressors.
Recently, twin-motor compressors have also been launched, with excellent control ranges. Maintaining these devices costs roughly the same amount as an oil-lubricated screw, but of course the investment cost and replacement of the screw elements is a costly operation. Of course, when investing, you must always remember how much it will cost if oil gets into the compressed air network.
For large needs over 250 kW, a turbo compressor may be the right solution. The advantage of turbo compressors is the best efficiency on all compression techinques, highly reliable technology and a very long service life.
As a rule, the control range of turbo compressors is 30-40%, so if we talk about consumption of air 50 m3/min, for example, the control range of the turbo compressor can be up to 20 m3/min. So, turbo compressor is not a base load compressor, although you often hear so said.
The initial investment cost of turbo compressors is usually high, but on the other hand, increasing the turbo compressor size, e.g. from 400 kW to 600 kW, no longer has a large effect on the price. For example, two 300 kW two-phase screw compressors are already a more expensive solution than one 600 kW turbo compressor. In addition, considering efficiencies, maintenance costs and service life, it is obvious that turbo compressor is the best choice in this compressor power range.
Sizing of a turbo compressor needs careful consideration and many things need to be taken into account; e.g. intake air temperature, required pressure, cooling water temperature, required control range, etc. All these variables have an effect on the compressor efficiency, maximum power, number of compression stages, etc.
The compressed air system is always a whole
When there is a shortage of compressed air, it may not be worth starting to buy a new compressor first, but the compressed air system should be considered as a whole. Sarlin AirAudit analysis of the compressed air system provides correct, measured information as a basis for decisions about the compressed air system and should be made before any additional capacity is acquired. The analysis does not focus only on the compressors, but on the functionality of the entire compressed air system, the quality of the compressed air produced, and the pressure level and its variations.
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