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Concept:
If you've ever used an electric drill in a piece of wood or metal, you may have noticed that the chips or spirals of metal follow the contour of the flutes up and out of the hole. Most of them, at least.
A similar sort of phenomenon occurs inside the rotary screw compressor housing.
At the wide end of the screw (sometimes there's one screw, sometimes more) an inlet valve allows free air into the screw housing when there's a demand. Free air flows into the housing from the outside as there's a partial vcuum formed inside the housing as the screw(s) rotate.
The air / oil mixture in the screw housing moves along the flutes from the wide end of the screw towards the narrow and pulling a vacuum behind, and drawing more air into the screw housing.
As the air / oil blend is pulled along the flutes of the screw the space in which the air is contained gets smaller and smaller. The diameter of the screw is larger at the inlet end, and smaller at the discharge end - thus compressing the air.
Manufacturers of rotary screw compressors, have their own ideas of what constitutes the ideal geometry of the screw within their air compressor. Rotary screw compressors may have just one screw (also sometimes known as augers), or there may be two or more. Single screw compressors function the same way as multiple screw units, with the air being compressed between the housing of the screw compartment and the screw itself, rather than between two or more screws.
This drawing's purpose is to give you an idea of how the rotary screw concept works with two screws. The actual "guts" of the rotary screw compressor will vary substantially depending on the designs of the company that manufactured that particular compressor.
The drawing shows the two screws which would be housed inside the screw compartment of the compressor, in a bath of oil.
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At the narrow end there's an outlet valve which feeds the compressed air /oil mixture into a separator. The separator will remove as much oil from the compressed air as possible, and then release the compressed into the compressor receiver and then into the plant main air lines.
Features / Comments:
Although rotary screw compressors are available in lower horsepower ratings, it would appear that it is in the 20-25 horsepower and-higher-range that many industrial compressor applications tend to move towards using a rotary screw compressor solution from other types of compressors.
One major manufacturer states that the rotary screw air compressor has become the most popular source of compressed air for industrial applications.
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It may be because of the need for a compressor with a continuous duty cycle. Some rotary screw compressor manufacturers claim a duty cycle of 24/7/365 , which is pretty incredible for any electro-mechanical device.
Rotary screw compressors are available with a direct motor-to-screw drive, others are belt driven. Each have their benefits best obtained from the actual manufacturer.
The perception may be, and one that is claimed by some manufacturers, is that rotary screw units have the least maintenance issues of all types of air compressors and are touted as being the easiest to maintain both in terms of regular maintenance and unscheduled downtime.
When you move up into the higher horsepower units, rotary screw units are reputed to be lower cost to purchase over a comparably sized reciprocating compressor and they boast lower operating costs than either vane or reciprocating units.
Others suggest their oil carryover from the compressor into the compressed air supply of the plant is calculated in parts per million per day, that rather than the ounces or more of oil that can enter the plant air-stream from older reciprocating models and some well-used vane models.
Other firms suggest that their rotary screw units boast a very low operating decibel rating, and claim noise output levels far below other types of compressors, an important issue to be considered for the benefit of workers in the area. It is our experience that the lower operating sound levels may not eliminate the need for a soundproof housing, unless the compressor is well equipped with sound-deadening cladding.
As noted in various locations in this site when you are moving towards selecting a rotary screw ( or any compressor type), you first need to know how much air you'll need in CFM at the PSI you need for your plant, your tools, and all ancillary equipment for now, and for the future. You'll want to build-in excess volume available, as one statistic we've seen says, on average, more than 10% of all compressor capacity is lost through leaks, despite the best efforts of the plant to reduce wasted air.
Once you got the compressor size figured out, and this link will help, then you'll want to review the up-front cost of compressors from a host of manufacturers. Check their mean-time-between-failure rates, their parts and service costs, the life expectancy of the unit with the duty cycle you will require, and don't forget to get the figures for operating costs.
Compressing air is expensive, and one compressor might provide lower up-front capital costs, yet end up being far more expensive in the longer term due to it's higher operating costs.
All factors having been considered, and certainly this is claimed by many of the manufacturer's of this type of compressor, the Rotary Screw compressor may surface as your best choice for your industrial application.
Here's more info on compressors in general rotary screw.
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