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[eryllich]

Hi all,

We are looking to replace one of our old desiccant air dryers (water cooled) with a newer model. We estimate that our current dryer uses 15-18% of the generated air. I found a brochure for a Atlas Copco MD dryer that uses heat of compression to remove moisture and does not use any of the generated air.

If you reduce the load by 15% on the air compressor, will the electrical consumption also decrease by 15%? Is the relationship linear, or is it exponential, similar to a VSD controlling a motor?

The compressor runs off of 480 V and is 350 HP, 1600 CFM, operating at 125 PSI.

Any help would be greatly appreciated.

Thanks.

[Art Montemayor]

Eryllich:

Your electrical consumption by the air compressor depends on how you are operating it. You may operate on an “on-off” capacity control or you can unload the compressor and keep it running. You haven’t given us any details on the way you are operating nor have you furnished any basic data, so it’s impossible to say if you will save any electrical consumption. You haven’t stated the type of compressor, although by the capacity you cite it is probably a centrifugal type. You also haven’t stated your dew point requirements and what you are using the air for. From the capacity and the dew point limits on the Atlas Copco MD type dryer, I would guess you are simply drying air for instrumentation use.

If you are, indeed, using a centrifugal compressor I doubt you will have “on-off” capacity control. And I also doubt you will see any electrical savings based on the type of dryer.

If your incentive for changing the type of dryer is to save air capacity for effective use rather than for dryer regeneration, then you can still employ a normal adsorption dryer rather than the Atlas Copco MD type. Please look at the attached sketch and you will see how this is conventionally done. With the illustrated type of arrangement you can obtain much dryer air than you would with the Atlas Copco MD type – and you don’t sacrifice any air for regeneration. I don’t know what Atlas Copco means by their capacity rating of liters/sec. The normal way to measure a gas or vapor is not by using liters (a normal liquid measurement), and you don’t state whether your 1,600 cfm are at standard conditions. I have to assume they are and that the MD size dryer would be the MD800 VSD which yields a -20 ^oC dew point at an ambient temperature of 30 ^oC.

If you are running a centrifugal, how do you intend to reduce its capacity in order to realize your electrical savings? I presume that you are aware that should you opt for the Atlas Copco dryer, you are locked in to buying only their silica gel packing. Also, you are locked in to whatever compressor discharge temperature you are getting in order to regenerate (& obtain a decent dew point).
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