aikmeam:
Clearance bottles (pockets) are vessels that are attached to, or built into, the cylinders on reciprocating compressors. These vessels (or compartments can be "fixed" or variable in the amount of volume they offer to the gases that originate in the compressor cylinders. Like clearance volume, they represent a volumetric inefficiency of the compressor. These volumes are normally segregated from the cylinder's working volume by either a manual or automatic valve that joins both volumes when opened.
As you can appreciate, if the clearance bottle (or chamber) is of the same volumetric size as the compressor's cylinder and the two are communicated then the piston can do no work (no compression takes place) and the volumetric efficiency is zero. The gas simply is pushed out of the working cylinder into the clearance pocket and back. There are no moving parts and the energy consumed at 100% unloaded capacity is simply that of keeping the machine moving through friction and mechanical inefficiency - sometimes as low as 5% of the design BHP. The compressor starts to do work as soon as the clearance pockets are closed.
Look at the following reciprocating compressor paper:
Go to page 13 of 78 and read up on Volumetric Efficiency there and on following pages. You will appreciate, I am sure, the simplicity of controlling your reciprocating compressor at zero capacity with such a simple system.
HOWEVER, there is a catch! (Nothing in engineering is that simple to achieve) You must have the cylinders bored and designed for this feature - or have the latent capability already in the design. Also, with double acting cylinders this becomes a physically bulky installation. This feature works best when the original compressor manufacturer is formally specified to furnish such a feature. It can become hairy on some retrofits.
You're exactly right; avoiding sucking vacuum is one of the reason recirculation control valves are present in the discharge of reciprocating compressors. There is no way to specify or maintain a constant speed reciprocating compressor at the
exact required volumetric capacity under normal production. The machine has normal, variable valve inefficiencies, wear & tear, ring leakage, clearance increases, V-belt and driver slippage, etc. The only way to overcome these troublesome features is to
oversize the required capacity by a contingent amount and normally make up the overcapacity by re-circulation from discharge-to-suction (capacity control). This is never taught in thermodynamics or in compressor text books, but it is the common sense portion of working with this type of machine. While centrifugals are also designed with a certain degree of over-capacity, they can be throttled or speed-varied within limits; recips, being from the positive-displacement tribe, will not tolerate throttling and so have to be capacity-controlled through other means. Unless you have a synchronous electric motor drive, you have to settle for a constant speed recip.
Regards,
Art Montemayor
Spring, TX