Wondor:
The basic data needed to give a full, correct, and concise response to your query is missing. You fail to give us sufficient data.
- What gas are you compressing? Is it toxic, inert, combustible, or hazardous?
- What gas are you proposing to use as purge? Is it compatible with your compressed product gas?
- How critical is it to have atmospheric air included or mixed with your compressed product gas?
- Do you purge to the atmosphere or do you flare the purge?
- What is the source pressure of the purge gas?
- How many stages of compression does your compressor have?
Purge gas can pass through the cylinders of an idle, multi-stage, reciprocating compressor from the 1st stage suction drum to the final stage discharge - BUT the source pressure of the purge gas must be sufficient to overcome the pressure drops through the entire compression and intercooling system. I have never seen this advocated, recommended, or employed. I've found that the time required and the steps to employ take too much time and an efficient displacement of all the pre-existing atmospheric air is just not practical. What I've always employed is a running purge where the compressor is manually operated with source gas and allowed to operate, pushing all the source gas through all the stages of compression and out to the final discharge. This takes a minimum of time and is the most efficient way to evacuate the compressor of all pre-existing air, while filling the same with the product gas and slowly bring up the various stages up to rated discharge pressure.
Check with your operating manual or with your compressor's manufacturer. I am sure that you will receive similar - if not exact advice.