I've visited the purge clean air system at LHO Yarm mid-station with John Worden.

The system is described C981637. There is a schematic in C981637-06-V (Vol.6).pdf although the schematic has some differences (or uncorrected mistakes).

This system is intended to provide positive pressure when a soft cover is attached to a chamber door. When the door is open, the purging does not help to keep the chamber clean because the flow is too slow. This protection has to be done with overhead HEPA filters (22x5000cfm). It may be possible that this purge air helps the tube not to allow dusts to come in. But before using this, the chambers and the tubes have to be cleaned, according to John.

- Here at the site, the purge air system is started up a day before the vent. This system is used for the vent air, the purge air, and turbo foreline filling.

- Air intake (attachment 1): At the site, the air is intaken from the VEA. We want to incorporate somewhat clean air instead of dirty, dusty, outside air.

- Initial filter (attachment 2): a high volume filter before the compressors.

- The compressors (attachment 3, 4) are 5x 6 horse power air compressor each goes up to 160 psi. They are turned on and off depending on the demand of the air. Which is turned on is revolved by the controller to equalize the compressor usage hours.

- The compressed air goes through the air cooler (heat exchanger) to remove the heat by the compressor work.

- This air goes through prefilters and accumulated in the air receiver (100psi) (attachment 5). This receiver tank has an automated vent valve for periodical water drainage at the bottom.

- The accumulated air is discharged to twin drier towers (attachment 6, blue). The tower is operated by the controller (attachment 7) alternately with a period of 4min (or 10min by setting). When one of the towers is working, a humid air comes from the bottom and the dry air is discharged from the top. A part of the dry air goes into the other tower from the top to the bottom and dries the tower. There is a vent at the bottom to discharge water periodically.

- The dried air goes through 4 types of filters. After the last filter, all of the plumbing should be made of stainless steel to keep cleanliness.

- The air goes to the pressure reducing regulator (attachment 8, gray). The final flow speed at the chamber side is 50cfm max, according to John.

- The lower pressure air goes through the final filter (attachment 8, blue). As the pressure is low, this filter is big in order to keep the volume of the air flow.

- The purge air is supplied to the chamber side with KF50 (attachment 9). There is a vent valve (attachment 10) for safety and also to run a dry air for at least a day before the use to clean up the supply line. The purge line is disconnected when no in use.

- The entire system (attachment 11) and size comparison (attachment 12).