Air Dryer Midland Grau

The Midland Pure Air Plus series air dryer is a desiccant type (moisture absorbing) air dryer. It uses two filtration elements to remove moisture and contaminants. A spin-on desiccant cartridge and a coalescing filter is used. Each requires replacement at recommended intervals. Refer to Maintenance Schedule.

In the charge cycle, contaminated, wet compressed air from the vehicle's compressor enters the air dryer through the inlet port. Air then is forced through the separate coalescing filter, where water droplets and contaminants are removed from the air stream. Next, the compressed air moves through the desiccant cartridge bed and the remaining moisture is removed. Some of the clean, dry air is diverted to the purge area to dry the desiccant bed. The balance flow through the outlet port into the vehicle's air system.

In the purge cycle, when the reservoir reaches the governor cut-out pressure, the air pressure from the governors unloading port enters the air dryer through the control port. This signal causes a valve to shift, close and block the inlet, while simultaneously causing the purge valve to open. The sudden opening of the purge valve permits the pressurized air in the coalescing filter sump to flush moisture and contaminants from the housing. Via rapid decompression it also backflushes the desiccant bed and coalescing filter. Air trapped in the purge reservoir is metered through the purge orifice and allowed to slowly pass over the desiccant bed at slightly higher than atmospheric pressure. Since low pressure air can retain more moisture per given volume, the relatively small volume of air can easily absorb the water previously absorbed by the desiccant bed during the charge cycle. The moist air is then expelled through the purge valve.