Dew point controls on desiccant dryers should definitely be considered. If not controlled, the purge air circuit on a desiccant dryer will consume constant flow even through it might not be required. This represents a significant area of waste in your compressed air system, but something can be improved with a retrofit.
Purge air is required on twin tower desiccant dryers to regenerate desiccant that has been saturated by water vapor during the processing phase. The typical basic method of control on these dryers is a fixed cycle timer that switches the purge to one side or the other every 10 or so minutes (every 4 hours on heated dryers). This timer, and the purge flow, are set so that desiccant in the dryer will be completely regenerated during each cycle when the dryer is running at its full rated flow, and the compressed air being processed is at the dryer maximum rated inlet conditions (typically 100 psi inlet, 100° F temperature). For a heatless dryer, between 15 and 20% of the dryer rating is consumed as purge flow.
But in cooler ambient conditions, the air coming into the dryer usually contains less moisture. And during average plant conditions, the air flow processed by the dryer is often less than the dryer nameplate. These two things mean that the dryer, on average, will be consuming much more purge flow than it needs. Often this purge flow turns out to be the biggest compressed air demand in the plant during periods of light load.
For example, let’s calculate the cost of the purge air of a 1,000 cfm rated desiccant dryer sized for two 100 hp 440 cfm rated compressors, one main and one standby. Let’s say the average plant demand is 250 cfm. If the dryer is a fixed cycle unit, it will consume constant demand of 150 cfm, loading the air compressor to 400 cfm, near full load. The purge air percentage of the total compressor output would in this case be 60%, not 15%, the dryer consumes a constant 15% of the dryer nameplate rating, an important thing to consider. This purge would cost about $29,000 per year at $0.10 per kWh if run under these average conditions full time (8,760 hours per year).
If dew point controls were applied to this dryer—which would turn off the purge flow when not required—the average flow going to purge would be greatly reduced. Since the dryer would only be loaded to about 29% of its capacity (considering the actual required purge flow) the control would be expected to save around 65 to 70% of its uncontrolled purge costs, worth about $19,000 per year. Considering the cost of a dew point controller retrofit is in the $3,000 to $5,000 range, this would be an excellent project.
Not all dryers are lightly loaded like this, but typically dew point controllers bring attractive simple paybacks. The best time to purchase dew point controllers is when a new dryer is installed. Consider this, it may be worth your investment.
Quite often dew point controllers already exist on the dryer but have been turned off, or have never been activated. If this is the case, then it might be quite easy to get an attractive saving.