Although heatless dryers are simple in design, very reliable, and generally a low maintenance machine, the high cost to produce purge air for their use is a drawback to their use. Purge consumption may actually represent a higher percentage of actual air demand, as heatless dryers on a fixed time cycle will continue to use the same volume of purge air regardless of system demand. This excess purging will result in lower dew points which are usually not a requirement. As the dryers are normally over-sized to account for peak demand, and that demand may be reduced substantially throughout the production cycle, it is beneficial to invest in the option of “Purge Economizer Control” which will maintain a constant dew point control (within 2-4ºF), and will reduce purge flow through the monitoring of the outlet dew point. Purge consumption can be reduced substantially using this optional control system, and based on the size of the dryer, this option can show a short payback period. Other benefits of this system will be a digital dew point display, so the user is aware of the performance of his dryer, and a watchdog monitor which will show the amount of time in the “Purgeless” mode, and the volume of compressed air saved (over standard fixed time cycle).
Other types of regenerative dryers use heat as a method for regeneration. These include internal heat reactivated type, external heater assisted purge, and heated blower purge types. A study of power consumption will show that these types will operate at higher efficiency levels than heatless dryers. Based on the additional equipment required to make these dryers function, there will be more components to service over time, and capital cost for the initial purchase will be higher. Based on the desiccant being regenerated with heat, the activated alumina desiccant used will deteriorate faster, and the life span for desiccant is about 3 years instead of the 5-7 years which is normally achievable in heatless dryers. After all of these considerations for additional maintenance and desiccant replacement are made, these dryers will be more attractive for larger air volume applications.
In recent years, air compressors have become more efficient at partial loads, which was not the case with simple modulating inlet valve rotary screw compressors many years ago. When weighing the advantages of heated purge dryers against heatless dryers, the air compressor efficiency should be considered. If your air compressor cannot show considerable energy saving when operating at partial loads, or the saving of compressed air does not allow you to operate fewer compressors at one time, then it will be difficult to calculate true energy savings. Any energy study concerning regenerative dryers should take into account the efficiency of the source air compressor(s) at partial loads.
For small reciprocating air compressors (under 30 CFM) operating on an auto start/stop basis, a “Purge While Pumping Only” system should be considered. Normally, on a fixed time cycle, these dryer would purge continually which may cause the compressor to start up only for the purpose of supplying purge air, even though there may be no system demand for air. With the “Purge While Pumping Only” control system, the dryer will operate on a shortened time cycle, and when the compressor shuts off, the purging tower will re-pressurize so no further purging takes place. On the next startup of the air compressor, the towers switch, and the last tower drying will depressurize and purge. This system should only be applied to small compressors using auto start/stop controls, but will show considerable energy savings and reduced air compressor operation.