Regenerative desiccant dryers are used in compressed air systems that require dew points to be below the minimum that refrigerated dryers can produce (generally 40 degrees Fahrenheit). Three types of regenerative desiccant dryers are widely used throughout industry: heatless, heated and blower purge.
The following discussion doesn’t address heat-of-compression (HOC) desiccant dryers, even though they require the least amount of energy to operate. The use of HOC dryers is limited to lubricant-free compressors.
Many plants require air quality that only regenerative desiccant dryers can produce. Unfortunately, in too many cases, the decision about which type of regenerative dryer to purchase is based on initial capital cost alone. This decision basis ignores the cost of energy that will be required to operate the dryer. Including energy cost can alter the economics of a purchase decision dramatically.
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Regenerative desiccant dryers use a desiccant medium to remove moisture from the compressed air stream. Wet air passes directly through the desiccant medium, which then adsorbs the moisture. The desiccant medium has a finite capacity for adsorbing moisture before it must be dried out, or regenerated. To do this, the tower containing saturated desiccant medium is depressurized and the accumulated water is driven off using purge air, heat or a combination of both. Desiccant dryers are generally of a twin-tower construction, with each tower containing its own desiccant bed. This allows one bed to dry compressed air as the other undergoes regeneration.
The energy cost associated with operating a desiccant dryer depends primarily on how the desiccant is regenerated — using purge air, heat or a combination of both. Desiccant dryers are categorized by their method of regeneration, the three primary types of which are:
- Heatless, which uses only compressed air as a purge
- Heated, which uses both heat and compressed air to purge moisture
- Blower purge, which uses air from an external blower, heat and minimal compressed air
The energy cost to operate each of these dryers depends on the amount of purge air used, the heater size and the blower motor size.
Heatless desiccant dryer
Heatless desiccant dryers use treated, unheated compressed air to regenerate the desiccant bed. This dryer is the most expensive to operate because of the large amount of compressed air that’s consumed for purging during the regeneration cycle. Typically, 15% of the rated flow capacity of the desiccant dryer is consumed as purge air. This is a significant amount of expensive compressed air to be used in this manner. In many facilities that operate heatless units, the air dryers are the largest single user of compressed air at the facility. Although the cost to operate this type of dryer is high, the heatless type is often selected because it has the lowest initial cost of any desiccant dryer.
Heated desiccant dryer
Generally, heated desiccant dryers have the lowest energy operating cost of the three types. A heating element supplements the drying action of the purge air. The heater can be mounted internally to heat the desiccant bed directly or externally to heat the purge air that’s blown through the bed.
Internal heating elements shorten the life cycle of desiccant beds and may cause dew point spikes. Therefore, they are not recommended and are not covered here. Like heatless dryers, the purge is compressed air supplied from the system. However, the amount of air required is reduced because the heat supplements the air’s drying action.
The amount of purge air an externally heated dryer requires varies by manufacturer, but is typically about 7% of the dryer’s rated flow capacity. The combination of heat and purge air is more energy efficient than purge air alone, yielding lower energy costs.
Blower purge desiccant dryer
Like heated-type desiccant dryers, blower purge-type dryers supplement the drying action of purge air with heat. In a blower purge desiccant dryer, however, the purge air is ambient air supplied by an electric blower. The typical blower supplies the entire purge air load, which is typically 20% of the dryer’s rated flow. Depending on the dryer design, blower purge-type dryers may still consume a small amount of compressed air that cools the bed at the end of the regeneration cycle. Blower purge-type dryers are generally more expensive to operate than the heated type, but less expensive to operate than the heatless variety.
Determining your own energy costs
So, we know that heatless desiccant dyers are the most expensive to operate in terms of energy costs, heated dryers are the least expensive, and blower purge dryers are somewhere in between. But, how can plant management quantify these costs so they can be used to make an informed decision when purchasing a desiccant dryer? Table 1 shows the appropriate equations you can use to calculate the energy cost to operate each type of desiccant dryer.