The Application and Use of Heatless Regenerative Air Dryers
The use of heatless regenerative air dryers in the compressed air world has increased because of the demand for clean, dry compressed air. Heatless dryers are named as such because they do not use any outside sources for heat. Not only in traditional industrial applications but also in clean industries such as medical, electronics, food, semiconductor, and packaging.
There are many types of regenerative air dryers, but they all use different methods of regeneration. The two basic categories of regenerative air dryers are heatless and heat reactivated. In this post, we will focus on heatless.
The Advantages
There are many advantages to using a heatless regenerative air dyer. The greatest one of all is the lower initial cost of the dryer versus other ones. Here are a few more:
- Low maintenance costs
- Lower initial cost
- Capable of achieving very low dew points (-100 °F pdp)
- High reliability due to simplified design
- Capable of handling inlet air temperatures to 120°F (at reduced capacity)
How These Dryers Work
You may be wondering how exactly these dryers work. Below we will explain how these dryers function and operate.
Maximum Inlet Air Temperature
The maximum inlet air temperature is a very critical factor in using the dryer. The compressed air entering the dryer is normally 100% saturated. Therefore, two factors determine the maximum moisture load that a regenerative air dryer sees:
- Inlet Air Temperature – If the temperature of the compressed air entering the dryer exceeds the design rating of 100°F, the moisture loading capacity of the dryer will be exceeded at maximum load (and the dryer performance will be degraded). The extent of the degradation of performance will depend on what degree the inlet temperature exceeds 100 °F, compressed air flow rate and inlet pressure level.
- Maximum Inlet Air Flow – The design rating for regenerative air dryers is 100 psig. Air pressure above 100 psig will actually increase the capacity of the dryer, while pressures below 100 psig will decrease the capacity.
Basic Operation
The heatless regenerative air dryer, like all regenerative air dryers, is a dual tower arrangement. Each tower is filled with a desiccant material used to absorb moisture from the compressed air in the on-line tower while regenerating, driving moisture off the desiccant in the off-line tower. This process is continuous and when properly applied will provide pressure dew points (pdp) in the range of -40°F to -100°F. The towers are switched from an on-line to off-line state based on a fixed cycle timer which is normally set for 5 minutes or a total 10 minute cycle. The system then expels moisture from the system in the form of water.
Desiccant
The desiccant used most often in the heatless regenerative air dryer is activated alumina. It is the desiccant of choice due to the following reasons:
1. Economical
2. High affinity for water
3. High abrasion resistance
4. Good adsorptive capacity (at higher RH)
5. High crush strength
6. Resistance to degradation by liquid water
In cases where dew points (pdp) of -100°F are required manufacturers of heatless regenerative air dryers will usually make the following design changes:
1. Utilize a split desiccant bed of activated alumina and molecular sieve, usually an 80/20 ratio.
2. Utilize a bed of activated alumina but reduce the dryer cycle time from a 10 minute complete cycle to a 5 minute complete cycle.
Absorption Process
Compressed air leaving the compressor discharge is 100% saturated at discharge temperatures with the potential to introduce gallons of water into the compressed air system. As the compressed air enters the heatless regenerative air dryer, it is exposed to the desiccant bed in the on-line tower. The desiccant has a very high affinity for water. Upon contact, the water vapor from the compressed air condenses and is absorbed. As a result, moisture within the compressor is reduced. Latent heat of condensation is released as water vapor is condensed during the adsorption process (exothermic reaction), raising the temperature of the desiccant bed 10°F – 20°F.
The duration of the adsorption phase in the on-line tower is a fixed time of five minutes, matching the duration of the regeneration phase on the off-line tower. Just before tower switchover, the off-line tower will be pressurized to line pressure and the purge air will be off. Upon completion of the re-pressurization of the offline tower, the towers will be switch to provide continuous drying.
A Few Things To Keep In Mind
As with the advantages, there are usually a few things you need to keep in mind when installing and using new compressed air equipment.
Design Ratings & Maximum Inlet Flow Rate
The maximum inlet flow rate needs to be determined. Inlet flow rates above the design rating will have a detrimental effect on the performance of the dryer. This performance degradation is due to the following:
1. A reduction in resident (contact) time of the process air through the desiccant bed
2. Increase in pressure drop across the dryer
3. Decrease in desiccant life due to fluidization of the desiccant bed as velocities exceed 60 fpm
4. Exceeding the moisture load capacity of the dryer
Most manufacturers of regenerative compressed air dryer rate their dryers based on the inlet conditions which conform to CAGI Standard ADF 200.
1. 100 PSIG inlet pressure
2. 100 °F inlet air temperature
3. 100 % relative humidity
Regeneration Process
The regeneration process is instrumental in the release of moisture from the desiccant in the off-line tower in order to continuously provide dry compressed air. The regeneration process in a heatless regenerative air dryer is accomplished by two methods:
Dry Process Air (Purge Air)
The primary method of regeneration is the utilization of a portion of dry process air diverted from the on-line tower. This portion of dry process air, referred to as purge air, is generally set at 15% of the nameplate capacity of the dryer. Typically the purge air is diverted to a pressure regulating device, reduced in pressure to between 35 – 55 psig and expanded to atmospheric pressure through a fixed orifice. On a -40 °F pdp designed system, the expansion of purge air to atmospheric pressure will result in an actual purge air pressure dew point (pdp) of approximately -70 °F. The ultra-dry purge air easily draws the liquid moisture off the desiccant, regenerating the desiccant and expelling the moisture from the system in the form of a vapor.
Latent Heat of Condensation
The secondary method aiding in the regeneration of the desiccant is the utilization of the latent heat of condensation. In order to fully regenerate the desiccant, it is important to maintain the temperature of the purge air to near inlet temperature. Due to the fact that some cooling of the purge air takes place during expansion across the orifice to atmospheric pressure, it is necessary to make use of the latent heat of condensation generated during the drying (adsorption phase). The latent heat of condensation will add approximately 10 to 20 °F to the process air temperature aiding in reducing the cooling effect due to expansion.
Dew Point Demand Control
The purpose of dew point demand is to match the dryer cycling to the actual operating demand. Instead of controlling the dryer on a fixed cycle time (5 minutes), the dryer cycling can be optimized by only regenerating when the desiccant bed is saturated. This will result in reducing the amount of purge air consumed and consequently a reduction in operational costs. When a heatless regenerating compressed air dryer is operated at a reduced load without dew point demand control, the purge air required—and consequently, the operational costs — are not reduced. The two most common methods of monitoring the dew point and reducing purge air usage are:
1. Dew point monitoring at dryer discharge
2. Capacitance probes located in desiccant beds
Summary
This post was meant to give you some basic information on the advantages you would receive from a Heatless Regenerative Air Dryer and to give you an idea of how they operate. We always recommend that you consult a Compressed Air Specialist to ensure that you choose the right equipment for your system and company. Our specialists are able to identify the right products for you and install the system to create a turn-key solution made specifically for you.
If you would like more information on regenerative air dryers, contact us by phone or email.
Adapted From 2009 Satellite Newsletter Written By Ron Nordby