Selecting and sizing a desiccant dryer
So you’ve read our previous blog post on desiccant dryers and decided you really do love ‘em and need ‘em in your compressed air system. In this blog post, we’ll help you figure out what size dryer you need, what accessories you might want to add, and how to take care of it so that it continues to do its job well (hint, we have seen desiccant dryers happily purging air, but not drying it).
What's the air demand?
First, you need to know the demand downstream of the dryer. How much compressed air volume with the extra-low dew point is needed to feed the plant, section of the plant or specific point of use. If only a particular point of use or a particular part of the plant needs the extra-low dew point, you will save capital, maintenance and energy costs by sizing the dryer just for that part of the system and use a refrigerated dryer for the main compressed air supply.
Demand has two facets: the amount of air needed, and the quality of the air needed. The amount of air is commonly expressed either as a volumetric flow rate (cubic feet per minute or cfm), or a mass flow rate (standard cubic feet per minute or scfm). Dryers are rated in standard cubic feet per minute; if the demand is given as cfm, then it needs to be converted to scfm.
➡➡ A standard cubic foot of air is 0.075 pounds of air in one cubic foot at 68°F, 14.7 psia, and 36% relative humidity. To convert cfm to scfm you need to know the location’s ambient conditions.
Desiccant dryers have two sizing parameters that determine the actual capacity of the dryer: inlet pressure and temperature. The capacity of the dryer increases with pressure, but the opposite is true for temperature: the higher the temperature, the lower the dryer capacity.
The effect of temperature and pressure is simplified into capacity correction factors (CCF) that are multiplied by the rated flow. These factors are relative to a set, or rated point: the point at which the dryer is operating at 100°F, 100 psig, and 100% relative humidity. The correction factor at this point is 1.00.
From here, the factor could increase or decrease depending on the temperature and pressure combination. When you find the temperature and pressure of the air entering the dryer, multiply the combined correction factors by the rated dryer capacity to determine the actual capacity. Correction factors are available from the dryer supplier (they may be on the product literature).
With heatless dryers the inlet temperature minimally affects the outlet pressure dew point, so you need only consider the pressure correction factor (see Chart 1). Heated desiccant dryers require the use of both the temperature and pressure correction factors (see Chart 2).
Heatless dryer sizing example:
If inlet pressure for a heatless dryer is 125 psig, correction factor is 1.10 (per Chart 1). If the dryer’s capacity at standard conditions is 250 cfm, the effective capacity at your conditions is 275 cfm (250 cfm X 1.10). Or if you only need to dry 250 cfm, you can use an 230 cfm dryer. Make sure to size for lowest pressure the dryer will see.
Heated dryer sizing example:
If inlet pressure is 125 psig, and the inlet temperature is 90°F, the combined correction factor is 1.39 (per Chart 2). If the dryer’s capacity at standard conditions is 1500 cfm, the effective capacity at your conditions is 2085cfm (1500 cfm X 1.39). Or if you only need to dry 1500 cfm, you can use an 1100 cfm dryer. Keep in mind that if inlet temperature varies seasonally, you should size for the highest inlet temperature.
Filtration
Desiccant dryers require proper pre- and post-filtration to deliver high-quality air. Filtration is essential to ensure drying effectiveness, prolong desiccant life, and remove particulate. Very often, the supplier will offer the dryer with the filter set. Don’t treat this as an unneeded option.
Pre-filtration
Oil and particles can contaminate the desiccant. To prevent this, pre-filters are used to filter out the contaminants so only air with water vapor can enter the desiccant bed. All desiccant dryers need robust coalescing pre-filtration to remove traces of compressor fluids or other hydrocarbons/oils that are ingested by the compressor from ambient air. Because these substances are hydrophobic (they repel water), contaminated desiccant coated will not remove moisture from the compressed air. Once contaminated, the desiccant cannot be cleaned and must be replaced (expensive and time consuming). The cost of maintaining good filtration is far less than the cost of replacing desiccant, not to mention productivity losses from not drying the air and the downtime during dryer service. If you want to extend the life of your coalescing filter, stage a general air line filter in front of it to remove larger particulates and oil droplets. This has the added benefit of reducing pressure drop.
Post-filtration
Desiccant dryers also create contaminants. Because air moves through the desiccant vessels in opposite directions during dry and regeneration, cycle after cycle of drying and regenerating will erode desiccant into dust. Large dust particles can be caught in a strainer located at the desiccant dryer outlet and smaller dust particles will escape downstream. Particulate “after-filters” are needed to capture these finer fugitives. When ordering dust filters for use downstream of a desiccant dryer, be sure to specify the purpose. Some manufacturers have specific reverse flow filters for this application. And for the heated dryer types, specify a high-temperature after-filter. Standard filters may not be designed for the high heat coming off the exhaust or blower purge dryer.
Maintenance
Filters
This may be obvious but filters elements must be replaced periodically to protect the desiccant, remove dust, and minimize pressure drop. Our colleagues on the service side recommend doing this on a schedule. Differential pressure gauges atop filters may be used as an indicator of when to change filter elements, but don’t rely on them alone. If a filter gets clogged the air pressure may break through the filter material. If it’s been a while since you looked at the gauge, you may not realize it went from green to yellow to red and back to green. The gauge will happily report little or no pressure drop while your desiccant gets coated with oil or fine dust clogs up instruments and production equipment.
Desiccant
Over time, bed fluidization, reversing air flows, and heat (from heated dryers) will break down the desiccant. At some point, too much of it has been reduced to dust and must be replaced. Expect three to five years of life for heatless dryers and two to three on heated dryers. The desiccant’s life expectancy is determined by the use of the dryer and inlet air quality. If the desiccant has been contaminated with oil, however, it is “game over.”
Check and empty the strainers on the dryer quarterly. This is a good way to check the condition of the desiccant. The presence of a lot of desiccant pieces may mean it is breaking down fast and it’s time to replace it. Does it smell or feel oily? Same. And it’s another indicator for checking the after filter.
The quality of desiccant is important for life span. Cheap desiccant doesn’t last long and doesn’t work as well as a high-quality desiccant.
When ordering replacement desiccant, be sure to specify the model of the dryer, some desiccant dryers require multiple sized desiccants.
Valves
Desiccant dryers have several valves that are vital to the performance of the dryer. Consult the supplier on how often these should be tested/inspected and services. Dryer maintenance kits will likely include valve rebuild kits.
Summary
If you’ve reached this point, you’ve probably realized that desiccant dryers offer superior drying but at a price. The trade-off includes higher purchase and operating costs (maintenance and energy) compared to a refrigerated dryer. For these reasons, it really pays to validate the need for the dryer and size it just for the applications where it’s really needed. And finally, invest in the maintenance that will protect your equipment/processes and minimize pressure drop and downtime.
Additional resources: