Refrigerated dryers physically chill compressed air to condense moisture vapor to a liquid state so it will precipitate out of the compressed air. This means that liquid moisture won’t form in the compressed air unless the compressed air temperature is cooled below the dryer’s effective dew point. The common target for a properly sized refrigerated dryer is a pressure dew point near 40°F. In contrast, a desiccant dryer reduces the moisture in compressed air by adsorbing moisture onto a desiccant material. Depending on the design, a desiccant dryer can provide dew points across a range from 0°F to -40°F or even -100°F.
In some cases, the pneumatic equipment or process requires extremely dry air. In other cases, the dew point from a refrigerated dryer would suffice, but the installation environment doesn’t allow for it. If, for example, the compressed air piping passes through a very cold/freezing area, moisture vapor will condense to liquid or freeze downstream of the refrigerated dryer. Another example is an extremely dirty or hot room which would not allow the refrigerated dryer to perform without excessive maintenance or being greatly over-sized.
So if your plant is in Green Bay, Wisconsin and has piping running outdoors, using a refrigerated dryer in colder months could be a problem. The dew point after the dryer will be somewhere around 40°F, but the outside air chills the compressed air piping and eventually, the pipe will cool the air stream below 40°F and transform the water vapor into liquid water. Starting with a desiccant dryer would lower the dew point so that the outside temperature would not affect air quality.
For reasons that will become clear below, desiccant dryers are considerably more expensive to buy and operate than refrigerated dryers, but if you truly need the low dew points, a desiccant dryer is a good solution.
Desiccant keeps things dry by adsorbing moisture in the surrounding air. There are many kinds of desiccants. Desiccants are hydrophilic materials including silica gel, activated carbon, molecular sieve, and activated alumina. You have probably seen packets of silica gel in the packaging of various consumer products to keep them dry during storage and transportation.
Activated alumina is the most commonly used desiccant compound for compressed air dryers, formed into beads (usually ⅛ inch to ¼ inch in diameter). Activated alumina beads look like spheres but are in fact highly porous and have a high surface area to volume ratio. This enables them to adsorb a lot of water vapor within a small volume.
There are several different types of desiccant dryers. They range widely in size and cost, but all are based on the use of desiccant material that will attract and trap water molecules from the compressed air passing over it.
Single tower desiccant dryers
Single tower desiccant dryers are primarily used for low, intermittent flows at a specific point of use that needs more drying than other uses in the facility. Applications include precision instruments, machinery that demands extra protection from contaminants, and high-quality painting. After a certain amount of air passes through them, the desiccant must be either replaced or taken out and manually regenerated (usually by baking). As you can imagine, there is a cost and inconvenience to doing this. The upside is that single tower desiccant dryers are very simple and don’t require electricity to operate.
Regenerating desiccant dryers
For larger flows, users typically choose some form of regenerating desiccant dryer to reduce the cost, inconvenience, and downtime associated with replacing or baking desiccant. Most have two towers of desiccant, which take turns drying and regenerating based on timers and moisture sensors.
Technically speaking, when desiccant captures moisture vapor, it releases energy in the form of heat. An equal amount of heat energy must be applied to regenerate the desiccant. This is why there are several designs for regenerative dryers (described below). Compressed air dryer engineers have found different techniques to deliver this energy, some more efficiently than others.
A critical aspect of some regeneration dryers is purge loss. This refers to the amount of dry compressed air that the dryer diverts to the moisture saturated tower to pull the water out of it and prepare it for the next drying cycle. After the moisture is stripped from the desiccant into the passing air, the wet air is released into the atmosphere. This is the purge.
When sizing air systems with regeneration dryers that purge, you must consider the dryer as one of the uses for compressed air. This can be a significant air demand depending on the size of the system since the purge rate (cfm) is based on the capacity of the dryer, not on the amount of air going through it. So oversizing a heatless dryer will purge/consume extra air that could be used for production (or that doesn’t need to be made at all). The higher the air flow, the higher the operating cost.
The more efficient dryers employ different mechanisms to reduce or eliminate purge air. They tend to be more complex and have higher first costs. The charts that follow the descriptions provide some perspective on the long term costs of different types of regenerating dryers.
Pressure swing (heatless) dryers
Pressure swing dryers regenerate desiccant by storing thermal energy when adsorbing and reusing that energy during regeneration to desorb the moisture from the desiccant beads. Pressure swing dryers alternatively cycle the compressed air through each of two desiccant vessels (AKA “towers”). As the vapor-laden air flows through a dry tower, the moisture is adsorbed onto the desiccant. Meanwhile, a portion of the dried compressed air is channeled through the wet tower, evaporating the water on the desiccant, and carrying it out of the tower as vapor. Though they rely on the heat of adsorption, they are sometimes called “heatless” dryers because they don’t have any supplemental heat devices to assist in regeneration. Heatless desiccant dryers have the greatest purge losses–as high as 15-20% of dryer capacity. Many brands offer optional controls that reduce the purge loss a few percentages.
The cycle time length determines the outlet dew point. More frequent cycles yield lower dew points (drier air) but increase the maximum purge flow which reduces the dryer’s outlet air flow. Among regenerative dryers, these dryers are the least expensive to purchase but usually the most expensive to operate due to purge losses of compressed air. These dryers can get to dew points as low as -100°F but this increases the purge losses and overall cost of operation. See examples in the charts below.
Heated desiccant dryers
To reduce the purge losses, heated desiccant dryers introduce heat to assist in regenerating the desiccant bed. They employ different methods to provide heat. Each presents a different trade-off of initial cost vs long term operating costs. See comparisons in the charts below.
Externally heated dryers
These dryers (also known as exhaust purge dryers) still use purge air to regenerate the bed but because the purge air is being heated, the added energy from the heat reduces the amount of purge air needed. By heating the purge air, the relative humidity of the air decreases, allowing the hot air to hold more water vapor as the purge air passes through the tower. The purge rate of externally heated dryers is around 6 or 7% at rated conditions. Externally heated dryers typically cost less to run than heatless dryers but cost more to purchase. They can reach dew points as low as -40°F.
Heated blower purge dryers
Heated blower dryers are similar to externally heated dryers as they both use a heater to heat the air used for regeneration and can reach dew points as low as -40°F. Blower purge dryers use a blower to move ambient air through the heater and desiccant bed. Because heated blower dryers use ambient air, there is no purge loss per se, but they may use up to 1.25% of the dry air from the drying tower to cool the regenerated tower. If cooling is disabled, then there is no air loss. Blower purge dryers are more expensive to purchase than heated purge dryers but are cheaper to run. See examples in the charts below.
Heat of compression (HOC) dryers
HOC dryers effectively recycle the heat coming off the compressor to regenerate the desiccant bed. They are the most energy-efficient but rely on very hot air generated by two-stage oil-free compressors. Twin tower HOC dryers differ from other heated desiccant dryers in that the hot inlet air stream flows first through the wet bed being regenerated, next through a heat exchanger, then the drying bed, and finally to an after filter. These dryers typically have the lowest operating cost, with a little electric power consumption for the controls (i.e., the controller, valves, and sensors); however, they have the highest initial cost. Note that some HOC dryers employ a design that has a single rotating drum of desiccant material instead of two towers. The operating principle is the same, as air is channeled through sections of the drum that are in different phases of drying.
Hopefully, we’ve made the point that while desiccant dryers can deliver much dryer air than the more common refrigerated dryer, they cost more to buy and operate. So it makes sense to consider where and why to apply them. Here are cases where they make good sense:
- in highly sensitive applications with low dew point specifications that cannot be met with a refrigerated dryer alone
- where there is a high-cost risk if product or equipment is exposed to moisture
- if piping is exposed to freezing temperatures (in this case, use the desiccant dryer seasonally if possible)
- in bad environments for refrigerated dryers (high dust, high heat)
Unlike refrigeration dryers, there is a significant penalty to over-sizing heatless desiccant dryers. This includes the costs of more purge air, more desiccant, and larger filters. If only some points of use or a part of the plant needs the extra-low dew point, size and place the dryer just for these and use a refrigerated dryer for the main compressed air supply.
And here’s your final tip: don’t buy one on eBay that’s too large just to get a good deal; it will end up costing you. Yes, it happens.