If you use desiccant air dryers in your plant, you probably know they can consume a lot of energy dollars. Much of this money often is wasted. However, you likely can take a number of steps to reduce your desiccant dryer costs and improve the dryer operation, leading to better air quality. Here, we’ll look at five factors worth attention: specifications check, dryer control, desiccant protection, adjustments and maintenance.
First let’s consider operating costs, though. Compressed air is one of the most expensive utilities to use in doing anything in your plant. This is because of the poor “wire to work” conversion efficiency of the compression process. Only ≈10–15% of the energy put into a compressor actually converts to mechanical energy output at the end user. The percentage is even less if the desiccant dryer consumes some of the air before it gets to the end of the line.
The heatless desiccant dryer (Figure 1) is the most common style of unit in compressed air systems that require dew points at less than 35°F. Such a dryer can be a costly energy hog because it gulps down expensive compressed air just to keep operating. A purge of compressed air must flow in this dryer to reactivate the desiccant once it becomes saturated.
A typical well-maintained heatless dryer using fixed cycle control (i.e., operated via a simple timer) constantly consumes a flow of compressed air of ≈15–20% of the dryer nameplate rating. A 1,000-cfm-rated dryer running fulltime (8,760 h/y) would use roughly 300,000 kWh of energy annually — worth $30,000/y for energy costing $0.10/kWh. If the dryer has standard controls, this cost is constant even if the unit has no plant flow to process. A dryer in poor condition might consume more than rated flow, increasing the costs even more, yet providing poor air quality.
One of the first and most obvious measures to implement to reduce dryer operating costs is to change the style of air dryer. Sometimes a desiccant dryer is installed in a location or on a process that doesn’t need air dried to -40°F levels. This can happen if a plant has a “standard” design practice that is blind to the actual needs of the end use. Consider, for example, a system located indoors in a heated plant. A refrigerated air dryer would consume no compressed air for purge. The refrigerated dryer uses power for its internal cooling circuit but the cost, comparatively speaking, is only a fraction of that of the desiccant dryer when the energy content of the purge flow is considered. A desiccant dryer consumes ≈6–8 times more energy than a refrigerated dryer. A refrigerated dryer that cycles on and off in proportion to moisture loading might use even less power.
One important thing to check is the flow being processed by the air dryer. If an uncontrolled desiccant dryer only is drying, for instance, 10% of its rating, perhaps due to poor sizing choice or design change, because the purge flow remains constant, the dryer will be consuming ≈150–200% of the actual flow being processed, which is extremely inefficient.
So, for lightly loaded dryers, perform a careful engineering analysis of the flows being processed. If you determine the dryer is oversized, then changing the size of the air dryer can greatly reduce the operating costs. Don’t know the flow? Reasonably priced flow meters are available; go buy some.
If you plan to change the dryer but still require a desiccant style, then consider selecting a more-efficient unit that consumes less compressed air for purge. A heated dryer that uses less compressed air or ambient air for desiccant purge or even applies vacuum to remove moisture may cost more to purchase but in the long run can pay for itself in reduced electrical costs.
You can gain savings by controlling a desiccant dryer using a dew-point-dependent switching strategy. An onboard dew-point sensor will turn off the dryer purge when it’s not required to flow based on the quality of air on the output of the dryer. If the air dryer is processing less than its full load, this strategy will reduce the purge flow roughly in proportion to the percentage dryer loading, purge flow included.
Most modern desiccant dryers can use dew-point control and many have the sensors already installed. If your dryer doesn’t have a sensor, you can retrofit one. Often, plants never have activated installed dew-point controls or have disabled them due to calibration issues. Installation or reactivation of dew-point control on a dryer rated at 1,000 cfm, but loaded at 50%, would save about $15,000 in operating costs per year for an expenditure of about $5,000. As with all sensors, you must not ignore the testing and calibration of dew-point probes; simple calibration errors easily can undermine the most sophisticated dryer control.
Another very common control issue is allowing a desiccant dryer to keep purging when no air at all needs to be processed — for example, a dryer associated with a specific compressor that has been turned off. In such a case, consider controls that will stop the dryer purge when the associated compressor is shut down.