Instrument air dryers are a necessary part of the compressed air system in a typical chemical plant. These devices and their associated filters remove water and lubricant from the compressed air that goes to sensitive instrumentation and control actuators. Such dryers act to pull down the dew point of the compressed air, even when ambient temperatures are low, so the air remains dry and contamination free. However, the units can experience problems. So, here, I’ll discuss some interesting issues I’ve encountered and ways to prevent them from occurring in your plant.
Compressed air occupies about one-eighth the original volume and can’t hold as much water vapor. The free water condenses out, leaving 100% saturated compressed air.
This saturated air is a problem. As it cools while passing through the main distribution piping, more water will condense out. That water will pick up pipe scale and rust and carry them downstream to the final end uses. Such contamination can ruin expensive instruments and cause malfunctions in control circuitry. In addition, if the pipes are exposed to freezing temperatures, ice will form — in some cases, building up in large enough quantities to restrict the flow of air.
To prevent these problems from happening, plants use instrument air dryers to reduce the dew point of the compressed air to well below the lowest temperature at the coolest location on the site. When we use the term “instrument air,” the typical requirement is to keep the pressure dew point at the discharge of the air dryer at least 18°F below the lowest temperature the pipes may encounter. Therefore, most plants with outdoor piping require air dryers that produce dew points well under freezing. This usually means using desiccant style dryers. However, if a plant doesn’t experience these conditions, it can gain significant cost savings by opting for refrigerated air dryers producing dew points between 35° and 40°F.
The Cost Of Air Drying
One of the issues with desiccant air drying is the cost of operation. About 15 to 20% of the flow rating of a fixed-cycle heatless desiccant air dryer (the most common type installed) is consumed by the dryer itself, through regeneration purge and the pressurization and depressurization of the dryer towers between cycles. Heatless desiccant dryers have twin towers filled with an adsorbent (usually activated alumina) to strip the water vapor from compressed air. One tower dries the air — usually for a 10-min cycle. Meanwhile, the other undergoes regeneration, which involves depressurizing it and passing some already dried compressed air through to drive off the adsorbed moisture.
However, compressed air isn’t free, consuming about 20 kW/100 cfm in a typical 100-psi system, which equates to about $2/hr/100 cfm at $0.10/kWh. Thus, operating a fixed-cycle heatless desiccant dryer sized at about 1,000 cfm costs around $31,000/yr. In contrast, the same size refrigerated dryer costs about $7,000/yr to run.
Consider if this massive extra outlay really is necessary. Most dryers are rated for input conditions of full flow, 100°F and 100 psi — but actual conditions often differ widely from these levels. Such variance can have significant impact. For example, every 20°F reduction in input temperature cuts the amount of water vapor in the compressed air by about half. A dryer with an 80°F inlet temperature and handling 50% of rated flow would process only 25% of the amount of moisture at rated conditions. Yet, in an uncontrolled dryer using a fixed timer cycle to switch between towers, the unit still would consume the rated compressed air purge flow, wasting nearly 75% of it.
You can minimize wasted purge air by detecting the dew point at the discharge of the air dryer and turning off the purge when the dryer is producing air at lower dew points than rated (indicating the desiccant isn’t saturated yet). Opting for different desiccant dryer designs also can reduce dryer purge flow.
Externally heated purge dryers use an electric heating element and a 4-hr cycle to better regenerate the desiccant while consuming only 7.5% of the dryer rating. In addition, some blower purge dryers use no compressed air for purge but instead pass ambient air through the heater element. Most require a flow of cooling air, about 2% of the dryer rating, to reduce desiccant temperature after the heating cycle. Some newer designs feature a closed-cycle cooling loop with radiant or water cooling to save cooling purge.
A heated dryer requires cooling because hot desiccant won’t adequately dry the air, causing a dew point spike. Heated dryers are deemed more efficient than heatless units; they indeed use less compressed air for purge but usually are more complex, more expensive and physically larger than the heatless type. And, of course, the heating element takes power. A typical 1,000-cfm heated blower dryer would incur about $17,000/yr in electricity costs at $0.10/kWh (including for cooling purge), $14,000 less than a heatless unit running uncontrolled at rated flow.