Your desiccant dryer purges even when the plant is idle

Desiccant dryers are there for a good reason: they give the very dry air that instruments, paint lines, outdoor piping and many processes need. But the most common kind, the heatless or pressure-swing dryer, pays for that dryness with compressed air. Most plants never look at how much, or at when it is spent.

How a heatless dryer uses air

A heatless dryer has two towers of desiccant. One dries the air while the other is cleaned, or regenerated, by blowing some of the dry air back through it and out to the atmosphere. That blown-off air is the purge. Every few minutes the towers swap.

The purge on a heatless dryer is typically about 15% of the dryer's rated flow. Heated and blower purge dryers use less air (roughly 2 to 7%), but they add heater and blower electricity.

What the purge costs

Take a heatless dryer rated for 1,000 scfm. Its purge is about 150 scfm. At about 20 kW of compressor power per 100 scfm, that is 30 kW of compressor power just to clean the desiccant. At $0.12 per kWh:

• Over two shifts (4,160 hours a year): about $15,000 a year.
• If it runs every hour of the year: about $31,500 a year.

Try your own dryer: Desiccant dryer purge cost calculator

Why the purge doesn't slow down

Most heatless dryers run on a fixed cycle. They purge the same amount every cycle, sized for the dryer running at full rated flow and the worst inlet conditions. The dryer doesn't know the plant is only drawing 400 scfm on second shift, or that it is a cool, dry day. It purges 150 scfm either way.

So the lighter the load, the bigger the purge's share. At full flow the purge is 15% of the air. At 40% load, the same 150 scfm is more than a quarter of what the compressors make. Dryers are often oversized for growth that never came, which makes this worse.

The idle plant problem

The costliest hours are often the ones no one is watching.

The dryer runs when the compressors don't

Many dryers are wired to run whenever they have power, not just when a compressor is running. When the compressors stop at the end of a shift, the dryer keeps cycling. It purges the stored air out of the receivers and piping until the pressure is gone. At the next start the compressors make all that air again, and the dryer gets a pressure swing it never needed.

The Sample Plant on our Examples page shows this. A dryer recorded as shutting down with the compressors kept cycling after every compressor had stopped.

The dryer keeps a compressor running

If a compressor stays on through nights or weekends, for leaks or a few small loads, the dryer's fixed purge goes on top. At 150 scfm, the purge alone can keep a compressor loaded. Over the hours a two-shift plant isn't producing, that is about $16,600 a year of purge in the example above, for air that dries nothing anyone uses.

What to do about it

• Interlock the dryer with the compressors. When the last compressor stops, the dryer should stop and hold its pressure. It is usually a simple control change.
• Add dew-point demand control. A dew-point sensor at the outlet lets the dryer stay on a tower until the desiccant actually needs cleaning. At light load and in dry weather, the purge falls a long way. Many dryers can be fitted with this control.
• Check the purge setting. The purge is often set by a valve or orifice. Make sure it matches the dryer's rating and your operating pressure, not a setting left over from start-up or a past repair.
• Size to the real flow. When a dryer is replaced, size it to the flow it will actually see. The rated flow sets the purge.
• Ask if you need that dew point everywhere. If only one area needs very dry air, a small desiccant dryer at that point, with a refrigerated dryer for the rest, can cost far less to run.
• Look at other dryer types. Blower purge and heat-of-compression dryers use little or no compressed air for purge. They cost more to buy, so compare them on the purge you would actually save.

How to tell if yours is doing it

Stand by the dryer when the plant is down and listen. A heatless dryer purging makes a loud, regular blast of air at each tower switch. If you hear it with the compressors off, or with only one compressor running for a near-empty plant, you have found waste you can stop.

Better still, log the system over a week. Pressure and compressor power, logged over nights and a weekend, show when the compressors run with no production, and how much of that air goes to the dryer.

Typical purge by dryer type: Compressed Air Challenge, Best Practices for Compressed Air Systems. The examples assume 100 psig, 20 kW per 100 scfm and $0.12 per kWh, as in our calculator.