Specifying a desiccant dryer for an application that needs refrigerant drying is one of the more consistent ways to waste money in compressed air. The running cost penalty alone, typically 15-20% of compressor output consumed by desiccant regeneration, adds up to thousands of pounds per year on a medium-sized system that never needed that level of drying in the first place.
The choice between refrigerant, desiccant and membrane dryers should be driven by what dewpoint your application requires and what your ambient conditions are. Here's a clear account of each technology and when it actually makes sense.
Refrigerant dryers: the right answer for most sites
A refrigerant dryer chills the compressed air to 2-3°C, causing moisture to condense and drain away. The air leaving the dryer has a pressure dewpoint of approximately +3°C.
For most industrial applications, general manufacturing, automotive, fabrication, engineering workshops, this is entirely adequate. At +3°C dewpoint, water won't drop out of the air stream as long as the pipework stays above about 5°C. In a heated industrial building, it does.
Modern cycling refrigerant dryers, where the refrigerant compressor cycles with demand rather than running continuously, use almost no energy when air flow is low. Capital cost for a unit matched to a 15-55kW compressor ranges from £1,500-5,000. Maintenance is straightforward and parts are inexpensive.
The limit is what you can achieve. You cannot reliably get below +3°C pressure dewpoint with refrigerant alone, regardless of how cold the refrigerant circuit runs. For applications that need drier air, you need a different technology.
Desiccant dryers: when you actually need them
A desiccant dryer passes the air through silica gel or activated alumina, which adsorbs moisture. The saturated bed regenerates periodically, either using a portion of the dried air stream (heatless) or with externally applied heat. This cycle produces outlet dewpoints of -40°C to -70°C routinely.
Desiccant is the correct choice for:
- Instrumentation and control air in outdoor environments where pipework can see freezing temperatures
- Applications requiring ISO 8573 water Class 1 or 2 (-70°C or -40°C dewpoint)
- Some pharmaceutical applications with very low moisture requirements
- Any process where even small amounts of moisture cause failures, certain catalyst processes, some electronics testing
The running cost is considerably higher than refrigerant. Heatless desiccant dryers typically consume 15-20% of the compressor's air output for bed regeneration, that 15-20% of compressed air energy is simply spent on drying. On a 37kW compressor running 4,500 hours at 14p/kWh, that's roughly £4,500-6,000 per year in regeneration energy. Heat-of-compression or electrically heated desiccant dryers reduce this, but carry higher capital cost.
Desiccant beds also need periodic replacement, every 3-5 years typically, at a cost that varies with system size but is meaningful on larger machines. And a desiccant dryer failure, particularly a failed switching valve between beds, can pass saturated air rather than dry air. On some designs there's no immediate indication that this has happened.
Don't specify desiccant for a standard industrial application that will work fine at +3°C. The additional cost has no return.
Membrane dryers: the niche technology with a clear use case
Membrane dryers use hollow-fibre membranes through which water vapour migrates out of the compressed air stream. No moving parts, no electricity required, no condensate for disposal. They can achieve dewpoints of -20 to -40°C depending on configuration and flow rate.
The limitations are real: suitable for low flow rates only, consume 10-25% of compressed air as sweep air, and are not well-suited as main system dryers on anything above a small installation.
Where they work well: point-of-use drying for a specific instrument, a portable application, or a dedicated process where the compact footprint and no-power operation are genuine advantages. As a site-wide drying solution, they're rarely the right choice.
The two installation mistakes to avoid
Fitting a refrigerant dryer in a compressor room that sees sub-zero temperatures in winter is a common and expensive mistake. If the ambient around the dryer drops below 0°C, the condensed water inside can freeze, blocking the drain and eventually causing heat exchanger damage. If the distribution system also reaches sub-zero temperatures, a +3°C dewpoint outlet air will produce condensation in the pipework. This situation needs either a desiccant stage or a heated enclosure for the dryer.
Neglecting the auto-drain on a refrigerant dryer separator is a slower-moving problem that causes moisture carryover downstream. Float drains need the same monthly verification as any other auto-drain on the system. This one gets missed because it's physically inside the dryer housing and easy to forget about.