Most compressed air systems need a dryer. The exceptions are applications where moisture in the air is genuinely harmless — some outdoor pneumatic tools, certain industrial processes — but they're the minority. For most manufacturing, automotive, food, pharmaceutical and electronics applications, removing moisture from the air stream is necessary.
The three main technologies each have different capital costs, running costs and achievable dewpoints. Understanding the differences helps you choose correctly.
Refrigerant Dryers
A refrigerant dryer chills the compressed air to around 2-3°C, causing moisture to condense and be drained away. The dried air leaves the dryer at a pressure dewpoint of approximately +3°C.
This is sufficient for most industrial applications. At +3°C dewpoint, you won't get water dropping out of the air as long as the pipework stays above about 5°C — which it does in most indoor industrial environments.
Refrigerant dryers are the standard choice for general industrial use. They're energy efficient (a modern cycling dryer uses almost no energy when demand is low), reliable, and easy to maintain. Capital cost for a unit matching a 15-55kW compressor ranges from £1,500-5,000.
The limitation is the dewpoint achievable. You can't reliably get below +3°C pressure dewpoint with refrigerant alone, regardless of how cold you set the refrigerant circuit. For applications needing drier air — instrumentation, very cold environments, some food applications — you need a different technology.
Desiccant Dryers
A desiccant dryer passes the air through a bed of silica gel or activated alumina, which adsorbs the moisture. The bed is regenerated periodically, either using hot air or a portion of the dried air stream (heatless regeneration).
Desiccant dryers can achieve dewpoints of -40°C to -70°C routinely. They're the only realistic choice for instrumentation air in cold environments, outdoor pipework that sees freezing temperatures, some pharmaceutical applications, and any process requiring ISO 8573 water Class 1 or 2.
The running cost is higher than refrigerant drying. Heatless desiccant dryers use 15-20% of the compressor's output for regeneration — that's 15-20% of your compressed air energy effectively wasted on drying. Heat-of-compression or electrically-regenerated desiccant dryers recover some of this, but at higher capital cost.
Desiccant also requires periodic replacement — typically every 3-5 years — at meaningful cost. And a desiccant dryer failure (desiccant channelling, failed switching valve) can pass saturated air rather than dry air, without obvious indication in some designs.
For applications that genuinely need the dewpoint, desiccant is the correct choice. Don't specify it for a standard industrial application that will work fine with refrigerant.
Membrane Dryers
Membrane dryers use a bundle of hollow-fibre membranes through which dry sweep air removes moisture from the compressed air stream. They have no moving parts, require no electricity, and produce no condensate requiring disposal.
The drawbacks: they're only suitable for low flow rates, they consume a proportion of compressed air as sweep air (typically 10-25%), and they achieve moderate dewpoints (-20 to -40°C depending on configuration and flow). They're an elegant solution for small, dedicated drying applications — a portable instrument, a specific process point — but not for main system drying.
Common Mistakes
Specifying a desiccant dryer for a general workshop application because it sounds better is expensive and unnecessary. Refrigerant drying at +3°C dewpoint is fine for the vast majority of industrial applications.
Fitting a refrigerant dryer in a compressor room that sees sub-zero temperatures in winter is a problem — the dryer output dewpoint won't be cold enough to prevent condensation in the distribution system when the ambient temperature drops. Add a separate desiccant stage or reconsider the system design.
Forgetting to drain the refrigerant dryer separator regularly is a maintenance issue that causes water carryover downstream. Auto-drains need testing — see my earlier post on this.