Short answer: A refrigerated dryer typically holds ISO 8573-1 moisture Class 4 at +3°C pressure dew point, while reaching Class 1–2 (−70°C to −40°C) needs a desiccant stage whose 12–18% purge typically costs about 15 kW more on a 20 m³/min duty — roughly 7,500 USD a year at 0.08 USD/kWh · 6,000 h/yr. The fix is usually not the dryer model; it is putting the desiccant stage only where Class 1–2 is genuinely required.
By Johnny Wayne, General Manager, AirSpace Machinery Co., Ltd. — 20 years in industrial compressed air systems. Reviewed by our engineering team.
Most desiccant-versus-refrigerated arguments we get pulled into start the same way: someone has specified Class 1–2 air for an entire plant and is comparing heatless desiccant against refrigerated units on purchase price. In the field we audited that pattern across iron, container-glass and textile plants, and the answer barely changes — one or two branches need the dry air, and every other cubic metre pays for it.
What does the ISO 8573-1 moisture class actually measure?
It measures the pressure dew point (PDP) at the dryer outlet — the temperature at which moisture starts to condense while the air is still under pressure. A Class 1 stream holds a PDP of −70°C or better, per ISO 8573-1:2010; Class 2 is −40°C, Class 3 is −20°C, and Class 4 is +3°C.
The distinction that trips people up is between PDP and atmospheric dew point. Air leaving a refrigerated dryer at +3°C PDP expands to a much lower dew point once it drops to atmospheric pressure, which is why Class 4 is adequate at the point of use in most plants and still fails a Class 1 specification.
Where does each dryer type land on the ISO 8573-1 scale?
The limit of the technology decides this, not the brand:
| Dryer type | Typical pressure dew point | ISO 8573-1 moisture class | Purge / regeneration cost |
|---|---|---|---|
| Refrigerated, non-cycling | +3°C | Class 4 | none, 1.5–3 kW electrical per 20 m³/min |
| Refrigerated, cycling | +3°C at load | Class 4 | none, 0.5–1.5 kW at typical part load |
| Heatless desiccant, twin tower | −40°C to −70°C | Class 2 to Class 1 | 12–18% of rated flow as purge |
| Externally heated or blower-purge desiccant | −40°C | Class 2 | 5–8% purge equivalent |
| Membrane, point of use only | +3°C to −40°C, flow dependent | Class 3–4 typical | sweep loss rises sharply at low flow |
If a process truly needs Class 1 or Class 2, a refrigerated dryer cannot deliver it at any price — the air has to be regenerated, and every regeneration method costs air, heat or both.
How much energy does a desiccant dryer’s purge really take?
More than the dryer label suggests, because the purge is not electrical load — it is already-compressed air the compressor has to make, so costing it needs the compressor’s specific power, not the dryer’s nameplate.
That is the trap. A desiccant dryer listing a 1 kW heater and a 15% purge rate is really consuming 15% of compressor output, and on a 20 m³/min system at 5.5 kW per m³/min that is about 16.5 kW of generation.
What does the comparison look like on a real 20 m³/min duty?
Take a plant logging 20 m³/min (about 700 CFM) at 7 bar, 6,000 hours a year, 0.08 USD/kWh, with average specific power of 5.5 kW per m³/min per the ISO 11011 assessment method:
- Compressor generation: 20 m³/min × 5.5 kW per m³/min = 110 kW average input.
- With a refrigerated dryer: 110 kW + 2.5 kW dryer draw = 112.5 kW total.
- With a heatless desiccant at 15% purge: 110 kW + (3 m³/min purge × 5.5) + 1 kW dryer = 127.5 kW total.
- Difference: 15 kW, which is 90,000 kWh and 7,200 USD a year at 0.08 USD/kWh.
- If Class 1–2 is genuinely needed plant-wide, that 15 kW is the price of the specification — the waste is applying it where nobody needs it.
What did the numbers look like in a real installation?
The installation we point to is not a dryer project, deliberately: it shows what happens when a specification is applied to the whole plant rather than the part that needs it.
Our 75 kW PMV VSD installation case study documents an iron processing plant in Ho Chi Minh City — a 15,000 m² facility with more than 200 employees running 16 hours a day, where compressed air cost more than 3,200 USD a month in electricity and maintenance ran at 850 USD a month on the old fixed-speed unit. Pressure there swung ±0.5 bar before the change. The plant was paying for a system-wide condition only a fraction of the load required. In practice the pattern repeats on the drying side: one branch drives the specification and the rest inherits it.
How do desiccant and refrigerated dryers differ across the textile, ceramic, glass and cement industries?
A low-pressure air compressor for the textile / ceramic / glass / cement industry is specified with one shared rule: identify the moisture class each process needs, then buy the cheapest technology that reaches it at that point of use, instead of drying the whole plant to the strictest requirement on site.
| Industry | Where moisture does damage | Typical moisture class needed | Dryer that reaches it |
|---|---|---|---|
| Textile | water marks on warp and fabric, nozzle wear on air-jet looms | typically Class 3–4 | refrigerated, plus filtration |
| Ceramic | caking in glaze lines, unstable spray-dryer atomisation | typically Class 3–4 | refrigerated, plus filtration |
| Glass | spots on ware, mould and blow-off nozzle corrosion | typically Class 3–4 | refrigerated, plus filtration |
| Cement | condensate in outdoor conveying and pulse-jet lines | typically Class 4–5 | refrigerated, with drain management |
Textile and ceramic are where a drying fault shows up in product quality first; cement is where condensate does its damage far from the compressor room. The low-pressure compressor series covers the air side.
Where should the desiccant stage sit — plant-wide or at the point of use?
At the point of use, wherever the strict class covers less than about a third of plant flow. On the 20 m³/min duty above, drying only 2 m³/min to Class 2 costs roughly 0.3 m³/min of purge — about 1.85 kW of generation — against 17.5 kW plant-wide, a difference of about 15.6 kW or 7,500 USD a year at 0.08 USD/kWh · 6,000 h/yr.
Two constraints decide whether the split is practical: the strict branch must be hydraulically separate, and it needs its own filtration and drain.
How do you decide which class you actually need?
Ask three questions, in this order, and the specification usually writes itself:
- Does the air touch the product? If not, Class 3–4 covers almost everything downstream.
- Does the air touch a critical instrument or analyser? Then the instrument’s own requirement governs, and it is often drier than the process needs.
- Is the pipe run outdoors or subject to freezing? Then the driver is line temperature, and insulation plus drainage may be cheaper than a desiccant stage.
Our published refrigerated compressed air dryers are rated from 1.0 to 100 m³/min at ISO Class 4–5 dew point, which answers more than 80% of the specifications we review.
What should you ask for in writing before choosing a dryer?
Four numbers, stated at your duty rather than at nominal flow:
- Pressure dew point at rated flow and rated ambient, tested to ISO 7183:2007, not a catalogue value at 25°C.
- Purge percentage or regeneration power at your flow — heatless desiccant purge typically rises as a share of delivered flow at part load.
- Pressure drop at rated flow, in bar, because it is charged to the compressor every hour of the year.
- Rated ambient temperature, since a refrigerated dryer’s dew point is only valid up to its design ambient, and in tropical plants that is often the binding constraint.
When is a refrigerated dryer alone the right answer?
When nothing downstream needs better than +3°C PDP and the pipework stays above the dew point, which covers most textile, ceramic, glass and cement process air. Two checks pay for themselves first: measure dew point at the point of use rather than at the dryer outlet, and fix condensate traps before buying equipment.
Key takeaways
- Refrigerated dryers stop at about +3°C PDP, which is ISO 8573-1 Class 4. Class 1–2 requires a desiccant stage — there is no refrigerated shortcut.
- A heatless desiccant purge of 15% costs about 15 kW on a 20 m³/min duty, or roughly 7,200 USD a year at 0.08 USD/kWh · 6,000 h/yr.
- Dry the branch, not the plant. Moving a desiccant stage to a 2 m³/min point of use saves about 15.6 kW — around 7,500 USD a year.
- Ask for dew point, purge, pressure drop and ambient at your own duty, and test the dew point claim against ISO 7183:2007.
Frequently asked questions
Is a desiccant dryer worth it if we only need dry air in one room?
Yes, when that room is a small share of plant flow. Drying 2 m³/min to Class 2 on a 20 m³/min site costs about 1.85 kW of generation, against 17.5 kW for a plant-wide heatless desiccant — roughly 7,500 USD a year at 0.08 USD/kWh · 6,000 h/yr.
Can a refrigerated dryer reach ISO 8573-1 Class 2 if we oversize it?
No. The limit is thermodynamic, not a matter of size: a refrigerated dryer cannot drive the pressure dew point below the evaporator temperature without freezing its own condensate, so Class 4 at about +3°C is its practical floor.
Does a desiccant dryer need a refrigerated dryer in front of it?
Usually yes. Removing bulk water first keeps the desiccant bed from being saturated by liquid carryover, the most common cause of short desiccant life and high purge rates we are called in to diagnose.
How much pressure drop does each type add?
Typically 0.1–0.2 bar for a refrigerated dryer and 0.2–0.4 bar for a heatless desiccant unit at rated flow, per the manufacturer’s data — and at 7 bar every 0.1 bar is paid for on every cubic metre, all year.
Sources and standards
- ISO 8573-1:2010 — Compressed air — Contaminants and purity classes. Defines the moisture classes and pressure dew points used here.
- ISO 7183:2007 — Compressed air dryers — Specifications and testing. The method behind a verifiable dew point claim at rated flow and ambient.
- ISO 11011:2013 — Compressed air — Energy efficiency — Assessment. Basis for the specific-power method behind the 5.5 kW per m³/min figure.
- ISO 9001:2015 — quality management certification held by AirSpace Machinery Co., Ltd., with CE marking under Machinery Directive 2006/42/EC.
- Our published references — the refrigerated dryer range rated 1.0–100 m³/min at Class 4–5, and the guide on choosing a refrigerated air dryer.
About the author
Johnny Wayne (Wei Zhuang) is General Manager of AirSpace Machinery Co., Ltd., the Shanghai manufacturer behind ChinaCompressor.org. The company builds oil-injected and Class 0 oil-free screw compressors from 2 to 540 HP (1.5–400 kW), including the low-pressure series at 15–300 HP for 3–5 bar duty, from a 4,000 m² production facility with 20 years of engineering history. Products carry CE marking and ISO 9001:2015 certification, and we sell through distributors rather than around them.
Send your flow, discharge pressure, running hours and the ISO 8573-1 class your process needs to sales@chinacompressor.org, or start with the free compressed air kWh forecast and system review.
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