I'm the quality and brand compliance manager at an industrial equipment sourcing company. I review every spec sheet and supplier package before it reaches a customer — roughly 200 a year, across dryers, filters, and the piping that makes a compressed air dryer system actually work. In 2024 I rejected about 18% of first deliveries on documentation alone. Not because the machines were bad. Because the paperwork couldn't prove what the machine would do at our customer's site.

Here's the checklist I run now, before anyone signs a purchase order. Eight steps, maybe two hours of work. It won't choose your supplier. It will catch the four failures I see most: the wrong dew point, flow ratings that don't survive a hot compressor room, delivery dates nobody wrote down, and performance guarantees nobody can enforce.

When this checklist applies

Use it any time you're sourcing a new compressed air dryer — refrigerated, desiccant, or a combined system with pre- and post-filtration — and the unit will sit downstream of a compressor you don't fully control. If you're replacing a dryer in a system you've run for ten years and you already know the inlet conditions, you can skip to Step 6.

If you're new to this, one thing first. "Compressed air dryer types" gets used loosely. Refrigerated dryers, desiccant dryers, and membrane dryers are not interchangeable options at different price points. They produce different pressure dew points, and that difference drives everything downstream.

Step 1: Write down the air quality class before you look at a single dryer

This is the step most people skip, and it's the one that causes the most expensive rework.

ISO 8573-1:2010 defines air quality by three numbers — particles, water, oil — written as a class triplet like 1:4:1. The middle digit is the one that decides your dryer. A water class of 4 corresponds to a pressure dew point of +3°C; class 2 gets you to -40°C; class 1 is -70°C. (Confirm the current edition and class table at iso.org before you commit — the standard has been revised more than once and a supplier quoting from memory will get it wrong.)

From the outside, choosing a dryer looks like a capacity question. The reality is that it's a dew point question first and a capacity question second. Get the class wrong and you can buy a perfectly good machine that ruins your product.

Two things to pin down in writing: whether the class applies at the dryer outlet or at the point of use (they differ, and a long uninsulated pipe run in winter will change the answer), and who is responsible for the filtration that supports that class. Dryer vendors love to quote the dryer and leave the coalescing filter to someone else.

Step 2: Match dryer type to dew point, then sanity-check the operating envelope

A refrigerated compressed air dryer is the default for general plant air. It's robust, cheap to run, and holds roughly +3°C pressure dew point. That's fine for pneumatics, most tools, and anything that isn't exposed to freezing temperatures.

A desiccant compressed air dryer goes colder — typically -40°C or -70°C — because it removes water vapor instead of condensing it. You need it for outdoor lines, instrument air, food and pharma contact, powder conveying, or any pipe run that sees sub-zero ambient. If the line can freeze, a refrigerated dryer is not a cheaper substitute. It's the wrong product.

Now the part that gets missed: desiccant dryers consume purge air to regenerate. Depending on the design that can be a meaningful share of the dryer's rated flow — heatless designs are the thirstiest. If you spec 10 m³/min of dryer and run 9.5 m³/min of demand, you've just bought yourself a moisture problem. Always ask for purge air consumption at your design conditions, and subtract it before you compare against compressor output.

Step 3: Correct the flow rating for your actual inlet conditions

Dryer capacity is rated at reference conditions — usually something like 38°C inlet air, 7 bar working pressure, 25°C or 35°C ambient. Your compressor room is probably not that room.

Hotter inlet air carries more moisture and punishes the dryer. Lower pressure moves more volume through the same element. A 45°C summer afternoon in an unventilated plant room can drop effective capacity well below the nameplate. I won't quote a correction percentage here, because the curves are model-specific and vendors publish them inconsistently — and honestly, that inconsistency is the point.

Ask for three numbers for each quoted unit: rated flow at reference conditions, the correction curves or tables for inlet temperature and pressure, and the resulting flow at your worst-case month. Then re-check Step 2's purge air subtraction against the corrected number, not the nameplate.

Never expected the biggest source of quote variance to be the reference conditions themselves. Turns out three suppliers can quote "10 m³/min" at three different baselines and all be technically truthful.

Step 4: Demand a CAGI-format data sheet, not a brochure

The Compressed Air and Gas Institute publishes standardized data sheet formats for dryer performance so buyers can compare like with like. Ask for the CAGI-format sheet. If a supplier can't produce one, that tells you something about how their ratings were derived. Check cagi.org for the current format and what fields are mandatory — this is worth ten minutes.

It's tempting to think unit price per unit of flow settles the comparison. It doesn't. Two dryers at the same nominal flow can differ on dew point at part load, pressure drop, refrigerant type, control turndown, and whether the quoted number is at 25°C or 35°C ambient. The unit price is the least informative number on the page.

Step 5: Put the pressure drop in writing

Every dryer adds resistance to the air stream, and your compressor pays for it in energy, forever. Ask each supplier for the rated pressure drop at your design flow, in bar or psi, and require it as a contractual figure.

Rule-of-thumb numbers get thrown around for how much a given pressure drop costs in compressor energy. Treat those as directional only — the exact figure depends on your compressor type and control. What you can do without argument is compare: if one unit quotes a materially lower ΔP at the same flow and the same dew point, it's worth a follow-up conversation. If a supplier won't state a ΔP at all, that's a red flag.

Also ask where the pressure drop is measured — across the dryer alone, or the dryer plus the filters they're bundling into the "compressed air dryer system." Bundled numbers hide things.

Step 6: Treat the delivery date as a specification, not a hope

Here's where I've changed my thinking, and it cost us to learn.

Looking back, I should have paid for the guaranteed date on the very first dryer we imported. At the time, the standard lead time looked safe on paper, and the expedite premium felt like padding. It wasn't. The unit slipped three weeks, commissioning moved, and the customer's line sat idle. Nobody remembers the $2,300 we saved. Everybody remembers the four dark days.

In a later project we paid an expedite fee to lock a commissioning window to a shutdown. That fee bought us certainty, not speed — the dryer would have arrived only eight days sooner. Those eight days were the whole point, because we'd scheduled labor against them.

So now the delivery date goes in the spec, with three components: the ex-works date, the freight mode and transit allowance, and the on-site date. Each one separately committed. "Approximately" doesn't count. For an industrial compressed air dryer sourced from overseas, the freight leg is usually the most variable part and the least discussed part of the quote.

To be fair, expedite fees are real money and not every project justifies one. But be honest about what you're comparing. An uncertain cheap date and a committed expensive one aren't the same product.

Step 7: Book the factory acceptance test before you need it

If the unit is going into a process where moisture damage is expensive, witness a factory acceptance test. Two items on the test sheet matter more than the rest:

  • Dew point at full load. Measured with a calibrated hygrometer, at rated flow and rated inlet conditions, with the instrument's calibration certificate attached to the report. A certificate of conformity is not a measurement.
  • Pressure drop at rated flow. Compare it against the figure from Step 5. If the tested number is materially worse, you want to know that in the factory, not on site.

So glad we started requiring the hygrometer printout. We were one shipment away from accepting a unit whose delivered dew point was measured at 30% load — a number that would have looked beautiful and meant nothing.

Step 8: Make the performance guarantee enforceable

"Meets specification" is not a guarantee. A guarantee names the measurement, the instrument, the conditions, the timing, and the remedy.

Write down: who measures the dew point after installation, with what instrument and calibration, at what load and inlet temperature, how long after commissioning, and what happens if it fails — repair, replacement, or credit, and within what window. Also fix the spare parts lead time in the same document. A dryer that holds dew point perfectly but needs six weeks for a valve is still a downtime problem.

One more clause people forget: the condensate drain. Specify the type and the failure behavior. A blocked drain turns a good dryer into a wet system, and nobody puts it in the warranty conversation until it happens.

Common mistakes

Buying on unit price per unit of nominal flow. Wrong baseline, wrong comparison, wrong answer.

Accepting "industry standard" as a specification. If the contract doesn't name the ISO class and the reference conditions, you've agreed to nothing measurable.

Forgetting purge air on desiccant units. The most common capacity shortfall I see, and the easiest to prevent.

Leaving the delivery date out of the contract. Everything else can be right and the project still fails on timing.

Skipping the pre-filtration scope. A dryer can't deliver a class it wasn't given the inlet air to achieve.

Assuming compatibility. No reputable supplier should promise a dryer will drop into any existing system without checking inlet conditions, piping, drainage, and electrical. If they do, ask why.

The short version

Air quality class first. Dryer type second. Corrected flow third. Everything else is verification. If you do only two things from this list, do Step 1 and Step 6 — one protects the product, the other protects the schedule.

Context note: U.S. Department of Energy material on compressed air systems — including its Compressed Air Sourcebook — is still the reference most auditors reach for, and DOE has estimated compressed air at roughly 10% of U.S. industrial electricity use, with leaks wasting a substantial share of compressor output. Figures vary by source and vintage; verify current numbers at energy.gov before citing them in a proposal. Dryer-specific performance figures should always come from the manufacturer's CAGI-format data sheet, not from general industry averages.