Who this checklist is for

I'm not an engineer. I'm the person who gets an email that says "we need a dryer for the new line" and has to turn that into a purchase order.

If that sounds familiar, this checklist is for you. It covers sourcing a compressed air dryer—refrigerated, desiccant, or a full compressed air dryer system—for a facility where you're not the technical expert. Seven steps. Each one has a checkpoint you can actually verify.

None of these steps require you to become an engineer. They require you to ask the right questions and write down the answers. That's it.

The 7-step compressed air dryer sourcing checklist

Step 1: Get the air quality requirement in writing

No supplier can quote you the right dryer until you know what the air is for. "Compressed air for the shop" isn't a spec. A paint booth needs dry air. A pneumatic conveyor tolerates more moisture. A food packaging line has its own rules.

Ask three questions of whoever requested the dryer:

  • What is the air used for?
  • Does anything touch the product, or is it only powering equipment?
  • What does the equipment manufacturer recommend?

Get the answer in writing, even if it's just an email. Verbal specs have a way of changing when something goes wrong.

Step 2: Match the dryer type to the application

There are three main compressed air dryer types: refrigerated, desiccant, and membrane. Each hits a different dew point range.

Refrigerated compressed air dryers typically deliver a pressure dew point around +3°C to +10°C. They're the lowest-cost option to run and fine for general industrial air.

Desiccant compressed air dryers use a desiccant material to adsorb moisture and can reach -40°C to -70°C pressure dew points. More expensive to run, but necessary for critical applications like instrument air or outdoor piping in cold climates.

Membrane dryers sit between the two—drier than refrigerated, less than desiccant, no moving parts.

How do you know which to ask for? Look at the required pressure dew point. If the spec calls for -40°C, a refrigerated dryer won't get you there no matter how big you size it. This is the single most common sourcing error I've seen—someone picks refrigerated because it's cheaper, then finds out it doesn't meet spec.

Step 3: Size by actual CFM, not compressor horsepower

Compressor HP is not the same as dryer sizing. Dryers are rated by CFM (cubic feet per minute) of air they can process at specific conditions—inlet temperature, inlet pressure, and ambient conditions all matter.

A 100 HP compressor moves roughly 400-450 CFM at 100 psig, but your actual consumption might be 300 CFM at one shift and 500 CFM at another. Dryers should be sized for peak flow plus a margin, not average.

Ask operations for a measured CFM reading if anyone has one. If not, size for peak demand plus 15-20%. And check the dryer's rated CFM at your actual inlet conditions—not the headline number on the datasheet.

It took me a few years of coordinating these purchases to understand that the spec sheet matters more than the price sheet.

Step 4: Check the dew point against ISO 8573-1

ISO 8573-1:2010 is the international standard for compressed air quality. It classifies air by particles, water, and oil. The water class is expressed as pressure dew point:

ClassPressure dew point
1-70°C
2-40°C
3-20°C
4+3°C
5+7°C
6+10°C

Reference: ISO 8573-1:2010, current revision as of January 2025. Verify at iso.org as revisions occur.

When you see a spec sheet, match the dryer's rated dew point to the class your application needs. Don't accept "meets ISO 8573-1" without the specific class. That's like saying a car "meets safety standards" without telling you which ones.

Step 5: Check the install requirements (purge air is the trap)

Refrigerated dryers need power and a drain. Desiccant dryers need power, a drain, and—this is the one that catches people—purge air.

Most heatless desiccant dryers use 15-20% of the rated flow as purge air to regenerate the desiccant bed. That means if you buy a 500 CFM dryer to handle 500 CFM of demand, you may come up short. Size for demand plus purge.

This is easy to miss because the datasheet leads with CFM and buries the purge percentage in a footnote. Ask the vendor directly: what is the purge air consumption at rated conditions?

Also confirm inlet air temperature. Dryer performance drops as inlet temperature rises—a dryer rated for 500 CFM at 100°F inlet may only handle 400 CFM at 120°F.

Step 6: Verify the vendor's documentation and invoicing

Here's a story I'd rather not tell twice. In 2022 I found a good price on a small refrigerated dryer through a supplier I hadn't used before. Ordered it. Delivered on time. Then I went to submit the invoice.

They sent a handwritten receipt. No line-item invoice, no tax number, no purchase order reference.

Finance rejected it. I spent two weeks chasing a proper invoice, and by the time it arrived the expense report was three weeks late. I had to explain the delay to my director.

Now I verify three things before placing any order: can they issue a proper invoice with the right tax details, do they accept PO-based payment, and do they provide a delivery note with the shipment. Any supplier that can't handle all three is a red flag, no matter how good the price.

Step 7: Plan consumables and maintenance from day one

Compressed air dryers are not maintenance-free. Consumables are part of total cost of ownership, and the quote alone doesn't show them.

  • Refrigerated: inlet filters, drain maintenance, occasional refrigerant service
  • Desiccant: desiccant replacement every 3-5 years, filter elements, purge valve service
  • Membrane: filter housings, membrane replacement every 3-5 years

Ask the vendor for the annual consumable cost at your usage level. If they can't tell you, ask for part numbers and price them yourself. A dryer that's cheap to buy and expensive to run isn't a deal.

Common mistakes to avoid

The five I see most often:

  1. Sourcing by price sheet instead of air quality spec
  2. Forgetting purge air on desiccant units
  3. Sizing by compressor HP instead of measured CFM
  4. Skipping the invoice and documentation check
  5. Assuming a small order means worse service

On that last one—your 1-unit or 5-unit order is not a burden. A supplier who treats it as one is doing you a favor by showing you who they are. The vendors I've kept for years are the ones who took my first small order seriously, before there was any reason to.

None of this is complicated. It's just boring. The engineers get excited about the internals; your job is to make sure the spec, the paperwork, and the invoice all line up before you sign.