Sizing a compressed air dryer correctly comes down to five measured inputs, not a catalog number. To choose the right air dryer for your compressor, gather the compressor's free air delivery (FAD, the volume of air the compressor actually delivers at its rated discharge), the working pressure, the inlet air temperature at the dryer, the ambient temperature around it, and the required pressure dew point (PDP, the dew point measured at operating pressure rather than atmospheric). CAGI's air drying selection guide treats all five as prerequisites, because a dryer must be corrected for pressure and temperature before it can be matched to a required PDP. That is why a unit rated at nominal flow drifts off dew point in summer: as inlet temperature rises, the published correction factor falls, and KAESER publishes the correction tables that quantify the gap. Work through the six steps below and you will finish with a defensible specification, the arithmetic shown, and every figure traced to a named source.
Sollant Treatment develops and manufactures industrial air and gas treatment equipment, including refrigerated and desiccant air drying technologies and combined dryer solutions. Its work centres on compressed air purity and quality for industrial sectors such as manufacturing, electronics, healthcare and food processing. This article draws on that product and application background to explain how an air dryer is matched to a compressor and its operating conditions.
Step 1: Gather the Five Inputs a Dryer Selection Needs
Before you open a single datasheet, collect five numbers from your own system: compressor FAD, working pressure, inlet air temperature at the dryer, ambient temperature at the installation, and required pressure dew point. This compressed air dryer selection guide starts there because flow alone is not enough. The dryer has to be corrected for pressure and temperature, then matched to the dew point you actually need (CAGI Air Drying Selection Guide).
Free Air Delivery (FAD) is the volume of air your compressor delivers at its discharge, measured in cfm or m³/min. Select to compressor FAD, not motor horsepower, because the water load is set by the volume of air delivered rather than by input power (Atlas Copco, Choosing a dryer, 2026-06-18).
Key Takeaway
- FAD in cfm or m³/min, from the compressor datasheet
- Working pressure range in psig or MPa
- Inlet air temperature at the dryer, not at the compressor
- Ambient temperature at the installation site
- Required pressure dew point for your end use
Time: about 30 minutes with the compressor log and a site thermometer. Difficulty: beginner.
Step 2: Map Your Required Dew Point to an ISO 8573-1 Class
Pressure dew point (PDP) is the temperature at which water vapour in your compressed air begins to condense at the system's working pressure, and it is the number a dryer is specified against. ISO 8573-1:2010 turns that number into a class you can buy against. In the ISO 8573-1 water class table, Classes 1 through 6 are defined by PDP: Class 1 at ≤ -70 °C, Class 2 at ≤ -40 °C, Class 3 at ≤ -20 °C, Class 4 at ≤ +3 °C, Class 5 at ≤ +7 °C and Class 6 at ≤ +10 °C.
|
Water class |
Limit |
Unit |
|---|---|---|
|
1 |
≤ -70 °C |
Pressure dew point |
|
2 |
≤ -40 °C |
Pressure dew point |
|
3 |
≤ -20 °C |
Pressure dew point |
|
4 |
≤ +3 °C |
Pressure dew point |
|
5 |
≤ +7 °C |
Pressure dew point |
|
6 |
≤ +10 °C |
Pressure dew point |
|
7 |
≤ 0.5 g/m³ |
Liquid water concentration |
|
8 |
≤ 5 g/m³ |
Liquid water concentration |
|
9 |
≤ 10 g/m³ |
Liquid water concentration |
In our export projects, ISO 8573-1 Class 1 or Class 2 requirements are commonly requested by customers in electronics, pharmaceutical, and precision manufacturing industries, while general manufacturing applications often use Class 4 requirements depending on production needs.
Classes 7 to 9 switch from dew point to liquid-water concentration in g/m³ because liquid water is already present in the air at those levels, so a condensation temperature no longer describes the condition.
Once you have the class, the ISO class mapping to dryer technology narrows your options: Class 4 (+3 °C) is the standard refrigerated target, while Class 3 (-20 °C) and better generally requires desiccant or adsorption drying.
Warning: Class 0 is not a numeric class. It means "as specified" by the buyer, so it is only meaningful when you write the test conditions and limits into the specification yourself.
Step 3: Match Dryer Technology to That Dew Point
Once the class is fixed, the technology choice is largely determined. Refrigerated dryers typically reach +3 °C to +7 °C pressure dew point, with non-cycling units landing at +5 to +7 °C and cycling or variable-speed designs holding +3 °C (Atlas Copco, Selecting the best compressed air dryer, retrieved 2026-06-18). That covers Class 4 and nothing drier.
Desiccant dryers reach −40 °C to −70 °C, and membrane dryers reach about −40 °C (Atlas Copco, retrieved 2026-06-18). Membrane suits a point-of-use instrument line; it cannot match a large desiccant system's full capability.
The real decision is operating cost. Heatless desiccant dryers use 16–25% of compressed air as purge (Atlas Copco, retrieved 2026-06-18), while heated purge dryers cut purge air to under 10% and blower purge designs cause minimal compressed-air loss (Atlas Copco, retrieved 2026-06-18). The widely repeated 15% heatless figure traces back to the same handful of OEM and DOE-derived sources, so treat it as one upstream, not five (KAESER Talks Shop, undated trade column).
Our Engineering Recommendation Experience
Based on our experience supplying screw compressors, centrifugal compressors, and customized compressed air systems, dryer selection should not only consider dew point requirements but also consider:
- Compressor operating hours per day
- Seasonal temperature changes
- Future production expansion plans
- Maintenance capability at the installation location
Step 4: Correct the Rated Capacity for Your Actual Site Conditions
A dryer's nameplate rating is measured at reference conditions, so the capacity you actually get is the nominal figure multiplied by the manufacturer's correction factors. The correction is multiplicative: for refrigerated dryers, corrected capacity = nominal rated flow × inlet-temperature factor × ambient-temperature factor × pressure factor (KAESER sizing procedure). Desiccant correction sets normally omit ambient temperature, which is consensus OEM guidance rather than a standard requirement.
The factors move more than most buyers expect. KAESER publishes the correction tables for this exact reason: at 120 psig the factor falls from 1.32 at 75 °F inlet to 0.50 at 115 °F inlet, and at 100 psig with 100 °F inlet it is 0.69. Ambient factors run from 1.09 at 75 °F to 0.92 at 95 °F (KAESER, Compressed air dryer sizing). This is why the rating must be corrected for the real installation ambient: refrigerated pressure dew point depends on ambient heat rejection, so an air-conditioned test bench overstates what the unit does in a hot compressor room.
A worked sizing example at 200 cfm makes the arithmetic concrete. At 120 psig, 95 °F ambient and 110 °F inlet, the combined factor is 0.82 × 1.05 = 0.861, so the unit delivers only 86% (172 cfm) of nominal. Required rated capacity is 200 ÷ 0.861 = 232 cfm, satisfied by a 240 cfm unit (KAESER).
Then add headroom. Atlas Copco's dryer selection guide puts the margin at 10–15% above corrected required capacity (Atlas Copco, Selecting the best compressed air dryer, 2026-06-18); independent OEM guidance elsewhere allows 10–20%.
Pro Tip: The example above uses assumed site conditions and is illustrative only. Substitute your own measured inlet temperature, ambient temperature and working pressure, and take the correction tables from the datasheet of the exact model being quoted. Correction tables are manufacturer-specific and are not interchangeable between brands.
For desiccant duty, Sollant Treatment's SLT series spans 1.5–850 m³/min, which covers most plant-scale flows; the refrigerated range runs from small through large industrial sizes.
Real Capacity Calculation Example
A customer required a dryer for a 75kW screw compressor system.
Site conditions:
- Air flow requirement: 12 m³/min
- Ambient temperature: 40°C
- Inlet compressed air temperature: 45°C
- Working pressure: 7 bar
After applying the correction factors, the required dryer capacity was higher than the standard catalog value.
Instead of selecting the dryer only according to nominal flow rate, our engineers recommended selecting a model with additional capacity margin to ensure stable dew point performance during high-temperature operation.
Step 5: Price the Energy and Pressure-Drop Penalty Before You Commit
Two dryers can both hit your required pressure dew point and still differ by thousands of dollars a year in operating cost. The gap comes from two places: purge loss on desiccant designs, and pressure drop on every design.
Purge loss is the bigger of the two on heatless desiccant dryers, because a share of your compressed air is consumed to regenerate the bed rather than delivered to production. That air was already paid for at the compressor, so the purge percentage you compared as a specification in Step 3 is really an electricity line item. Multiply your compressor's annual kWh by the purge share and price it at the rate you actually pay.
For pressure drop, use the DOE rule of thumb of about 1% per 2 psi, roughly 0.5% per psi, and treat it as a rule of thumb rather than a measured result. US industrial power averaged 9.17 cents per kWh in June 2026 (EIA Electric Power Monthly Table 5.3, released 2026-08-26), so a 4 psi dryer-plus-filter penalty on a system spending USD 50,000 a year on electricity illustrates roughly 2% extra energy, about USD 1,000 a year. That is an illustration of the rule, not a benchmark.
The penalty cuts both ways. Grossly oversizing a dryer wastes electricity and capital, while undersizing degrades air quality (KAESER, Compressed air dryer sizing). Size to the corrected capacity from Step 4, then compare the two candidates on annual energy, not on purchase price alone.
Step 6: Verify the Supplier and the Certificate Before You Order
A specification is only as good as the documentation behind it, and the most common gap is a certificate that names a product family rather than the exact model on your quotation. Treat the paperwork as a second selection step, not an afterthought.
Ask for five documents and check each one against the model number you intend to buy:
- A CE Declaration of Conformity naming the exact model, not a series.
- PED 2014/68/EU documentation where the pressure equipment falls in scope.
- A valid ISO 9001 certificate whose scope covers the manufacturing site.
- A third-party ISO 8573-1 test report stating the test method and sampling conditions.
- Model-specific certificates throughout, rather than family-level approvals.
This checklist mirrors the documentation requirements used in federal compressed air specifications, which call for model-level declarations rather than blanket conformity statements (Unified Facilities Criteria, UFGS 22 15 26.00 20, undated).
Refrigerant status is the second thing to confirm. The AIM Act HFC phasedown is the federal driver moving refrigerated dryers away from R-134a, which has a global warming potential of 1,430, toward lower-GWP R-513A. The EPA states that compressed air dryers sit outside the scope of the 2023 Technology Transitions Rule, so the phasedown reaches your unit through refrigerant supply and service rather than a direct equipment ban (EPA, Technology Transitions HFC Restrictions by Sector, 2023-09-19). Confirm the current position with your supplier before publication, since this is a regulatory area that shifts.
Warning: Three red flags should stop a purchase. An unquantified "Class 0" claim with no test report behind it. Missing third-party test documentation. A certificate that names a different model than the one quoted.
Common Mistakes to Avoid
Sizing from the nominal rated flow. The most consequential error is picking a dryer whose nameplate capacity matches compressor output on paper, then discovering it under-delivers once summer inlet temperatures arrive. Rated capacity is published at a reference condition, so the number only holds after the correction factors in Step 4 are applied. Apply them before you compare models, not after the dryer fails to hit its dew point.
Selecting to motor horsepower instead of FAD. Buyers often match a dryer to the compressor's horsepower rating because it is the number printed largest on the nameplate. Horsepower describes the motor, not the air the system actually delivers. Size to the compressor's free air delivery in SCFM or m³/min, which is the flow the dryer has to dry.
Treating the 15-18% heatless purge figure as independent corroboration. That range appears across many vendor pages, but the sources trace back to a shared origin rather than separate measurements. Treat it as one claim, not five agreeing ones. Confirm purge loss against the specific dryer's published data for your operating pressure.
Accepting a "Class 0" claim with no stated test conditions. A class rating is only meaningful alongside the test method, inlet conditions, and measurement point behind it. Ask for those, and for the certificate that names them. A class number without conditions is a marketing label, not a specification.
Specifying a dryer without pricing its pressure drop. Every dryer adds resistance to the line, and that drop is paid for continuously by the compressor. Include the pressure-drop penalty in the total cost before you commit, as Step 5 sets out. A cheaper unit with higher drop can cost more over its service life.
Results: How to Verify Your Selection Is Correct
You should now hold a written specification you can defend: a required pressure dew point expressed as an ISO 8573-1 class, a corrected capacity in cfm with the three factors and their sources shown, a technology choice justified against that class, and an annual energy estimate at your own electricity rate.
Run one last check before you commit. Take the winning model's own datasheet correction factors and re-run the arithmetic from Step 4. The corrected capacity must still exceed your required capacity by the 10–15% margin, with the same three factors applied. If it does not, the model is undersized for your site, not for the catalogue.
As a stretch goal, compare a fixed-speed refrigerated unit against a VSD or cycling design on part-load energy. The dew point advantage of cycling and VSD machines only pays back where the load profile is genuinely variable. If your demand is flat, that premium buys little.
When the numbers hold, have an engineer review the specification against your actual load profile before you order.
Talk to a Sollant Treatment engineer: send your completed specification and get a selection review that confirms capacity, dew point class and energy estimate before you commit to a model.
Frequently Asked Questions
Q: How do I choose between a refrigerated and a desiccant dryer when I need something between Class 4 and Class 3?
A: Refrigerated dryers reach a pressure dew point of roughly 3 °C, which maps to Class 4 for water. Desiccant dryers go far below that, to -40 °C and lower. If your process genuinely needs better than Class 4 but you do not need -40 °C, the decision usually comes down to whether the extra dryness justifies the desiccant unit's purge loss and higher operating cost. For most plant air that sits just inside Class 3, a refrigerated dryer paired with a properly sized coalescing filter is the lower-cost route. Confirm the exact class your process requires before you commit, because the gap between Class 4 and Class 3 is where most overspending happens.
Q: Can a membrane dryer replace a small desiccant unit?
A: Membrane dryers are compact and have no moving parts, but they are limited in flow capacity and typically deliver a dew point in the range a small desiccant unit beats. They suit point-of-use applications with low flow and moderate dryness needs, not a substitute for a desiccant dryer where -40 °C is required. If your flow is small and your dew point target is modest, a membrane unit can work. If you need deep dryness at any real volume, it cannot.
Q: What if the manufacturer does not publish correction tables for the model I am quoting?
A: Ask the supplier directly for the correction factors for your inlet temperature, ambient temperature, and working pressure. A supplier who cannot produce them is a risk. If no tables exist, apply the standard correction approach yourself and document your assumptions, then hold the supplier to the corrected capacity in writing.
Q: Is oversizing by more than 10 to 15 percent ever justified?
A: Rarely. A 10 to 15 percent margin covers normal variation in load and conditions. Beyond that you pay for capacity you will not use, and on a refrigerated dryer you also lose efficiency at part load. The exception is a known future expansion, where the extra capacity is planned rather than speculative.
Conclusion
You now have the five inputs a dryer selection needs, a target dew point mapped to an ISO 8573-1 class, a technology matched to that class, and a capacity figure corrected for your own site conditions. That last step is where most specifications quietly fail. A nominal 200 cfm rating corrected down to 172 cfm of delivered capacity is the difference between a dryer that holds its dew point in August and one that does not, and the arithmetic is the only thing standing between the two.
The energy and pressure-drop estimate tells you what the unit will cost to run, not just to buy, and the supplier checklist tells you whether the certificate behind the number is real. Work through those six steps in order and you can defend the selection to procurement, to operations, and to whoever signs the purchase order.
If you would rather have a second set of eyes on the numbers before you commit, talk to an engineer about a selection review. Bring your FAD, pressures, and temperatures, and we can check the corrected capacity and the class mapping against your actual duty cycle.