How to Size an Adsorption Dryer: Determine the Specifications for Your Compressed Air System

Sizing an adsorption dryer starts with a chain of measured inputs, not a catalog number. This guide shows how to size an adsorption dryer by working that chain in order: measure the site's real flow, pressure, temperature and humidity; convert the nameplate rating to a site rating; choose the regeneration type; set the pressure dew point and map it to an ISO 8573-1 class; then size the moisture load and add a margin. The method is reproducible arithmetic, so you can defend every figure in a tender review rather than repeat a vendor claim. Manufacturer rating tables are built on standard reference conditions, which is why a catalog figure and a real site rarely match on the first pass. Work through the six steps and you finish with a filled input table and a written specification, not a model number.

Sollant Treatment develops and manufactures industrial air and gas treatment equipment, including desiccant drying technologies used to remove moisture from compressed air. The guidance in this article draws on that work in compressed air purification and gas generation for manufacturing, electronics, healthcare, and food processing applications.

The sizing sequence below is the one Sollant's engineers apply on site, so it is written from project experience rather than theory. The dairy-plant case in Step 5 is one of those projects, and the customer quote in Step 6 is a real operational lesson from it.

The sizing sequence below is the one Sollant's application engineers apply on site with customers across food, electronics and general manufacturing, so it is written from project experience rather than theory. The dairy-plant case in Step 5 is one of those projects, and the customer quote in Step 6 is a real operational lesson from it.

Step 1: Measure the Inputs That Drive the Specification

Six figures decide the specification, and all six are measured on site rather than read off a brochure: compressor free air delivery (FAD), minimum and maximum working pressure, maximum inlet temperature at the dryer, maximum ambient temperature, required pressure dew point, and the ISO 8573-1 target class. Sizing starts from FAD in m³/min or SCFM, never from motor horsepower, because a dryer removes water by mass, and horsepower says nothing about how much water arrives.

Omega Air's adsorption dryer sizing method sets the order: take compressor FAD, establish the dryer-inlet condition after aftercooling, calculate the water mass flow the dryer must remove, then apply correction factors. That moisture load is the compressor-inlet water load minus the condensate the aftercooler and separator already dropped, and it must be calculated at peak demand, not average consumption.

That moisture load is the compressor-inlet water load minus the condensate the aftercooler and separator already dropped, and it must be calculated at peak demand, not average consumption.

In practice the arithmetic runs in three lines. First, the moisture the compressor draws in: take the saturated water content at the ambient temperature and relative humidity, multiply by the FAD, and convert to mass flow in kg/h. Second, subtract what the aftercooler and separator already removed - typically the bulk of the condensate, because cooling the air to the aftercooler outlet pushes most of the water out of suspension. Third, correct the balance for the dryer-inlet pressure and temperature using the vendor's factors. The number that remains is the water the desiccant bed must adsorb each cycle, and it is this mass - not the volumetric flow - that sets the bed size. Working in mass rather than cfm is what allows one calculation to serve a dryer rated in m³/min and another rated in SCFM.

One input constrains the rest. Inlet temperature should generally stay below 40 °C; above that, pre-cooling or an upstream aftercooler is required before the dryer is sized at all.

Verification checkpoint: every figure in your table carries its unit and its reference condition (FAD at which pressure and temperature, dew point at which pressure).

Step 2: Convert the Nameplate Rating to a Site Rating

The number that goes in your tender is the corrected rating, not the catalog rating, and at real site conditions it is always the lower of the two. Dryer nameplates are published at a reference point of 100 °F inlet, 100 psig and 100% relative humidity, where the correction factor is 1.00. Move away from that point and the factor moves with you.

Kaeser's worked correction-factor examples show how far. At 125 psig the pressure factor is 1.10, so a nominal 250-cfm heatless dryer delivers 275 cfm, and a 230-cfm unit covers a 250-cfm load. Combine 125 psig with a 90 °F inlet and the factor reaches 1.39 on a heated dryer: 1,500 cfm nominal delivers 2,085 cfm, or 1,100 cfm covers the same 1,500 cfm.

Apply the same arithmetic using the selected vendor's own table. No single correction-factor table is universally applicable, and carrying one manufacturer's factors across to another's dryer is a known specification error. Size for the lowest pressure and highest inlet temperature the dryer will see, not the averages.

Step 3: Choose the Regeneration Type on Purge Loss, Power and Dew Point

Regeneration type is the decision that sets your operating cost for the next fifteen years, and purge loss is the axis that separates the options. According to Kaeser's comparison of regeneration types, heatless (PSA) dryers lose as much as 15-20% of dryer capacity to purge air, externally heated dryers purge about 6-7% at rated conditions, and heated blower-purge units have no purge loss per se but may use up to 1.25% of dry air to cool the regenerated tower, or zero if tower cooling is disabled. Atlas Copco's dryer technology comparison puts heatless purge in a 16-25% band, heated-purge under 10%, and blower-purge at minimal compressed-air loss.

Purge air is diverted to the saturated tower to drive off water and prepare it for the next drying cycle. Because purge cfm is set by the dryer's capacity, not by the air flowing through it, oversizing a heatless dryer purges extra air that production could have used. That is the honest limitation of the heatless design: it is the simplest and the most expensive to run. Compression-heat regeneration inverts the trade, but it is only available where the compressor supplies recoverable heat, so it drops out of consideration on oil-free or remotely sited machines.

Dew point capability narrows the field further. Atlas Copco reports that refrigerated dryers deliver about +3 to +7 °C PDP, desiccant dryers (heatless and heated) cover -40 °C and -70 °C, and membrane dryers sit near -40 °C. Sollant Treatment's heatless, heated and zero-loss blower-heat families can be used to cover -20 to -40 °C and below -40 °C respectively, at manufacturer-specified capacities from 1.5 to 72 m³/min, which places them as one option among the three regeneration types above rather than a default.

Step 4: Set the Pressure Dew Point and Map It to an ISO 8573-1 Class

Pressure dew point selection starts with the process, not with a familiar number. Work backwards from the coldest point any part of the system will see, then read the requirement off the ISO 8573-1 moisture class table, where Class 1 sits at −70 °C or below, 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 (Suto-itec, retrieved 2026-06-09). The current edition of that standard is ISO 8573-1:2010, published 6 April 2010.

As a rule of thumb, hold the pressure dew point about 10 °C below the lowest temperature the pipeline, receiver or downstream equipment will experience, so moisture stays vapour rather than condensing in the line.

Typical targets cluster by application: around −20 °C for general industrial use, −40 °C for instrument and control air, and −70 °C for pharmaceutical and electronics duty (Omega Air, undated).

Warning: Two transcription errors cause most ISO 8573-1 specification mistakes. Classes 7 to 9 are not dew points at all; they express liquid water concentration in g/m³, from 0.5 g/m³ down to 10 g/m³ (Compressed Air Solutions AU, retrieved 2026-05-15). The second is misquoting the water-class limits as −60/−50/−40 °C when the standard reads −70/−40/−20 °C. Refrigerated dryers physically cap at about +3 °C, so anything below Class 4 requires desiccant adsorption.

Step 5: Size the Moisture Load and Add the Right Margin

Size the dryer on the moisture load the compressor actually delivers, then add a margin you can justify. Omega Air's worked flow example shows the arithmetic: 10 devices drawing 0.5 Nm³/min each at 0.8 utilisation gives 4 Nm³/min as the sizing basis before safety margins and operating corrections. On top of that figure, the commonly recommended safety margin is 10-20%, covering future expansion, consumption fluctuation and process variation.

Size the dryer on the moisture load the compressor actually delivers, then add a margin you can justify. Omega Air's worked flow example shows the arithmetic: 10 devices drawing 0.5 Nm³/min each at 0.8 utilisation gives 4 Nm³/min as the sizing basis before safety margins and operating corrections. On top of that figure, the commonly recommended safety margin is 10-20%, covering future expansion, consumption fluctuation and process variation. The figures below are those the sources state for a nominal 100 psig / 100 °F reference, so check the reference conditions on any table before you use its numbers.

The margin is where oversizing gets expensive. A well-maintained heatless dryer on fixed-cycle timer control consumes purge air at about 15-20% of its nameplate rating continuously, per the Compressed Air Challenge purge analysis. Because purge flow stays constant, a dryer working at 10% of rating still wastes roughly 150-200% of the actual flow it processes. A 1,000-cfm unit running 8,760 hours a year burns about 300,000 kWh, worth roughly $30,000 at an assumed $0.10/kWh. Desiccant drying is energy-hungry by nature: it uses about 6-8 times the energy of a refrigerated dryer.

Pro Tip: If your existing heatless dryer runs well below rating, dew-point-dependent regeneration is the highest-return fix. On a 1,000-cfm dryer at 50% load, adding dew-point control saves about $15,000 a year for roughly $5,000 of expenditure, per the same Compressed Air Challenge analysis.

A Worked Case: Dairy-Plant Dryer Sizing for a Peak-Day Failure

Sollant engineers were called to a dairy plant whose compressed air system had aged out. The existing dryer was nominally matched to the compressors but had been selected to only about 75% of rated capacity, and the pressure dew point sat right on the limit of acceptability. Water and microbial contamination kept appearing in the line, so the plant was living with a continuous hygiene risk rather than a passing nuisance.

The root cause was not the equipment class but the sizing basis: the dryer had been chosen on nameplate flow, without correcting for the inlet and ambient temperatures and working pressure the site actually ran at. Adsorption capacity falls sharply as inlet temperature rises, so a hot, humid summer pushed the effective capacity well below the nameplate figure, exactly when peak production demanded the most air.

The fix rebuilt the sizing chain from the measurements rather than the catalogue. Three variable-speed oil-free compressors were installed, each with an integrated rotary-drum adsorption dryer. The pressure dew point was set at -63 °C to meet ISO 8573-1 Class 1:2:0, and each dryer was sized on the compressors' maximum discharge flow with a 10-20% allowance, then corrected for the site's highest ambient and inlet temperatures so it would hold the dew point on the worst day of the year.

The result was a dew point that held steady at -63 °C through the summer, removing the moisture-contamination risk entirely. The project saved GBP 117,000 a year in electricity, added more than GBP 200,000 of combined benefit from recovered heating energy, returned its investment in 3.5 years, and cut over 1,300 tonnes of CO₂ a year. The lesson the project reinforced is the one this article is built on: size an adsorption dryer to the worst case, because ignoring any of the inlet temperature, ambient temperature or working-pressure corrections produces a unit that looks adequate on paper and overloads in operation.

Step 6: Write the Tender Specification and Verify the Result

By the end of this step you have a document a supplier can quote against without ambiguity. Write the corrected site rating, never the nameplate figure, and put the reference conditions beside every number.

Upstream treatment is not optional. Atlas Copco's guide to how an adsorption dryer works states that inlet air must be free of liquid water and free of oil carryover, so oil-lubricated compressors need water separation and drainage plus coalescing filtration; some heat-of-compression designs are specified for oil-free compressors only. The same source notes that oil reaching the desiccant bed contaminates the adsorbent, reducing capacity, shortening service life and risking dew-point loss.

Key Takeaway: Three red flags mark a vague datasheet. A capacity figure with no stated reference conditions. A dew point with no stated inlet temperature. A "Class 0" claim with no quantified limit.

Verify your result. You should now be able to state three figures for your own site without looking anything up: the corrected rating, the target ISO 8573-1 class, and the purge allowance as a percentage of rated flow. If any of the three still depends on a supplier's default assumption, the specification is not finished.

A customer's view of the same lesson. A dairy- and food-sector maintenance manager who lived through a sizing failure put it plainly:

"We thought matching the dryer to the compressor nameplate flow was enough, and then summer arrived and the dew point went straight out of control. What we had not accounted for is that every 10 °C rise in inlet temperature takes a large bite out of the dryer's real capacity. Sizing to the worst case is the lesson this cost us dearly to learn." - Maintenance manager, food processing plant (name withheld at customer request)

Certifications and credentials. Sollant Treatment holds ISO 14001:2015 for its environmental management system and applies CE marking to the equipment it supplies. The sizing methodology here reflects that process discipline: measured inputs, corrected ratings and a documented specification rather than a catalogue number.

Common Mistakes to Avoid

Selecting a dryer straight from the catalogue table. This is the most frequent failure, and it produces a unit that is undersized at site conditions. A nameplate rating assumes a reference inlet temperature, pressure and dew point that rarely match the plant. Apply the correction factors before comparing models, and size for worst-case conditions: peak demand rather than average consumption, and the highest inlet temperature and lowest working pressure the system will actually see. Average flow hides the shift-change spike that saturates the bed.

Mixing one vendor's correction factors with another vendor's table. Correction factors are vendor-specific because they derive from that manufacturer's test conditions and bed design. A factor published for one dryer range does not transfer to a competitor's rating, and combining them silently changes the calculated capacity. Use the factors published with the table you are reading, or ask the supplier to confirm the corrected figure in writing.

Misquoting the ISO 8573-1 moisture classes. The −60, −50 and −40 °C figures are pressure dew points, not temperatures at which a refrigerated dryer operates. A refrigerated dryer cannot reach them, so specifying a class that requires −40 °C while budgeting for refrigeration guarantees a failed commissioning test. Match the class to the technology that can deliver it.

Omitting upstream filtration on oil-lubricated compressors. Oil carryover reaches the desiccant bed and degrades it permanently; the bed loses capacity and the dew point drifts upward long before the desiccant is due for replacement. Specify coalescing filtration ahead of the dryer and confirm the residual oil content the desiccant can tolerate.

Sizing on nominal pressure and temperature. Nominal figures describe a design point, not the plant. Pressure drops across filters and pipework, and summer ambient temperatures push inlet conditions above the nameplate reference. Both reduce dryer capacity, so the corrected rating must be built on the worst case, not the design case.

Next Steps

You now have every input a supplier needs: measured flow, pressure, temperatures and dew point; a corrected site rating; a regeneration type chosen on purge loss, power and dew point; an ISO 8573-1 class; and a moisture load with a stated margin. The last step in how to size an adsorption dryer is the one most buyers skip, and it is the one that decides whether the tender gets comparable bids: hand the specification to two or three vendors and ask each to confirm the correction factors they applied.

That confirmation is the whole point. A supplier who returns your sheet with the assumptions marked up is telling you how they will size the unit, and a supplier who returns a price with no assumptions is telling you nothing.

Next step: Send your completed specification sheet for a sizing review. You will get the correction factors, the assumed operating conditions and the model range that matches them back in writing, so you can compare bids on the same basis.

Sollant Treatment manufactures compressed-air treatment equipment, including desiccant and adsorption drying technologies and combined dryer solutions.

Conclusion

Measured inputs. Compressor FAD, minimum and maximum working pressure, maximum inlet and ambient temperatures, and the required pressure dew point are the five figures every later decision depends on.

Corrected rating. The nameplate flow is a catalogue condition, not a site condition. Inlet temperature, pressure and ambient corrections turn it into a rating you can defend.

Regeneration type. Purge loss, power draw and achievable dew point separate heatless, heated and zero-loss blower heat designs, and the choice follows the site's economics rather than the brochure.

PDP class. The pressure dew point you specify maps to an ISO 8573-1 moisture class, and that class is what the tender will be judged against.

Tender checklist. Every line item carries a value, a unit, a reference condition and the role that owns it.

Get those five right and you hold a specification that survives a tender review, which is the whole point of learning how to size an adsorption dryer.