How to Size a Compressed-Air Dryer Correctly

To size a compressed-air dryer correctly, start with the maximum delivered airflow through the dryer—not compressor horsepower or pump displacement. Correct that flow for the worst credible inlet pressure, inlet temperature, ambient temperature, and required pressure dew point. Apply the selected manufacturer's correction factors in the stated direction, then choose a model whose corrected capacity meets or exceeds the requirement.

The sizing inputs you need#

Record these conditions before requesting a quote:

Input	Use in the calculation
Maximum system flow	Highest coincident airflow that can pass through the dryer
Minimum dryer-inlet pressure	Lowest pressure expected while that maximum flow occurs
Maximum dryer-inlet temperature	Highest compressed-air temperature expected at the dryer
Maximum ambient temperature	Highest cooling-air temperature around an air-cooled dryer
Required pressure dew point	Dryness needed at operating pressure
Duty cycle and simultaneity	Determine which intermittent loads can occur together
Planned additions	Add identified future demand rather than an arbitrary percentage
Manufacturer and dryer series	Determine the applicable rating conditions and correction table

Use conditions that can occur together. Combining an impossible low pressure from one operating mode with a peak flow from another can produce an unnecessarily large result. Conversely, using annual averages can hide the hot, high-flow condition that limits dryer performance.

Rated capacity is not corrected capacity#

A dryer's published SCFM is a rated capacity at specified reference conditions. It is not a promise that the dryer will process that flow under every permitted condition.

The CAGI refrigerant-dryer data sheet uses ISO 7183 Option A2 rating conditions: 100°F compressed-air inlet temperature, 100 psig inlet pressure, 100°F maximum ambient temperature, and saturated inlet air. It defines SCFM at 14.5 psia, 68°F, and zero relative water-vapor pressure. Other catalogs use different reference conditions, so two identical-looking nominal ratings are not necessarily based on the same operating point.

Corrected capacity is the flow a rated dryer can process at the installation's actual conditions while meeting its stated outlet performance. Manufacturer documents commonly use one of two conventions:

1. Capacity-factor convention

   Corrected capacity = rated capacity × pressure factor × inlet-temperature factor × ambient factor × dew-point factor

   Therefore:

   Required rated capacity = design flow ÷ combined capacity factor

2. Required-capacity convention

   Required rated capacity = design flow × pressure factor × inlet-temperature factor × ambient factor × dew-point factor

The Parker KA-MT 10–95 document uses the second convention. It calls the result Minimum Drying Capacity and instructs the reader to select a listed flow rate equal to or greater than that result. Kaeser and Atlas Copco examples use capacity factors that divide into design flow. Never infer the calculation direction from whether a factor is above or below 1.0. Follow the equation supplied for the exact series.

Step 1: Establish delivered airflow#

Use measured flow when reliable data are available. Otherwise, use the compressor's documented full-load delivered capacity or free air delivery at the applicable discharge pressure. For multiple compressors, add the delivered airflow of every unit that can load at the same time and feed the common dryer.

Do not substitute piston displacement. Displacement describes the volume swept by a compressor mechanism; it does not account for all losses between intake and delivered output. Atlas Copco defines free air delivery, or FAD, as air actually delivered at the outlet and expressed as an equivalent volume at stated inlet conditions. Its published piston-compressor data also list displacement and FAD as different values.

Likewise, do not compare an unqualified ACFM measurement directly with a dryer rating in SCFM:

• ACFM is volume at the stated actual pressure and temperature.
• SCFM is volume normalized to stated reference conditions.
• FAD represents delivered mass flow as an equivalent volume at specified free-air conditions.

A flow meter may report mass flow already normalized into SCFM. Verify its configured standard temperature, absolute pressure, and humidity basis. If it reports actual line volume, have the supplier convert it to the reference basis used by the dryer catalog.

Step 2: Apply duty cycle without hiding peak demand#

Duty cycle is the percentage of a period during which a load operates. It is useful when estimating demand from intermittent tools:

Average load contribution = full-load flow × duty cycle

That calculation alone does not establish the required dryer size. A dryer receives the flow that passes through it at each moment. Several low-duty-cycle tools can still overlap and produce a high short-term flow.

Use duty cycle in one of these ways:

• For independent loads that can overlap, calculate the credible maximum coincident demand.
• For sequenced equipment, use the maximum allowed by the documented sequence.
• For a compressor dedicated to a common dryer, use the maximum compressor combination the controller can load.
• If a receiver and flow controller demonstrably limit downstream flow, use that controlled maximum and document the receiver assumptions.

CAGI recommends applying usage factors when determining system demand, and its sizing guidance permits a safety factor. Model the real demand pattern rather than a shift-wide average that cannot represent a peak.

Step 3: Use the lowest inlet pressure#

Dryer capacity changes with pressure. At a given free-air mass flow, lower line pressure produces a greater actual volume through the dryer. Manufacturer tables therefore commonly show less usable capacity, or a larger required-capacity factor, at lower inlet pressure.

Use the minimum pressure at the dryer inlet while design flow is present. Account for upstream filters, separators, piping, and compressor control bands. Do not use the compressor's maximum nameplate pressure unless it is also the dryer's minimum operating pressure.

As one manufacturer-specific example, Kaeser's TX-series table assigns a 0.79 capacity factor at 80 psig and 110°F inlet temperature, versus 1.00 at 100 psig and 100°F. Parker's KA-MT 10–95 modular adsorption-dryer table expresses the pressure effect with a required-capacity factor: its minimum-inlet-pressure factor rises as minimum inlet pressure falls. The Parker table gives a factor of 1.60 at 58 psig, 1.00 at 100 psig, and 0.53 at 232 psig.

These Parker values apply only to the KA-MT 10–95 range covered by the cited document. They must not be attributed to Parker CDAS dryers or transferred to another Parker family.

Step 4: Use the maximum inlet temperature#

Inlet temperature matters for two related reasons. First, warmer air can hold more water vapor. When saturated compressed air enters a dryer at a higher temperature, the dryer receives a larger moisture load. Second, a refrigerated dryer must remove sensible heat before it can cool the air enough to condense water.

Use the temperature after the aftercooler and moisture separator, at the dryer inlet, during the hottest operating condition. A clean, correctly drained aftercooler can materially reduce the load before air reaches the dryer.

The effect can be large. Atlas Copco's F25–355 refrigerated-dryer table gives an inlet-temperature capacity factor of 1.00 at 95°F, 0.82 at 104°F, and 0.69 at 113°F under that series' reference framework. Those figures apply to that documented series; they are not universal factors.

The Parker KA-MT 10–95 table uses the opposite calculation direction. Its maximum-inlet-temperature factor is 1.00 at 95°F, 1.15 at 104°F, and 1.22 at 113°F. These factors increase the calculated Minimum Drying Capacity and apply only to that documented KA-MT range.

Step 5: Use the maximum ambient temperature#

An air-cooled refrigerated dryer rejects heat into the surrounding air. As ambient temperature rises, its refrigeration system has less temperature difference available for heat rejection, so usable airflow capacity can fall.

For the Atlas Copco F25–355 series, the published ambient factor declines from 1.00 at 77°F to 0.81 at 95°F and 0.62 at 113°F. Kaeser's TX table uses a different reference point and gives 1.00 at 100°F ambient. This difference is why factors cannot safely be transferred between manufacturers—or between different series from one manufacturer.

Measure the air entering the dryer's condenser. Poor clearance or recirculation of hot exhaust air can make that temperature higher than the general compressor-room reading.

Water-cooled dryers use cooling-water conditions instead. Obtain the manufacturer's correction method for inlet water temperature and flow.

The cited Parker KA-MT 10–95 table lists an ambient correction factor of 1.00 from 77°F through 122°F. That result belongs to the KA-MT adsorption-dryer calculation. It does not establish how a refrigerated dryer responds to ambient temperature.

Step 6: Set the required pressure dew point#

Pressure dew point is the temperature at which water vapor begins to condense while the air remains at system pressure. It is not interchangeable with atmospheric dew point.

A typical refrigerated dryer is suitable when the required pressure dew point remains above freezing. Applications that require a substantially lower pressure dew point generally need desiccant or another drying technology. Start with refrigerated vs. desiccant air dryers, then map the required contaminant class with our ISO 8573-1 air-quality classes guide.

Some manufacturers include a dew-point correction factor. A lower requested pressure dew point can reduce capacity. Include that factor when the table requires it; do not assume that every dryer has the same rated dew point.

For example, the Parker KA-MT 10–95 table gives an outlet-dew-point factor of 1.00 for −13°F and −40°F, and 2.00 for −100°F. The same document identifies −40°F as standard, −100°F as an option, and −13°F as another option. These are product-family-specific pressure-dew-point values and factors.

Step 7: Calculate the minimum catalog rating#

Assume the following documented design condition:

• Maximum delivered flow: 125 SCFM
• Minimum inlet pressure: 80 psig
• Maximum inlet temperature: 110°F
• Maximum ambient temperature: 100°F
• Dryer family: Kaeser TX, using its published table
• Applicable pressure/inlet-temperature factor: 0.79
• Applicable ambient factor: 1.00

The combined capacity factor is:

0.79 × 1.00 = 0.79

The minimum nominal rating is:

125 SCFM ÷ 0.79 = 158.2 SCFM

Select a model rated at least 158.2 SCFM under that table's reference conditions. In practice, this means moving to the next available catalog size that also satisfies the pressure-dew-point, pressure-drop, electrical, temperature-limit, and installation requirements.

This calculation illustrates the Kaeser TX table. It is not a reusable set of factors.

For the Parker KA-MT 10–95 range, use Parker's documented multiplication equation instead:

Minimum Drying Capacity = system flow × inlet-temperature factor × ambient-temperature factor × minimum-inlet-pressure factor × outlet-dew-point factor

Then select a KA-MT model with a listed inlet flow rate equal to or greater than the calculated Minimum Drying Capacity. Do not use this equation as evidence for the CDAS family or any other Parker dryer range.

How much sizing margin is appropriate?#

There is no evidence-backed universal percentage that fits every system. CAGI says a safety factor can be included, but the appropriate allowance depends on uncertainty and expected demand.

Build margin from identifiable items:

1. Add documented future equipment demand.
2. Add measured flow uncertainty if the instrument or estimate warrants it.
3. Include compressor combinations that controls can actually load.
4. Use worst credible seasonal conditions.
5. Select the next catalog size above the corrected requirement.

Avoid applying a large blanket percentage after already combining impossible extremes. Oversizing can add capital cost, and part-load behavior differs between cycling, non-cycling, and desiccant dryers. Ask the supplier to show the corrected capacity at both peak and typical conditions.

Final quote checklist#

Require the quote to state:

• Dryer model and nominal rating
• Rating standard and reference conditions
• Design flow and its SCFM, ACFM, or FAD basis
• Minimum inlet pressure
• Maximum inlet and ambient temperatures
• Required pressure dew point
• Every correction factor and the calculation convention
• Corrected capacity at design conditions
• Maximum pressure and temperature limits
• Pressure drop at design flow
• Purge or regeneration demand, if applicable
• Drain, prefilter, and afterfilter requirements

Once the required corrected capacity and drying technology are known, compare suitable units in the best refrigerated air dryers for small shops or the best point-of-use desiccant air dryers. Verify that each comparison uses evidence for the exact model under consideration.

Evidence limits#

The cited manufacturer tables prove how to correct capacity only for the named product families and document versions:

• The Kaeser example applies to the TX-series table reproduced in its installation guide.
• The Parker factors and Minimum Drying Capacity equation apply to the KA-MT 10–95 modular adsorption-dryer range.
• The Atlas Copco examples apply to the F25–355 refrigerated-dryer brochure.
• The CAGI documents provide standardized terminology and general sizing guidance but do not rank individual products.

The evidence does not support transferring Parker KA-MT factors to CDAS dryers. It also does not support a broad best-of ranking across manufacturers. Obtain the current data sheet for the exact model and market before purchasing or approving a final selection.

Sources#

• Rated SCFM, reference conditions, and pressure-dew-point testing: CAGI Refrigerant Compressed Air Dryer Data Sheet
• Capacity-factor method and Kaeser TX pressure and temperature example: Kaeser Compressed Air System Installation Guide
• KA-MT 10–95 Minimum Drying Capacity equation, correction factors, flow ratings, and dew points: Parker KA-MT 10–95 modular adsorption-dryer product information
• F25–355 pressure, inlet-temperature, ambient, and dew-point factors: Atlas Copco F25–355 refrigerated-dryer brochure
• Free air delivery and delivered-versus-intake airflow: Atlas Copco guide to compressor flow measurements
• Usage factors and allowance for a documented safety margin: CAGI Technical Brief—Sizing Compressed Air Equipment