Dryer correction factor worksheet

What does the dryer correction worksheet calculate?

The worksheet starts from the published rated capacity of a specific dryer model and applies the correction method that the same manufacturer publishes for that model. It returns the corrected usable capacity at your conditions, the margin against your design flow, and, if needed, the minimum catalogue size that would meet the duty.

The calculator on this page does the arithmetic. The explanation below covers the inputs, the method and the traps that cause undersized dryers.

Why rated capacity is not usable capacity

Dryer makers rate capacity at defined reference conditions. For refrigerated dryers these are commonly based on ISO 7183, typically around 7 bar(g) (102 psig) working pressure, about 35 °C compressed-air inlet temperature and about 25 °C ambient or cooling medium, with a stated pressure dew point. Every maker states its own basis, so read the data sheet rather than assuming.

Real plants rarely match those conditions:

• Inlet air temperature rises when the compressor aftercooler is fouled or the room is hot, and every extra degree carries more water vapour into the dryer.
• Ambient temperature in Telangana and Andhra Pradesh often reaches 40–45 °C in summer, reducing condenser performance on air-cooled refrigerated dryers.
• Lower working pressure means a larger actual volume for the same free air, and more water per cubic metre of free air at saturation.
• A lower required dew point, or a desiccant dryer specified for −40 °C rather than −20 °C, reduces capacity.
• Cooling-water temperature affects water-cooled refrigerated dryers.

The supplier’s method governs

There is no safe universal correction table for all dryers. Two makers may both publish an “inlet temperature factor”, yet the numbers, the reference point and the way factors combine can differ. Some multiply factors; some use a combined chart; some publish a selection program only. Desiccant dryers also depend on regeneration method, cycle time and purge arrangement.

If the method for the model is missing, unclear or outside its published range, the worksheet marks the result as incomplete pending supplier confirmation, rather than inventing a factor.

The method step by step

1. Establish the design flow in FAD (m³/min or cfm), including expected growth and any compressors that may run together.
2. Record actual conditions: working pressure at the dryer inlet in bar(g) and psig, compressed-air inlet temperature, ambient or cooling-water temperature, and required pressure dew point.
3. Take the model’s rated capacity and rating conditions from the current data sheet.
4. Look up each correction factor from the same data sheet for the actual conditions.
5. Calculate corrected capacity = rated capacity × F(pressure) × F(inlet temperature) × F(ambient) × F(dew point), or use the maker’s combined method if that is what they publish.
6. Calculate margin (%) = (corrected capacity − design flow) ÷ design flow × 100.
7. Or calculate required rated capacity = design flow ÷ (product of factors), and choose the next catalogue size up.
8. Check that every condition lies inside the dryer’s operating envelope.

Worked example

A plant needs to dry 15.0 m³/min (about 530 cfm) at 6.5 bar(g) (about 94 psig). In May the compressed-air inlet reaches 45 °C and the room ambient 42 °C. The candidate refrigerated dryer is rated 17.0 m³/min. The factors below are hypothetical, used only to show the arithmetic; real values must come from the chosen model’s data sheet.

The dryer that looked comfortably sized on paper would be heavily overloaded on a summer afternoon. The pressure dew point would rise, water would pass into the distribution system and drains would struggle. Two practical alternatives exist alongside choosing a larger dryer: reduce the inlet temperature by servicing the compressor aftercooler and improving room ventilation, or move the dryer to a cooler location. Both change the factors, so recalculate after any change.

Refrigerated and desiccant dryers: different correction issues

For heatless desiccant dryers, the purge air comes out of the delivered flow, so the dryer inlet must be sized for design flow plus purge, using the maker’s figure for that model and condition. A separate worksheet covers purge consumption.

Common sizing mistakes we see on site

• Selecting the dryer to match the compressor nameplate without correcting for summer conditions.
• Placing the dryer behind the compressor where it breathes hot exhaust air.
• Ignoring a fouled compressor aftercooler that raises dryer inlet temperature.
• Adding a second compressor later without rechecking the dryer.
• Reading the dew point indicator on a refrigerated dryer as proof of air quality, rather than measuring pressure dew point.
• Specifying −40 °C dew point where +3 °C would serve the process, increasing cost and energy.

Recheck after any change on site

Correction is not a one-time exercise. Adding a compressor, raising the set pressure, moving the dryer, enclosing the room or simply reaching a hotter season all change the factors. Rerun the worksheet whenever one of these changes, and compare the corrected capacity with measured pressure dew point. If the measured dew point drifts upwards while the calculation still shows margin, look for fouled condensers, failed drains or a bypass valve left partly open.

How to read the result

A positive margin means the dryer should meet the duty at the stated conditions, provided those conditions are realistic for the worst season. Most engineers keep a reasonable positive margin to cover fouling, future demand and measurement uncertainty; the size of that margin is a design decision to record, not a fixed rule. A negative margin means the dryer is undersized for that case. An incomplete result means the maker’s data does not cover the condition and must be confirmed before selection.

How Reddy Compressors helps

Reddy Compressors, at Tarnaka, Secunderabad, since 2002, sells new and used/refurbished refrigerated and desiccant dryers, rents dryers with compressors from our own fleet, and services dryers of most makes as an independent multi-brand specialist (not an authorised dealer of any brand). We have tie-ups with all major and reputed brands.

• Dryer selection at real site conditions, using the chosen maker’s published correction data.
• Site checks of inlet temperature, ambient, pressure and pressure dew point.
• Service of refrigerated and desiccant dryers: condensers, drains, valves, desiccant and filters.
• Spare parts identified from the dryer nameplate and confirmed before quoting.
• Service coverage across Telangana and Andhra Pradesh by over 20 service engineers and technicians; supply across India.

What to send us

• Make, model and nameplate photo of the dryer and each compressor feeding it.
• Working pressure and the required pressure dew point or ISO 8573-1 water class.
• Peak summer room temperature and, if known, dryer inlet air temperature.
• Current and planned compressed-air demand.
• Any symptoms: water in lines, dryer alarms or high-pressure trips.

Frequently asked questions

Why is my air dryer not removing water in summer?

Most often the dryer is overloaded by hot inlet air and high ambient temperature, which can reduce usable capacity far below the nameplate. A fouled compressor aftercooler, a blocked condenser or a failed drain makes it worse. Measure dryer inlet temperature and pressure dew point, clean coolers and condensers, check drains, then recalculate capacity with the maker’s correction factors.

How do I size a refrigerated air dryer for my compressor?

Start with the total FAD the dryer must handle, then apply the chosen model’s correction factors for working pressure, inlet air temperature and ambient temperature at the hottest expected conditions. Divide design flow by the product of the factors to get the minimum rated capacity, and choose the next catalogue size above that figure.

Can I use one correction factor table for all dryers?

No. Each maker defines its own rating conditions and correction factors, and they are not interchangeable between brands or even between model ranges. Using the wrong table can oversize or undersize a dryer significantly. Always take factors from the current data sheet of the exact model being selected, and ask the maker if a condition is outside the published range.

What dew point does a refrigerated dryer give?

A refrigerated dryer typically delivers a pressure dew point of about +3 °C under its rating conditions, which suits most general plant air. When it is overloaded by hot inlet air or high ambient temperature the dew point rises. For lower dew points such as −20 °C or −40 °C, a desiccant dryer is normally used.

Does lower air pressure reduce dryer capacity?

Generally yes. At lower pressure the same free air occupies a larger actual volume and holds more water vapour per unit of free air at saturation, so the dryer must work harder. Makers publish a pressure correction factor for this; the factor falls below 1.0 at pressures under the rating point. Use the exact model’s figure.

Should the dryer be installed before or after the air receiver?

Both layouts are used. A receiver before the dryer lets air cool and drops out some water, and smooths flow surges, which helps dryer performance. A receiver after the dryer stores dry air for peak demand. The right choice depends on demand pattern and room layout; in either case the dryer must be sized for the peak flow it will see.

Related pages

• Refrigerated compressed-air dryers: selection and sizing
• Desiccant adsorption compressed-air dryers
• Adsorption dryer purge consumption worksheet
• FAD and reference condition conversion tool
• Dryer service and dew-point performance
• Dew-point measurement