Adsorption dryer purge consumption worksheet

What purge air is and why it matters

A twin-tower desiccant dryer dries air in one tower while the other tower is regenerated. In most designs, regeneration uses some of the dried air: it is expanded to near atmospheric pressure, passed back through the saturated desiccant and vented. That vented air was compressed at full cost and never reaches a single user.

Desiccant dryers are chosen where a low pressure dew point is needed, typically −20 °C or −40 °C, and in some critical duties −70 °C. Pharmaceutical plants, instrument air, paint lines, outdoor pipework and cold-store applications are typical. The dew point is valuable; the purge that produces it is a running cost that is often invisible because nobody meters it.

Purge matters for three decisions:

• Capacity. The compressor must deliver production demand plus purge, so purge can push a plant into needing an extra machine.
• Energy. Purge air carries the same kW per m³/min as any other compressed air.
• Technology choice. Heatless, heated, blower-purge and heat-of-compression dryers trade purge air against electrical energy very differently.

Regeneration types compared

Use the manufacturer’s figures for the specific model. The worksheet does not invent a purge rate; it makes the figure you have comparable with others.

Why the purge basis must be stated

A purge figure is meaningless until you know what it is a percentage of, and at what condition. Suppliers state it in different ways, and mixing them is the most common error.

A percentage stated against rated flow does not fall automatically when the dryer runs at part load. On a fixed-cycle heatless dryer, purge continues at almost the same rate whether the plant is busy or quiet, which is why lightly loaded dryers waste a larger share of their air.

The calculation method

The worksheet follows a fixed sequence so that each quantity keeps its own basis.

1. Purge flow on the reference basis: Q purge = Q basis × purge fraction, where Q basis is the rated flow the supplier’s percentage refers to.
2. If the flow is given as actual volume, convert it: Q ref = Q actual × (P actual ÷ P ref) × (T ref ÷ T actual), with pressures absolute and temperatures in kelvin.
3. Active fraction: f active = purge-active time ÷ total cycle time.
4. Average purge: Q avg = Q instantaneous × f active, used only when the figure is instantaneous.
5. Annual purge volume = Q avg × operating hours × 60, in m³ per year.
6. Net dry air = dryer inlet flow − Q avg − any other verified dryer losses.
7. Energy equivalent: P purge (kW) = Q avg × specific power of the compressors supplying it; annual kWh = P purge × operating hours.
8. Electrical regeneration: heater kWh = heater kW × heater-active hours; blower kWh = blower kW × blower-active hours.
9. Total regeneration energy = purge energy equivalent + heater kWh + blower kWh.

Gauge pressure must always be converted to absolute pressure first. If humidity correction is not applied, label the result as a dry-basis approximation.

Worked example

A heatless dryer is rated for 10 m³/min FAD at 7 bar(g) (about 100 psig). The supplier states the average purge for this dryer as 1.5 m³/min over the cycle. The compressors supplying it have a measured specific power of 6.5 kW per m³/min. The dryer runs 8,000 hours a year.

• Average purge = 1.5 m³/min, taken directly from the supplier’s figure (no further multiplication, because it is already a cycle average).
• Annual purge volume = 1.5 × 8,000 × 60 = 720,000 m³.
• Energy equivalent = 1.5 × 6.5 = 9.75 kW, or 78,000 kWh a year.
• Net dry air available = 10 − 1.5 = 8.5 m³/min.

Now take a measured purge of 0.5 m³/min actual at 2 bar(g) and 30 °C, to be restated at 1.013 bar a and 20 °C. Q ref = 0.5 × (3.013 ÷ 1.013) × (293.15 ÷ 303.15) ≈ 1.44 m³/min. The measured line flow nearly triples when put on the same basis as compressor FAD, which is why actual flow can never be compared directly with a FAD figure.

The figures above are illustrative. Enter your own dryer data, measured specific power and hours to get a result for your plant.

How to interpret the result

The energy equivalent is a screening figure. It tells you what the purge air would cost if every cubic metre were produced at the stated specific power. The real saving from reducing purge may be smaller or larger, because compressor controls respond in steps:

• If the trim compressor is a variable-speed machine, saved purge usually translates closely into saved kW.
• If the fleet runs on load–unload control, saved purge may simply increase unloaded time unless a compressor can be switched off. Check with the load–unload cycle energy calculator.
• If purge reduction removes the need for one compressor at peak, the capacity benefit can be worth more than the energy.

Also read the net dry air line carefully. A dryer that is correctly rated for inlet flow may still leave production short once purge is subtracted, especially when ambient temperature is high and the dryer has been derated using the dryer correction factor worksheet.

Common purge problems we find on site

• Purge set higher than the design figure after a past dew-point complaint, and never reset.
• Fixed-cycle timers on a dryer serving a plant that runs well below rated flow on night shifts.
• Leaking purge or exhaust valves venting air continuously, often audible as a constant hiss from the silencer.
• Blocked exhaust silencers raising back pressure and starving regeneration, which then prompts operators to increase purge.
• Desiccant dusting or channelling, so the dryer needs more purge to hold dew point.
• Wet air reaching the dryer because the pre-filter drain has failed, overloading the desiccant.
• Dew-point dependent switching fitted but the sensor out of calibration, so the controller falls back to fixed cycle.

Validation checks built into the worksheet

How Reddy Compressors helps

We service and supply spares for desiccant and refrigerated dryers of all major brands, alongside the compressors that feed them. Working from our office at Tarnaka, Secunderabad, our engineers cover the whole of Telangana and Andhra Pradesh, and we supply dryers, compressors and spares across India. We are an independent multi-brand specialist, not an authorised dealer of any brand. We have tie-ups with all major and reputed brands.

For purge-related problems we can:

• Measure dew point and check whether the dryer is over-purging to compensate for a fault.
• Inspect purge, switching and exhaust valves, silencers and desiccant condition.
• Check pre-filters and drains so wet or oily air is not reaching the beds.
• Advise on dew-point dependent switching or a different regeneration type where the numbers justify it.
• Supply new or used compressors and dryers, or rental air while a dryer is overhauled.

See our dryer service page for what a service visit covers.

What to send us

• Dryer make, model, serial number and a nameplate photo.
• Regeneration type and the controller’s cycle or purge settings.
• Required pressure dew point and any recent dew-point readings.
• Working pressure, flow and running hours of the compressors feeding it.
• Symptoms, such as wet air, high pressure drop or a constant exhaust hiss.

Frequently asked questions

How much purge air does a heatless desiccant dryer use?

A heatless dryer uses a significant share of its dried air as purge; the real figure depends on the model, working pressure, cycle settings and dew-point target. Lower working pressure increases the purge share. Always take the purge figure from the manufacturer’s data for your model, note its basis, and enter it in the worksheet rather than relying on a generic percentage.

Why is my desiccant dryer using so much compressed air?

The usual causes are a fixed-cycle controller running at full purge while the plant is lightly loaded, a purge rate increased in the past and never reset, leaking purge or exhaust valves, or degraded desiccant that needs more purge to hold dew point. A blocked silencer can also disturb regeneration. Measuring dew point and listening at the exhaust during the drying phase are good first checks.

Is a heated desiccant dryer cheaper to run than a heatless one?

Often, but not always. Heated and blower-purge dryers use much less compressed air for regeneration, but they add heater and blower electrical energy and usually cost more to buy and maintain. The answer depends on flow, running hours, dew-point duty and your compressors’ specific power. Enter both options in the worksheet on the same basis and compare total regeneration energy, not purge air alone.

Does dew-point dependent switching really reduce purge?

It can, when the dryer regularly runs below its rated flow or inlet conditions are milder than design. The controller extends the drying time until the desiccant actually needs regeneration, so fewer purge cycles occur. The saving depends on your load profile and on the dew-point sensor being calibrated. A plant running near full load around the clock will see far less benefit than one with quiet shifts.

How do I convert purge flow measured in the pipe to FAD?

Multiply the actual measured flow by the ratio of absolute line pressure to reference pressure, then by the ratio of reference temperature to line temperature, both in kelvin. For example, 0.5 m³/min at 2 bar(g) and 30 °C becomes about 1.44 m³/min at 1.013 bar a and 20 °C. The FAD and reference condition conversion tool performs this step with humidity correction where data is available.

Should I size my compressor for purge air as well as production demand?

Yes. A desiccant dryer’s inlet flow equals the dry air your users need plus the average purge and any other dryer losses. If purge is left out, the compressor may be undersized, pressure may sag at peak, and an extra machine may start more often than expected. Use the net dry air result from the worksheet when checking compressor capacity.

Related pages

• Desiccant and adsorption air dryers
• Dryer correction factor worksheet
• FAD and reference condition conversion tool
• Specific power comparison calculator
• Dew-point measurement
• Compressed air knowledge and calculation centre