How much does heatless dryer purge cost you?
Shivchandra Shrivastav, AIRS, Ahmedabad · Updated October 2026
A heatless desiccant dryer typically sends 15–20% of its rated air back to atmosphere to regenerate its desiccant. On a 37 Nm³/min dryer running 24×7 in Gujarat, that is roughly ₹29–38 lakh of electricity a year. Most plants never measure it, because the loss leaves through a silencer and never shows on a meter.
What purge is and why heatless dryers need it
A heatless (pressure swing) dryer has two towers of desiccant. While tower A dries the air, tower B is depressurised and a stream of already-dried compressed air is blown through it to strip out the moisture. That stream is the purge. It is paid for by your compressor, then thrown away.
Calculate your own purge cost
Purge cost per year (₹) = dryer flow (Nm³/h) × purge fraction × specific power (kWh per Nm³) × running hours × tariff (₹/kWh)
Worked example:
| Input | Value |
|---|---|
| Dryer flow | 37 Nm³/min = 2,220 Nm³/h |
| Purge fraction | 18.5% (measured on a Gujarat chemical plant) |
| Specific power of compressed air | 0.10 kWh per Nm³ |
| Running hours | 8,400 h a year |
| Tariff | ₹10.3 per kWh |
| Purge cost | about 3.45 lakh kWh = ₹35.5 lakh a year |
At 8% purge the same dryer costs about ₹15.4 lakh a year; at 2.6% about ₹5 lakh.
How to measure purge on your dryer
- Read the nameplate flow and the design purge percentage in the manual.
- Measure compressor kW and flow with the dryer in and, briefly, bypassed (if your process allows), or put a flow meter on the purge line.
- Log the dryer cycle: a fixed timer that never changes with load is a sign of wasted purge.
- Measure outlet pressure dew point. If it is far drier than you need (for example -60 °C when you need -40 °C), you are over-purging.
Five ways to cut purge loss
- Load-based purge control. Retrofit a controller that runs regeneration to the real load. AIRS purge controller
- Replace with a modular heatless dryer with lower purge. On the Gujarat site above, the replacement dryer ran at 2.6–8% purge and delivered -43 to -58 °C PDP against a -40 °C design, with about 0.1 bar pressure drop.
- Switch to a zero purge (blower reactivated) dryer on large, 24×7 duties. Zero purge dryers
- Lower the dew point target to what the process needs. Not every line needs -40 °C; general plant air may only need a refrigerated dryer.
- Fix the basics: a failing pre-filter or a stuck purge valve raises purge and ruins desiccant.
Heatless vs zero purge vs refrigerated
| Dryer type | Typical PDP | Compressed air used for regeneration | Best for |
|---|---|---|---|
| Refrigerated | +3 °C | None | General plant air |
| Heatless | -40 °C | 15–20% of rated flow | Small flows, instrument air |
| Heatless with purge control | -40 °C | Varies with load | Existing heatless dryers at part load |
| Zero purge (blower reactivated) | -20 °C / -40 °C | None (blower and heater use power) | Large 24×7 flows |
FAQ
Is 15–20% purge normal?
It is normal for fixed-cycle heatless dryers at full flow. It is wasteful at part load.
Does the purge silencer tell me anything?
A silencer that blows hard all cycle, or is coated in desiccant dust, points to over-purge or desiccant breakdown.
Can I just reduce the purge valve opening?
Not safely by hand; dew point can rise and the desiccant can saturate. Use a controller that proves dew point.
What dew point does pharma need?
Often ISO 8573-1 Class 2 for water (-40 °C PDP); confirm with your QA.
How quickly does a purge fix pay back?
On 24×7 plants usually within months; it depends on load and tariff.
Does monsoon humidity change purge need?
Higher inlet temperature and moisture raise the dryer's load; size and set up for Indian monsoon conditions.
Who can measure it for me?
AIRS measures it in a one-day Energy Saving Assessment; the fee is adjusted against any order placed within 90 days. Call or WhatsApp +91 84888 42227.