Estimate dryer purge-air loss, compressor energy, and annual electricity cost from a published SCFM value or a purge percentage.
Compressor and electricity
The rate is a 2025 US industrial planning estimate. Replace it with the energy charge from your local electric bill.
Advanced options
Table of contents
How to use our Compressed Air Dryer Purge Loss and Annual Energy Cost Calculator
- Choose How is purge listed? to match the dryer data sheet: use a published flow or a percentage plus airflow.
- Enter the purge specification in the visible field or fields. In percentage mode, make sure Dryer airflow used for the percent (SCFM) uses the same basis as the published percentage.
- Enter Purge operation (hours per year), using hours when the system stays pressurized or the dryer regenerates.
- Enter Compressor specific power (kW per 100 SCFM) and the usage-based Electricity energy rate, then select the matching Electricity rate unit.
- Click Calculate. Sanity-check that Purge flow used in the estimate matches the dryer specification and that annual cost falls to zero if purge operation is zero.
Definitions
SCFM: Standard cubic feet per minute. This is an airflow rate referenced to stated standard conditions, so it may differ from actual CFM.
Purge flow: Compressed air a dryer uses during regeneration or operation. The compressor must replace this airflow.
Purge loss: Purge flow written as a percentage of a stated dryer airflow. A 15 percent loss on 500 SCFM equals 75 SCFM of purge flow.
Compressor specific power: Compressor input power needed to deliver airflow, entered here as kW per 100 SCFM.
Electricity energy rate: The usage-based charge for electricity in $ per kWh or cents per kWh. The default is a 2025 U.S. industrial planning estimate and should be replaced with a local bill rate. [1]
Annual purge-air volume: Total standard cubic feet consumed as purge over one year, shown in millions of standard cubic feet.
Common mistakes and quick fixes
Mistake: Entering a published flow and also treating Published purge loss (percent) as an extra loss.
Fix: Choose one path under How is purge listed?. The two inputs are alternate ways to find the same purge flow.
Mistake: Copying actual CFM into Published purge flow (SCFM).
Fix: Enter only flow stated in SCFM, and check the standard reference conditions on the source document.
Mistake: Using production time for Purge operation (hours per year) even though the dryer or compressor stays pressurized when production stops.
Fix: Use an hour meter or schedule that includes all hours when purge air is consumed.
Mistake: Entering 8.62 in Electricity energy rate while Electricity rate unit is set to $ per kWh.
Fix: Select Cents per kWh for 8.62, or enter 0.0862 when using $ per kWh.
Mistake: Using compressor nameplate kW as Compressor specific power (kW per 100 SCFM).
Fix: Use measured or published input power per 100 SCFM at the relevant operating pressure.
Limitations & Key Assumptions / Boundary Conditions
- The estimate includes compressor electricity used to replace purge air. It excludes direct dryer heater, blower, and control electricity.
- It uses only the usage-based electricity rate. Fixed customer charges and demand charges are excluded because they need tariff and peak-demand details.
- Flow and compressor specific power should use compatible standard-flow reference conditions and the relevant operating pressure. A source labeled only CFM should not be assumed to be SCFM.
- Purge percentage must apply to the entered dryer airflow. Manufacturer ratings may use rated flow, inlet flow, or another stated condition.
- Annual cost is an average based on one energy rate and annual hours. Monthly utility rates, compressor loading, pressure, and dryer regeneration behavior can change actual cost.
- A zero value for Purge operation (hours per year) correctly produces zero annual volume, energy, and cost, while estimated compressor power for a nonzero purge flow can still be shown.
Methodology
Calculation method
The calculator first finds purge flow from the selected input path. It uses only the fields visible for that path, so a published purge flow is never added to a percentage-based value.
purge flow (SCFM) = published purge flow
purge flow (SCFM) = dryer airflow (SCFM) * purge loss (percent) / 100
It converts the selected electricity-rate unit to $ per kWh, then estimates compressor input power needed to replace the purge airflow.
compressor power (kW) = purge flow (SCFM) * specific power (kW per 100 SCFM) / 100
annual compressor energy (kWh) = compressor power (kW) * purge operation (hours per year)
annual electricity cost ($) = annual compressor energy (kWh) * electricity rate ($ per kWh)
annual purge-air volume (million scf) = purge flow (SCFM) * 60 * annual hours / 1000000
average monthly cost ($) = annual electricity cost / 12
Worked example
With 15 SCFM of published purge flow, 20 kW per 100 SCFM of compressor specific power, 8,000 annual hours, and $0.0862 per kWh, compressor power for purge is 3 kW. Annual energy is 24,000 kWh, annual cost is $2,068.80, annual purge-air volume is 7.2 million standard cubic feet, and average monthly cost is $172.40.
In percentage mode only, airflow remaining after purge is dryer airflow minus calculated purge flow. At 500 SCFM and 15 percent purge loss, this is 425 SCFM.