Compressed air energy monitoring
Electricity for the whole package, delivered flow at a stated reference condition, and header pressure, so the ratio survives the next audit.
- Measure
- Compressor and dryer electricity, delivered air flow, header pressure, loaded and unloaded state
- Calculate
- Compressed air specific energy, as kW per cubic metre per minute
- Sensors
- ZIO-20, reads the flow transmitter's 4-20 mA output ZIO-2X datasheet (PDF, opens in a new tab)
- On site
- ZGW-20 Gateway, compressor power monitoring and the ratio, on site ZGW-20 datasheet (PDF, opens in a new tab)
Hardware for a compressed air measurement
Meter every electrical input you intend to include, and read the flow and pressure the system already measures.
The compressor, and the dryer and auxiliaries when they are inside the boundary
- Choose the CT rating per supply: 120 A split-core, or 300 A, 1 kA and 3 kA Rogowski coils
Class 0.5S to IEC 62053-22 with its CTs, 0.1 A to 3 kA
The flow transmitter on the header, which is the denominator
0 to 20 mA at 0.001 mA resolution, up to 36 V DC, mains powered
Holds the state, the flow and the power, and calculates specific energy from them
If the pressure signal is a voltage
A pressure transmitter with a 0-10 V output instead of a current loop
If the controller has a published register map
Loaded, unloaded and sequence state from the compressor controller
ZIO-20 and ZMB-31 share one enclosure with different terminals inside, so the ordering code is what you specify.
What the ratio is made of
The numerator is easy. The denominator is where compressed-air comparisons usually fall apart, because delivered flow depends on the reference conditions it was measured at.
| Position | What it tells you | Reference | Sensor |
|---|---|---|---|
| Package electricity | What the compressor draws loaded, unloaded and idling | One meter per supply, and the same set of supplies in every comparison ZEM-63 datasheet (PDF, opens in a new tab) | ZEM-65 (on this page) |
| Delivered flow | The air the system actually supplied | Record whether the flow is actual, standard, normal or free air delivery ZIO-2X datasheet (PDF, opens in a new tab) | ZIO-20 (on this page) |
| Header pressure | The service condition the flow was delivered at | Measured where the header is controlled, kept as a band and not only an average | ZVO-20 (on this page) |
| Loaded and unloaded state | How much of the electricity produced no air at all | Controller state, or the power signature of an unloaded run ZMB-3X datasheet (PDF, opens in a new tab) | ZMB-31 (on this page) |
Reference conditions are part of the number. Standard, normal, free air delivery and actual conditions give different cubic metres. Two figures in the same unit are not comparable until the reference matches.
Say what is inside the boundary. Dryers, drains, cooling fans and a standby compressor each move the ratio. List them with the result.
How to calculate compressed air specific energy
Divide the electrical input inside the boundary by the air delivered over the same period.
specific energy = Ptotal ÷ V̇
Ptotal = Pcompressors + Pdryer + Pauxiliaries- Ptotal
- electrical input inside the boundary, kW
- V̇
- delivered flow at the stated reference condition, cubic metres per minute
- specific energy
- kW per cubic metre per minute
Worked example
A compressor house drawing 75 kW while delivering 11.5 cubic metres per minute is at 6.5 kW per cubic metre per minute. Add a 4 kW dryer to the boundary and it becomes 6.9, for exactly the same air.
In Edge
One calculated sensor sums the electrical inputs, a second divides by flow with a guard for zero flow, and the loaded state stays as its own point so unloaded periods can be separated.
Commissioning checks
Most of these are about the flow signal, because that is where the error usually is.
-
Every included supply is metered
List the compressors, dryers and auxiliaries inside the boundary and find each one's meter.
Pass when the list and the meters agree, and anything excluded is written down.
-
Flow scaling matches the transmitter
Compare the transmitter's configuration with the scaling in Edge, and check the units and reference condition.
Pass when both ends agree, including whether the reading is normal, standard or actual.
-
States are visible in the power
Watch a load and unload cycle in the electrical trend and in the controller state.
Pass when the state changes when the power signature does.
-
Pressure is measured where it is controlled
Compare the transmitter with an independent gauge at the controlled header under steady and changing demand.
Pass when the two agree, and the trend shows the control band rather than one average.
-
The boundary is recorded
Write down which machines, dryers and fans are inside the calculation, and which are standby.
Pass when a later comparison can be checked against that list.
What this measurement does not tell you
Specific energy compares periods. It does not diagnose the system.
- Flow at different reference conditions is not comparable, even when the units look the same.
- Specific power does not quantify leakage, useful production, air quality or receiver storage behaviour.
- A pressure reduction is a system decision that needs a competent compressed-air assessment and the equipment's own limits.
- Unloaded running shows up as a poor ratio, which is a symptom and not a cause.
Sources and related guides
Sources
- Improving compressed air system performance, a sourcebook for industry (opens in a new tab) US Department of Energy and the Compressed Air Challenge. System assessment, measurement boundaries and the reference conditions a flow figure depends on.
- Minimise compressed air leaks, compressed air tip sheet 3 (opens in a new tab) US Department of Energy, Advanced Manufacturing Office. What leakage does to specific energy, and how a leak load is measured.
- ZEM-63 Wireless 3-Phase Electricity Monitor datasheet (opens in a new tab) EpiSensor. Specifications, ranges and ordering codes.
Related
- Pump performance monitoring Application guide Electrical input, flow and differential pressure on the same clock, so a change in pump power can be read as speed, demand or degradation.
- 4-20 mA remote monitoring for process instruments Application guide Bring current-loop, voltage and temperature signals onto the network with their range, scaling and fault behaviour intact, and prove each one on site.
- Equipment performance monitoring boundaries Knowledge Base Define input, useful output, operating state and time boundaries for pumps, compressors, boilers, chillers and heat pumps.
- 4–20 mA loop voltage and headroom calculator Engineering tool Check a 4–20 mA loop has enough supply voltage at its design current.
Reviewed by EpiSensor Engineering on . Revision 4.
Find out whether your flow signal can carry a ratio
We will agree the boundary, check the flow signal can be trusted, and size the electricity meters.