Flow Measurement

Accurate compressed air flow measurement is fundamental for audits, control, and system optimization.

Measurement Technologies

Thermal Mass

The most common method for compressed air:

                    Temperature
                    sensor
                         │
    ═══════════════════[●]═══════════════════
                    Heated
                    element
                         │
                    ┌────┴────┐
                    │ Control │
                    │   ΔT    │
                    └─────────┘

Principle: Measures the energy required to maintain a constant temperature difference. Higher flow equals more cooling, more energy required.

Advantage	Disadvantage
Measures mass flow directly	Sensitive to humidity
Wide range (100:1)	Sensitive to gas composition
Low pressure drop	Requires calibration
No moving parts	Moderate response time

Vortex

                    Bluff body
                        │
    ═══════════════════[▼]═══════════════════
                        │
                    Vortices
                     ○   ○
                      ○   ○
                       ○   ○
                        │
                    Pressure/piezo
                    sensor

Principle: A body breaks the flow creating vortices. Vortex frequency is proportional to velocity.

Advantage	Disadvantage
Very robust	Requires minimum flow
Low maintenance	Sensitive to vibrations
Long-term stability	Straight runs required
Wide temperature range	Not for low flows

Differential Pressure

    P1                 P2
    ▼                  ▼
    ═══════[╳]═════════════
           Orifice
           plate

$$\Delta P = P_1 - P_2$$
$$Q \propto \sqrt{\Delta P}$$

Advantage	Disadvantage
Economical	Limited range (4:1)
Simple	High permanent pressure drop
Well understood	Orifice wear
Easy verification	Sensitive to upstream conditions

Ultrasonic (Time of Flight)

              Transducer A
                   ╱
    ═════════════╱═══════════════
                ╱
               ╱
    ═════════╱═════════════════
            ╱
      Transducer B

    Time A→B ≠ Time B→A (due to flow velocity)

Advantage	Disadvantage
Non-invasive (clamp-on)	High cost
No pressure drop	Sensitive to deposits
Wide range	Critical installation
Bidirectional	Complex calibration

Meter Selection

By Application

Application	Recommended Technology
Temporary audit	Ultrasonic clamp-on
Permanent monitoring	Thermal mass insertion
Billing	Thermal mass inline
High temperature	Vortex
Wet gases	Vortex
Low flow	Thermal mass

Selection Table

Factor	Thermal Mass	Vortex	ΔP	Ultrasonic
Range	★★★★★	★★★	★★	★★★★★
Accuracy	★★★★	★★★★	★★★	★★★★
Cost	★★★	★★★★	★★★★★	★★
Installation	★★★	★★★	★★★★	★★★★★
Maintenance	★★★★	★★★★★	★★★	★★★★

Installation

Straight Run Requirements

              Elbows, valves, etc.
                    │
    ════════════════╪════════[M]════════════
                    │        │
                    ├──10D───┤──5D──┤
                    │        │      │
              Upstream     Meter  Downstream

Upstream Disturbance	Straight Run Required
90° elbow	10-15 D
Two elbows in plane	15-20 D
Partially open valve	20-25 D
Reduction	10-15 D
Expansion	15-20 D
Compressor	25-30 D

D = Pipe internal diameter

Location in System

    Dry Side vs. Wet Side:

    Compressor ══> Aftercooler ══> Dryer ══> Distribution
                       │             │
                  Wet side       Dry side
                  (avoid)        (preferred)

Location	Advantage	Disadvantage
After dryer	Dry air, stable reading	Doesn't include purge
Before dryer	Total flow	Humidity may affect
At each compressor	Individual control	Multiple meters
At header	Total flow produced	Includes internal leaks

Units and Conversions

Reference Conditions

Standard conditions (SCFM, Nm³/h):
  - Pressure: 1 atm (14.7 psia, 1.01325 bar)
  - Temperature: Varies by standard

  SCFM (US): 60°F (15.6°C)
  Nm³/h (Europe): 0°C (32°F)
  FAD: Conditions at compressor inlet

SCFM ↔ Nm³/h Conversion

• $1 \text{ SCFM} = 1.699 \text{ Nm}^3\text{/h}$ (approximate)
• $1 \text{ Nm}^3\text{/h} = 0.589 \text{ SCFM}$

For exact conversion:

$$\text{Nm}^3\text{/h} = SCFM \times \frac{460 + 60}{460 + 32} \times \frac{28.32}{1000}$$

Correction for Conditions

$$Q_{\text{corrected}} = Q_{\text{measured}} \times \frac{P_{\text{actual}}}{P_{\text{std}}} \times \frac{T_{\text{std}}}{T_{\text{actual}}}$$

Example:

• Measured: 1000 CFM at 100 psig, 100°F
• Corrected to standard (14.7 psia, 60°F):

$$SCFM = 1000 \times \frac{114.7}{14.7} \times \frac{520}{560} = 7{,}248 \text{ SCFM}$$

Calibration and Verification

Calibration Frequency

Meter Type	Recommended Interval
Thermal mass	Annual
Vortex	2-3 years
ΔP with orifice	Annual (orifice inspection)
Ultrasonic	Annual

Field Verification

Method	Accuracy	Complexity
Comparison with another meter	±2-5%	Low
Tank capacity method	±5%	Medium
Pump-up test	±3%	Medium
Accredited laboratory	±0.5-1%	High

Tank Method

1. Isolate tank from system
2. Depressurize tank
3. Measure time to fill from $P_1$ to $P_2$
4. Calculate flow:

$$Q \text{ (SCFM)} = \frac{V \times (P_2 - P_1)}{14.7 \times t}$$

Where:

• $V$ = Tank volume (ft³)
• $P$ = Pressures (psia)
• $t$ = Time (minutes)

Common Errors

Error	Consequence	Solution
Insufficient straight runs	Erroneous readings	Use flow conditioners
Meter on wet side	Drift, damage	Relocate after dryer
Incorrect units	Incomparable data	Standardize to SCFM or Nm³/h
No P/T compensation	Error up to 20%	Use compensated meter
Incorrect orientation	Measurement error	Follow manual

Quick Verification

Compare measured flow with compressor's theoretical flow. If it differs by more than 20%, investigate meter installation or possible leaks in the system.