FLOW METERING

Selection of fluid flow meters is based on four factors: pressure drop, accuracy, turndown, and cost. Turndown defines the flow range over which the meter is accurate. For example, a turndown of 4 indicates that the minimum flow at which accuracy is maintained is 25% of the meter's range or rated maximum flow. At a turndown of 30:1, the meter is accurate down to about 3% of its flow range. The following table summarizes these factors for each flow measurement technology:

Technology / Fluid(s) / Relative
PD / Typical
Accuracy / Turndown / Relative
Cost
Hot wire / Air / Very low / 1% / 50-12,000 fpm velocity / Moderate
Pitot tube / Air/liquids / Low / 5% / 4:1 / Moderate
Orifice plate / Liquids/steam / High / 3% / 4:1 / Low
Venturi tube / Liquids/steam / Low / 3% / 4:1 / High
Flow nozzle / Liquids/steam / Medium / 3% / 4:1 / High
Vortex shedding / Air/steam/
liquids / Medium / 0.5% / 30:1 liquids
20:1 steam / High
Positive displacement / Liquids / High / 0.1% / 100:1 / Low to Moderate
Turbine or propeller / Steam/liquids / Low / 1% / 30:1 / Turbine: High
Prop: Low
Magnetic / Liquids / Very low / 1% / 30:1 / High
Ultrasonic / Dirty liquids / Very low / 1-5% depending on liquid / 20:1 / Moderate
Target / Liquids/steam / Medium / 1% / 20:1 / Low

The accuracy of most meters is defined in terms of percent of full range. Therefore, it is important that designers select flow meters that have a maximum rated flow relatively close to the anticipated HVAC process flow. Often, this introduces a pressure drop trade-off that must be evaluated.

The turndown of differential pressure meters (Pitot tube, orifice plate, Venturi tube, and flow nozzle) can be improved to between 10:1 and 16:1 by using dual transmitters, one for the low range of flow and one for the high range.

HVAC Design Guidelines

FLOW METERING 1