An old truism, “if you can’t measure it you can’t manage it”. If your power bill to generate compressed air in your facility is in excess of $ 100,000 per year, it might be time to consider adding a flow meter or meters to your air system to identify opportunities to reduce your annual operating bill. Some primary causes of higher than normal compressor operating costs include:

• Operating at higher than required air pressures.

• Leaks in piping and point of use equipment.

• Faulty drain traps.

• Desiccant dryer purge air consumption.

• Using compressed air to generate vacuum or to cool equipment.

• Lots of other issues which you can find on other articles on our website.

If you are considering a flow meter, what types should you consider? Four common air flow meters include orifice plates, vortex meters, turbine meters and thermal flow meters.

• Orifice plates use a calibrated plate with a precise hole in the plate to generate a pressure drop at a specific flow rate. The pressure drop is measured with a differential pressure measuring transmitter and a reading is based on the change in this pressure drop across the plate. Although simple in operation both pressure and temperature needs to be also measured to get a precise fix on the air density which adds considerable cost to the system. Turn down, which is a measure of the accuracy range, is poor and typically 4:1. This means a meter calibrated for a full scale flow 10,000 SCFM will read to only 2500 SCFM.

• Vortex meters use a shedder bar (wing like plate) in the air flow which generates vortices in the downstream air flow and generate vibrations on the shedder bar. A transducer reads the vibrations and generates a signal. Pressure and temperatures still need to be read to correct for density but turn down is improved to 10/15:1. Cost is about the same as an orifice plate.

• Turbine meters use air flow velocity to turn a fan blade which in turn generates an electrical signal which is proportional to the air or gas flow rate. Turbine meters are offered in both inline as well as insertion models. Pressure and temperature both need to be measured to correct for density and turn down is in the 10/15:1 range.

• Thermal mass flow meters use a heated element which is then cooled by the air or gas flow with a reading taken to measure the amount of power used in the cooling process. The heating element is very small and the power changes in the mill watt range are used to provide a flow measurement signal. Sensor placement is not critical and should be about the center of the pipe. Since the cooling effect measured is based on the mass of compressed air or gas flow no pressure or temperature compensation is required. Turn down ratios are in excess of 100:1.

From an accuracy, turn down, and cost point of view we suggest using a thermal mas flow meter for measuring compressed air, natural gas and other gas flows. Typical savings for equipment is about 50% when compared to the other meter choices discussed in this article.