Options for air handling

With the need to reduce energy consumption, there is a growing focus on flow sensing in HVAC systems as a key technology to help monitor and so reduce energy use. A new alternative to the differential pressure sensor is the mass flow sensor, which uses micro electrical mechanical system (MEMS) technology to produce much higher sensitivity and repeatability while being even easier to use. These sensors use thermopile chips together with micro-machined structures to measure flows from 1mm/s to 40 m/s.

Well-placed flow sensors and flow meters can have a huge impact on the operating cost of a HVAC system, if they are correctly selected and installed. In choosing the sensor, the designer must take into account a number of factors.

The most important is the accuracy and repeatability of the flow sensor in the context of the gas that is being monitored. Most manufacturers state their performance figures for a specific gas, and they may vary significantly between a flue gas environment, for example, and a clean air duct in an air-conditioning system. The flow rate is also a key parameter in this context – ideally the change in the output voltage of the sensor should be very steep at the likely median flow rate, giving high resolution and high sensitivity.

Second, consider the location of the sensor. Flow measurement devices need a minimum length of unobstructed straight-run pipe to achieve the specified performance figures. Finally, some devices need more frequent recalibration, especially in a dusty environment.

MEMS sensors

Omron’s solution is based on a sensitive flow chip that is only 1.5mm2 in area and 0.4mm thick. The chip has two thermopiles either side of a tiny heater element, and measures the deviation in heat symmetry caused by a passing air or other gas flow in either direction. A thin layer of insulating film protects the sensor chip from exposure to the gas.

When there is no flow present, temperature distribution concentrated around the heater is uniform, and the differential voltage of the two thermopiles is 0V. When even the smallest flow is present the heat symmetry collapses. The temperature on the side of the heater facing the flow cools, and the other side warms. The difference of temperature appears as a differential voltage between the two thermopiles, allowing the flow direction, velocity and the mass flow rate to be measured.

This technology offers highly repeatable accuracy at flow rates from 1 litre/minute to 50 litres/ minute – higher if used in bypass configuration. Omron’s MEMS sensors can detect mass flows with repeatability down to +/-0.1% available on the most precise devices. A simple adjustment to the amplifier circuit allows the sensor to be calibrated for different gases and flow rate, allowing customers to specify desired characteristics for each application.

The key benefit of MEMS mass flow sensors over conventional differential pressure sensors is that at low flow rates the output signal change is much more rapid for a mass flow sensor, giving a clear electrical response to even relatively small changes in the flow rate – and higher accuracy and repeatability in the whole system as a whole.

Dust segregation

One of the most popular contexts for flow sensors is in the detection of clogged filters – which by definition entails installation in a dusty environment. Clearly, a sensor that is itself impaired by dust is useless.

Omron’s MEMS flow sensors have a patented dust segregation system. Cyclonic air flows are set up inside two centrifugal separation chambers, separating the dust and discharging it through the exhaust vent regardless of the installation direction of the chamber.

Any dust that does accumulate is held at the bottom of the chamber by gravity, and does not enter the detection path of the sensor. This patented 3D dust segregation chamber removes up to 99.5% of dry air-borne particles, based on Omron’s FEM analysis.

Applications

The basic principle of mass flow measurement of air or another gas using a thermopile associated with a MEMS structure can be applied in a great variety of applications including domestic gas boilers, ventilators, medical instruments, fuel cells, pneumatic systems etc.

The same basic chip is suitable for air, oxygen, city gas, LPG and other non-corrosive gases. This means changes to the circuit constants can easily tune the chip to different environments.