Chris King
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Anhydrous Ammonia Control for Nitrogen Oxides Reduction

As a technique to reduce the level of Nitrogen Oxides (NOx) in boiler or furnace exhaust gases, Selective Catalytic Reduction (SCR) has been around for years. SCR is a technology which converts Nitrogen Oxides (NOx) with the aid of a catalyst into diatomic Nitrogen (N2) and Water (H2O). A reductant agent is injected into the exhaust stream through a special catalyst. A typical reductant used here is Anhydrous Ammonia (NH3).

A customer of Bronkhorst, who has been selling and servicing boilers and pumps for commercial and industrial applications for over 50 years, had been using a mass flow controller (MFC) which was not reliable and robust enough for the application and thus their customers were suffering from poor ammonia measurement and control.

Selective Catalytic Reduction example

Why use mass flow measurement in Ammonia Control?

Some NOx reduction systems are liquid ammonia based, and others are gas based ammonia. Whatever the state of the ammonia in the NOx reduction system Bronkhorst can offer accurate ammonia measurement and control. Systems in the field today are using the MASS-STREAM (gas), IN-FLOW (gas) and Mini CORI-FLOW (liquid) to accurately control the ammonia being injected into the exhaust gas stream so that proper reaction takes place without ammonia slip. Ammonia slip is when too much ammonia is added to the process and it is exhausted, un-reacted, from the system; effectively sending money out the exhaust stack.

There are very strict federal and state air quality regulations that specify the allowable level of NOx which can be released into the atmosphere and there can be very heavy fines if those levels are exceeded. The company needs to provide their customers with a reliable and robust solution. The application demands a robust and repeatable mass flow controller that is at home in industrial environments.

What kind of Mass Flow Meter or Controller can be used here?

In the NOx reduction system serviced by our customer the mass flow controllers are used to control the flow of anhydrous ammonia (ammonia in gas state) into the exhaust gas of a boiler or furnace where it is adsorbed onto a catalyst. The exhaust gas reacts with the catalyst and ammonia which converts the Nitrogen Oxides into Nitrogen and Water.

Bronkhorst recommended a mass flow controller – from the MASS-STREAM series - using the CTA (Constant Temperature Anemometer) technology which is ideal to avoid clogging in potentially polluted industrial gas applications.

MASS-STREAM mass flow meter

Let me explain a bit about the working principle of this kind of mass flow controller and why it is suitable for an application like this.

The CTA (Constant Temperature Anemometer) principle is essentially a straight tube with only two stainless steel probes (a heater and a temperature sensor) in the gas flow path. A constant temperature difference between the two probes is maintained with the power required to do so being proportional to the mass flow of the gas. This means the MASS-STREAM is less sensitive to dirt, humidity, or other contaminants in the gas, as compared to a by-pass type flow meter that relies on a perfect flow split between two paths. The thru-flow nature of the CTA technology is ideal to avoid clogging in potentially polluted industrial gas applications. The straight flow path and highly repeatable measurement and control capability, combined with the robust IP65 housing, allows the MASS-STREAM to thrive in tough applications.

  • Watch our video animation, explaining the functions and features of the Bronkhorst Mass Flow Meters and Controllers for gases using the CTA principle.
  • Check out the top 5 reasons why to use mass flow controllers with CTA measurement.
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Chris King
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How does it compare to conventional CTA (Constant Temperature Anemometer) measurement technologies?

For over 35 years Bronkhorst High-Tech has brought a revolutionary flow technology to the market, and in today’s blog I would like to discuss one such example, called MASS-STREAM™. This device leverages Constant Temperature Anemometer (CTA) thermal mass flow technology, though differently than conventional meters in this category. I will describe what CTA thermal mass flow meters are, the conventional type and their applications, and what makes MASS-STREAM™ technology different.

What is a conventional Constant Temperature Anemometer (CTA) thermal mass flow meter?

The CTA thermal mass flow meter works with a sensor with probes, which are inserted into the gas stream in order to directly contact the flowing gas. One of the two sensors is designed as a heater, and the other one is designed as a temperature probe.

When the instrument is powered up a constant difference in temperature (ΔT) is created between the two sensor probes. The heater energy required to maintain this ΔT is dependent on the mass flow. The working principle is based on King’s Law of the ratio between the mass flow and heater energy. What that means is the higher the flow, the more energy is required to maintain the chosen.

A conventional CTA thermal mass flow meter is installed by inserting the long probes through the pipe wall and into the gas stream. The probes are passed through an insertion port (hole) in the pipe. The “head” of the instrument is above the outer wall of the pipe.

Common characteristics of CTA thermal mass flow meters are having no moving parts, a low pressure drop across the instrument, and no need for additional temperature or pressure compensation.

Where are conventional Constant Temperature Anemometer (CTA) thermal mass flow meters used?

As you might imagine, processes where gas flows in pipes are places one will find CTA thermal mass flow meters. The rugged nature, no moving parts, and low pressure drop are beneficial for measuring gas flow in industries like midstream Oil & Gas, upstream Oil & Gas, Wastewater treatment, and Steel.

The types of applications where these flow meters are used include applications with gases such as methane, propane, argon, compressed air, coal emissions, carbon dioxide, ammonia, and others as well.

Typically a CTA thermal mass flow meter is a good choice when the gas has the potential to be dirty or includes some moisture as the through flow nature of the technology can be more forgiving to contamination than other flow meter technologies.

CTA esnsor of thermal mass flow meters

MASS-STREAM™ Mass Flow Meters

Also based on CTA thermal mass flow technology, the MASS-STREAM™ flow meter/controller differentiates itself from conventional CTA flow meters on several points.

1. In-Line

The MASS-STREAM™ flow meter/controller is not installed using an insertion port through the wall of a pipe. Rather, the MASS-STREAM™ is an in-line flow instrument. That means that the instrument itself is connected between two ends of the tube or small pipe and effectively becomes part of it.

Unlike the conventional CTA thermal mass flow meter, the MASS-STREAM™ is a compact instrument where the main circuit board housing and sensor sit on top of the flow body through which the sensor probes project. An inline instrument allows for the use of CTA technology in applications using tubes and small pipes.

2. Flow Rate

As mentioned earlier the MASS-STREAM™ flow meter/controller is an instrument which is mounted in the line of the tube or small pipe, and applications which use tubes for flowing gas are ones where the flow rate is low.

Of course “low” is a subjective term, so as an example the MASS-STREAM™ lowest flow range is 10 -200 mln/min, but the flow meter family can go as high as 5000 ln/min.

3. Control

Perhaps the most important difference between the MASS-STREAM™ and the conventional CTA thermal mass flow technology is that the MASS-STREAM™ is available as a meter (as are all the others) or as a thermal mass flow controller.

The MASS-STREAM™ thermal mass flow controller is a complete control loop. It measures the gas flow, it has an onboard PID algorithm, and it provides a control signal to an electrically and mechanically connected control valve. All it needs is a setpoint signal and it will precisely control of the gas flow.

It is a complete control loop that can easily fit in one’s hand.

MASS-STREAM flow meter

In Conclusion

While other Constant Temperature Anemometer (CTA) thermal mass flow meters well serve the applications for which they are best suited, none are designed for low flows or as a complete control loop like the MASS-STREAM™. This meter serves applications ranging from process industries to food and beverage to pharmaceuticals to medical and chemical and beyond. MASS-STREAM™ technology is usable for virtually any kind of gas or gas mix, provides precise control, and is very compact and robust.

In our blog ‘How mass flow meters can help hospitals save on medical gases’ an application of MASS-STREAM technology used in the medical industry has been explained.

Check our video of the principle of operation of MASS-STREAM.

To learn more about MASS-STREAM™, and whether it’s the right meter for your application, please contact our office.

James Walton
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Thermal mass flow instruments that make use of a bypass (capillary bypass or bypass sensor) are what most people have in mind when they think of thermal mass flow instruments. What are the differences?

In instruments based on the thermal principle, power is applied to heat the sensor tube. Accordingly the temperature of the tube is measured at two points. With no flow measured, the temperature differential between the two points will be zero.

When the flow increases, the temperature at the first measuring point will decrease, as fluid carries away the heat. At the same time the temperature at the second measuring point will increase as the fluid carries heat to it. More flow will result in a greater temperature differential and this temperature differential is proportional to the mass flow.

Another technology used to measure mass flow is CTA (Constant Temperature Anemometry). In a CTA (through flow, straight tube) instrument there are two measurement “probes” inserted into a straight tube flow path. The first “probe” both heats and measures the temperature of the fluid, as the second “probe” measures the temperature of the fluid.

Again, as the gas flow increases the gas will carry heat from the first measuring point to the second one. In a CTA, however, the power is varied to keep the temperature between the two measuring points constant, and it is this power level that is proportional to the mass flow.

Each technology has its advantages and disadvantages which generally are application specific.

A clean, dry gas application where higher accuracy is as important as repeatability, may be a better application for a bypass instrument like the Bronkhorst EL-FLOW series.

An application with a dirty or slightly moist gas, or where lower accuracy but high repeatability and robustness is required, may be a better application for a CTA instrument like the Bronkhorst MASS-STREAM™ series.

Curious about using a thermal Mass Flow Meter or Controller? Or the top 5 reasons why we use Mass Flow Meters with CTA measurement?.

For more information, please visit our website

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Nicolaus Dirscherl
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Following on our previous blog, ‘Top 5 reasons to use Mass Flow Meters (MFM) and controllers (MFC) with thermal inline CTA measurement’, we now focus on the measurement principle of these mass flow meters using inline CTA measurement (Constant Temperature Anemometry).

King’s Law and CTA meters

The working principle of these CTA flow meters and controllers is based on King’s Law. King’s Law can be attributed to L.V. King, who in 1914 published his famous King’s Law, mathematically describing heat transfer in flows. He used a heated wire immersed in a fluid to measure the mass velocity at a point in the flow. This can be described by the following formula:

  • P = P0 + C · Φmn
  • P: Heater power
  • P0: Heater power offset at zero flow
  • C: Constant (device-dependent)
  • Φm: Mass flow
  • n: dimensionless figure (type 0.5)

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Wheatstone bridge and CTA meters

According to King’s Law, the greater the velocity of the gas across the probes, the greater the cooling effect. The electronics are realized with a Wheatstone bridge, which is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. Its operation is similar to the original potentiometer.

The two probes of the CTA sensor act as the legs of the Wheatstone bridge and as the heater probe is cooled by the fluid, the resistance of the probe is decreased and more energy is required to maintain the temperature difference.
The CTA sensor is aiming to keep this temperature difference (delta-T) between the two probes at a constant level. The flow rate and the heater energy required to maintain this constant delta-T are proportional and thus indicate the mass flow of the gas. The actual mass flow rate is calculated by measuring the variable power required to maintain this constant temperature difference as the gas flows across the sensor.

Video explaining the operating principle of a Mass Flow Meter based on CTA measurement

Video explaining the operating principle of a Thermal Mass Flow Controller