As working as a field service engineer for many years now at the company Bronkhorst, I have seen a lot installations in the field and I often get questions regarding the influence of the pipe length on the performance of a mass flow meter.
In today’s blog I will try to explain why the correct choice of piping is essential for an optimal performance of your installation using thermal mass flow meters or controllers and why this has an influence on:
• Deviation in measurement of the thermal mass flow meter
• Frozen pipes
Deviation in measurement of the thermal mass flow meter
Deviation in the measurement data can be caused by using a too short pipe length, because the pipe length is a parameter for the gas temperature. For an optimal performance we advise to avoid excessive temperature fluctuations during commissioning and process operation as much as possible, especially in a process with thermal mass flow meters and controllers. If you use mass flow meters based on the Coriolis principle, temperature fluctuations have no influence on the measurement data, as this measurement principle has been based on measurement of real mass.
In case of a high velocity of the gas flow, the temperature of the gas can change really quickly. In general it can be said, that the higher the flow rate, the more the gas temperature will change. This can interfere with the temperature of your instruments, as the temperature of a gas will lower much faster than the temperature of the instrument itself. This can cause a deviation in your measurement data.
Therefore, for optimal performance of a thermal mass flow meter the gas temperature should be equal to the instrument temperature. Choosing the appropriate length of piping can help you here. If the piping is long enough, the gas has the ability to cool down gradually, more at the same pace as the instrument. This will help you minimize the temperature deviation.
Another effect which I encounter in the field is frozen pipes. How do frozen pipes occur?
When a cooled gas flows with a high velocity through the piping, the temperature of the piping will lower, especially when restrictions in the piping are used, such as narrowing of pipe diameter or the use of (shut-off) valves in the piping. As a result the piping will attract moisture. If the ambient temperature lowers beneath zero degrees the moisture will freeze. This can also happen within the pipe when the medium (gas) contains moisture.
In this case, using a refrigeration dryer can offer you a solution to make sure the gas which is used in the process is dry, to avoid freezing as well.
The pipe length in practice
I talk about “too short” and “long enough”. But what is long enough? Generally we advise to use a minimum pipe length of:
For gas flow rates between 100-1500 l/min it is common to use a 12mm or ½” pipe, and we advise a larger pipe diameter for gas flows > 1500 l/min.
The two effects discussed here are very common in all kinds of processes with high gas flow rates (>500 l/min), such as:
- Plasma vapour deposition technique; used to provide rotor blades with a coating to make them suitable for high temperatures
- Blast furnaces; to make stainless steel out of conventional steel
If you need any advice in this matter, contact your local UK Customer Service Department, we will gladly assist you and offer help and guidance 24/7!
Website Bronkhorst UK
Bronkhorst is the industry leader in thermal mass flow meters and controllers. Veco has been a reliable partner for Bronkhorst for 35 years, co-developing key components and pushing boundaries to enable the next generation of world leading products. Learn about Veco’s contribution to an award nominated next generation thermal mass flow meter/controller in this blog. Vote for our partner if you agree with us that the EL-FLOW Prestige series with Veco's precision technology deserves this innovation award.
Innovation award nominations preview
Gas flow measurement in general
Flow measurement is recognised as one of the ‘need-to-know’ process parameters, alongside temperature, pressure and level measurement. Accurate mass flow measurement of gases is critical in operations and control of many industrial and laboratory processes. In the food and beverage sector, the chemical industry and semiconductor fabrication, flow meters accuracy is often the determining factor between optimum quality and rejected products. In areas like laboratory research, pilot plants and custody transfer, precise and repeatable measurement is equally critical.
Thermal mass flow meters
Thermal mass flow meters measure the mass flow of gases, employing a combination of heated elements and temperature sensors, with thermodynamic principles used to derive actual flow. They need limited correction for changes in temperature, pressure or density and are extremely accurate, especially when measuring low and very low flow rates, and are no longer regarded as high cost.
The Laminar Flow Element as seen in Picture 1 is a critical component of the thermal mass flow meter using the bypass principle. The functionality of the Laminar Flow Element (LFE) is to create a shunt. In the case of a thermal mass flow meter based on a bypass principle the mass flow is measured in a bypass of the main flow channel. Due to the specific dimentions of the LFE the same linear pressure/ flow characteristics as the capillary sensor tube can be created.
Picture 1: Exploded view of Prestige MFC
The LFE consists of thin discs with etched channels. A combination of multiple thin discs allow the measurement of very small (less than 1 mln/min) as well as medium (20 ln/min) flow rates. These flow discs are created by Veco by using the Chemical Etching process.
Picture 2: Shunt principle of the Laminar Flow Element; only a part of the gas flow will flow through the sensor. The rest flow will flow through the LFE.
Creating industry-leading products with Chemical-Etched metal parts
Miniturisation of products – and therefore components – is a necessity for manufacturers in order to create industry-leading products. For the flow discs in LFE we applied the Chemical etching process, a subtractive manufacturing method for micro-precision parts.
Chemical Etching (also known as Photo-Chemical Machining (PCM) and Chemical Milling) is as precise as it is quick and economical. Complicated, multi-level, multi-feature, high precision parts can be produced without the need for expensive tooling or machinery.
Compared to ‘traditional’ machine processes such as CNC Machining, Stamping, Pressing, Wire Erosion and even more ‘contemporary’ processes such as Laser Cutting and Electro Chemical Machining, Chemical Etching with its flexible tooling and scalability provides you with an extremely competitive alternative right through from prototyping to large scale production. Rapid prototypes can be produced from your drawings in a matter of days.
- Stress and burr free parts
- Micron sized features
- Tight tolerances
- Wide range of materials
- Thicknesses from 25um to 2mm
- Round holes, sharp edges, straight or profiled edges
- Rapid prototyping
- Cost effective manufacturing
- Flexible tooling allowing easy modifications
EL-FLOW Prestige is the next generation of Bronkhorst Mass Flow Meters / Controllers for gases. Nearly all core components have been redesigned and many improvements and innovations have been incorporated. With this new series Bronkhorst introduced the “Differential Temperature Balancing” technology, ensuring a superb sensor stability.
You can sign in for the White Paper ‘A holistic view based on data to design components which offer optimal performance of a thermal mass flow meter/controller’, written by Bronkhorst, which describes the innovations integrated in the EL-FLOW Prestige.
EL-FLOW Prestige brochure
EL-FLOW Prestige video
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) ans Water (H2O). A reductant agent is injected into the exhaust stream through a special catalyst. A typical reductant used here is Anhydrous Ammonia.
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 or robust enough for the application and thus their customers were suffering from loss of ammonia measurement and control.
Why using mass flow measurement in Ammonia Control?
In the NOx reduction system the mass flow controllers are used to control the flow of anhydrous ammonia 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 NOx into Nitrogen and Water.
There are very strict federal and state air quality regulations that specify the allowable level of NOx which can released into the atmosphere and there can be very heavy fines if those levels are exceeded. The company needed 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.
The Bronkhorst solution was to recommend a mass flow controller using the CTA (Constant Temperature Anemometer) technology which is ideal to avoid clogging in potentially polluted industrial gas applications, like the Bronkhorst MASS-STREAM mass flow controller. 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.
The customer began installing the MASS-STREAM from a point when the old Mass Flow Controllers (MFCs) failed.
Within the medical arena there is increased pressure on budgets and financial accountability, with a significant trend for the sector to look again at how resources are used and where savings can be made.
One of the largest expenditures in most hospitals is the cost of purchasing or producing the various medical gases needed, such as Medical Air, Nitrogen, Oxygen and Nitrous Oxide. Often the usage and consumption of these gases is neither monitored nor measured or, whenever it is done, it is often a crude estimation, inaccurate and recorded only by pen and paper.
Most hospitals rely on the rate at which the cylinders (in which the gas is supplied) empty to determine the amount and rate of gas used. There are of course many issues associated with this method, such as:
- The amount of gas in a particular sized cylinder can vary greatly, even when directly delivered by the gas supplier
- Total gas consumption and peak times of consumption cannot be accurately determined
- Leaks can go undetected
- Specific point of use consumption is impossible to determine
This makes it very difficult to manage costs overall and to assign invoicing costs to individual departments and sections.
A company specialising in the design, installation and maintenance of gas systems was asked to install the medical gas network in a new hospital. An approach was made to Bronkhorst UK Ltd for the supply of gas meters which could then be communication-linked to the building maintenance system.
Thermal mass flow Instruments with integrated multi-functional displays were offered to fulfil both the accuracy and reliability requirements . As a result of their through-flow measurement (Constant Temperature Anemometry - CTA technology) the thermal mass flow instruments offered the additional benefits of no risk of clogging, no wear as there are no moving parts, minimal obstruction to the flow of the gas and hence ultra-low pressure drop, all based upon the fact that the instrument body is essentially a straight length of tube.
In addition to the local integrated displays both 4…20 mA and RS232 output signals were available ensuring integration with the Building Management System (BMS). This gave the end user real time continuous data logging and remote alarming should the gas supply enter low- or high-flow status for any given event. As a double failsafe the instrument offers both on-board flow totalization and further hi/lo alarms.
The installation of the mass flow instruments for this hospital application provided the following benefits to the client:
1. On primary networks:
- Separated invoicing for hospital/clinic/laboratory departments sharing the same source of medical gas
- Monitoring and acquisition of consumption data
- Leak detection within gas line, safety vent and medical gas source
2. On secondary networks:
- Independent gas consumption invoicing between the health institution departments
- Over-consumption detection
- Monitoring and acquisition of consumption data
- Leak detection within gas line
Subsequent installations across Europe have followed the trend of increased accountability by installing a Mass Flow Meter for the incoming bulk delivery, obtaining a totalized flow reading and cross matching this to the bulk invoice. This could be useful in the event of inadvertent errors or typos when a bulk delivery invoice is being raised.
More and more companies in varying industries are trying to make the transition to low flow solutions. Especially in the chemical industry and food & pharma market the trend is to focus on continuous manufacturing, waste reduction, lower downtime and more flexibility. In these industries the availability of ultrasonic flow meters for liquids suitable for 1” pipe lines or larger are enormous, but it is much harder finding solutions for smaller line sizes. Conventional ultrasonic flow meters use either the Doppler Effect or Transit Time measurement. These techniques are practically suitable for large bore sizes.
But what about ultrasonic flow meters for flow rates lower than 1500 ml/min or even 200 ml/min?
Due to the complexity of physics and technology there are not many measurement principles present in this particular flow area, especially ultrasonic flow meters. Therefore the big challenge was to find a solution to use ultrasound in tubes with very small diameters. In close collaboration with TNO (Netherlands organization for applied scientific research) Bronkhorst managed to develop an innovative instrument using Ultrasonic Wave Technology. This technology is applied in the new ES-FLOW™ series for measuring liquid volume flows between 4 to 1500 ml/min independent of liquid density, temperature and viscosity with an accuracy of 1% of rate ± 1 ml/min.
How does Ultrasonic Wave Technology work?
The ES-FLOW™ is based on ultrasonic wave technology. Measuring is done in a straight stainless steel tube with an inner diameter of 1.3 mm, without obstructions or dead spaces. At the outer surface of the sensor tube multiple transducer discs are located which create ultrasonic waves by radially oscillation. Every transducer can send and receive, therefore all up- and down-stream combinations are recorded and processed. By accurately measuring the time difference between the recordings (nanosecond range) the flow velocity and speed of sound is calculated. Knowing these parameters and the exact tube crosssection, the ES-FLOW™ is able to measure liquid volume flows.
The distinctive character of this flowmeter is that it’s capable to measure the actual speed of sound, meaning that the technology is liquid independent and calibration per fluid is not necessary. Next to that the speed of sound can be used as an indicator of the type of fluid present in the flowmeter.
Four reasons why to use the ES-FLOW™ Ultrasonic flow meter:
- One sensor for multiple liquids
Many companies have changing process conditions and make use of various liquids like additives or solvents. As the ES-FLOW™ technology is fluid independent, recalibration is not needed with liquid changes. Also non-conductive liquids as demi water can be measured.
- Easy to clean and reduced risk of clogging
Cleaning processes are often time consuming. Due to the straight sensor tube design with no dead volume, particles have reduced chance of clogging the instrument. Cleaning can be done in a few minutes therefore the amount of down time will be limited.
- Vibration insensitive
Ultrasonic measurement is not sensitive for vibrations as it doesn’t rely on frequencies or rotations. It is also irrelevant if the flow is laminar or turbulent.
- Integrated PID controller and fast response
The on-board PID controller can be used to drive a control valve or pump, enabling users to establish a complete, compact control loop with fast response time.
Bronkhorst’s new ultrasonic flow meter, the ES-FLOW™, will be launched June 2017.
First introduction to the market will be at the Sensor + Test in Nürnberg.
We invite you to visit us at stand 244 in hall 5.
Watch the ES-FLOW Video on YouTube
ES-FLOW On-Line content
For many years, Mass Flow Controllers (MFCs) and Mass Flow Meters (MFMs) have been used in Analytical instrumentation. There are some distinctive applications like carrier gas control or mobile phase control in Gas Chromatography (GC) and Liquid Chromatography (LC). I discovered that there are a lot more applications of Mass Flow Controllers in analyzers then I could imagine when entering the world of Mass Flow Controllers after many years working in Analytical Chemistry.
One application I would like to focus on in this blog is Mass Spectrometry or shortly, as chemists like to use abbreviations, MS. Mass Spectrometry comes in many forms and is often coupled to Gas Chromatography and Liquid Chromatography. A Mass Spectrometer coupled to a Gas Chromatography (GC) is called a GC-MS and a Mass Spectrometer coupled to a Liquid Chromatography (LC) is called a LC-MS.
Where are Mass Spectrometers applied?
The market for Mass Spectrometers is huge and expanding. The instruments are used for Analytical Research in general but increasingly important in Food Research. Research concerning aging of whiskey and fingerprinting of red wine to determine the origin of the grapes are some examples. Another emerging market is Biopharmaceutical Research where Mass Spectrometers are used to study proteins and how these proteins are digested in living organisms. There are even Mass Spectrometers on Mars (!), where the martian soil is studied.
Figure 1: Mass Spectrometer(schematic)
What is a Mass Spectrometer?
The Mass Spectrometer is often compared with a weighing scale for molecules. Every molecule is built up from atoms and every atom has its own atomic mass and this is “weighted” by a Mass Spectrometer. Before it can weigh the different atoms that are present in one sample, the atoms have to be separated from each other. This is done by charging the atoms (to form ions) and using a magnet to deflect the path that the ion is following. The lighter the ion, the more influence the magnet has and the bigger the deflection. The detector detects where the ion hits and this is a measurement of the weight.
The place where the ionization takes place is called the ion source and there are a lot of different types of ion sources, depending on the matrix of the sample and on the ions that you want to form. The ionizing part is the most interesting part from a Mass Flow point of view because in this part different gases are used, depending on the technique of ionization.
There are two main techniques: hard ionization and soft ionization. With hard ionization techniques, molecules in the sample are heated and fragmented down to atomic levels giving information about the atomic structure of the molecule. With soft ionization techniques the molecule stays more intact giving mass information of the molecule. This is used in Food and Pharma research and has become very popular in the last decade.
Let’s look into detail to one of the most popular soft ionization techniques, the Electrospray Ion Source. The EIS vaporizes the liquid (coming from a Liquid Chromatograph, for example) by leading gas alongside a charged needle to form an aerosol spray. Leading a counter gas flow through the formed spray will evaporate most of the liquid that you do not want to measure, leaving the charged droplets going into the Mass Spectrometer.
Figure 2+3: Electrospray ionization (ESI)
Mass Flow Controllers and Evaporation used in Electrospray Ion source
The interesting part is that the flow needs to be very constant as you want the process of forming droplets and evaporating solvent to be the same, day after day and at different locations with different circumstances. An important parameter in this reproducibility is the gas flow. By using Mass Flow Controllers for Nebulizer gas and Evaporation or Drying gas, the ion source will always have reproducible gas flows.
Our solutions department can design compact gas modules for analytical applications to supply gases for ion-source combined with other gas flows with high accuracy and good reproducibility. Combining components like pressure switches and/or shut-off valves with the flow channels can give a compact gas handling module to fit in the small footprint demanding designs of the Mass Spectrometers. Furthermore, the changes on leaks are decreased significantly as the whole manifold can be leak and pressure tested before it is shipped to the customer.
If you would like to learn more about Bronkhorst customized flow solutions, you can watch this Video or visit our website.