Hot Air Flow Measurement

Hmm, sounds like our spray dryer. We don't measure air flow though. We just measure the pressure of the hot air.

I immediately thought "pitot tube" but then I got to the "annubars don't work". I don't know why it won't work but if you say so. Shame though, it would have been a viable solution if you combine it with temperature compensation.

If your system is anything like ours, you have a fan that feeds outside air to an air heater. If your fan has speed control, you can calculate the theoretical air flow from the speed of the fan.

The next idea that comes to mind is a vortex flow meter but they don't make them that big.

Hot-wire absolutely not; with propeller, also; carefully you may use Pitot/D'Arcy.

Visit www.schiltknecht.com. All the best.

I don't see a problem with hot-wire (thermal mass) type, except possibly the cost. There are quite a few companies who make them e.g. FCI. According to some fairly old literature I have, temperature 410°C is a special, but available. Pressure and velocity are well within range. Insertion type so fitting in duct no problem.

Big advantage of thermal mass type is readings in mass or std flow units are not affected by actual pressure or temperature (unlike ΔP types).

Sounds good. but will there be any inaccuracy due to variation of Temperature ( 230 to 410 deg C). what will be the accuracy of these kind of instruments in reality?

Accuracy data I've seen quoted is +/- 1% fullscale, +/- 3% reading. You'd need to check with supplier for details, but in principle change in temperature does not affect reading (see my previous).

Another advantage I forgot to mention is that instrument output varies linearly with gas flow, whereas ΔP vares as Q², causing very low ΔP at low flow, specially with a Pitot. With an orifice plate meter, you can at least choose what ΔP you want, but it's not practical for your duct, and ΔP adds to system pressure loss.

Hot wires instruments while actually measuring "mass" (through transfer of heat between the hot wire and the gas molecules that hit the wire), are designed and calibrated to deliver a velocity measurement. Therefore, the correlation between mass to velocity should take into account the kind of gas under test and the density (affected by temperature and pressure)

Reply to #8 - sorry Mario, I disagree. Hot-wire meter has to be calibrated for the gas being handled, but after that the readings in mass or std volume flow are unaffected by temperature and pressure.

I once had a major discussion on just this point with an end-user engineers, and with the help of an instrument supplier (Captor, Germany) eventually convinced them.

Dear Codemaster: I have no doubt that if you measure mass by means of a hot wire calibrated for mass with a certain gas, you have a measurement not affected by density.

The situation is different when you go a step further and want to measure velocity. As the mass of gas contained in a fixed flow rate varies with density, you should take into account the variation of density due to changes in temperature and pressure to convert a true mass measurement into a velocity measurement. This is taken into account in every modern hot wire anemometer with a feature known as "temperature compensation" to allow the user to know if the velocity of a gas is steady when temperature changes up and down.

Hello Mario - I accept that if you want to know actual velocity/flow you need to correct for gas density (temp and pressure) since as you acknowledge the instrument measures std flow.

But this is quite unusual, the vast majority of applications eg compressed air systems, biological treatment... refer the flow to std conditions. And the original post referred to mass flow, 51000kg/h. OK compressor outputs often given as "free air delivered" which may not be rigorously to (any) std conditions, I suppose it means referred to local atmospheric pressure and ambient temp, but it's not far off, and definitely isn't at discharge pressure, say 7 barg. Come to think of it, it works OK in that if the compressor is up a mountain where atm pressure = 0.5 bara, FAD (assuming definition above) is unchanged, but std flow is halved.

What about an acoustic system, which uses the doppler-effect? No parts inside the duct, no moving objects!

You are saying to measure hot air flow. The best way is to use the fan in the flow path and couple it to a small dc generator.

The principle how it works is the flow of air is directly proportional to the output of the dc generator and you have to convert the output dc voltage or current to the required form.

Before putting the instrument into practical use it needs to be calibrated with a standard value. This method is the best suitable if you use interface it with digital system, computer or embedded system.

Reply to #5 - is there a fan type flowmeter available good for 410°C ?

Most "fan" anemometers work detecting the rotation of the fan wheel by means of a electromagnetic device which sends pulses proportional to the rotation. The instrument is calibrated to get a constant that represents velocity / gas speed. This is much easier, less bulky and more precise than having a DC generator.

The problem is that it is really though to design such a anemometer head capable to withstand the very high temperatures in this application and also with the capabilities to deal with the corrections due to the many changes in the gas properties at these harsh conditions. (vane materials, bearings, cables, materials expansion, etc.)

How long will a vane anemometer work accurately before losing accuracy and deteriorate under these conditions?

yes Mario you are correct and iam constructing such type of measurment device for my project but the important thing is that you can use it with digital system only but not with analog meters or any other system

........

It would be wonderful if you could actually build an instrument capable to deal with extreme environmental conditions. Regarding the analog / digital conversion, this is today quite easy and inexpensive. Just get the help of a qualified electronics designer and he will have a standard chip to deal with this in the measuring circuitry.

Thanks Mario,

No need of any Electronics Design Engg. I am Electronics and Electrincal Engineer and i have did almost 75% of my project work completed and as soon as i finish it completly i will post it in the net .

........

1 ) I have found some of those type of instrument in the net. mostly thermoset type plastics is used in this type of flow devices its very costly but if you need such one then you may fabricate a one in metal . and the metal you are choosing should meet the property of your require temp and it will be possibly alloy . for which type of material try in the Chemical and Material Science Section in the this Discusion form.

2) about the question of the Accurancy this will be the most accurate one avaliable method and if you use it with the embedded system or computer interfaced the you can use a temprature sensor and use internal algorithm to correct such accurancy variation due to temprature changes

3) the importand thing is that you must isolate the fan and DC generator to avoid serious damage. you should use a coupler made of thermal resistive material

.....

Use a Magnetrol TA2 Thermal Dispersion Mass Flowmeter. Or if the velocity profile is irregular and you require high accuracy you may use multiple TA2's and average the outputs.

http://www.magnetrol.com

Aerofoil can be used for hot air flow measurement in a Duct. This differential pressure type flow measurement using dp type transmitter. The temperature of the hot air can be measured and correction to be applied to the output of the dp transmitter.

R.Thiyagarajan