I don't have any experience with the company (see link below) or their equipment but I found them while surfing today. They mention that there is minimal impact of changes in moisture content. Although it's designed for free fall conveyor transfer point applications it may be adaptable to your pneumatic conveying application.

http://www.berthold.com.au/industria...lk%20flow.html ■

I have found a system from Promecon that measures the mass flow of pulverised coal. I will test this equipment in a biofuel application this spring. You are welcome to contact me in this matter. I am not sure you can get an accuracy of 1 % but in what application is this necessary?

Regards

Marcus Svanberg ■

At The Wolfson Centre we've been working on this problem, in collaboration with many others around the globe, for about twenty years. We have tested just about every principle ever thought of.

As the other respondent says, you need a velocity measurement and a density measurement. The velocity measurement is easy - electrostatic or capacitive are good enough, others are equally possible.

However, getting the density measurement is very difficult. Anything electrical for measurement of density (e.g. capacitance tomography, electrostatic measurement, microwave absorption etc) is bady affected by moisture content. Audio (e.g. sound attenuation or reflection) is badly affected by small changes in particle size. The only method NOT affected by moisture or particle size (or both) is radiometeric (e.g. X-ray attenuation) but you need a high density in the flow for it to work - it's fine at say 200 kg of solids per cubic metre of air (local value, not cross-sectional average), but at 10 forget it! So basically, no good for your application. Also, it's expensive if you use an X-ray generator, whereas if you use a radioactive source (Strontium 90 isotope for instance) you have a safety issue.

The only really reliable method of determining the flow rate is to measure it by loss in weight at the feeder. A second best might be to calibrate the feeder if it is a volumetric system such as a screw, but the accuracy can be poor if the density varies.

Notwithstanding all of the above, you have to look at the reason fror wanting the measurement, in the context of the overall system. You may be aware that the other thing which is a problem when feeding a combuster with a biomass fuel is the water content - even if you know the mass fed, this isn't the whole answer to your control problems because what you are really trying to control is the heat generation rate and this is determined by the feed rate of dry solids, modified by the change in nett calorific value as a result of moisture content. These problems compound each other - if you feed at a higher moisture content, the heat you are feeding reduces because in a given mass, there is less dry solids, but reduces even further because the nett CV is also less! So with the typical wide moisture content of biomass fuels, even if you had a reliable mass flow measurement system, it really wouldn't help you very much!

There are other means of control which could be more helpful, for example based on measurement of heat generation rate. The problem with this is the time lag - which can make the control dynamics difficult. I would suggest a study of the fuel variation (density and moisture content) would be a good starting point to understanding what can be achieved in the way of both open-loop control of feeding and also the dynamics of closed-loop control through heat generation.

I hope this helps. Email me if you want more info, bm08@gre.ac.uk. ■

I wanted to know if anyone has tried measuring a pneumatically conveyed powder stream using a coriolis meter. It would have to be a "strainght tube" type. In theory it should give a velocity and a density regardless of electrical characteristics?

I am trying to avoid using a measuring pot and weigh cells to confirm flow. ■