The Ten Most Commonly Asked Questions
About Conveying with Venturi Eductors
1. Don’t they need evenly metered feed or they’ll clog? Wouldn’t a full ‘head’ of product ‘overfeed’ the eductor?
Eductors do not require evenly metered feed. They cannot be overfed or clogged with a freely flowing particulate. Eductors are self-metering. As momentum transfer devices, they entrain only as much product as they can accelerate and convey through the discharge line.
2. Don’t eductors need compressed air to work? Don’t they make a lot of noise?
People often confuse conveying eductors, which are quiet and use low pressure air, with steam ejectors, which can whistle loudly. Solids eductors flood-fed with product or fed from closed bins are very quiet, certainly below 85 dB.
3. What conveying velocity does Fox design around? What controls velocity?
Fox designs its eductors to convey product at about 3500 to 4500 ft/min. This is high enough to convey most bulk materials with plenty of margin (fluidization velocity is not a direct function of bulk density) and minimizes line losses and degradation. (See technical discussion in Fox Bulletin 350 for conveying fragile products at lower speeds to reduce degradation.) The nozzle orifice size – carefully chosen – precisely regulates air flow in the line.
4. What if the blower fails and the eductor is conveying product? Can it simply restart?
Almost always. Many users install a slide gate valve between the hopper and eductor so the convey line can be purged, but it is not always necessary. Unless there’s a long vertical rise (50+ ft), the eductor will start right up.
5. We tried eductors before. They didn’t work. What’s different?
Many companies have, at some point, built a homemade venturi. They welded together some tubing, some reducers, some piping and some sheet metal and made the best venturi they’d ever seen. Usually, it didn’t work and they’ve never tried eductors again. Fox eductors have been installed in over a thousand plants, handling hundreds of products in dozens of industries, in over twenty countries, since 1963.
6. Why don’t the big system designers install eductors in their conveying systems if they work so well?
Simple answer – They don’t make eductors and they don’t like to sell products that they don’t make. They manufacture rotary airlocks, and want to sell rotary airlocks, even if inappropriate for a given system. Reducing maintenance in your plant is not their priority.
7. Don’t eductors need to be evenly fed or they’ll clog? Wouldn’t a surge ‘over-feed’ the eductor?
(This question is usually asked twice – with slight variations). It hasn’t happened yet. A surge simply accumulates at the product inlet as the eductor continues to convey product at it’s maximum rated capacity.
8. How does an eductor meter or control the feed rate?
It does not. Eductors cannot be used to regulate convey rates. They have a maximum capacity or rating for a given product in a given system (i.e. a 3″ eductor will convey 3 t/h) but do not control rates.
9. Why do eductors require more energy than airlocks?
Eductors perform a radical manipulation of the pressure in your convey line, enabling product to be fed into a region at 0 psig with no blowback. This results in some energy loss but enables improved solids handling. See above illustration comparing an airlock and an eductor.
10. How much headroom do they need?
Fox eductors can be installed beneath outlets only 7″ above the ground, enabling use handling ‘overs’ and ‘unders’ from screeners. For a color copy of above, ask us to mail you a copy of Bulletin 301.