1) What's the deepest vacuum I can pull with an air ejector?

Since most users wish to vent or suck vapors, the maximum vacuum level - which is obtained at the no-flow, or shut-off condition - is usually irrelevant. Using air at 80-100 psig, our off-the-shelf ejectors can pull down to about 10 psia. Other, more expensive, custom built ejectors can pull down to about 3 psia. Two stage ejectors can reach 1 psia.

2) Can I use an air ejector to handle dusty or corrosive gasses?

No problem. That's the reason ejectors are used - to eliminate use of the very expensive fans or blowers which, despite their high cost, will nevertheless fail often when handling dusty or corrosive vapors. Air ejectors should be used in such applications - particularly if you'd like your process to run reliably without regular equipment failures. Any material of construction can be specified - stainless, TFE, Monel, titanium, ceramic-lined, etc.

3) Must we use air at 60 - 100 psig? Our plant management hates any new equipment that uses compressed air, yet I know that if we try to use a blower to vent these explosive or corrosive or dusty gasses, we'll have no end of blower failures and shutdowns. So what can we do?

Using plant air is convenient for small ejectors or ejectors that just run intermittently. However, for continuous gas venting in a process, its completely unnecessary to use air at 60 - 100 psig when all the ejector needs to do is vent gasses at low pressure and exhaust to a vent or scrubber or duct at low pressure. The best solution is buy a matched blower/ejector combination from Fox. A small, quiet Rotron blower running at 2-4 psig provides all the air most air ejectors need. (Note - these are not pd blowers that require silencers, lubrication, etc.) Only the ejector - with no moving parts - sees the dusty, corrosive, or explosive gas and the blower can be located 50 - 100 ft away from areas that require X-Proof or special motors. Since Fox buys over 400 blowers a year, Fox's blower prices may be cheaper than your cost for buying one from your local blower rep - who knows nothing about integrating it with ejectors.

4) What gas temperatures can a Fox air ejector handle?

Plain old stainless is ok for gasses up to about 1000° - 1200° F. Inconel ejectors are good for gasses up to about 1500°F. Other nickel-based alloys can be used up to about 1800°. Above this - inlet gasses must be quenched or cooled before the ejector.

5) We need the ejector to handle a large gas flow rate but, in order to protect process equipment on the suction side, the ejector must be designed so it never pulls a vacuum deeper than 10 inches of water. Can Fox do that?

Any air ejector that can suck in a large gas flow rate will pull a significant vacuum level if that gas flow rate, or suction load, stops. An air ejector cannot be designed with a limited shut-off vacuum level. The solution is a very simple control scheme that diverts some motive flow to the ejector suction port and therefore limits shut-off vacuum level to whatever value is needed to protect your process from implosion

6) We need to use an gas ejector to blend two gasses in a precise mixture ratio. Can that be done?

Nothing could be easier. Since the motive flow goes through a sonic choke in the ejector nozzle, the motive flow is held absolutely constant if motive pressure is constant (use a pressure regulator.) If suction inlet pressure is also fixed (such as atmospheric at 14.7 psia), placing a Fox sonic choke on the suction port of the ejector will establish a fixed suction flow rate. Since both flows are fixed, mixture ratio is established. This scheme has no turndown.

7) Can we control the suction flow rate by changing the motive flow with a valve?

No. Suction flow rate should be controlled with a valve on the suction line.

8) We need to use as little compressed air as possible. What's the smallest air ejector Fox makes?

Fox makes a complete line of Mini-Eductors that use as little as 1/4 SCFM of air. They are used in thousands of sampling, analyzer, and instrumentation systems. Request or download our Mini-Eductor brochure

9) Can an air ejector suck up water from a sump using compressed air?

No. Only eductors driven by steam or water can work. Request or download our Liquid-Eductor brochure

10) How much motive air to I need to drive an air ejector?

This all depends on how much work you want the ejector to accomplish, which is defined as compression ratio. If all you want the ejector to do is suck gasses at near 0 psig and vent to near 0 psig, your required motive air flow may be only 1/2 or 1/4 your suction flow. However, if you want the ejector to perform some real compression work, the required motive flow can be equal to - or exceed - the suction flow rate. The more compression work you want the ejector to perform, the higher the required mass flow rate must be. If your compression ratio is over 1.3, plan on the required motive flow rate being about equal to the suction flow rate.