The Ten Most Frequently Asked Questions About Fox Air and Gas Jet Ejectors
1. What Motive Flow rate is required? Is there a simple ‘Rule of Thumb’ we can use to estimate air requirement?
There is no simple rule or equation or chart that is useful for predicting motive flow for air or natural gas or hydrogen at all temps and all pressures. The best way to get the right answer, fastest, is to complete the Fox Data Sheet data sheet or otherwise describe the pressures, temps, and flow rates – and email to firstname.lastname@example.org. We can then input into our proprietary computer algorithms that will predict performance, motive flow, etc. You will usually get a response within 24 hours, if not that same day and a quote in one day. Or you can call us and speak with an engineer. Anyone you may speak with at Fox will be a graduate engineer.
2. How fast can we get one?
Fox has a large inventory of stock air ejectors that are stored both assembled and unassembled, in line sizes from 1/2” to 6” in carbon steel, stainless, and PVC (to 3” only). If your application has a low discharge pressure, the chances are good that components of stock ejectors can be quickly modified to become an optimized venturi jet for your specific application – and can ship in a week or less. Completely custom ejectors, for deep vacuum or higher discharge pressures may require 4 – 8 weeks – which can often be expedited with a rush premium.
3. Can we control the suction gas flow rate by adjusting the Motive flow rate?
In certain applications, and only when discharging to low pressure, this can work over a limited range. However, as a general rule it is always best to maintain a constant motive gas flow rate and adjust the suction gas flow with a dedicated valve on the suction line.
4. What’s the deepest vacuum I can pull with an air ejector?
Most applications need to simply vent or exhaust vapors. The maximum vacuum level—at the no-flow, or shut-off condition—is usually irrelevant. But here is a quick guide with air at 80 – 100 psig, off-the-shelf ejectors can pull down to about 10 psia (or -20” Hg vacuum). Custom-built single stage ejectors can reach 2 – 3 psia. Two stage ejectors can reach 1 psia.
5. How and where are Fox ejectors manufactured?
Fox ejectors below a 5” line size are machined from bar at our plant in New Jersey. We are not locked into fixed geometries by castings made overseas or anywhere else, and we do not simply thread together stock components. We have complete machining and welding capability in-house, which is why we can usually offer an expedited option for rush projects.
6. 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 ejector that can suck in a large gas flow rate will pull a significant vacuum level if the suction gas flow rate, or suction load, stops. An ejector cannot be designed with a limited shut-off vacuum level. There are various solutions. An easy one is to install a vacuum break valve on the suction line, that opens to bleed in free air, thus protecting the process vessel from vacuum.
7. We need to use an ejector to blend two gasses in a precise mixture ratio. Can that be done?
Since the motive flow goes through a sonic choke in the ejector nozzle, the motive flow is held 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 ration is established. This approach can only be used for fixed, constant flow rates, and will not work to maintain a constant mixture ratio with varying flow rates.
8. Can an ejector suck up liquids from a sump using compressed air?
Yes, but this approach is only used for limited liquid flow rates—like emptying very small sumps that collect refrigerants, hydraulic oil leaks, or fuel.
9. Can we get a straight-through ejector so the suction gas does not need to make a 90° turn.
Yes, in-line gas ejectors are available for certain applications, but they are:
limited to low discharge pressure applications.
significantly less efficient than standard ejectors.
not typically in stock at Fox Valve.
more expensive that stock ejector.
This is usually a high-cost penalty for avoiding installation of one elbow to accommodate a stock ejector.
10. What gas temperatures can a Fox ejector handle?
Stainless is okay for high gas temps up to about 1200° F. Inconel is used for gases up to about 1600° F, and other nickel alloys can be used up to 2000° F.
11. What size ejectors are available from Fox?