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# Calculation of the ejected flow line diameter

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## Calculation of the ejected flow line diameter

(OP)
Hello!
How can I calculate the diameter of the ejected flow line?
Ejected flow is air.
Volume = 1500 m3.
Air is ejected from atmospheric pressure to 0.02 MPa abs.
Ejector type - steam ejector.
Ejection time - 30 minutes.

### RE: Calculation of the ejected flow line diameter

What does the eductor manufacturer recommend?
These are fairly standardized.

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P.E. Metallurgy, consulting work welcomed

### RE: Calculation of the ejected flow line diameter

(OP)
Without the Supplier, it is impossible to calculate the diameter of the pipeline through which the air flow enters the ejector?

### RE: Calculation of the ejected flow line diameter

Is this a transient thing?

I.e are you trying to get a 1500 m3 space from 1bara to 0.2bara in 30 minutes?

This will make any calculation very difficult.

Do you mean the line from the ejector or air line into the ejector?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Calculation of the ejected flow line diameter

(OP)
Air line into the ejector.
The task is to pump out air, create a vacuum for a specified period of time.

### RE: Calculation of the ejected flow line diameter

It needs to be larger than the inlet to the eductor throat.
Too small and it restricts flow, too large and it just adds dead volume that has to be pumped.
It usually isn't too much of an issue since these lines are very short.
You want the system as compact as possible.
In vacuum distance is bad.

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P.E. Metallurgy, consulting work welcomed

### RE: Calculation of the ejected flow line diameter

Ask the ejector supplier to give you performance data for the ejector at some equidistant pressure points for the air line; say at 101kpa abs (starting pressure), 70kpa abs, 40kpa abs and 10kpa abs. Size the air line such that dp for the air line (for the length of line from source to ejector) is say no more than 10% of air pressure at that operating point. Repeat for all operating pressure points and choose the largest line size required for these operating points to meet this dp requirement.
For example at 40kpa abs, at the vendor quoted mass flowrate of air, choose line size such that line dp is no more than 4kpa, while at 10kpa, choose line size such that line dp is less than 1kpa. Mass flow of air from source would drop as air line pressure drops for a fixed steam rate.
If you keep dp at less than 10% of operating pressure, the simpler incompressible flow equation would be adequate if you want to avoid using the more complicated compressible flow calc equations, which would otherwise be necessary if you choose dp to be > 10% of operating pressure.
Presume you already have selected the ejector required for this evacuation time of 30min.

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