Steam Ejector versus Vacuum Pump

[joshhamilton]

Why do different people prefer one or the other? What are the advantages and disadvantages of both. ALso, If you are to design a vacuum pump, what/how do you go about it (any helpful online or physical resources?) Please help!!

 

[zdas04]

Why does anyone prefer anything? Economics, reliablity, experience, spares requirements, etc. If you have excess steam available then a steam-driven air ejector is really inexpensive (after pretty low capital expenditure, your cost is the fuel to generate the steam used in the ejector). If you have to add boiler capacity to run a steam ejector then it can be really expensive. Vacuum pumps come in many shapes and sizes so it is hard to compare technologies with regard to a "vacuum pump".

David

 

[rmw]

You asked this in a Chemical forum and not a power forum where folks might have a different view.

IN addition to what David has stated, other issues deal with how deep a vacuum you need to reach. If you use a water based LRVP, your vacuum is dictated by your ability to cool the water in the pump. If you use oil or other fluids you have the same consideration and your vacuum is limited by the vapor pressure of the fluid at that temperature.

You have the separator and pump and cooling systems for the LRVP fluid to deal with and that is moving parts and pieces. And the electricity bill to go with it.

With steam, you can stage the jets (I've never seen more than 5 stages) and pull some really deep vacuums if the process will allow it. You still have to have the cooling water or fluid for the inter and after condensers in the jet train.

And, you say vacuum pump which I immediately launched off on a disertation about LRVP's but there are non-liquid vacuum pumps that will get into really deep vacuums as well. But they are spendy and special purpose.

So what you need and what your resources are as to which method you choose to pull a vacuum on whatever process it is that you have.

rmw

 

[unclesyd]

As posted above the process of selecting a motive force for a vacuum system gets a little involved rapidly. Also as posted above you have to get all your physical parameters togather before you can even start comparing systems.

If you have all your data the application engineers at both links are very knowledgeable about their products. Fox has some good graphs and schematics.

http://www.foxvalve.com/l2_steam_ejectors_howmany.aspx

http://www.s-k.com/pages/pro_06_00.html

A size too big is not problem, but an undersized system doesn't work.

 

[rmw]

When I mentioned other than LRVP's, here is one I had in mind. When it finishes, it looks like there are some others to see as well.

http://www.youtube.com/watch?v=0z1zIug6m3M

Here is a site that will give information on both jets and LRVP's. They used to have a connection to a H&C but I don't see it there now.

http://www.graham-mfg.com/

Enjoy.

rmw

 

[joshhamilton]

how about sizing of a vacuum pump to replace a steam ejector. i know you have to consider the piping requirements and flow rates and air leakage and amount of fluids. But how? and what are the first steps for sizing? Anyone? Thanx

 

[rmw]

Your question is not clearly understood. A vacuum pump, no matter what style it is, jet or pump is a compressor. It compresses air (or whatever it is handling) from a low pressure (deep vacuum) to a higher pressure (atmospheric).

Any compressor, therefore has a design flow rate, a design fluid (or combination of fluids if air/water vapor/other non-condensibles for example) and a design starting and ending point.

The pipe sizing is then a function of the fluid flow and pressure losses along the loop.

You haven't been specific about your application, but as an exapmple, for surface condensers for power plant applications, HEI has design parameters that help with vacuum pump sizing.

If you already have a steam ejector, then someone has already done the pipe sizing calculations. If on the other hand you are replacing the steam jets because they are now undersized (in-leakage exceeds original design) then your piping may be as well.

If you don't know your non condensibe rate or air in-leakage rate, you are going to have to put a measuring device on the non condensible exhaust of your current jet system and measure it.

Can you state why you are considering replacing a steam jet system with a vacuum pump? If I had a similar choice, and no limitations, I probably try to do the opposite.

rmw

 

[joshhamilton]

Yes, rmw
We are trying to run our crystallizer condenser more efficiently and have a payback in steam savings/energy. So I need to perform some equipment specifications to meet the process design requirements...

I know that steam injector is a safer bet (operators think so too). But im the intern and thats why im here, to replace it and not ask why (you know). Thats why I need help and the steps to take, what to do first? and so on....

 

[25362]

If you can get hold of Process Design and Engineering Practice by Donald R. Woods, PTR Prentice Hall, you"ll find good information. For example, Figure 2-20 gives the general applicability for "vacuum" gas moving equipment, as function of non condensible gas exhausted at steady state, (kg/h) vs absolute pressure, (kPa); plus worked examples.

 

[PISA]

Ejector or vacuum pump? Eventually the best choice will be an “hybrid system”: a multistage system comprising a pre-condenser (always the most economical equipment to handle vapours) and then 1 – 3 ejector stages and a liquid ring vacuum pump as a last stage. The vacuum ejectors have many advantages for the higher vacuum (large volumic flow but small mass flow) but the motive steam/gas consumption becomes high for higher discharge pressure. The vacuum pump as a volumic pump is more economical for higher inlet pressures.
Pisa,

http://www.vacuum-guide.com