8 replies to this topic

[redtitan777]

Hi, everyone.

I'm a new chemical engineer working in a bulk chemicals manufacturing plant and I'd like some help in understanding the steam ejector which makes use of superheated steam to deliver powder into a milling system.

Some of the technical specifications of the steam ejector:

Primary nozzle throat diameter: 15.8 mm
Primary nozzle exit diameter: 22.6 mm
Venturi throat diameter: 101.6 mm

Ratio of specific heat capacities: 1.31 (steam)

Mach number calculated from area-mach relation = 2.1889

Process conditions:

Steam (stagnation) pressure: 12.2 kg/cm²g = 1.298 MPa
Steam (stagnation) temperature: 271.88 C = 545.07 K

If I use isentropic equations to solve for the nozzle exit pressure and temperature based on the above mach number and process conditions, I get:
P = 1.241 MPa
T = 312.68 K

What I don't quite understand from the final results is that the absolute pressure is above atmospheric and the temperature is below saturation temperature, which doesn't seem to be the case as suction pressure is created in the actual process and superheated steam shouldn't be condensing.

Perhaps I have done it the wrong way and I'd appreciate if someone could point it out for me.

Thank you!

[Bobby Strain]

Best you get performance data from the manufacturer.

Bobby

[ankur2061]

redtitan77,

Steam jet ejectors are still designed using a lot of empirical correlations and a lgreat deal of information related to their design is still not available in the public domain. Steam jet ejector manufacturer's (obviously the reputed ones) are still few in number and the design is generally customized for each application. Some reputed manufacturer's of steam ejectors are listed below:

1. Graham Manufacturing Company
2. Croll Reynolds
3. GEA jet pumps
4. Schutte & Koerting

Perhaps the most comprehensive literature on steam ejectors is published by the "Heat Exchange Institute" (HEI) is in the form of a standard called the "Standards for Steam Jet Vacuum Systems" and if you are enthusisatic about finding design fundamentals related to steam ejectors than it is a must own document. This document among other information has steam ejector nozzle coefficients and charts for ejector nozzle capacity based on dry air.

Suction to a steam ejector can be above atmospheric specially if you are using a steam ejector as a thermocompressor.

In multi-stage steam ejectors (for high vacuum applications - 1 torr (mm Hg) or below) inter-condensers and an after-condenser are employed to reduce the load on the subsequent stages by partly condensing the mixed process stream between the two stages. The after condenser condenses almost entirely the residual process stream before discharging at atmospheric pressure.

Please note that steam ejectors inherently require superheated steam since wet steam can cause erosion of ejector nozzles.
Steam can be imparted superheat either by reducing high pressure steam to a lower pressure (required pressure for the ejector) by a letdown system or by providing a superheater (generally an electrical superheater).

Since you have not provided your calculations it is difficult to provide any comment on the correctness of your calculations. I do believe that you are trying to do these calculations for academic interest and not for any design purpose. I would advise against any design related activities for ejectors due to the reasons I mentioned in the first paragraph of my post.

Regards,
Ankur.

[redtitan777]

Hi Ankur,

Thanks for your informative reply.

My purpose of carrying out the study on the steam ejector is mainly to understand the effects of a change in the primary nozzle throat diameter on the process conditions. We recently encountered some difficulties in plant operation relating to powder flow into the milling system. One of the proposed solutions is to change the nozzle in the steam ejector to another with a smaller throat diameter, which would hopefully increase the suction pressure and improve powder flow into the milling system. It is a single-stage ejector which has been in use for quite a long time and I currently do not have any information on the manufacturer.

I have read some literature regarding compressible flow through a nozzle and applied the equations for isentropic flow to determine the flowrate, exit pressure and temperature of superheated steam through the nozzle. I'm not too sure about the calculations for the mixing section so I'm assuming for a rough estimation that the change in suction pressure would be the same as the change in exit pressure.

I've only done some simple calculations which I'm attaching for your perusal. Please let me know if any more information is required.

 Ejector Calculations.doc   178KB   289 downloads

[jrtailor09]

redtitan777

Find below reference for the steam jet ejector.

Power, R.B., "Steam-Jet Air Ejectors", Hydroc. Proc. and Petr. Ref., 43(3), 1964
Aerstin, F., and Street, G., "Steam Ejectors for Vacuum Service", Ch. 15 in APPLIED CHEMICAL PROCESS DESIGN, Plenum Press, New York, NY, 1978
Perry and Green, "Perry's Chemical Engineers Handbook" p6-8, 7th Ed., McGraw-Hill, NY, 1997.

Regards,
Jatin

[ankur2061]

redtitan77,

I have not done the type of calculations you have done and cannot comment on them. The only calculations I have done for steam ejectors is the "Dry Air Equivalent" capacity (which is the term of reference specified in ejector datasheets) and for which the calculation is available at:

http://www.cheresour...e-calculations/

As I mentioned in my earlier post I would rely on the vendor to provide me troubleshooting guidance rather than trying to experiment myself with different nozzle sizes for your single-stage ejector.

I certainly do have some articles for reference related to steam ejectors which you may find useful and attached herewith.

Regards,
Ankur

Attached Files

•  ejector_fundamentals.pdf   133.24KB   365 downloads
•  Steam_Jet_Ejectors.pdf   6.05MB   452 downloads
•  Undertsanding_vacuum_system_Fundamentals.pdf   120.59KB   305 downloads

[redtitan777]

Hi everyone,

Thanks for all your helpful advice.

I thought that there might be some fundamental equations that describe the process in the steam ejector but it seems that I have to obtain some empirical data from the manufacturer.

I have also thought of using the chart given by DeFrate and Hoerl in Perry's Chemical Engineers' Handbook to determine the effect of the change in nozzle throat diameter, but I'm just a little uncertain.

Based on my current understanding, for the same motive pressure, a smaller nozzle throat gives a smaller steam flow rate with higher supersonic exit velocity and lower suction pressure (more vacuum effect) than a larger one. From the chart, it seems that if I want to decrease the primary nozzle diameter but keep the entrainment ratio the same (same steam and powder flow) as before, suction pressure would decrease (more vacuum effect). The only problem is I can't quantify the amount of change in suction pressure or how much better the powder flow will be.

Please correct me if I'm wrong. I'll check to see if I can get hold of any information on the manufacturer and contact them.

Attached Files

•  Fig 10-102.png   114.76KB   46 downloads
•  Fig 10-103.png   15.09KB   28 downloads

[Bobby Strain]

If you use superheated motive steam, you might gain some capacity by desuperheating the steam. You ejector supplier can advise you.

Bobby

[narendrasony]

 Vaccuum Ejector Calculation.xlsx   523.78KB   304 downloadsDear Redtitan777,

 

       

Based on my current understanding, for the same motive pressure, a smaller nozzle throat gives a smaller steam flow rate with higher supersonic exit velocity and lower suction pressure (more vacuum effect) than a larger one. From the chart, it seems that if I want to decrease the primary nozzle diameter but keep the entrainment ratio the same (same steam and powder flow) as before, suction pressure would decrease (more vacuum effect). The only problem is I can't quantify the amount of change in suction pressure or how much better the powder flow will be.

 

 

Sounds counter intutive. For acheiving greater vacuum primary nozzle area must be increased so as to acheive the same discharge pressure . Please refer Case-2 & 3 in the attachment. Also refer Case-4 when a primary nozzle with smaller area is selected. There is a possibility of pressure fluctuation also in this case. More insight is required from learned forum members.

These are only theoretical calculations, for actual sizing and design one must rely on ejector manufacturer. 

 

Also due to irreversible frictional losses nozzle calculations are done with some isoentropic efficiency (empirical). Again manufacturers are best to tell.

 

Regards

Narendra