EDUCTORS & JET PUMPS

Performance for steam-air eductors is determined by tests using gases of a specific molecular weight and temperature. The term Dry Air Equivalent (DAE) is a way of presenting data so that corrections can be made for temperature and molecular weight. The actual gas being pumped will generally be at some temperature and composition other than air at 70°F. As it is not practical to maintain testing facilities for an infinite number of gases and temperatures of those gases, the method described here has been devised to correct all gases to a standard set of conditions. This allows eductors to be designed under given circumstances, then applied to the actual process conditions they will work under.

This method is described by the Heat Exchange Institute HEI in the book "Standards for Steam Jet Vacuum Systems, Fourth Edition 1988" and is synopsized below. If a more detailed explanation is needed, please refer to the previously mentioned publication.

This method deals with the gases in terms of weight of flow in a given period of time. The most commonly used is Lb (of gas)/Hr. To proceed, the units of each gas component should be converted to Lb/Hr. (Conversion Factors for several different units are found on the back cover of this brochure.) Water vapor (steam) is handled as a separate component in this calculation because the temperature correction factor is different for condensable gases.

To find the average molecular weight for the gas components, take the Lb/Hr flow for each gas component and divide it by the molecular weight of that component. This will result in the number of moles of each component. Then add together the Lb/Hr flows of each component (except the water vapor).

This will result in the total Lb/Hr flow of gas to be pumped. Then add together the total moles of each component. The result will be the total moles of gas to be pumped. Finally, divide the total Lb/Hr flow to be pumped by the total moles to be pumped. The result will be the average molecular weight of the mixture. The average molecular weight of the mixture is then used to obtain a gas weight correction factor from the Molecular Weight Entrainment Ratio Table.

The correction factor of the water vapor is obtained by reading the correction factor from the "Molecular Weight Entrainment Ratio Table."

For the more common gases the molecular weight of each component may be obtained from this page. If your gas is not here, you can calculate it by adding the weight of the atoms of each element in the gas. (Then if the gas is diatomic, multiply by 2, etc.)

Take the total flow rate (Lb/Hr) of the gases to be pumped minus the flow rate (Lb/HT) of the water vapor, divide this by the gas flow rate (Lb/Hr) correction factor times the gas temperature correction factor. The result will be the non-condensable gas load for the ejector.

Then take the weight of the water vapor, divide it by the molecular weight correction factor times the temperature correction factor for steam.

Then add the results of these two calculations together. The final result will be the total gas load Lb/HT required of the eductor in Dry Air Equivalent. This number is used as the desired suction flow (Qs) in the table.

50 Lb/Hr of a mixed gas and steam at a temperature of 300°F contains 15 Lb/Hr of 0 2,10 Lb/Hr of Air, 5 Lb/Hr of H 2, 5 Lb/HT Of CO 2, and 15 Lb/Hr of steam.