An ejector curve requires four things in order for the curve to be valid:

1)      The ejector’s motive steam conditions must be at or better than design. This includes a motive steam pressure that is -0%/+20% of the design pressure and no more than 50 deg F hotter than the design temperature. The motive steam must not be wet.

2)      The ejector needs to be operating with a discharge pressure that is better than its design.

3)      The ejector must be in good physical condition and free of debris, build up, or internal mechanical damage.  Any internal damage to the ejector's motive nozzle, diffuser, or internal motive steam leaks will impact the ejector performance in a way that would not be accounted for in the performance curve. Any buildup of debris in the motive nozzle or process build up in the ejector will also impact an ejector’s performance.

4)      The load must be in vapor phase only and not including any liquid droplets from the upstream process.

The molecular weight of the process load directly impacts an ejector’s capacity. In order to graph an ejector’s suction pressure the loading needs to be expressed in common units. Traditionally this is either an air equivalent loading or a water vapor equivalent loading.   Process loads that are of a smaller molecular weight are harder to entrain into an ejector than process loads that are of a larger molecular weight. This relationship between the load’s molecular weight and the ease in which an ejector can handle that process load is referred to as an entrainment ratio. This entrainment ratio is standardized by the Heat Exchanger Institute or HEI. The entrainment ratio curve is shown below for reference and would be used to convert each load component into an equivalent load component. As mentioned previously this is normally either an equivalent air load (MW 29) or equivalent water vapor load (MW 18).

The temperature of the suction load directly impacts an ejector’s capacity. Since temperature directly impacts the specific volume of the process load, increasing the temperature of the ejector’s suction flow causes it to take up additional volume. This means the ejector can handle less flow as the process inlet temperature gets hotter. If the ejector’s suction flow is colder the process flow will be more dense and the ejector will be able to handle more of that load. Ejector curves are traditionally normalized to a process inlet temperature of 70 ºF. If the process flow differs in temperature, than a correction factor must be implemented to correct for temperature difference. This temperature correction is standardized by the Heat Exchanger Institute or HEI. The temperature correction curve is show below for reference and would be used to convert the flow to the normalized temperature of 70 ºF.

Once all of the load components have been converted into an equivalent load and the temperature correction has been taken into account, the loading can then be compared to the performance curve. Each load component should be corrected individually. Using an average MW can lead to errors when determining an equivalent load. A performance curve typically shows the ejector’s loading in lb/hr on the X-axis and the ejector’s corresponding suction pressure in mmHgA on the Y-axis, as shown below. As the loading is increased, the corresponding ejector’s suction pressure will degrade. As the loading is decreased, the corresponding ejector’s suction pressure will improve.

Every ejector performance curve should indicate the design motive steam conditions and the ejectors maximum allowable discharge pressure. When these are satisfied, and no internal damage is present, the ejector can be expected to operate on its curve.

Understanding how to read and interpret an ejector performance curve is important when reviewing an existing ejector system. A performance curve can be a powerful tool when evaluating a new set of load conditions or troubleshooting an existing system. While the curves contain a lot of valuable information about a specific ejector, they do require a certain level of understanding to utilize. Hopefully this article gives you the tools you need to use ejector performance curves. If you have any questions or concerns regarding a specific system or require help understanding your system’s performance curves, please contact the Graham Service department.

Reference

The entrainment ratio curve and temperature correction curve are published by the Heat Exchanger Institute or HEI, Steam Ejector Vacuum Systems 7th edition.