Our topic for this article is wet steam or poor steam quality and its impact on ejectors and their operation. Ejectors utilize motive steam to generate vacuum. Understanding how steam quality impacts ejectors is important for both the performance and mechanical reliability. This discussion is going to cover both of these topics.
First, let’s define wet steam in a closed system. When liquid water reaches its boiling point, additional heat energy will cause a portion of that water to start boiling off. The liquid water will continue to boil off as additional heat energy is added until all of the water has become steam. When this happens, the steam has reached what is known as the dry and saturated point. Additional heat energy added to the steam beyond that causes the steam to become superheated. On the other hand, if steam at the saturation temperature is allowed to cool, a portion of the steam will condense back into liquid water. Such steam is known as “wet steam”. Steam quality is a measurement of the amount of steam vapor in a saturated liquid / vapor mixture. Steam quality of 100 indicates 100% vapor. Wet steam will have a quality below 100, dry or superheated steam will have a quality of 100. It’s important to note that steam that is dry or superheated at the boiler may not necessarily be dry by the time it reaches the ejectors. This difference is often overlooked in the field.
Wet motive steam has a negative impact on ejector performance. Ejectors have what is called a motive nozzle and that nozzle expands the high pressure motive steam, which creates high velocity steam. The kinetic energy in that high velocity steam is what drives the ejector performance. Steam vapor is compressible, however liquid water is not. As liquid droplets in the wet steam pass through the ejector nozzle, they displace some of that motive steam. Since those liquid droplets can’t be expanded, it ultimately reduces the amount of energy passing through the nozzle which can negatively impact an ejector’s suction pressure and the overall system performance.