The point at which one needs to worry about ice build-up is therefore directly related to the ejector’s operating pressure. Ejectors that have a design suction pressure approaching the triple point pressure of 4.59 mmHgA are normally constructed with a steam jacket. That jacket could be a fabricated jacket or coiled tubing, both of which apply heat to the outside of the ejector’s diffuser to prevent ice build-up. In most systems only the first stage ejector or first couple of ejectors operate at a deep enough vacuum to start freezing, which is why not all of the ejectors in a system are steam jacketed.
Ejectors at low loads will pull a deeper vacuum than what they would at their design loads. It’s possible for an ejector that isn't normally going to be operating at a deep enough vacuum to build ice, to pull a deeper vacuum because of low loads and ice up. Increasing the ejector loading normally corrects this issue but if the user intends to regularly operate at these very low loading points, steam jacketing may be required.
How do I know if my ejector is freezing up? Taking a suction pressure and comparing it to the triple point pressure of 4.59 mmHgA can identify if you are at risk of icing. An external diffuser temperature can also be taken to determine if it’s below 32 ºF. If your external diffuser temperature is cooler than 32 ºF than the internal temperature is also cooler than this. If your diffuser is cooler than 32 ºF you may also start to freeze moisture in the air to the outside of the diffuser. The ejector behavior will also start to become erratic as it freezes up. Normally the ejector will build up ice over a period of time as the pressure degrades. Then the ejector will break performance and heat up causing that ice to melt off. At that point the ejector suction pressure will pull back down and the process repeats itself. This is normally seen as a pressure spike in the system that repeats itself on a cyclic basis.