Help with Heating Eductor

[justonwo]

In cooperation with an eductor vendor, I spec'ed an eductor for what is probably an unusual purpose. We have designed the eductor to pull vacuum on a falling film evaporator on one of our skids. The goal is to pull the maximum level of vacuum possible so we can operate our evaporator at low pressure/temperature. So the eductor will essentially be pulling water vapor into the motive fluid (where it then condenses). This should work well because the evaporative load on the evaporator is so low.

At our motive flow of 80 gpm and because we are starving the eductor for water vapor by putting a low heat load on our evaporator, the eductor we spec'ed is supposed to pull 25 inHg vacuum with the help of the condensing water vapor. This seems fairly unambiguous as I look at the performance tables for our eductor.

We have been installing the skid this past week and have gone through various commissioning procedures. My engineer on site is telling me that, with the suction side of the eductor blocked off (and the vapor space full of air), he is alarmed that he's only getting 5 inHg vacuum. We won't know until we start up the evaporator this weekend, but I'm not so troubled by this news. At least, I think I'm not. Here are a couple of questions the eductor vendor didn't seem to understand well enough to answer (sadly).

1) We haven't started our evaporator yet so the entire evaporator and suction line is full of air. To me, if that line were instead full of water vapor and the control valve on the suction line was then completely blocked off, you would expect to see a much higher level of vacuum in the small vapor space between the valve and the suction inlet to the eductor. Why? The water wants to condense in the motive fluid and air doesn't. Same reason that you get a much higher capacity with steam than with air for a given eductor. Does that make sense?

2) After convincing the eductor vendor that steam and air in that vapor space would produce different results, he referred to one of his tables and insisted that even with air, the dead-headed suction inlet should see a vacuum around 15 inHg. However, I think he's referring to one of his tables where he still has some flow (they don't list dead-headed vacuum levels in their tables). And I think all he really knows is how to read his tables. We're only getting 5 inHg with the suction side dead-headed. Short of a leaking valve or mis-installation, what gives?

So here's the question: if you completely block off the suction side of an eductor, does it still efficiently produce vacuum or do you need some amount of flow. In other words, do you reach the maximum amount of vacuum with an eductor just before you completely choke off flow (at which point you then see a decrease in suction)? Does this level of vacuum then drop off as you completely choke off the flow (i.e. some kind of dynamic fluids effect is required to give max vacuum)?

Obviously, we'll know if the eductor is up to the task when we start up our evaporator, but I'd like to get a jump start and make sure everything is up to snuff before we start the evaporator (and relax my brain so I can stop thinking about it).

 

[zdas04]

It needs flow. I've had this discussion in the field a bunch of times and the suction pressure on an ejector (i.e., a thermocompressor with a compressible power fluid) will often increase when you shut the suction (probably because of leakage into the steam chamber from the fringes of the power nozzle stream), but it will never decrease much.

An eductor (i.e., a thermocompressor with an incompressible power fluid) behaves a bit differently, but it still won't work well with a shut suction. In fact, I have often seen severe cavitation in the entrance to the throat when you shut off suction on an eductor.

From your discussion it sounds like your "eductor" is really an "ejector" powered by steam, but I'm not sure of that since you say your motive flow is 80 gpm and people don't typically state steam flow in gpm. If it is actually an eductor then there is is good chance you have destroyed the throat with your test.

David

 

[justonwo]

David, just to be clear the motive fluid is liquid water (or, more
specifically, a stream that's mostly water) which is why I list the flow in gpm. I'm guessing if you have severe cavitation that you'll lose much of your vacuum.

 

[MikeHalloran]

The only eductor I've ever seen get well into the twenties of inches of mercury was a lab-scale version driven by a faucet tap. It had a very unusual spiral baffle in the diffuser. It was used to pull vacuum on a small tank of water that was agitated continuously. Both were supplied as part of the patented Nold Dearator.

I've home-brewed more nearly classical eductors from sheetmetal and machined parts, none as beautifully tapered as commercial eductors, but they were made for draining pipes that were sometimes wet, and sometimes full of fire, so high vacuum performance was not a major design consideration, and I didn't have time to generate performance curves.

None of those showed any weird dynamic performance issues, so long as the discharge was not restricted.

The little data that you've provided suggests that your eductor's discharge is partially restricted, or you've got a leak into the evacuated area.

Could you possibly reveal the pressure and temperature of the motive fluid at the eductor port, and the manufacturer and model number?



Mike Halloran
Pembroke Pines, FL, USA

 

[justonwo]

Remember, the fact that you are pulling water vapor is the KEY to the high level of vacuum. If the gas is condensible, you get much better performance. Temp of the motive fluid is 185 F. 40 psig upstream and 0 psig downstream. It's a 2.5" MLE from Jacoby Tarbox.

 

[unclesyd]

If it's an ejector there should be a no flow pressure table.

Years ago I tested ejectors by putting a blind on the suction side and checking the pressure against a table.

 

[rmw]

First off, years ago I was a design engineer on desalination equipment for boats and oil platforms with no steam available so water driven eductors were the vacuum equipment of choice and we operated at relatively deep vacuums (for water based processes, that is).

Second off, yes your eductor does pull some water vapor, but its primary purpose is to pull off non condensables so that the evaporation process (and the condensing of the water vapor there in) can create the vacuum needed for the process to operate. So, yes, some water vapor comes over with the non condensables, but it is really the "air" (if that is what your non condensable is) that the eductor is designed to transport. If you process had no non condensables involved, then the condensation of the process would create a complete vacuum and vacuum equipment (jets, eductors, liquid rings, etc.) wouldn't be required at all.

Third off, it is a old field service engineer trick with steam jets at least to block the jet suction and check that it pulls down to the deepest vacuum attainable for that jet. I have never seen that done with a water driven eductor, but according to comments made above by those who have, the process may not be comparable.

Fourth off, large equipment that operates at a deep vacuum such as power plant condensers that require vacuum equipment to maintain that vacuum creation capability (by removing the non condensables that would otherwise block heat transfer surface) usually has a hogging jet to remove the bulk of the initial air at start up as the operating jets just don't have the capability to remove all that air very quickly. Once the hogging jets get the vacuum down to some managable level, the operating jets can take over and take the volume down to the deeper vacuums required for the process to be able to start up. Your field guy may be trying to get this eductor to operate like a hogging jet while it is designed for much lower flows of non condensables. I don't know what plans you have for getting the air out in order to start up the evaps. If it is only the operating jets, then he may have to wait a while. Some of our evaps mentioned in "first off" above were large enough to require hogging equipment.

rmw

 

[Compositepro]

I've also "home-brewed" eductors. I would say that if your eductor is not working now there is no reason it will work any better later. It sounds more like your eductor was designed to pump liquids. The motive jet must mix with the pumped liquid to fill the diffuser nozzle. Gas pumping requires a tight clearance between the jet and the diffuser nozzle to prevent backflow around the jet.

An eductor really seems like a really wrong choice for a thin film evaporator. A small mechanical pump would provide much lower vacuum with far less energy. Are you using a high flow of stripping gas to get evaporation rather than high vacuum?

 

[justonwo]

rmw, thanks for the detailed response. In our case we have no condenser other than the motive liquid. So the eductor will need to keep pulling steam to maintain the vacuum. These eductors are billed as "heating eductors" and are designed to heat the motive stream with the steam being pulled in. But yeah I agree you won't get deep vacuum until the air is evacuated.

 

[rmw]

Justinwo, somehow I didn't pull that out of the OP. Then too you have to remember that the maximum vacuum you can pull is a function of the saturation temp of the water present at the time of the attemp to pull vacuum. If you are going to operate with 25 in vacuum, you have to be somewhere near 130-140 degF (I am not about to go look it up any closer than that). Any higher temperature than that, and the vacuum will just be trying to boil off the fluid at the boiling point for that vacuum at that sat temp/press. You can't lower the vacuum until you boil off enough fluid to reduce the temp of any water present to that which will permit a 25" vacuum.

rmw