Venturi Traps

[Craig Jensen]

We are researching the possibility of converting our 4 kilns (84 feet, double track) from inverted bucket traps to venturi style with strainers upstream.  We are being told that we can shorten drying times, possibly lower supply steam pressure, and tighten our standard deviation, due to maintaining only saturated steam in our coils, while at the same time reducing our losses in flash steam at the condensate tank.

At our mill, our drying times are significantly short enough that our up time per kiln is under 60%.  (due to not having wood in front of them to dry)  So shorter charges do not immediately mean increased revenue from more throughput.  We could run longer runs of the long drying species if the drying times were shortened more, so there is a flexibilty advantage - but hard to quantify that.  Boiler is hogged fuel, with an abundant supply, so no real savings there.  Less flash steam would mean lower chemical cost. . .


To make a long story short, tighter standard deviation in our HemFir loads, resulting in being able to run a higher target, hopefully driving drying defect down is our only real path to more revenue in this scenerio.

Current economy percent due to drying defect in HemFir is 2%.  Would have to cut that in half for a 1 year payback in grade recovery alone.

An added benefit would be that the HemFir would run throught he planer more efficiently if the Mean was higher.  This reduction in overtime at the planer, as well as increased recovery of those overdry boards that blow up altogether is probably the real money maker in the scenerio.

So,
Does anyone have experience changing over from inverted bucket traps to venturi?  What are your experiences?  Good, bad, indifferent??

Pitfalls to avoid?

Anything that you documented that you can share?

Craig J.

[MichaelM]

Most people probably do not know what a venturi trap is.  Most people are probably thinking orafice trap which it is not.  I found the comments below (pasted at end of my comments) discussion at http://www.eng-tips.com/viewthread.cfm?qid=130048 .
If someone does have experience with venturi traps, it would be nice to hear.  I am not aware of any in kilns.

My comments -

Bucket traps are pretty good at passing condensate and not live steam.  You would, as you say, have to lower the pressure in the coil to reduce flash steam. This way less of the condensate would change from a liquid to a vapor after passing through the trap.  Remember, however, if you lower the pressure in the coil, you will also lower the temperature and slow the drying rate.

Changing traps only -
It is not clear to me why changing traps would reduce the standard deviation of MC in the wood or have much impact on drying time if the current bucket traps are sized correctly and working correctly.  There should be all steam in the coils (no condensate back up) with any properly operating trap. 

Reducing the pressure -
If there is saturated steam ahead of the control valve, then the steam in the coil should be saturated or will be a short distance after the valve (depending on the pressure drop).  As you say, if the steam pressure is lowered, the steam entering the coil will be closer to saturation (less superheat) and will reach saturation in less distance and there might be a more uniform temperature in the coils.  Whether this will significantly impact the drying, I don't know. 

In any event, you might be able to lower the steam pressure with the existing traps.  Two cases -

First case - control valve is fully open for much of the drying cycle.  The pressure in the coil is close to the header pressure.  Lowering the steam pressure in the header will lower the pressure in the coil and lengthen the drying time because the temperature will be lower in the coil

Second case - control valve is modulating during most of the cycle.  In this case when the pressure is lowered in the header the valve will open more at any given time in the cycle.  This will maintain the pressure (and temperature) in the coil so you can have the same drying time. If you lower the header pressure too much you get to case one.

Remember also that steam at a lower pressure occupies a larger volume.  You have to be sure that the rest of the system (pipes, prvs) is designed large enough to operate at a lower pressure.  You might use fewer pounds of steam at a lower pressure becuase the latent heat goes up as temperature decreases, but the volume increase will more than offset this.

A lower steam pressure in the header may make conditioning easier to accomplish.


Pasted from http://www.eng-tips.com/viewthread.cfm?qid=130048
ncstate86a (Mechanical)
25 Jul 05 17:45
We are considering using a steam trap that utilizes an ISO standard venturi nozzle to remove the condensate from the steam system. It has no moving parts and appears to be very energy efficient. Our testing shows that it works well under varying loads (above 15% of rated max load).  The venturi design should not be confused with orifice traps; I have experimented with orifice traps and was disappointed by their performance under varying loads. We have tested the  veturi design rigorously on a weighted condensate test rig and have found it to work well under varying loads. Can anyone (other than steam trap manufacturers) provide me with their experiences using venturi (NOT ORIFICE) steam traps?   

cooleyjo (Mechanical)
27 Jul 05 18:49
so basically what you want is a scrubber to get fine particulate out of the air correct.  this works extremely well venturipak by envirocare is an example of this.  the faster the air goes the more particulate matter that is extracted you want a mach number of 4+ at the venturi throat.
Unless I have completely misinterpreted what you are trying to do

ncstate86a (Mechanical)
28 Jul 05 8:01
The veturi type steam trap is a steam trap. It is designed to removed condensate that forms in steam lines and heating equipment as the steam gives off its latent heat. Our testing shows the purpose of using a venturi style trap over a conventional mechanical steam trap is three fold:

1) No moving parts to fail
2) As the condensate forms and travels through the throat of the venturi nozzle, the pressure head of the condensate is transferred to velocity, causing the pressure of the saturated condensate to drop, which in turn causes the fluid to flash back into steam. During the flashing process, the fluid volume expands several hundred times, eliminating the available flow path for escaping steam. The result is a highly efficient condensate removal device under varying condensate loads.
3) The nozzle is very efficient at moving condensate and very inefficient at moving steam due to sonic choking. The result is minimal steam loss even at loads under 15%. The result is the inability to "fail open" as other condensate removal devices do. I hope this clarifies.
Any practical info from users would be helpful.

arto (Mechanical)
28 Jul 05 8:52

Look @ the Jet-Vac info @ the bottom here - might be useful:
Artisan Industries Inc. - World Leader in Equipment Manufacturing and Separation Technologies since 1934

katmar (Chemical)
28 Jul 05 9:16
I have not seen a steam trap based on a venturi before, only those using a series of stepped orifices.  But I can't see that there would be much difference in performance.

Do you have a reference to a web site where we can learn more about them?  From the lack of relevant comments here it seems that not many know about these things.

You said specifically that you were not interested in experiences with orifice steam traps, but on the off-chance that you mean you are not interested in single orifice traps here is a reference to some comments I posted on my experience with stepped orifices  some time back

thread391-117776
ncstate86a (Mechanical)
28 Jul 05 10:37
The reason I was not interested in orifices (sharp edge) is that orifices tend not to choke as compared to a venturi nozzle. Secondly, orifices tend to creates eddy currents on the downstream side of the orifice plate as the condesate exits the orifice and flashes back to steam. The result is a build up of chemicals/precipitate that results in plugging. A venturi nozzle doe not suffer this problem. Orifices also do not work well on varying loads. Our limitted testing and Navy testing validate these statements.

[Craig Jensen]

Thanks Michael,
We are concerned that we may be getting too much back pressure in our condensate line from bucket traps that are leaking by and introducing live steam into the condensate system. 
The issue we seem to have is that at a high load, the zones furthest from the condensate tank seem to be lagging behind.  We have tried to check with a heat gun when the kiln is down and the steam valves are open, but we are not exchanging much heat and everything seems fine then.  We did not have this problem when every trap in the system was brand new, so that is why I suspect internally leaking traps.  Not all the way blowing steam all the time, just not fully closed when they are supposed to be closed.

Also, I would suspect that we would have heat exchange efficiency loss if we are not keeping all of the steam in the coil until it condenses.  Those btu's got away, and have to be replaced with more, resulting in less heating capacity, and longer drying times, more time to hit set point, reduced heating control. . . .

Trying to find the cause of a slow creep downward in efficiency and control, and a creeping upward of drying defects.

Craig J.

[bmason]

Like Craig, I have been aggressively approached by a supplier to make the conversion to a Venturi steam trap.  They claim energy savings, lower standard deviation,
and a reduction of 10-15% in cycle times.  Does anybody have experiences (good or bad) with a Venturi type steam trap?  And is the claim of a 10-15% reduction in cycle
times a legitimate one when dealing with North American softwoods?

[HencoV]

Consider this: The three kiln factors that influence a kilns performance (all else remaining unchanged) are   1. Air flow, 2.Heating capacity, 3. Humidity control. If you can do something to improve either of these significantly, and you  then re-look your drying schedule to balance the schedule with the kiln's new abilities and it makes sense...go for it. Personally I think changing steam traps, if you know your old inverted buckets worked well is not going to make a huge difference, if any. Challenge the salesman....make sure your figures are right and convert one kiln only and see if it is worth it.

[MichaelM]

Kind of an outdated reply, but this thread recently popped up in my e-mail and I wanted to make one more comment.

If the bucket traps are releasing steam into the condensate return system, then they are not working properly.  The bucket drops when the low density steam enters the trap and closes the orafice. 

[PaulBallard]

Craig: Regarding the possibility of lowering the pressure you mentioned in our original post, we had a specific exercise in mind.

Once a plant is fully converted to our venturis, we offer to consult with the client on possible ways to make the most of the improved consistency of drying that usually results in a tighter bell curve - wets to drys.

Usually first on the list are things like sorting the wood by species and MC. Once optimal charge times are achieved then we can look at some other possible measures.

One of these includes playing with the boiler set point (where practicable) to see if the improvements in charge times being experienced can be maintained at a lower set point. This could be a simple matter of lowering the boiler set point by a small amount - say 5 psig - and then monitoring charge times to see if they hold. After dropping the pressure a few pounds for a few rounds, if productivity falls off we then recommend raising the boiler set point back up to the previous setting.

If you have any questions about this, feel free to contact me.