Author Topic: Wet bulb control  (Read 5789 times)

Offline apalmer

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Wet bulb control
« on: September 10, 2014, 08:28:57 AM »
     The last charge of 5/4 RO in one of our two older 120,000bfm kilns suffered extensive honeycomb. I am suspecting that the wet bulb performance may have been a contributing factor. (We use a wick over a RDT. Rinsed out before use so that is soaks properly.)

I have read that WB feed water temperature can influence readings. We have had issues with water flow control, using a basic ball valve tap to adjust flow. Since the last damaged charge, a needle valve was added to gain more flow control. I have also read however, that water being too cold or too hot can affect readings. What temperature is right? Should there be overflow from the reservoir or should it only fill at the rate that evaporation occurs?

As a secondary question, should I just say 'screw it' and put in a recommendation to upgrade our kilns to computer controls with wafers? Is it worth the cost of the upgrade?

Offline MichaelM

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Re: Wet bulb control
« Reply #1 on: September 11, 2014, 09:15:27 AM »
Your red oak is probably a 28-day or longer cycle.  Change the sock, do not rinse and reuse.  In our workshop (How to Dry Lumber for Quality and Profit, Dec. 8-10, 2014) we recommend changing it every week to 10 days on longer kiln cycles or changing it every charge on shorter cycles.  We also recommend getting your socks from a kiln manufacturer or other reliable source so that they wick well.  Throw a dry sock into a bucket of water and is should sink.  If it floats, then it does not wick well enough.

The wet-bulb sock should be wet to the tough.  Put your fingers on it when you are getting the kiln samples and you should see liquid water on your fingers.  If it is just damp, then it is probably not wicking as well as you would like.

The needle valve is a good step, but I would suggest a flow meter (see Dwyer catalog) that is in the range of 0-100 cc/min.  Regulate the flow to 50 to 75 cc/min and keep it the same for every charge.  Most of the meters have a meter built into them.  Use one flow meter for each tray.

Yes, you should have an overflow.  In theory you could control the flow in and make it match the evaporation rate out, but the evaporation rate changes during the kiln cycle because the rh and T change.  Water should flow into the wet-bulb sock reservoir, there should be a weir to keep the level constant, and there should be an overflow tube that allows you to see the amount overflowing.  I usually tell people to have an overflow (drip, drip, drip) of about a drip every second or two at a minimum, and a slow steady flow at a maximum.  You are probably fine as long as there is overflow.  Being able to see it allows you to ensure that there is water in the tray (the gage tells you there is water going in, but the tube could have failed).

Water temperature may have a little effect, but I would not worry about it unless you are putting in ice water or boiling water directly into the tray.  Wet-bulb theory says that the water supplied to the sock should be at the wet-bulb temperature.  Of course, in the simple way we use the wet-bulb in a kiln this is not possible nor am I aware that anyone does it.  You probably have a length of 1/4" od tubing that passes through the kiln wall and is exposed to the kiln's dry-bulb temperature between the wall and the tray.  If the tube was long enough, the water would reach the dry-bulb temperature (it would either warm or cool).  If you have room temperature water entering the tubing there is probably condensation on the outside of the tube which enhances the heating.  Most people use clean fresh water and the water is heated somewhat before it reached the tray and it seems to work.  Unless you are doing something strange, I wouldn't worry about this.  If you are using condensate (which I do not recommend) it probably cools enough.  Keep in mind that the water also has a chance to change temperature while it is sitting in the tray.

If you are not using computer controls, then you are one of the few.  However, people have successful dried red oak for 100 years without computers.  The circular chart should tell you if the wet-bulb sock dried out.  It might not tell you if the water flow is insufficient.  Example - suppose you want a 10F depression.  The 10 F depression is usually measured with a fully wetted sock.  If the sock is mostly wetted, but not fully wetted, the indicated Tw will rise and the vents will open to bring it back to 10F.  When this happens the real depression might be 12F or 15F.  The same thing would happen if you had a computer.  You as the operator would have to notice unusual vent operation to catch this early or see that there is no overflow from the tray.  As an operator  I would not use one bad charge to justify computer controls.  I would

Lastly - are you sure this was a wet-bulb problem?  Honeycomb can also be caused by raising Td too soon while the wood is still too high in MC - even if you have controlled the wet-bulb depression well.  You could also increase your chances for honeycomb (which will happen midway through the schedule) by pushing the dry-bulb up a little early.   Even worse - you control Tw, not Td-Tw.  If the dry-bulb temperature is not correct then the rh or wet-bulb depression will not be correct even with the best wet-bulb in the world.  Is it possible that the dry-bulb sensors are in need of calibration?

Mike Milota

Offline PhilM

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Re: Wet bulb control
« Reply #2 on: September 12, 2014, 12:45:37 PM »
I agree with everything that Mike M. said so very well.  I have tried to add just a few additional thoughts.

The wet bulb temperature that your controller is reading needs to be verified using a hygrometer.  A low temperature hygrometer can be obtained from Conway Cleveland Corp. and other sources as well.  This first step will cost you far less than a new control system.
 
Another relatively inexpensive item would be to install a view flow control valve on the WB feed water infeed.  This has a glass or plastic sight tube that lets you see the drip rate to the WB.  Our supplier in North Carolina is no longer in business, however, and I have not located another supplier.  It may be that Mike’s suggestion is better since he gives a source!

Ideally the wet bulb pan should be supplied with exactly the amount of water that is evaporated.  The wet bulb pan can be equipped with a constant-level device that incorporates a float valve to maintain water level in the wet bulb pan.

Get a hygrometer (or two !) to check that wet bulb temperature (and dry bulb) before you rush out and buy a new controller.

Offline HencoV

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Re: Wet bulb control
« Reply #3 on: November 28, 2014, 06:35:28 AM »
When it comes to wet bulbs, I strongly believe in keeping it simple. The attached pics are of such a WB assembly, though not high tech, it has exactly the attributes needed by a WB:

1. No spilling
2.You can at a glance see if there is water in it
3.Evaporation is matched 100%
4.No issues of feed water temp influencing reading
5. Nothing can get stuck, like a valve or solenoid ect....no moving parts
6. Only replaceable part is the WB sock, and the plastic cold drink bottle....cheap
Only down side, it must be manually filled, which may not be so bad, because it forces the operator out of his air conditioned office:)
« Last Edit: November 28, 2014, 09:20:22 AM by HencoV »

Offline HencoV

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Re: Wet bulb control
« Reply #4 on: November 28, 2014, 06:36:31 AM »
Outside kiln WB

Offline HencoV

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Re: Wet bulb control
« Reply #5 on: November 28, 2014, 06:37:39 AM »
Inside Kiln WB

Offline MichaelM

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Re: Wet bulb control
« Reply #6 on: December 05, 2014, 07:40:04 AM »
I agree that is a good set up.  We do something almost like that on our 4'kiln at OSU.

Instead of a bottle that we fill manually, we have a small water tank with a float switch on the outside.  The level is maintained constantly.

I would caution the use of this type of wet-bulb in a high velocity kiln where the pressure difference across the load can be an inch three across the load.  The differential air pressure can affect the height of the water column (pushing it down on the entering-air side and pulling it up on the leaving-air side).  The sock can dry if it pushes down.  Don't ask how I know.

Offline apalmer

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Re: Wet bulb control
« Reply #7 on: January 08, 2015, 04:44:56 AM »
This advice was very helpful. Thank you!

Offline kilnguy

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Re: Wet bulb control
« Reply #8 on: February 10, 2015, 09:44:40 AM »
SII sells a great product with a float assembly similar to what's in the back of a toilet.  We have had great success with these. Here is a sideview of one with the sock pulled off so you can see it.

 


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