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Wet bulb facts

Started by GeneWengert-WoodDoc, July 17, 2018, 10:04:05 AM

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GeneWengert-WoodDoc

Wet bulb and wicks...a few facts.  The wick should be muslin.  The bulb should be about 1-1/2" away from the water supply, with the wick bridging the gap and positioned so that the wick remains soaking wet.  The wick should have about 600 fpm air flow.  The water should be distilled or steam condensate.

You can make your own muslin wicks.  However, new fabric from the fabric store usually has sizing on it, so the wicks need to be washed and rinsed well before use.  I therefore suggest washing and rinsing well any commercial wicks or wick fabric that you purchase.  Some commercial wicks are too thick...more like a wash cloth...bad idea.

The active part of a long bulb is the last 1" of the bulb.

All these are bad:  I did see one operation where the operator put the bulb in the water supply directly to keep the bulb wet. I have seen many where the bulb is too close and too far from the water.  I have seen city water used.  I have seen hot water used.  I have seen many bulbs with poor air flow, as the bulb is put in a location where it is protected from getting bumped, which also means poor air flow.   Note that 600 fpm is about 7 mph.  Many kilns use under 300 fpm blowing through the lumber load.  (An auxiliary fan on the bulb is a great idea.)  Poor air flow means the true RH is lower than measured.

Note that small errors in WB temperature are only important when drying wood, especially oak and other check-prone wood, that is green or quite wet.   For wet wood, the drying rate is proportional to (100 - Actual RH).  So with green wood when the initial conditions might be 87% RH, a one degree WB error can mean the actual RH is 84% RH, so the Lumber will dry about 25% faster than desired or expected.  A 5 F error under 20% MC is of little importance.

Because of WB errors, the drying rate of the lumber (the lumber's response) is more useful much of the time.

To check the WB, take the wick off and make it a dry bulb.  It should be reading the same as the regular, adjacent DB.

As air passes through a load of lumber, the energy from the air is given to the lumber to provide the energy needed to evaporate water.  This loss of energy from the air means the air cools and the RH increases, meaning that drying slows as the air moves through the pile.  The slower the air flow, the greater this effect. This is not a big issue with a small narrow load, but when over 6 feet wide, most kilns will reverse the air flow direction through the load every 2 hours to achieve more uniform drying.  It is also why we target 300 fpm or even higher in most kilns.

When we first start drying with wet lumber, the surface temperature of the lumber is very close to the WB temperature...the surface is actually a large WB.  As drying continues, the surface temperature approaches the DB temperature.  I have developed mathematical expressions or models for all this.

Technically speaking, because this is an adiabetic process, the WB temperature in the kiln is uniform, from one side of the load to the other.  Hence, only one WB is needed, even in very large kilns.

Because of the change in DB as air goes through the load, we measure the incoming air temperature and not the exit air temperature.  The DB temperature drop across the load, TDAL, is used in kilns with fast drying lumber to estimate drying rate and MC.  With this info, kiln conditions can be altered for slow drying or high MC lumber to get more uniform final MC.  In fact, I developed zone kilns for southern pine using TDAL many years ago.  This approach does not work for hardwood kilns, as with hardwoods we want the MC of the wettest Lumber pieces, and not the average behavior of the entire bundle or stack of lumber.

A final comment...because of the possible issues with a WB many kilns use thick paper EMC wafers (Lignomat has them) and measure the EMC of the air.  The resistance of these wafers is used, just like a pin moisture meter.  This resistance is then be converted to EMC, RH and WB at the known DB temperature.  A few kilns also use electronic RH sensors, but their calibration after a few months is poor in the dusty, humid, acidic kiln atmosphere.

:o Zzzzzzzzz
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

btulloh

Good info Gene.  Thanks.  I stayed awake for the whole thing. :D  

The air flow info answered a nagging question.  I don't have 600 fpm anywhere in the kiln, so I need a little wb fan I guess.  My goal is to measure rh before and after the air enters the stack.  My sensors are placed that way now.  I'm using capacitance-type rh sensors, but they seem to be suffering from the conditions and I don't have confidence in them.  The temperature sensors I'm using are of a form factor that lend themselves to adding a wick.  With any luck and the right air flow they may provide accurate readings.  TBD.  I may have to bite the bullet and buy some industrial grade sensors, but . . . 

Your post was very timely and helpful.  Thanks again.

HM126

btulloh

A side note - I have finally developed an appreciation for DEW POINT TEMPERATURE after converting all my rh and temp readings to dew point as an intermediate step for other calculations.  It's funny what can turn on the switch in my head. (Sometimes it never does turn on, but that's a different problem  :D)

I have found enough info and equations to do all the psychrometric calculations I need.  Some assembly required.  One of the benefits of something like the Pi over a microcontoller enviroment is it has enough resources to do a lot of complex math.
HM126

GeneWengert-WoodDoc

As the WB is the same everywhere, a kiln needs one WB measurement and then with a DBs entering and exiting, it is easy to calculate dew point, EMC, RH, etc.

Close to behind or in front of the fans would be ideal location for WB, shaded from sun, etc.  Velocity is high enough and the WB there is the same throughout the kiln. As we have to change wicks often, the location needs to be accessible.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

btulloh

Good point / suggestion.  Placing the wb in front of one of the fans takes care of the airspeed requirement. 

I've been sort of stubbornly working toward checking the rh in both the collector area and also as the air leaves the stack.  With the circulation in the kiln, it doesn't really make sense to do it that way.  

I think checking the temp in both places is useful though.  I find it interesting to see the drop in temp caused by evaporation.  
HM126

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