Proportionate venting

[HencoV]

I am tasked to set up a drying schedule for a kiln that uses proportionate venting. Having a preference for quick open/close venting, because I believe it is more energy efficient, I will be feeling my way through this one.

In my mind, and in practice here in SA I have not seen an installation where proportionate venting have been setup correctly - referring to the drying schedule - , meaning that except for during warm up, conditioning or equalizing, vents would be 100% open, or 100% closed for extended periods of time. This to me means that set points at that stage in drying is not correct - the schedule is either to harsh (100% open) or too soft (100% closed) for the amount of moisture coming off the timber at the time. Also when vents stay 100% open for long periods of time,the main and booster steam valves also stay open for extended periods - wasting boiler energy. 9 out of 10 times, after a sawmill install a fancy new kiln with proportionate venting, 6 months down the line they have to buy a new boiler...because operators, and sawmill engineers for that matter, do not understand the effect that badly setup proportionate venting has on steam usage.

My question: is there a rule of thumb I can use, to balance effective proportionate venting, and energy loss and quality/efficient drying? I am thinking that moving to the next phase in drying, proportionate vents could be 100% open for a couple of minutes, but then should start to close slightly/slowly. Maybe when vent reaches ±80% closed, it is time to move to the next phase with a slightly bigger depression, and starting again with 100% open vents...closing slowly until the drying gradient at that set point needs to be made harsher?

[MichaelM]

Henco,
You have lost me.  Many of the things you have said make perfect sense – with on/off venting the conditions in the kiln will change a lot, steam demand will be less even, and it is less energy efficient.  For several reasons, I do not understand the part about having to buy a new boiler –
1.  If proportional venting is more efficient, then steam demand should be less.
2. If steam use is more even, then the boiler should work better.
3. If the conditions in the kiln are more even, the lumber should dry with higher quality.
Please tell us more.  Is this hardwood or softwood?  It’s hard to answer because on/ff controls were phased out almost 50 years age.  I have not seen this in the U.S. for over 25 years.  And yet, we do not experience problems.  Contact me off line if you want to at Mike dot Milota at OregonState dot edu.  We can put a reply on line after figuring this out more.
Mike M.

[HencoV]

Have been working on one of Europe's big brand kilns the last 48 hours...I'm having some trouble getting my head around the proportionate venting on the control system.

In my mind, proportionate control, means, having the controlling variable (valve/vent) open 100% until It gets close to the Setpoint, and then start closing gradually until setpoint is reached and the valve/vent is closed. And the reverse when opening. Obviously with PID calculations, like Ziglor Nichols ect, depending on how the manipulated variable (temp or rh or emc) reacts, this will determine the PID settings and reaction speed of the control loop

If a specific condition, calls for the controlling variable to be at a 50% open position, and this value is forced to 80%. The PID loop should bring it back to 50% provided the conditions did not change.

Bringing this back to proportionate venting. On this system venting is controlled by EMC, and has a hysteresis setting of + and - 0.5. The actual EMC would sometimes go as high as setpoint +2.5. Especially after fan direction change (which is normal) This should call for vent to be 100% open, as quickly as possible, however, on this system it could take as long as 15 minutes to get there. By now the EMC has gone up some more, resulting in the Vents to stay open at 100% for anywhere between 60-120 minutes before it starts closing slightly. Could someone that has programmed proportionate venting please explain  what reaction they would require the vents to do in a PID loop control?

Apart from the fact that I think the programmer of this control set missed the correct application of PID, I think the vents by design is also too small. This could be the reason why PID doesn't work, because dead time almost runs into infinity, and the controller takes for ever to decide to which position the vent must move. The kiln takes ± 210m3 at a time, and has a total vent capacity of only 1.15m2. If the old rule of thumb still applies of 0.03m2 / m3 softwood, this kiln would require ± 6.3m2 of venting capacity.

ps. Micheal, I send you an email...let me know what you think

[MichaelM]

I agree with you that it appears that the PID is not properly implemented.  If it takes that long for the vents to open, either the P or the I or both are too low.  It doesn’t make sense, however, that the P term alone doesn’t cause the vents to open when the error is 2.5%.

I’m not that familiar with wafers, but the wafer response might be slower than a wet-bulb and this might be a factor in the slow control.  Is it possible that liquid water is wetting the sensor and causing control issues?

As far a vent area, if the vents are 100% open and the humidity won’t come down or takes a long time to do so, then they are too small.  If you dry multiple species, this might be normal if only happens with the wet, fast-to-dry species.  Control might be poor on slow-to-dry species if the vents are large to accommodate the fast case.

[HencoV]

The system uses a Relative humidity probe and a dry bulb. It then calculates EMC. There is no water dripping on the probe.

I am starting to see the light with proportionate venting.

I think one needs to start with the question: What are you aiming to achieve with prop venting?

Obviously , to control the air moisture in the kiln, as close to, or on EMC/RH/ Wet bulb set point as possible.

The software controls for this kiln have lots and lots of controller settable variables.  The strategy is to compare actual EMC with EMC SP, and open or close the damper/vent slightly until these two are equal.

The biggest stumble block (lack of settable variable) is the lack of the ability to decrease the sample rate time between running the algorithm to decide whether to open/close more. This coupled with an extremely slow opening damper (163 second to open) renders what was the original aim of prop venting, useless. The controller controls the EMC actual on a + 1 to -0.5 EMC% band. At a DB SP of 85 deg C and SP EMC of 5%, this equates an RH% variance of 44% RH max over to 31% RH max below…..this is not control and is the reason why the client cannot achieve optimum drying. The controller eventually gets the EMC actual and SP the same, sometimes by the time it is time to change fan direction, it is still not there, and the gap is wider after direction change.

My open/close control with a 0.3 degC variance on WB temp is way more accurate. (At 85 deg C DB 62 deg C WB ± 0.3 deg C = RH% Variance of 35.5%RH over and 34.4 % RH below)….Less than 1% RH variance vs 13% RH Variance on this specific proportionate control.

The Ideal would be for the algorithm to run, calculate error between actual and SP, make an error adjustment (as quickly as possible with quick responding vent motor), wait to see the effect the adjustment had on the error (dead time) and rerun the algorithm. This could result in controlling the RH at close to 0% RH diff. Is this really worth all the trouble?

It should be interesting to do a study of different kilns and their control systems, to analyse the control strategy behind venting and calculate the actual RH% variance band that it operates on.

A well designed proportionate venting strategy could be beneficial ---- for now I still believe in quick Open/close venting.

[drykilned]

MY NAME IS ED ANDERSON. I AM NOW RETIRED BUT STILL STAY ACTIVE WITH
THE DRYKILN FIELD.  TO ADVISE YOU I NEED TO KNOW WHAT PRODUCT YOU ARE DRYING AND WHETHER YOU HAVE SINGLE TRACK OR DOUBLE TRACK KILNS.  ARE YOU USING ONE BOILER OR MULTIPLE BOILERS TIED TOGETHER.
WHAT IS YOUR BOILER CAPACITY IN RELATION TO STEAM REQUIREMENT FOR
THE BOILERS.  THIS MAY BE A VERY SIMPLE SOLUTION WITHOUT HAVING TO
REPROGRAM ANYTHING.  WHOSE CONTROL SYSTEM IS OPERATING THE KILNS?

I AM NOT A SALESMAN OR PAID CONSULTANT. I HAVE MANAGED/MAINTAINED/OPERATED STEAM, DIRECT FIRED, SLOPE GRATE
GREEN FUEL, AND DIRECT FIRED VORTEX KILNS FOR ALMOST 30 YEARS BEFORE RETIRING IN 2012. I WOULD RATHER DISCUSS THIS OVER THE PHONE
AFTER RECEIVING THE ANSWERS TO THE ABOVE QUESTIONS.  YOU CAN PROBABLY FIND PEOPLE WHO REMEMBER ME AT USNR, ED BAHR SERVICES, KILNTEK, ATLANTIC FIREBRICK, MCCONNELL INDUSTRIES, HURST BOILER, BEADLES LUMBER COMPANY, BALFOUR LUMBER COMPANY, AND SPIB.

CELL PHONE 229-921-0095

IF YOU PREFER, WE CAN DISCUSS IT ONLINE, JUST CANNOT DO IT AS WELL.

ED

[HencoV]

After spending ± 200 hours with this kiln, I believe I fully understand what proportionate venting is about.

The controls on this kilns has many fancy settable features. In what seems to be fail safe mode, where the controls uses and average of dry bulb probes (on the enter and exit side of the stacks) to control temperature, and the average of RH sensors (on the enter and exit side of the stacks) to control humidity, the kiln controls according to the schedule "sort of"....running on EMC variance of -0.5% to +1.0% around set point. This way of control however, is not ideal or optimal, because in an attempt to reach average values, the condition of air entering the stack is extremely harsh. eg. trying to control on 85 Deg C with EMC of 5.6%, the actual condition of air entering is pushed to 95 Deg C + and EMC of ± 3. Especially during the beginning of drying this will only stuff up pine. There is a setting to have the kiln control the conditions based on the condition of air entering the stack, using only the probes on that side when fan direction is blowing into the stack. However, some fail safe internal programming causes the main steam to close when EMC goes too high. Of course after direction change, and now using the probes on the other side of the stack, from which the fans now blows, it is normal for EMC to be high for a few minutes....the main steam closes, vents open, temp drops and of coarse this causes EMC to increase even further. The result--the kiln shuts down due to a "too high EMC reading". Of coarse the local distributor of the kiln has never come across this, as they always set the kiln to use average values to control.

For complete and accurate control, you cannot use the average readings of probes on the opposite sides of a stack to control (air enter side and air exit side)...this renders the accuracy of modern control systems useless. 

To conclude: After sending all my findings, graphs and comments to the kiln supplier, and not getting a proper response in roughly 5 weeks, I can only conclude that they either just don't give a damn, or they don't understand the fundamentals of drying. The control is fancy enough to baffle with brilliance. It just lacks one thing and that is to actually control the drying process efficiently.