I am a software developer and have been approached by a kiln owner to develop an application for calculating wood kiln schedules for various hardwoods, softwoods and exotic species. After considerable research on the topic my questions are: How come this application does not already exist and is there a market for this application?
(Commercial Kiln Drying Forum)
From contributor B:
Basic USDA FPL book schedules are available via the FPL website. There have also been several standalone programs over the years which can develop book based schedules. Most experienced operators use these schedules as a base, then modifying such as initial and ending temp
For example my DH kiln has a max operating temp of 135F so I have to convert the USDA book schedule to fit my kiln and hope that I didn't make a calculation error. This program would also be able to incorporate a precise adjusted RH to EMC at any temp automatically instead of manually looking it up on a chart. It also would be able to formulate the theoretical kiln schedule by incorporating all the operators’ personal parameters and allow the kiln operator to precisely plot the actual schedule to make sure that the schedule is on track and not drying too fast for a poor outcome or drying too slow for time and money efficiencies.
Another large area for my needs is to track my cost. I have a small kiln and I don't always fill it with every load, and as any operator knows outside air temp and initial lumber temp changes the cost of initial heat up drastically. This program would be able to calculate the cost to dry the particular load based on species, size of charge, outside air temp, starting MC, ending MC, etc. As with any software program once the operator tailors it to their own needs, all the results, graphs, charts, etc. are instantly calculated resulting in a better time efficiency and limits human error. So, we are looking for input on if there is a market out there that is also looking for all this info that I am looking for.
We do not know how to factor in many of the variables you mention, such as air velocity and barometric pressure. We do not know how to plot the schedule as a function of time (hardwoods use MC based schedules; time depends on thickness of the lumber, green MC, air flow and so on) and compare to actual to see if it is going too fast or slow. Most kilns have plenty of power so R-factor is not an issue in time, but it does affect energy use a few percentage points. Equations do exist for calculating EMC, RH, etc. As energy cost is a large component (50%) in drying costs, it is easy and probably essential to measure energy use to help calculate costs. Much energy is lost through the doors, vents, floor and frame, so Wall or door R-value alone is not enough. Drying cost is based primarily on drying time and time is based on the initial and final MC. However, knowing the schedule is not enough to calculate and predict drying time. With all the unknowns, a prediction would be filled with uncertainty, so the usefulness would be limited to a commercial operation.
So, based on my views as listed above, I do not see a market for such a program. In my experience, I seldom see human error as causing problems in drying. Rather, problems are more often due to lack of management direction or lack of understanding.
Between all the charts, graphs, schedules that are published, all of this has come from some type of mathematical equation and/or formula based on either scientific proof or simply from real-life trial and error. So we already have all this info that is scattered all over, where a software program would compile everything including the variables. For example, someone (maybe or most likely even you) at some time already calculated the max rate that water can be drawn out of each type of species of wood at a safe rate as to not collapse the cells, etc. The equations for relating temperature, humidity, and moisture content have already been published in the USDA Dry Kiln Operator's Manual. If there is variable in that formula due to Vapor Pressure due to change in elevation or barometric pressures, and being the relationship between vapor pressure and temperature is non-linear then the software program would also be able to use the Clausius–Clapeyron relation formula to recalculate for these variables (I'm just using this as an example).
Even for energy efficiency portion of a software program that I'm interesting in, once I enter in the volumetric air volume, lumber volume along the R-values of the walls ceilings, doors, floor, I know my own R-values, but if I didn't I could look them up, someone somewhere has already calculated all those numbers. Once all values have been inputted the software would do its thing and calculate everything needed instantly according to all values inputted; outside temp, outside RH, specific gravity, initial MC, initial wood temp, volumtric area of wood, thickness of lumber, amount of water, cost of KW/hour, etc. It could even be linked to your own local weather station to get accurate real-time temps, outside RH and atmospheric pressures etc. and current electrical rates, etc. However I understand there would have to be an estimation on efficiency lost through door seals, cracks, vents etc. That I understand, but so far that is the only questionable area that I can't come up with a formula for other than a standard 8, 10, 12, or 15% loss by guessing. In other words everything that I have read so far can be calculated from a formula, including all the variables, and once the operator enters these variables the software will update everything. So I guess my overall question to you is; what variables in your professional experience cannot be explained in a chart/graph/formula?
I think one of the big needs in our industry is for a spread sheet to accept moisture section and sample board weights and then automatically calculate MC, speed, suggested kiln settings. Maybe even make it into an App for I-phone, I-pad, etc. I have seen a few of these, but they are not for sale or are not stand alone products but come with a new kiln package.
If you are drying hardwoods, you need Drying Hardwood Lumber instead of the manual you mention. You can find a copy here at WOODWEB. Although we can calculate a lot of different variables and include effects like barometric pressure and heat loss, the results will not change how we run a kiln. We run the kiln based on the MC of the wettest samples, along with knowledge about wood quality and drying defects. These factors are the big factors and they are quite variable.
Now as for those spreadsheets you mentioned, I will talk to the software programmer of ZINCASTLE Software Systems offline. I know he can meet your needs on all of those. As to if it would be financially reasonable I'll let him ask you and other operators all those pertinent questions.
I am somewhat confused by your control based on the rate of change of the WB. So, do you set a given WB and DB and then run the kiln, controlling only the DB at a constant value and let the WB change over time? Then you look at this rate of change? It would seem that if the rate of change is something like the WB is increasing at 0.1 degree per 15 minutes (meaning the RH is increasing), then you may have the correct WB setting (and a reasonable EMC so that you do not over stress the lumber when above 50% MC) but no venting. With venting that will bring the rate of change down to zero. That is how a typical kiln operates and dries lumber efficiently and with good quality - most of the time the rate of change of the WB is zero; it is only for a short period (30 minutes) for five times in the schedule that the WB is changed (and also sloppiness of the control system will cause small changes plus and minus, but average zero). However, even at zero rate of change, we could be drying the lumber too fast or too slowly or just right. Can you explain more completely what you are doing? Are you doing this with softwoods or hardwoods, green or air-dried?
In setting up custom drying schedules below 100 Deg C for freshly cut pine species found in South Africa (Pinus patula, P.teada, P.elliotii, and P.radiata) I spend a lot of hours observing the effect of active venting on the drying speed and quality. Starting with prescribed drying schedules as a guide, I specifically looked at the effect different settings had on venting. Even when using in kiln moisture meters, in conjunction with prescribed drying schedules, there were times when the vents would stay closed for hours, or open for extensive periods. This did not give the required results.
I came to the conclusion that when running at a single DB and WB setting, eg 80/63 the initial venting times would be long. i.o.w. while there was still a lot of moisture in the timber, vents would be open for long periods, and closed for short periods. As the timber dries out, this reverses to a state where the vents stay closed for long periods and only open for very short periods. The rate of change I referred to is the speed by which the WB actual temperature increases when vents are fully closed, or how long the vents remain closed before enough moisture has evaporated to reach vent setpoint. When the WB temp does not increase anymore, it would indicate a state of equilibrium, or evaporation equal to leaks in the kiln structure, or at worst case hardening which restricts the flow of moisture to the surface. In all three cases vents would stay closed.
Using these assumptions I programmed the PLC with only 3 basic phases:
1. Heat up – DB equals WB until wood core temp reaches my first drying phases WB/vent Setpoint.
2. Phase 1: Run a set DB & WB - 80/63 until MC content gets to ±27% (in kiln moisture meter).
3. Phase 2: Go into an auto vent phase that would decrease the vent temp if the vent stays closed for too long (settable variable) or increase if it stays open for too long (settable variable). End MC 12%.
The end result was flat timber with a very even moisture distribution and very little kiln defect (I did not find a necessary to run a conditioning phase after drying). I think that the time spend running at a steady RH, helped in smoothing out average MC in the load.
Has anybody tried something similar with schedules? Obviously this cannot be applied to all timbers and all kilns. This was done on a compartment/batch type kiln that dries ± 40m3 of 38mm thickness boards at a time. We have gone up to 95 Deg C, but kept the phase2 RH around 44%. Drying quality decreased slightly, but was still acceptable.
Also, to achieve uniform MC, we equalize (not condition) the load. Conditioning is the aggressive steaming of lumber to remove drying stress. It is done at 4% EMC above the target MC. So, in your case with 12% MC target, we would condition (if possible) at 16% EMC. Many U.S. kilns are not well enough insulated to develop such high humidity’s.
A load of lumber will release three-four U.S. gallons of water per 1% MC loss per MBF. So, in a large kiln, we might be looking at ten gallons of water loss per hour. If the lumber is wet and the kiln is hot and the humidity is well below 100% RH and the fans are working, how can the lumber not be drying? So, if the vents do not open, that means that either drying has stopped or that there is some other way that the evaporated moisture is leaving the kiln, perhaps through leaks or through condensation.
I have seen kilns that run on the basis of constant venting. The idea is that because the drying rate is constant (almost), then a control system that uses constant venting will be a good way to control the drying rate. The control system opens the vents just a little bit so that the vents are open all the time and change the amount they are open very little, hour after hour. Early attempts to do this used a control system that was not adequate, so overall the idea did not work out well in practice. A similar approach was used by Dallas Dedrick in his CRT (Constantly Rising Temperature) patented idea used for softwoods on the West Coast of the USA. The idea was also tried for southern pine, but the reports I received were not as good as hoped for.
I've worked through some of the calculations on how to convert relative humidity and temperature to absolute humidity. That's in units of mass, which makes it easier to account for where the water is going. On the issue of a software app, I'd be interested to be a user and developer if it was open-source. I think the first requirement is just to have a giant look-up-table for existing drying-schedules. After that, system modeling might be useful, but it might also be extraneous. I mean if the schedules are optimized, then there's little point in intentionally deviating from them, right? Maybe where models could be useful is where the drying-schedule is accidentally deviated from, through power failure or equipment failure, etc, and the operator wants to know what action to take to minimize degrade and eventually get back on schedule. It could be thought of as gracefully shutting down the kiln and putting it in standby or air-dry mode. This might also have use for solar kilns that make this transition every day, or for hybrid solar kilns that may have varying schedules that are influenced by resource management.
Another use for a model is in kiln-design and optimization. But that seems more like R&D experimentation activity rather than a production tool, because there are too many variables to accurately model, right? I suppose that role of theory here is to guide the kiln-design and operation in the right direction, but then testing and experimental refinement is required for fine-tuning. I've programmed some basic models from scratch, and that is how I see them being used. I wouldn't dare suggest that someone use a theoretical model in place of an extensively developed and proven schedule, unless the model was so accurate and extensive that it could actually predict the established schedules across all sorts of variables like species, etc.