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Next Generation Dry Kilns – Why Modern Computer Controls and Energy Efficient Kilns are the Future
Why Modern Computer Controls and Energy Efficient Kilns are the Future.
Date Posted: 2/1/2011
You may believe in global warming or not. You may discuss the necessity of alternative energies. You may even think that the oil and gas resources are endless. But one thing you can not dispute, wasting energy in sawmills has become more and more expensive, and it is likely to increase in cost.
The days are over where wood waste was basically free because there is an open market value for it now. You also have to consider the investment costs for your boiler. The more energy efficient your kilns are, the more you can downsize the boiler you need or you can connect more kilns to your existing boiler without the need to invest into a new one. It goes without saying that if you heat your kilns with gas or diesel energy, efficient kilns are even more important.
The answer could be to dry your lumber in a dehumidification dry kiln which uses a heat pump system to heat and dry the air, so no additional heating source is necessary. This technology has the following pros and cons:
• No boiler needed
• Gentle drying / good drying quality
• Low drying costs
• Limitations in the final kiln temperature (so not profitable for fast drying soft woods)
• Limitations in the kiln sizes
• Relatively slow drying process
• Uses electrical energy for heating which is likely to increase in cost
• Some systems need a gas or diesel back up burner to get to the starting temperature which allows the heat pump to work
As most of the bigger sawmills and furniture manufacturers in the United States and Canada already have a waste wood boiler they use conventional lumber dry kilns. In Europe the dehumidification drying is almost dead due to extreme costs for electrical energy.
If you want to know more about the Hildebrand dehumidification kilns (also for vacuum kilns) please contact us.
With this article (and the following ones) the authors will explain the importance of a well designed, fully integrated, conventional dry kiln system, where hard and software are perfectly balanced. Only by following this approach are next generation dry kiln results possible today.
Kiln, Control System and Software Should Form One Unit
Many times in the United States and Canada you will see dry kiln operations where the kilns were supplied from one company, but the controls were supplied from another. And if the kilns and the controls were supplied from a single company, it is likely that the in-kiln moisture and climate measurement system bears a different brand name. The reason for this is simple. A basic kiln alone (the box) is easy to design and build by any company involved in the sawmilling business and experienced in working with metal. The control system however, even for a very basic kiln, requires a technical know-how and experience. The more complex the kiln gets, the more difficult it is to control the drying process.
As kiln and control systems should form one homogenous unit, it is certainly less than optimal to have different suppliers for the sensitive control and measurement systems. Having two different contacts can create problems because servicing responsibilities can be unclear if something goes wrong. And certainly a company that can supply and service all facets of the kiln… the building, the controls, and the moisture measuring systems, will best be able to provide the optimal lumber drying experience that a mill is seeking.
Modern Controls Should Support the Kiln Operator
Another common practice in the United States and Canada is to take sample boards out of the kiln (for hardwood drying) and oven dry them to determine the moisture content. If a company has many dry kilns on the yard, this can consume most of the dry kiln operators time.
For softwood dry kilns, the situation is at least as bad. Here the kiln operator has to take hot spots with a moisture meter in order to determine the final moisture content to avoid over or under drying. To take the hot spots the operator needs to shut down the kiln (high temperature in softwood drying) go inside and do the measuring.
In hard wood lumber drying, it is pretty difficult to accurately measure the moisture content above fiber saturation. It is crucial to know the actual moisture content at any time to use the correct drying schedule for the different species. Otherwise, the valuable lumber suffers from drying defects and degrade. In softwood drying, it is more the moisture content below fiber saturation, which is important for the operator to determine the time to shut down the kiln and unload the lumber. If this is not done properly, the lumber might still be too wet and has to be re-dried, or the lumber is too dry which means unnecessary long drying times. Both effects result in increased drying costs.
The solution for the soft wood drying could be modern resistancy measurement systems as they are very accurate below fiber saturation, and give good readings even for thicker lumber (where capacitive meters still have issues). The problem in the past simply was that due to the fast drying times of softwood, setting the probes and connecting wires of the probes to the control system was a hassle and took too much time. With accurate and reliable wireless moisture meters as they are available nowadays, the probe setting and wiring can be reduced to a minimum.
The solution for the hardwood drying is more complex. Here, an exact and reliable reading of the moisture contents above fiber saturation is of the essence. Also the value of the lumber inside the kiln is very high, so chances should not be taken. However, the routine of taking samples every day could be reduced for the following reasons:
• Resistance moisture probes are now more accurate, even for moisture levels above fiber saturation (see description below).
• The operator can set the probes, take the initial samples, and recalibrate the probes accordingly if the probe readings are not in correspondence with the oven dry test. He then can reduce the amount of samples to a minimum, just to verify the probe readings.
• If the operator does not trust the probes at all, and does not want them to determine the drying schedule, he can at least set the probes, get the readings, and use them to double check the oven dry test. A modern control system will allow him to get the readings on the screen without influencing the drying schedule.
• Below fiber saturation, the probes are accurate and can be trusted, and no sample boards have to be taken anymore
The efforts of taking sample boards or using hot spots to get the correct moisture content can be reduced drastically. This occurs due to the above mentioned reasons and will raise a sawmill’s profits. In addition, the kiln operator can use his time to optimize the drying schedule and lumber handling on the yard.
The above described features and advantages can certainly only be achieved with a state of the art control system. With the new Hildebrand hardware THE FOX, and the corresponding software MERLIN, all this is now possible. The hardware components are sitting in the MCC and are controlled via touch screen. This touch screen is a fully functional control computer without any limitations.
All hardware components and touch screens can be connected wirelessly to a laptop or notebook allowing for off-site control of the kilns. It probably doesn’t happen very often that the operator wants to control the kiln during hunting season. But it is a real advantage to be able to carry the controls of the kiln with you on-site without the need to go in the control room any time you want to check the kilns. And while you are in a kiln association meeting, it may be a good idea to check the running schedule and ask the experts about a real-time problem.
Two Underestimated But
The reason why the measurement of lumber moisture content, even above fiber saturation, has recently become more accurate and reliable, is that Hildebrand also refers to two other important parameters in the lumber drying. Both of these factors are in correspondence with the moisture content of the lumber. They are the moisture gradient and the lumber temperature.
The moisture gradient is important, as it is responsible for the amount of tension in the lumber. The bigger the differential between core moisture content and surface moisture content becomes the greater the tension in the lumber gets. This leads to drying defects such as cracks and collapse. With the Hildebrand control system, the operator now gets the information about the level of moisture gradient in the lumber and places the probes in different board depths (surface, shell, core). The software takes the different readings and interferes automatically or displays it on the screen to enable the operator to adopt the schedule.
Knowing the lumber temperature is a must for the vacuum kiln drying. It is also important in conventional drying because there is a direct connection between moisture and temperature.
The law of thermodynamics says the following about the temperature behavior during lumber drying: At the beginning, the lumber temperature increases until it reaches the level of the wet bulb temperature. As soon as drying starts in the core of the board, the lumber temperature increases slowly until it gets to the dry bulb temperature by the end of the drying process. That means the less water evaporates in the core after heating up, the smaller is the difference between dry bulb temperature and lumber temperature. After reaching fiber saturation, the evaporation of water gets slower, and the temperature differential gets smaller. The change is measurable, and it can be used to correct faulty probe measurements.
The above mentioned possibilities and features are not theory. They exist and can help you decrease your drying costs per board feet by reducing your drying times and energy consumptions. These strategies can also increase your drying quality and improve lumber grades. The good news is that this approach only affects your controls, which are an easy fix to replace or retrofit. But this is not the end of the changes that are necessary to get the next generation of dry kilns. The mechanics of the kilns need to change to avoid the huge waste of energy which is still common practice these days. With future articles we will try to explain what changes are necessary and how they can be implemented.
Editor’s Note: The preceding is a paid advertorial submitted by Brunner Hildebrand.
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