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… for the drying of the wood, that make up about 80% of the total drying costs. With the new Mühlböck drying system Type 1003, which is already in operation at several sawmills, these costs are reduced by about half. The innovation here: the energy that is readily available in the exhaust air of a drying kiln is made available in its entirety for the drying in a different kiln.
Who doesn’t know them, the plumes of steam above the drying installations of all wood-processing companies? Plumes of steam above conventional drying kilns and sawdust dryers signal a high demand in heating capacity. The heating capacity brought into the drying kilns is converted directly into steam. This thermal energy is otherwise generated through the burning of bark, wood chips, shavings or biomass fuel in boilers. The expenses here are substantial.
Depending on the amount of sapwood and heartwood, the amount of water that needs to be dried out of one cubic meter of lumber, lies between 200 and 400 liters. For the drying of shavings from W50 to W10 the amount of water that needs to be dried out, is exactly 800 liter per ton (at W10). In conventional drying systems, depending on the time of year, 1 to 1.5 kW/hour is needed per liter of water. This means that per cubic meter of lumber, between 200 and 600 kW/hours of energy needs to be supplied. For the drying of shavings even up to 1200 kw/hours per ton are necessary. If one calculates 0.03 Euro for every kW/hour of thermal energy, the costs of thermal energy for lumber result in up to 18 Euro per cubic meter of lumber, and up to 36 Euro per ton of shavings.
Mühlböck Holztrocknungsanlagen GmbH makes it possible, to reduce these costs by about 50% and this with an amortization period of six months to a maximum of three years. Through the application of Mühlböck’s new drying system Type 1003, either the use of burning fuel can be cut in half, or the amount of drying material can be doubled, at the same rate of fuel usage and with the same boiler.
Difference with conventional drying kilns
Which is now the difference between a conventional drying kiln and a Mühlböck 1003 drying kiln? With conventional kilns the wood usually is heated up to about 70 degrees Celsius. The wood releases moisture into the passing air. This is then taken out of the amount of water that has been expelled from the wood. Mostly this is done through the exchange of the moist, hot air from the drying kiln with cold outside air. The moist hot exhaust air transports water and heat out of the drying kiln. The for the drying process needed new but cold air, has to be heated up through heating coils, thus becoming dry and ready to absorb moisture again. The heat in the drying air is reduced by the evaporation of the water. This is detectable by the difference in temperature of the air at the entrance and exit of the stack. At a temperature of 70 degrees Celsius at the entrance of the stack, a temperature of 60 degrees Celsius at the exit is a normal value. So reheating of this circulating air also needs to take place. In most installations the delivery of heat is done through hot water from the boiler, which is fueled with waste wood. If in addition a heat recovery system is implemented, then the needed heating capacity can be reduced. The possibilities however are modest, since the dry outside air can only absorb little energy from the moist hot drying kiln air. Mostly the fresh air is already at the same temperature as the exhaust air, and the exhaust air cooled down only a few degrees.
Mühlböck 1003 drying kilns
Mühlböck drying kilns of the Type 1003 are a closed system, in which the hot air always stays inside the drying kiln. Only in one area of the kiln exhaust air is expelled into the environment and is cooled down to almost the temperature of the outside air. Thus the applied heat is used optimally. These drying kilns are not heat recovery machines in the true sense. They are equipped with heat recovery, but that is only part of the system. The installations basically are made up of 3 parts: the pre-dryer, post-dryer and the heat recovery. They are built up in a group of at least 4 kilns. Every kiln is constructed in such a way, that they can operate as pre-dryer or post-dryer. With 4 drying kilns in one group, one kiln will function as pre-dryer and 2 or 3 kilns as post-dryers. All kilns are tied together through three different channel systems as well as the mutual heat recovery. This way the thermal energy that is implemented in the post-dryers, can get back to the pre-dryer and is used there a second time.
Mühlböck 1003 drying kilns are also used as continuous kilns. In continuous kilns the lumber is dried without interruptions. The slatted stacks of lumber are placed on trolleys in a loading zone. The trolleys are moved forward automatically at fixed time intervals. The trolleys move further in a front buffer zone and from there into the pre-dryer, heated only by the heat recovery. Following are two post-drying zones, which are heated. These dryers are also offered with a similar system as sawdust dryer. All Mühlböck 1003 drying systems have in common the approximately 50% lower thermal energy demand, the slightly smaller savings in electrical energy and the short amortization period.
‘With our newly developed Beltdryer Type 1003 and our wood chips dryer, we now offer the logical addition to our current range of lumber drying kilns’ explains Managing Director Kurt Mühlböck. ‘The gathered drying experience and knowledge out of over 12,000 lumber drying kilns has flowed into the development of these products’ according to Mühlböck.
*Internationaler Holzmarkt, 4/2016 – www.holzmarkt-online-at