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Understanding Vacuum Drying Technologies for Commercial Lumber Figure 3. Vacuum kiln drying schedule (VacuPress EASYDry Software 2019, Vacuum Pressing Systems, Brunswick Maine, USA). The upper plots show the temperature in Fahrenheit degrees inside the kiln as a function of hours of operation, and the lower plot shows the pressure in inches of mercury. At 57 hours, there was a spike in temperature, which resulted in an increase in pressure. The slight drop in temperature between water (red) and lumber core (violet) represents the energy used to remove water from the lumber (T°C = (T°F – 32)/1.8; 1 in. of mercury = 3.39 kPa). steam kiln in 12 days (288 hours) (Lyon and others 2021). This advantage is even more pronounced with thick stock, such as large table slabs or mantle pieces. A vacuum kiln can also be more energy efficient compared with conventional steam technology. This is partly due to the time efficiency previously described, but vacuum kilns can have significantly lower daily electrical use. In the hard maple study previously mentioned (Lyon and others 2021), estimated average daily electrical use for the vacuum kiln was 166 kWh (597.6 MJ) per day and the estimated average daily electrical use for the conventional steam kiln was 179 kWh (644.4 MJ) per day. At the current rate of $0.11 per kWh, the vacuum kiln dried its charge for approximately $44 and the conventional steam kiln dried its charge for approximately $236. Both the vacuum kiln and conventional steam kiln were powered entirely with electricity, with an electric boiler providing the steam. In most cases, biomass or natural gas is used to generate steam for conventional kilns. Vacuum kilns use lower drying temperatures, which saves energy and may allow wood to retain more of its original color. The recent study by Lyon and others (2021) found that there was no visual difference in wood color between vacuum drying and conventional steam drying; vacuum drying can produce industry-acceptable white hard maple that is comparable with that produced using a conventional kiln schedule. However, the recent use of low temperature schedules and higher fan speeds in conventional steam kilns can achieve superior color retention. Another advantage of lower drying temperatures is that wood strength properties are higher because the degrade caused by high temperature drying is avoided. Lower drying temperatures also provide an advantage by reducing drying stresses (Espinoza and Bond 2016). However, the lack of stress reversal systems on commercially available vacuum kilns makes stress removal difficult when it does occur. In vacuum kilns, organic compounds are extracted and condensed along with the water condensate and need to be treated before being discharged into the water supply (Espinoza and Bond 2016). Vacuum kilns might allow the operator to mix some wood species with similar specific gravities and moisture contents but of varying thicknesses in the same kiln charge. Lastly, because the kilns use hot platens, the dried lumber remains very flat. For example, the Vacupress system (Vacuum Pressing Systems, Inc., Brunswick, Maine, USA) has a flexible rubber membrane that allows atmospheric pressure to push down on the charge of lumber, creating up to 13 lb/in2 (8.96 MPa) of pressure. This results in a higher yield when resawing into components. Challenges of Vacuum Drying Vacuum kilns are fundamentally different than conventional steam kilns. Therefore, wood manufacturers need to rethink how these changes will impact product flow within a business. Loading and unloading, cycle times, and lumber volume must all be considered. First, vacuum kilns are loaded and unloaded differently than conventional kilns. In most systems, stickers are not used and the heating system, heating platens, must be incorporated into the lumber unit, as shown in Figure 4. For smaller vacuum kilns, the lumber and platens must be loaded and unloaded by hand from a forklift, as shown in Figure 5. For these kilns, lumber layers alternate with 3PDF Image | Understanding Vacuum Drying Tech for Commercial Lumber
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