Sprague, Mike

Western Dry Kiln Clubs

Western Dry Kiln Clubs. Meeting (31st : 1980)

Undetermined

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HELPING TO REDUCE ENERGY CONSUMPTIONMike SpragueLumber Systems, Inc.Portland, OregonSome of the information that I am going to give in this talkis taken from the U.S. Forest Research Note #FPL0228, published in1974. The title is "How to Reduce Energy Consumption in KilnDrying Lumber." Other publications I am using for reference arewritten by A. P. Green on surface temperature versus BTU loss and"Production Kiln Drying" by John Devine.You have all heard about the energy crisis, and I would liketo give you some suggestions on helping to reduce your energyconsumption around your dry kiln. In 1974, approximately $290.00in energy cost was required to dry a 50,000 board foot load ofgreen soft wood in a 100% efficient kiln. That was $240.00 fornatural gas and $50.00 more for 2,000 kilowatts of electricalpower for the fans. Today, that same load would cost $800.00 inenergy consumption, plus $60.00 cost for the 2,000 kilowatts ofelectrical energy. The energy consumed in this dry kiln to drythe 50,000 board feet of wood would heat seventeen to twenty homesfor one month with an average gas usage of 75 to 90 therms permonth. Following are some suggestions that I hope will help youaround your dry kiln.1. Start increasing your efficiency by air drying whenpossible. That is, when green lumber arrives and is first cut,immediately get it onto stickers, even if it is going to the kilnssoon. Store it where it can air dry, particularly during thesummer months. For each percent of moisture lost in air drying,the energy savings in subsequent kiln drying is roughly 50 to 85BTU's per board foot. For example, on a 50,000 board footcharge, for every percent of moisture lost in air drying, youcan save $17.50 in total heat cost.2. Do not use steam spray or water spray in the kiln,except during conditioning. Let the moisture coming out of theboard build up the humidity to the desired level. Steam may beused in leaky kilns, in cold weather, when very small wet bulbdepressions are required, or during the conditioning period.3. Repair and caulk all the leaks, cracks and holes in thekiln structure and doors to prevent unnecessary venting and lossof heat. Make sure doors are closed tightly, particularly at thetop, and temporarily plug any leaks around the doors with ragsor other new gaskets, shims, strips, or hangers if necessary. Indry kilns use rags and plug material to fill the holes around thebottom of the door near the track openings. Adjust and repairthe vents so that they close tightly.A. Kiln Doors: Assuming the kiln is running on anelevated schedule for hemlock to 2 x 4 dimensionwith 200° dry bulb and 170° wet bulb, air loss aroundthe doors is 700 feet per minute with 1/16 of an inchgap. The total kiln schedule is 54 hours. This54-hour schedule will lose 23,000,000 BTU's. Using47a gas-fired boiler that is seventy percent efficient,the equivalent loss is approximately $167.00 percharge for a 68' single track kiln.B. Vents: Total vents equals 14. Assume seven vents onone side open 1/8 of an inch on the same 54-hourschedule. Heat loss equals 1,350,000 BTU's at anestimated loss of $9.61. The BTU loss through thealuminum lids for all fourteen vents uninsulatedwould be approximately 610,400 BTU's during thecharge, which would be a $4.36 loss or a total lossof $13.97.C. Panel: Assume that the 68' single track dry kilnconstructed of aluminum panels, which would haveapproximately 4,860 square feet of aluminum housing,the total heat loss during the kiln charge would beabout 4,500,000 BTU's with steam supplied to thiskiln by gas blower. The gas consumption would beapproximately $31.82 loss of heat through thesewalls.D. Wood Roof: If this kiln had a wood roof, the heatloss through a 4.5-inch thick roof drying the samematerial, would be approximately 1,948,000 BTU'sduring the schedule. This converts to about $13.91for gas and pre-fab $8.90/charge 2".E. Hollow Concrete Blocks 8" Thick: If this single trackkiln was made out of masonry blocks for the sidewalls and end walls, the loss through these blocksduring the schedule would be about 16,000,934 BTU's.The cost would be approximately $120.00.1. If this masonry kiln had a wood roof, the totalloss would be approximately $134.87 in heat,plus a total of $139.23 including vents.2. If the masonry kiln had a pre-fab roof, thetotal loss would be $129.86.F. Doors are a point of major heat loss. Things toimprove are:1. Door frames are straight and not bowed.2. Door sill angles in place or missing, bent or insome way keeping the door from sealing.3. Door should close tightly around the jams. Ad-justments on the door should be made at the liftbracket and the mullion angles, which can helpthe door to seal tightly.4. Are the rail notches at the bottom of the doorexcessive? Generally the notches are much largerthan they should be and can be improved bycrowding around the rail or fastening the smallernotch conforming with the rail shape to thebottom of the door or lift-out rail. I have seensome doors on masonry kilns where the plywoodpanels have been eaten out as large as two footholes, so the air from the kiln is going rightout the hole in the door. I have also seen wherethe plywood panel has rotted out and they havereplaced it with steel. They have stopped the48air from leaving the kiln, but now they heat thesteel up and there is no insulating value.4. Brick and Block Kilns: Coat the inside of the kiln witha vapor resistant coating. This will prevent the walls and theroofs from absorbing water because dry walls conduct less heat tothe outside.5. In many kilns, more heat is lost through the roof thanthrough the walls. Much of this loss is due to wet insulation.To reduce the heat loss, consider installing a new roof or repairthe old one. Add additional insulation if necessary, making surethe interior vapor barrier or coating is intact. In order for anyinsulation to be effective, it must be reasonably dry. Roofs arealways a problem area, even on prefab aluminum roofs. Roofextrusions can become loose and allow the water to leak into thepanel. Rain or snow on the roof can cause greater heat lossthrough the panel because of the larger temperature differencefrom the inside of the panel to the outside. The vapor thencondenses out in the panel and runs down the inside on the insideskin. On flat roofs this can cause a problem. Water puddles upon the panel and remains in pockets created by the weight of thewater.6. Install or repair baffles to obtain uniform air flow inthe kilns to prevent waste by short circuiting air travel. Thispays off in energy savings. Reverse your fans approximately everysix hours if possible. Tighten up the tip clearances on your fansto improve the efficiency (split roller bearings and high per-formance fan).7. Research at the Western Forest Products Laboratory inVancouver, B.C. has shown that in the early stages of drying athigh air velocity (more than 600 feet per minute) can acceleratedrying. In the later stages, low velocity (250 feet per minute)is as effective as high velocity and uses less energy. Therefore,arrange to adjust fan speed if possible during the run. The laterstages of drying, when high velocity is not efficient, is general-ly defined as below the fiber saturation point or approximately25 percent moisture content. The two-speed fan system is moreimportant on schedules which require many hours drying from 25percent moisture content down to the final; for example, schedulesfor glue lam stock which has to be dried well below the 15 per-cent moisture content.8. Have the recorder controller calibrated for efficientoperation. The kiln should not oscillate between periods of anyventing and spraying and should not spray and vent at the sametime.9. Check the remainder of the equipment such as traps,valves, coils. Make sure that the traps are in good workingorder and are not blowing steam. Make sure the valves are seatingtightly. Check all steam lines and make sure there are no leaks.If you find a leak, repair it. Make sure that you have adequatewater to the wet bulb.10. Accurately determine moisture content of the wood youare drying. Do not waste energy by over-drying your material orby taking too long because the samples were not representative ofthe bulk. Try to plan your loads so that when they are49sufficiently dry, someone will be able to shut them off and unloadthe kiln if possible.11. When unloading and loading the kiln, do it as fast aspossible to save the loss of heat retained in the building.12. During non-use periods, close all valves and doorstightly; use small amounts of steam, if necessary, to keep thekiln from freezing up.13. Use accelerated schedules when possible. Note that thehigher the temperature for drying, the more efficiently energy isused. The drying efficiency at higher temperatures can beillustrated by the fact that 1730 BTU's per pound of water evapo-rated requires 640 cubic feet of air to be vented for each poundof water removed at 140 dry bulb temperature with a 40 degreedepression. With 240 degrees dry bulb temperature and 40 degreedepression, approximately 1,090 BTU's are required per pound ofwater evaporated and 36 cubic feet of air are vented for each onepound of water removed. Another example is at a 10 degreedepression. With 110 degree dry bulb, the grains of vapor perpound of dry air are 280, the BTU's per pound of dry air is 69,and the cubic feet of air vented per pound of water removed is489. Now look at 190 degrees dry bulb temperature and 180 degreeswet bulb. The grains of vapor per pound of dry air is 4,515, theBTU's per pound of dry air is 695, and the cubic feet of airvented per pound of water removed is 51.In conclusion, as you see by running your kilns, if possible,at higher temperatures, you will vent more moisture per pound ofair, which will save energy.50

Helping to reduce energy consumption

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Helping to reduce energy consumption

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