1、26.1CHAPTER 26AIR CONDITIONING OF WOOD AND PAPER PRODUCT FACILITIESGeneral Wood Product Operations 26.1Pulp and Paper Operations 26.2HIS chapter covers some of the standard requirements for airTconditioning of facilities that manufacture finished wood prod-ucts as well as for pulp and paper product
2、process operations.Special Warning: Certain industrial spaces may contain flam-mable, combustible, and/or toxic concentrations of vapors or dustsunder either normal or abnormal conditions. In spaces such as these,there are life-safety issues that this chapter may not completelyaddress. Special preca
3、utions must be taken in accordance withrequirements of recognized authorities such as the National Fire Pro-tection Association (NFPA), the Occupational Safety and HealthAdministration (OSHA), and the American National StandardsInstitute (ANSI). In all situations, engineers, designers, and install-e
4、rs who encounter conflicting codes and standards must defer to thecode or standard that best addresses and safeguards life safety.1. GENERAL WOOD PRODUCT OPERATIONSIn wood product manufacturing facilities, ventilation can be con-sidered a part of the process. Metal ductwork should be used andgrounde
5、d to prevent a buildup of static electricity. Hoods should bemade of spark-free, noncombustible material. A pneumatic conveyingsystem should be furnished to reduce the accumulation of wood dustin the collecting duct system. The airflow rate and velocity should beable to maintain the air-dust mixture
6、 below the minimum explosiveconcentration level. If dampers are unavoidable in the system, theyshould be firmly fastened after balancing work. Dust collectors shouldbe located outside the building. Fans or blowers should be placeddownstream of the dust collector and air-cleaning equipment, andshould
7、 be interlocked with the wood-processing equipment. When thefan or blower stops, the wood process should stop immediately andforward a signal to the alarm system.Deflagration venting and suppression should be furnished forwood-processing workshops and wood-processing equipment suchas vessels, reacto
8、rs, mixers, blenders, mills, dryers, ovens, filters,dust collectors, storage equipment, material-handling equipment,and aerosol areas. The deflagration suppression system must be dis-armed before performing any maintenance work to avoid possibleinjury from discharging the suppressant. Warning signs
9、should bedisplayed prominently at all maintenance access points.Finished lumber products to be used in heated buildings shouldbe stored in areas that are heated 10 to 20F above ambient. Thisprovides sufficient protection for furniture stock, interior trim, cab-inet material, and stock for products s
10、uch as ax handles and glue-laminated beams. Air should be circulated within the storage areas.Lumber that is kiln-dried to a moisture content of 12% or less can bekept within a given moisture content range through storage in aheated shed. The moisture content can be regulated either manuallyor autom
11、atically by altering the dry-bulb temperature (Figure 1). Some special materials require close control of moisture content.For example, musical instrument stock must be dried to a givenmoisture level and maintained there because the moisture content ofThe preparation of this chapter is assigned to T
12、C 9.2, Industrial AirConditioning.Fig. 1 Relationship of Temperature, Relative Humidity, and Vapor Pressure of Air and Equilibrium Moisture Content of Wood26.2 2015 ASHRAE HandbookHVAC Applicationsthe wood affects the harmonics of most stringed wooden instru-ments. This control may require air condi
13、tioning, heating, and/orhumidification, with or without reheating.Process Area Air ConditioningTemperature and humidity requirements in wood product pro-cess areas vary according to product, manufacturer, and governingcode. For example, in match manufacturing, the match head must becured (i.e., drie
14、d) after dipping. This requires careful control ofhumidity and temperature to avoid a temperature near the ignitionpoint. Any process involving application of flammable substancesshould follow the ventilation recommendations of the National FireProtection Association, the National Fire Code, and the
15、 U.S. Occu-pational Safety and Health Act.Finished Product StorageFinished lumber to be made into furniture, equipment parts,musical instruments, architectural woodwork, or other wood prod-ucts of value is stored and/or manufactured under controlled tem-perature and humidity to maintain proper wood
16、dryness. Improperdrying can cause laminated or glued joints to fail. Finished woodthat has changed dimension because of excess moisture gain or losscan cause fitting problems. Cracking, splitting, checking, warping,and discoloring are other problems with improperly dried and/orstored wood.Green, rou
17、gh, cut lumber is stacked end to end in layers, eachlayer being separated by wood strips to allow air circulation. Lum-ber can be stacked and left to dry naturally in open-sided sheds.Enclosed, heated kilns with steam coils and/or direct steam injec-tion, forced air circulation, makeup air, and exha
18、ust air vents couldbe used where faster, controlled drying is preferred. Drying (or addi-tion of moisture) can be accomplished by HVAC systems usingdehumidifying coils and/or desiccants, heating/reheat coils, humid-ifiers, makeup and exhaust air, distribution air ducts, and automaticcontrols. An ins
19、ulated dehumidifying/humidifying room could beconstructed and finished to minimize moisture migration fromhigher-humidity areas. Lumber can also be dried by solar kilns,microwaves, dielectric heating, superheated steam, and vacuum.Wood is composed of natural fibers and its moisture content var-ies a
20、ccording to its environment. Samples from the wood beingdried must be tested for moisture content at predetermined timeintervals to prevent overdrying and defects. Drying rates are deter-mined by the wood species. Final moisture content depends uponthe woods ultimate use.The formula for determining
21、moisture content isLumber/wood drying using HVAC systems can be accomplishedwith factory- or field-assembled systems. The quantity of lumber/wood to be dried, wood species, rate of drying, total moistureremoval, drying room construction, economics (cost and rate ofreturn on investment), fire and saf
22、ety codes, maintenance, and easeof use influence the type of HVAC system to be installed.2. PULP AND PAPER OPERATIONSThe papermaking process comprises two basic steps: (1) wood isreduced to pulp (i.e., wood fibers), and (2) the pulp is converted topaper. Wood can be pulped by either mechanical actio
23、n (e.g., grind-ing in a groundwood mill), chemical action (e.g., kraft pulping), ora combination of both.Many different types of paper can be produced from pulp, rang-ing from the finest glossy finish to newsprint to bleached board tofluff pulp for disposable diapers. To make newsprint, a mixture of
24、mechanical and chemical pulps is fed into the paper machine. Tomake kraft paper (e.g., grocery bags, corrugated containers), how-ever, only unbleached chemical pulp is used. Disposable diapermaterial and photographic paper require bleached chemical pulpwith a very low moisture content of 6 to 9%.Pap
25、er Machine AreaIn papermaking, extensive air systems are required to supportand enhance the process (e.g., by preventing condensation) and toprovide reasonable comfort for operating personnel. Radiant heatfrom steam and hot-water sources and mechanical energy dissipatedas heat can result in summer t
26、emperatures in the machine room ashigh as 120F. In addition, high paper machine operating speeds of2000 to 4500 fpm and a stock temperature near 122F producewarm vapor in the machine room.Outdoor air makeup units and process exhausts absorb andremove room heat and water vapor released from the paper
27、 as it isdried (Figure 2). Makeup air is distributed to working areas aboveand below the operating floor. Part of the air delivered to the base-ment migrates to the operating floor through hatches and stairwells.Motor-cooling equipment distributes cooler basement air to thepaper machine drive motors
28、. Wet and basement exhaust should be installed inside the room.Outdoor air intakes with insulated adjustable louvers should beinstalled on the outside wall to supplement the mechanical air sup-ply. In facilities with no basement exterior wall, sufficient mechan-ical air intake should be provided. Th
29、e exhaust, adjustable louver, ormechanical air intake should be furnished with modulating control.When the ambient temperature drops to near freezing, outdoor air-flow must be reduced to a minimum and the appropriate heaterstarted to prevent freezing.The most severe ventilation demand occurs in the
30、area betweenthe wet-end forming section and press section and the dryer section.In the forming section, the pulp slurry, which contains about 90%water, is deposited on a traveling screen. Gravity, rolls, foils, vac-uum, steam boxes, and three or more press roll nips are sequentiallyused to remove up
31、 to 50% of the water in the forming section andpress section. The wet end is very humid because of evaporation ofmoisture and mechanical generation of vapor by turning rolls andcleaning showers. Baffles and a custom-designed exhaust in theforming section help control the vapor. A drive-side exhaust
32、in thewet end removes heat from the motor vent air and removes the pro-cess generated vapor.To prevent condensation or accumulated fiber from falling on thetraveling web, a false ceiling is used with ducts connected to roofWeight of samplewhen cutWeight of oven-dried sample 100Weight of oven-dried s
33、ample-Moisturecontent, %=Fig. 2 Paper Machine AreaAir Conditioning of Wood and Paper Product Facilities 26.3exhausters that remove humid air not captured at a lower point. Atthe wet end, heated inside air is usually circulated to scrub theunderside of the roof to prevent condensation in cold weather
34、. Addi-tional roof exhaust may also remove accumulated heat from thedryer section and the dry end during warmer periods. Ventilation inthe wet end should be predominantly by roof exhaust.The large volume of moisture and vapor generated from thewet-end process rises and accumulates under the roof. To
35、 keepcondensation from forming in winter, the roof is normallyexhausted and hot air is distributed under the roof. Sufficient roofinsulation should be installed to keep the inside surface tempera-ture above the dew point. Heat transfer from the room to the inte-rior surface is(1)wheretr= room air te
36、mperature, Ftis= roof interior surface temperature, Fto= outdoor air temperature, FRris= heat transfer resistance from room air to roof interior surface. In winter, Rris= 0.61 ft2Fh/BtuRiso= required total R-value from roof interior surface to outdoor air, ft2Fh/BtuFor a given project, toand trhave
37、been determined and only tisneeds to be selected. For wet-end roof insulation and assuming96% relative humidity, tiscan be shown on a psychrometric chartto betis= tr 1.2F (2)Then Equation (1) can be simplified to find the required roofR-value asRiso = (tr to 1.2) (3)In the dryer section, the paper w
38、eb is dried as it travels in a ser-pentine path around rotating steam-heated drums. Exhaust hoodsremove heat from the dryers and moisture evaporated from thepaper web. Most modern machines have enclosed hoods, whichreduce the airflow required to less than 50% of that required for anopen-hood exhaust
39、. The temperature inside an enclosed hoodranges from 130 to 140F at the operating floor to 180 to 200F inthe hood exhaust plenum at 70 to 90% rh, with an exhaust rate gen-erally ranging from 300,000 to 400,000 cfm per machine.Where possible, pocket ventilation air (see Figure 3) and hoodsupply air a
40、re drawn from the upper level of the machine room totake advantage of the preheating of makeup air by process heat as itrises. The basement of the dryer section is also enclosed to controlinfiltration of machine room air to the enclosed hood. The hoodsupply and the pocket ventilation air typically o
41、perate at 200F;however, some systems run as high as 250F. Enclosed hood ex-haust is typically 300 cfm per ton of machine capacity. The pocketventilation and hood supply are designed for 75 to 80% of the ex-haust, with the balance infiltrated from the basement and machineroom. Large volumes of air (5
42、00,000 to 800,000 cfm) are requiredto balance the paper machines exhaust with the building air bal-ance.The potential for heat recovery from hood exhaust air should beevaluated. Most of the energy in steam supplied to the paper dryersis converted to latent heat in the hood exhaust as water evaporate
43、sfrom the paper web. Air-to-air heat exchangers are used where theair supply is located close to the exhaust. Air-to-liquid heatexchangers that recirculate water/glycol to heat remote makeup airunits can also be used. Air-to-liquid systems provide more latentheat recovery, resulting in three to four
44、 times more total heat recov-ery than air-to-air units. Some machines use heat recovered from theexhaust air to heat process water. Ventilation in paper machinebuildings in the United States ranges from 10 to 25 air changes perhour in northern mills to 20 to 50 in southern mills. In some plants,comp
45、uters monitor the production rate and outdoor air temperatureto optimize operation and conserve energy.After fine, bond, and cut papers have been bundled and/or pack-aged, they should be wrapped in a nonpermeable material. Mostpapers are produced with less than 10% moisture by weight, theaverage bei
46、ng 7%. Dry paper and pulp are hygroscopic and begin toswell noticeably and deform permanently when the relative humid-ity exceeds 38%. Therefore, finished products should be storedunder controlled conditions to maintain their uniform moisture con-tent.Finishing AreaTo produce a precisely cut paper t
47、hat stabilizes at a desirableequilibrium moisture content, the finishing areas require tempera-ture and humidity control. Further converting operations such asprinting and die cutting require optimum sheet moisture content forefficient processing. Finishing room conditions range from 70 to75F db and
48、 from 40 to 45% rh. Rooms should be maintained withinreasonably close limits of the selected conditions. Without preciseenvironmental control, the paper equilibrium moisture content var-ies, influencing dimensional stability, the tendency to curl, and fur-ther processing.Process and Motor Control Ro
49、omsIn most pulp and paper applications, process control, motor con-trol, and switchgear rooms are separate from the process environ-ment. Air conditioning removes heat generated by equipment, lights,etc., and reduces the air-cleaning requirement. (See Chapter 19 forair conditioning in control rooms that include a computer, a com-puter terminal, or data processing equipment.) Ceiling grilles or dif-fusers should be located above access aisle
