1、20.1CHAPTER 20PRINTING PLANTSDesign Criteria 20.1Control of Paper Moisture Content 20.2Platemaking. 20.2Relief Printing . 20.2Lithography . 20.3Rotogravure. 20.4Other Plant Functions. 20.4HIS chapter outlines air-conditioning requirements for keyTprinting operations. Air conditioning of printing pla
2、nts can pro-vide controlled, uniform air moisture content and temperature inworking spaces. Paper, the principal material used in printing, ishygroscopic and very sensitive to variations in the humidity of thesurrounding air. Printing problems caused by paper expansion andcontraction can be avoided
3、by controlling the moisture contentthroughout the manufacture and printing of the paper.1. DESIGN CRITERIAThe following are three basic printing methods: Relief printing (letterpress). Ink is applied to a raised surface.Lithography. Inked surface is neither in relief nor recessed.Gravure (intaglio p
4、rinting). Inked areas are recessed below thesurface.Figure 1 shows the general work flow through a printing plant.The operation begins at the publisher and ends with the finishedprinted product and paper waste. Paper waste, which may be asmuch as 20% of the total paper used, affects profitability. P
5、roper airconditioning can help reduce the amount of paper wasted.In sheetfed printing, individual sheets are fed through a pressfrom a stack or load of sheets and collected after printing. In webfedrotary printing, a continuous web of paper is fed through the pressfrom a roll. The printed material i
6、s cut, folded, and delivered fromthe press as signatures, which form the sections of a book.Sheetfed printing is a slow process in which the ink is essentiallydry as the sheets are delivered from the press. Offsetting, the trans-ference of an image from one sheet to another, is prevented by apply-in
7、g a powder or starch to separate each sheet as it is delivered from thepress. Starches present a housekeeping problem: the particles (30 to40 m in size) tend to fly off, eventually settling on any horizontalsurface.If both temperature and relative humidity are maintained withinnormal human comfort l
8、imits, they have little to do with web breaksor the runnability of paper in a webfed press. At extremely lowhumidity, static electricity causes the paper to cling to the rollers,creating undue stress on the web, particularly with high-speedpresses. Static electricity is also a hazard when flammable
9、solventinks are used.Special ConsiderationsSpecial 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 complete
10、lyaddress. Special precautions 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,
11、designers, and install-ers who encounter conflicting codes and standards must defer to thecode or standard that best addresses and safeguards life safety.Various areas in printing plants require special attention to pro-cessing and heat loads. Engraving and platemaking departments musthave very clea
12、n air: not as clean as that for industrial cleanrooms, butcleaner than that for offices. Engraving and photographic areas mayalso have special ventilation needs because of the chemicals used.Nitric acid fumes from powderless etching require careful duct mate-rial selection. Composing rooms, which co
13、ntain computer equip-ment, can be treated the same as similar office areas. The excessivedust from cutting in the stitching and binding operations must be con-trolled. Stereotype departments have very high heat loads.In pressrooms, air distribution must not cause the web to flutter orforce contamina
14、nts or heat (which normally would be removed byroof vents) down to the occupied level. Air should be introducedimmediately above the occupied zone wherever possible to mini-mize total flow and encourage stratification. High air exchange ratesmay be required where solvent- or oil-based inks are used,
15、 becauseof the large quantity of organic solvent vapors that may be releasedfrom nonpoint sources. Exhaust emissions from dryer systems maycontain substantial concentrations of solvent vapors, which must becaptured and recovered or incinerated to satisfy local air pollutionrequirements. Where these
16、measures are required, efforts should beThe preparation of this chapter is assigned to TC 9.2, Industrial AirConditioning.Fig. 1 Work Flow Through a Printing Plant20.2 2015 ASHRAE HandbookHVAC Applications (SI)made to maximize point-source capture of vapors to minimize thesize, cost, and energy requ
17、irements for vapor recovery/incinerationequipment. These efforts also minimize the effect of these require-ments on general ventilation systems.Conventional air-conditioning and air-handling equipment, par-ticularly rooftop equipment, may be unable to handle the high out-door air requirements of pre
18、ssroom applications effectively.Stratified ventilation may be used in high-bay installations to reducetotal system airflow and air-conditioning requirements. Pressroomsusing oil- or solvent-based inks should be provided with a minimumof 2.5 L/(sm2) of outdoor air to ensure adequate dilution of inter
19、-nally generated volatile organic compounds. Ventilation of storageareas should be about 0.5 air changes per hour (ach); bindery venti-lation should be about 1 ach. Storage areas with materials piled highmay need roof-mounted smoke- and heat-venting devices.In a bindery, loads of loose signatures ar
20、e stacked near equip-ment, which makes it difficult to supply air to occupants withoutscattering the signatures. One solution is to run the main ducts at theceiling with many supply branches dropped to within 2.5 to 3 m ofthe floor. Conventional adjustable blow diffusers, often the lineartype, are u
21、sed.2. CONTROL OF PAPER MOISTURE CONTENTControlling the moisture content and temperature of paper isimportant in all printing, particularly multicolor lithography. Papershould be received at the printing plant in moisture-proof wrappers,which are not broken or removed until the paper is brought to t
22、hepressroom temperature. When exposed to room temperature, paperat temperatures substantially below the room temperature rapidlyabsorbs moisture from the air, causing distortion. Figure 2 shows thetime required to temperature-condition wrapped paper. Printers usu-ally order paper with a moisture con
23、tent approximately in equilib-rium with the relative humidity maintained in their pressrooms.Papermakers find it difficult to supply paper in equilibrium with arelative humidity higher than 50%.Digital hygrometers can be used to check the hygroscopic con-dition of paper relative to the surrounding a
24、ir. The probes contain amoisture-sensitive element that measures the electrical conductivityof the paper. Intact mill wrappings and the tightness of the rollnormally protect a paper roll for about six months. If the wrapper isdamaged, moisture usually penetrates no more than 3 mm.3. PLATEMAKINGHumid
25、ity and temperature control are important considerationswhen making lithographic and collotype plates, photoengravings,and gravure plates and cylinders. If the moisture content andtemperature of the plates increase, the coatings increase in light sen-sitivity, which necessitates adjustments in the l
26、ight intensity or thelength of exposure to give uniformity.If platemaking rooms are maintained at constant dry-bulb tem-perature and relative humidity, plates can be produced at knowncontrol conditions. As soon as it is dry, a bichromated colloid coat-ing starts to age and harden at a rate that vari
27、es with the atmo-spheric conditions, so exposures made a few hours apart may bequite different. The rate of aging and hardening can be estimatedmore accurately when the space is air conditioned. Exposure canthen be reduced progressively to maintain uniformity. An opti-mum relative humidity of 45% or
28、 less substantially increases theuseful life of bichromated colloid coatings; the relative humiditycontrol should be within 2%. A dry-bulb temperature of 24 to 27Cmaintained within 1 K is good practice. The ventilation airrequirements of the plate room should be investigated. A plantwith a large pro
29、duction of deep-etch plates should consider locat-ing this operation outside the conditioned area.Exhausts for platemaking operations consist primarily of lateralor downdraft systems at each operation. Because of their bulkinessor mass, plates or cylinders are generally conveyed by overhead railto t
30、he workstation, where they are lowered into the tank for plating,etching, or grinding. Exhaust ducts must be below or to one side ofthe working area, so lateral exhausts are generally used for open-surface tanks.Exhaust quantities vary, depending on the nature of the solutionand shape of the tank, b
31、ut they should provide exhaust in accordancewith the recommendations of Industrial Ventilation by the Ameri-can Conference of Governmental Industrial Hygienists (ACGIH2010) for a minimum control velocity of 0.25 m/s at the side of thetank opposite the exhaust intake. Tanks should be covered to mini-
32、mize exhaust air quantities and increase efficiency. Excessive airturbulence above open tanks should be avoided. Because of thenature of the exhaust, ducts should be acid-resistant and liquidtightto prevent moisture condensation.Webfed offset operations and related departments are similar towebfed l
33、etterpress operations, without the heat loads created in thecomposing room and stereotype departments. Special attentionshould be given to air cleanliness and ventilation in platemaking toavoid flaws in the plates caused by chemical fumes and dust.A rotogravure plant can be hazardous because highly
34、volatilesolvents are used. Equipment must be explosionproof, and air-handling equipment must be sparkproof. Clean air must be suppliedat controlled temperature and relative humidity.Reclamation or destruction systems are used to prevent photo-sensitive hydrocarbons from being exhausted into the atmo
35、sphere.Some reclamation systems use activated carbon for continuous pro-cessing. Incineration or catalytic converters may be used to producerapid oxidation to eliminate pollutants. The amount of solventsreclaimed may exceed that added to the ink.4. RELIEF PRINTINGIn relief printing (letterpress), ro
36、llers apply ink only to the raisedsurface of a printing plate. Pressure is then applied to transfer the inkFig. 2 Temperature-Conditioning Chart for PaperPrinting Plants 20.3from the raised surface directly to the paper. Only the raised surfacetouches the paper to transfer the desired image.Air cond
37、itioning in newspaper pressrooms and other webfed let-terpress printing areas minimizes problems caused by static elec-tricity, ink mist, and expansion or contraction of the paper duringprinting. A wide range of operating conditions is satisfactory. Thetemperature should be selected for operator com
38、fort.At web speeds of 5 to 10 m/s, it is not necessary to control the rel-ative humidity because inks are dried with heat. In some types ofprinting, moisture is applied to the web, and the web is passed overchill rolls to further set the ink.Webfed letterpress ink is heat-set, made with high-boiling
39、, slow-evaporating synthetic resins and petroleum oils dissolved or dis-persed in a hydrocarbon solvent. The solvent must have a narrowboiling range with a low volatility at room temperatures and a fastevaporating rate at elevated temperatures. The solvent is vaporizedin the printing press dryers at
40、 temperatures from 120 to 200C, leav-ing the resins and oils on the paper. Webfed letterpress inks are driedafter all colors are applied to the web.The inks are dried by passing the web through dryers at speeds of5 to 10 m/s. There are several types of dryers: open-flame gas cup,flame impingement, h
41、igh-velocity hot air, and steam drum.Exhaust quantities through a press dryer vary from about 3300 to7000 L/s at standard conditions, depending on the type of dryer usedand the speed of the press. Exhaust temperatures range from 120 to200C.Solvent-containing exhaust is heated to 700C in an air pollu
42、tioncontrol device to incinerate the effluent. A catalyst can be used to re-duce the temperature required for combustion to 540C, but it re-quires periodic inspection and rejuvenation. Heat recovery reducesthe fuel required for incineration and can be used to heat pressroommakeup air.5. LITHOGRAPHYL
43、ithography uses a grease-treated printing image receptive toink, on a surface that is neither raised nor depressed. Both greaseand ink repel water. Water is applied to all areas of the plate, exceptthe printing image. Ink is then applied only to the printing image andtransferred to the paper in the
44、printing process. In multicolor print-ing operations, the image may be printed up to four times on thesame sheet of paper in different colors. Registration of images iscritical to final color quality.Offset printing transfers the image first to a rubber blanket andthen to the paper. Sheetfed and web
45、 offset printing are similar to let-terpress printing. The inks used are similar to those used in letter-press printing but contain water-resistant vehicles and pigment. Inweb offset and gravure printing, the relative humidity in the press-room should be kept constant, and the temperature should bes
46、elected for comfort or, at least, to avoid heat stress. It is importantto maintain steady conditions to ensure the dimensional stability ofthe paper onto which the images are printed.The pressroom for sheet multicolor offset printing has moreexacting humidity requirements than other printing process
47、es. Thepaper must remain flat with constant dimensions during multicolorprinting, in which the paper may make six or more passes throughthe press over a period of a week or more. If the paper does not havethe right moisture content at the start, or if there are significantchanges in atmospheric humi
48、dity during the process, the paper willnot retain its dimensions and flatness, and misregistering will result.In many cases of color printing, a register accuracy of 100 m isrequired. Figure 3 shows the close control of the air relative humid-ity that is necessary to achieve this register accuracy.
49、The datashown in this figure are for composite lithographic paper.Maintaining constant moisture content of the paper is compli-cated because paper picks up moisture from the moist offset blanketduring printing (0.1 to 0.3% for each impression). When two ormore printings are made in close register work, the paper at the startof the printing process should have a moisture content in equilib-rium with air at 5 to 8% rh above the pressroom air. At this condi-tion, the moisture evaporated from the paper into the air nearlybalances the moisture added by the press. In obtaining register, i
