ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 22-2012 HUMIDIFIERS.pdf

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1、22.1CHAPTER 22HUMIDIFIERSEnvironmental Conditions . 22.1Enclosure Characteristics. 22.2Energy Considerations 22.3Equipment . 22.5Controls. 22.9N the selection and application of humidifiers, the designer con-I siders (1) the environmental conditions of the occupancy orprocess and (2) the characteris

2、tics of the building enclosure.Because these may not always be compatible, compromise is some-times necessary, particularly in the case of existing buildings.ENVIRONMENTAL CONDITIONSA particular occupancy or process may dictate a specific relativehumidity, a required range of relative humidity, or c

3、ertain limitingmaximum or minimum values. The following classifications ex-plain the effects of relative humidity and provide guidance on therequirements for most applications.Human ComfortThe complete effect of relative humidity on all aspects of humancomfort has not yet been established. For therm

4、al comfort, highertemperature is generally considered necessary to offset decreasedrelative humidity (see ASHRAE Standard 55).Low relative humidity increases evaporation from the membranesof the nose and throat, drying the mucous membranes in the respira-tory system; it also dries the skin and hair.

5、 The increased incidence ofrespiratory complaints during winter is often linked to low relativehumidity. Epidemiological studies have found lower rates of respira-tory illness reported among occupants of buildings with midrangerelative humidity than among occupants of buildings with lowhumidity.Extr

6、emes of humidity are the most detrimental to human com-fort, productivity, and health. Figure 1 shows that the range between30 and 60% rh (at normal room temperatures) provides the best con-ditions for human occupancy (Sterling et al. 1985). In this range,both the growth of bacteria and biological o

7、rganisms and the speedat which chemical interactions occur are minimized.Prevention and Treatment of DiseaseRelative humidity has a significant effect on the control of air-borne infection. At 50% rh, the mortality rate of certain organismsis highest, and the influenza virus loses much of its virule

8、nce. Themortality rate of these organisms decreases both above and belowthis value. High humidity can support the growth of pathogenic orallergenic organisms. As shown in Figure 2, humidity levels around50% can be lethal to the Pneumococcus bacterium (Brundrett1990). Similar effects can be seen in o

9、ther microorganisms thatcause serious health issues. Consequently, relative humidity in hab-itable spaces should be maintained between 30 and 60%.Relative humidity also has a major role in the effects of differentbacteria. Figure 3 shows the mortality of mice exposed to influenzaunder varying degree

10、s of relative humidity (Brundrett 1990).Electronic EquipmentElectronic data processing equipment requires controlled rela-tive humidity. High relative humidity may cause condensation in theequipment, whereas low relative humidity may promote static elec-tricity. Also, rapid changes in relative humid

11、ity should be avoidedbecause of their effect on bar code readers, magnetic tapes, disks,The preparation of this chapter is assigned to TC 5.11, HumidifyingEquipment.Fig. 1 Optimum Humidity Range for Human Comfort and Health(Adapted from Sterling et al. 1985)Fig. 2 Mortality of Pneumococcus Bacterium

12、Maximum mortality for airborne Pneumococci comes when relative humidity is held at 55% rh.(Adapted from Brundrett 1990)22.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)and data processing equipment. Generally, computer systems havea recommended design and operating range of 35 to 55% rh. How-

13、ever, the manufacturers recommendations should be adhered to forspecific equipment operation.Process Control and Materials StorageThe relative humidity required by a process is usually specificand related to one or more of several factors:Control of moisture content or regainRate of chemical or bioc

14、hemical reactionsRate of crystallizationProduct accuracy or uniformityCorrosionStatic electricityTypical conditions of temperature and relative humidity for stor-age of certain commodities and manufacturing and processing ofothers may be found in Chapter 14 of the 2011 ASHRAE Hand-bookHVAC Applicati

15、ons.Low humidity in winter may cause drying and shrinking of fur-niture, wood floors, and interior trim. Winter humidification shouldbe considered to maintain relative humidity closer to that experi-enced during manufacture or installation.For storing hygroscopic materials, maintaining constant humi

16、d-ity is often as important as the humidity level itself. The design ofthe structure should always be considered. Temperature control isimportant because of the danger of condensation on productsthrough a transient lowering of temperature.Static ElectricityElectrostatic charges are generated when ma

17、terials of high elec-trical resistance move against each other. The accumulation of suchcharges may have a variety of results: (1) unpleasant sparks causedby friction between two materials (e.g., stocking feet and carpetfibers); (2) difficulty in handling sheets of paper, fibers, and fabric;(3) obje

18、ctionable dust clinging to oppositely charged objects (e.g.,negatively charged metal nails or screws securing gypsum board towooden studding in the exterior walls of a building that attractpositively charged dust particles); (4) destruction of data stored onmagnetic disks and tapes that require spec

19、ifically controlled environ-ments; and (5) hazardous situations if explosive gases are present, asin hospitals, research laboratories, or industrial clean rooms.Increasing the relative humidity of the environment reduces theaccumulation of electrostatic charges, but the optimum level ofhumidity depe

20、nds to some extent on the materials involved. Figure4 illustrates the voltage that can be accumulated in the human bodyat different humidity levels. Relative humidity of 45% reduces oreliminates electrostatic effects in many materials, but wool andsome synthetic materials may require a higher relati

21、ve humidity.Hospital operating rooms, where explosive mixtures of anesthet-ics are used, constitute a special and critical case. A relative humidityof at least 50% is usually required, with special grounding arrange-ments and restrictions on the types of clothing worn by occupants.Conditions of 22C

22、and 55% rh are usually recommended for com-fort and safety.Sound Wave TransmissionAir absorption of sound waves, which results in the loss of soundstrength, is worst at 15 to 20% rh, and the loss increases as the fre-quency rises (Harris 1963). There is a marked reduction in soundabsorption at 40% r

23、h; above 50%, the effect of air absorption is neg-ligible. Air absorption of sound does not significantly affect speechbut may merit consideration in large halls or auditoriums where opti-mum acoustic conditions are required for musical performances.MiscellaneousLaboratories and test chambers, in wh

24、ich precise control of rel-ative humidity over a wide range is desired, require special atten-tion. Because of the interrelation between temperature and relativehumidity, precise humidity control requires equally precise temper-ature control.ENCLOSURE CHARACTERISTICSVapor RetardersThe maximum relati

25、ve humidity level to which a building maybe humidified in winter depends on the ability of its walls, roof,and other elements to prevent or tolerate condensation. CondensedFig. 3 Mortality in Mice Exposed to Aerosolized InfluenzaNote that numbers of deaths and lung lesions were minimized when humidi

26、ty was held between 40 and 60% rh.(Adapted from Brundrett 1990)Fig. 4 Effect of Relative Humidity on Static Electricity from CarpetsBelow 40% rh, perceptible shocks are more likely.(Adapted from Brundrett 1990)Humidifiers 22.3moisture or frost on surfaces exposed to the building interior (visiblecon

27、densation) can deteriorate the surface finish, cause mold growthand subsequent indirect moisture damage and nuisance, and reducevisibility through windows. If the walls and roof have not been spe-cifically designed and properly protected with vapor retarders on thewarm side to prevent the entry of m

28、oist air or vapor from the inside,concealed condensation within these constructions is likely to occur,even at fairly low interior humidity, and cause serious deterioration.Visible CondensationCondensation forms on an interior surface when its temperatureis below the dew-point temperature of the air

29、 in contact with it. Themaximum relative humidity that may be maintained without con-densation is thus influenced by the thermal properties of the enclo-sure and the interior and exterior environment.Average surface temperatures may be calculated by the methodsoutlined in Chapter 25 of the 2009 ASHR

30、AE HandbookFunda-mentals for most insulated constructions. However, local cold spotsresult from high-conductivity paths such as through-the-wall fram-ing, projected floor slabs, and metal window frames that have nothermal breaks. The vertical temperature gradient in the air spaceand surface convecti

31、on along windows and sections with a highthermal conductivity result in lower air and surface temperatures atthe sill or floor. Drapes and blinds closed over windows lower sur-face temperature further, while heating units under windows raisethe temperature significantly.In most buildings, windows pr

32、esent the lowest surface tempera-ture and the best guide to permissible humidity levels for no con-densation. While calculations based on overall thermal coefficientsprovide reasonably accurate temperature predictions at mid-height,actual minimum surface temperatures are best determined by test.Wils

33、on and Brown (1964) related the characteristics of windowswith a temperature index, defined as (t to)/(ti to), where t is theinside window surface temperature, tiis the indoor air temperature,and tois the outdoor air temperature.The results of limited tests on actual windows indicate that thetempera

34、ture index at the bottom of a double, residential-type win-dow with a full thermal break is between 0.55 and 0.57, with naturalconvection on the warm side. Sealed, double-glazed units exhibit anindex from 0.33 to 0.48 at the junction of glass and sash, dependingon sash design. The index is likely to

35、 rise to 0.53 or greater only25 mm above the junction.With continuous under-window heating, the minimum index fora double window with a full break may be as high as 0.60 to 0.70.Under similar conditions, the index of a window with a poor thermalbreak may be increased by a similar increment.Figure 5

36、shows the relationship between temperature index and therelative humidity and temperature conditions at which condensationoccurs. The limiting relative humidities for various outdoor temper-atures intersect vertical lines representing particular temperatureindexes. A temperature index of 0.55 was se

37、lected to represent anaverage for double-glazed, residential windows; 0.22 represents anaverage for single-glazed windows. Table 1 shows the limiting rel-ative humidities for both types of windows at various outdoor airtemperatures.Concealed CondensationVapor retarders are imperative in certain appl

38、ications because thehumidity level a building is able to maintain without serious con-cealed condensation may be much lower than that indicated byvisible condensation. Migration of water vapor through the innerenvelope by diffusion or air leakage brings the vapor into contactwith surfaces at tempera

39、tures below its dew point. During buildingdesign, the desired interior humidity may be determined by thebuilding enclosures ability to handle internal moisture. This is par-ticularly important when planning for building humidification incolder climates.ENERGY CONSIDERATIONSWhen calculating energy re

40、quirements for a humidification sys-tem, the effect of dry air on any material supplying it with moistureshould be considered. The release of liquid in a hygroscopic materialto a vapor state is an evaporative process that requires energy. Thesource of energy is heat contained in the air. Heat lost f

41、rom the air toevaporate moisture equals the heat necessary to produce an equalamount of moisture vapor with an efficient humidifier. If properhumidity levels are not maintained, moisture migration from hygro-scopic materials can have destructive effects.The true energy required for a humidification

42、system must becalculated from the actual humidity level in the building, not fromthe theoretical level.A study of residential heating and cooling systems showed a cor-relation between infiltration and inside relative humidity, indicatinga significant energy saving from increasing the inside relative

43、 humid-ity, which reduced infiltration of outside air by up to 50% during theheating season (Luck and Nelson 1977). This reduction is apparentlydue to sealing of window cracks by frost formation.To assess accurately the total energy required to provide a desiredlevel of humidity, all elements relati

44、ng to the generation of humidityand the maintenance of the final air condition must be considered.This is particularly true when comparing different humidifiers. Forexample, the cost of boiler steam should include generation andTable 1 Maximum Relative Humidity in a Space for No Condensation on Wind

45、owsOutdoor Temperature, CLimiting Relative Humidity, %Single Glazing Double Glazing5416003525 23 4510 17 3915 12 3320 9 2825 6 2430 4 2035 3 17Note: Natural convection, indoor air at 23.3C.Fig. 5 Limiting Relative Humidity for No Window Condensation22.4 2012 ASHRAE HandbookHVAC Systems and Equipment

46、 (SI)distribution losses; costs for an evaporative humidifier include elec-trical energy for motors or compressors, water conditioning, andaddition of reheat (when the evaporative cooling effect is notrequired).Load CalculationsThe humidification load depends primarily on the rate of naturalinfiltra

47、tion of the space to be humidified or the amount of outdoorair introduced by mechanical means. Other sources of moisture gainor loss should also be considered. The humidification load H canbe calculated by the following equations:For ventilation systems having natural infiltration,H = VR(Wi Wo) S +

48、L (1)For mechanical ventilation systems having a fixed quantity ofoutdoor air,H = 3.6Qo(Wi Wo) S + L (2)For mechanical systems having a variable quantity of outdoor air,H = 3.6Qt(Wi Wo) S + L (3)whereH=humidification load, kg of water/hV=volume of space to be humidified, m3R=infiltration rate, air c

49、hanges per hourQo= volumetric flow rate of outdoor air, L/sQt= total volumetric flow rate of air (outside air plus return air), L/sti= design indoor air temperature, Ctm= design mixed air temperature, Cto= design outside air temperature, CWi= humidity ratio at indoor design conditions, kg of water/kg of dry airWo= humidity ratio at outdoor design conditions, kg of water/kg of dry airS = contribution of internal moisture sources, kg of water/hL = other moisture losses, kg of water/h = density of air at sea level, 1.20 kg/m3Design ConditionsInterior design condi