ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 25-2012 MECHANICAL DEHUMIDIFIERS AND RELATED COMPONENTS.pdf

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1、25.1CHAPTER 25MECHANICAL DEHUMIDIFIERS AND RELATED COMPONENTSMechanical Dehumidifiers 25.1Installation and Service Considerations . 25.8Wraparound Heat Exchangers 25.8HE correct moisture level in the air is important for health andTcomfort. Controlling humidity and condensation is importantto preven

2、t moisture damage and mold or mildew development, thusprotecting buildings and occupants, and preserving building con-tents. This chapter covers mechanical dehumidification using acooling process only, including basic dehumidifier models (withmoisture removal capacity of less than 1.4 kg/h) used for

3、 homebasements and small storage areas, as well as larger sizes requiredfor commercial applications.These dehumidifiers are used for applications where dew pointsof 1.7 to 4.4C and above are maintained. For applications requiringdew points below 1.7C and for other methods of dehumidification,see Cha

4、pter 24.Commercial applications for mechanical dehumidifiers includethe following:Indoor swimming poolsMakeup air treatmentIce rinksDry storageSchoolsHospitalsOffice buildingsMuseums, libraries, and archivesRestaurantsHotels and motelsAssisted living facilitiesSupermarketsManufacturing plants and pr

5、ocessesIn addition, an air-to-air heat exchanger (e.g., heat pipe, coil run-around loop, fixed-plate heat exchanger, rotary heat exchanger) maybe used to enhance moisture removal by a mechanical dehumidifieror air conditioner. The section on Wraparound Heat Exchangers dis-cusses how dehumidification

6、 processes can be improved by usingsuch a device. Other uses of air-to-air heat exchangers are coveredin Chapter 26.MECHANICAL DEHUMIDIFIERSMechanical dehumidifiers remove moisture by passing air over asurface that has been cooled below the airs dew point. This cold sur-face may be the exterior of a

7、 chilled-water coil or a direct-expansionrefrigerant coil. To prevent overcooling the space (and avoid the needto add heat energy from another source), a mechanical dehumidifieralso usually has means to reheat the air, normally using recoveredand recycled energy (e.g., recovering heat from hot refri

8、gerant vaporin the refrigeration circuit). Using external energy input for reheat iswasteful and is prohibited or limited in many countries (seeASHRAE Standard 90.1).A mechanical dehumidifier differs from a typical off-the-shelfair conditioner in that the dehumidifier usually has a much lowersensibl

9、e heat ratio (SHR). The dehumidifier starts the compressoron a call for dehumidification, whereas an air conditioner starts thecompressor on a call for sensible cooling. Typically, a room dehu-midifier has an SHR of 0.6 or less, compared to a standard air-conditioning system of 0.8 SHR. Dehumidifier

10、s must also allowcondensation from the cooling coil to drain easily from the coils.They may need air velocities over the cooling coil lower than thosefor a typical air conditioner, to improve moisture runoff and mini-mize carryover of condensed moisture.In addition, the need to introduce code-mandat

11、ed ventilation airmay require that outdoor air be treated to avoid introducing exces-sive moisture. Basic strategies include precooling outdoor air enter-ing the air-conditioning evaporator coil, or providing a separatesystem to provide properly conditioned outdoor air. For some low-dew-point (below

12、 7C) applications, mechanical dehumidificationmay be used as the first stage, with desiccant dehumidification forthe final stage to maximize efficiency and minimize installed cost.Although the main purpose of a mechanical dehumidifier is toremove moisture from the air, many features can be incorpora

13、ted forvarious applications, such asDehumidifying and cooling (no reheat)Dehumidifying with partial reheat (leaving dry-bulb temperatureis cooler than with a dehumidifier with full reheat)Dehumidifying with full reheatDehumidifying with heat recovery to various heat sinksDehumidification capacity mo

14、dulationReheat capacity modulationVentilation air introductionAuxiliary space or water heatingOften, mechanical dehumidifiers can be incorporated in a systemto use waste heat from mechanical cooling (e.g., heat rejection to aswimming pool, whirlpool, domestic hot water, heat pump loop,chilled-water

15、loop, or remote air-cooled condenser).Outdoor dehumidifiers should be protected against internal mois-ture condensation when winter conditions are severe, because of thehigher dew-point temperature of air circulating in the unit.Psychrometrics of DehumidificationAir enters the dehumidifying coil at

16、point A (Figures 1 and 2).The dehumidifying coil removes sensible heat (SH) and latent heat(LH) from the airstream. The dehumidified, cooled air leaves thecoil at its saturation temperature at point B. The total heat removed(TH) is the net cooling capacity of the system. In reheating, the refrigeran

17、t (hot gas) rejects heat it has obtainedfrom three sources. First, sensible heat absorbed in the air-coolingprocess is rejected to air leaving the cooling coil. This air is at pointC, which is the same dry-bulb temperature as the entering air minusthe moisture content. Second, the latent heat remova

18、l that causes themoisture to condense also adds heat to the hot refrigerant gas. Thisheat is also rejected into the airstream, raising the air temperature toThe preparation of this chapter is assigned to TC 8.10, Mechanical Dehu-midification Equipment and Heat Pipes.25.2 2012 ASHRAE HandbookHVAC Sys

19、tems and Equipment (SI)point D. Third, nearly all electric power required to drive the refrig-eration cycle is converted to heat. This portion of heat rejectionraises the air leaving temperature to point E.This process assumes that all heat is rejected by the refrigerantreheat coil. Depending on ref

20、rigerant system complexity, any part ofthe total heat rejection can be diverted to other heat exchangers(condensers/desuperheaters).Dehumidifier supply air temperatures can be controlled between10 and 35C. However, system design should not rely on a mechan-ical dehumidifier as a dependable heat sour

21、ce for space heating,because heat is only available when the unit is operating.Residential DehumidifiersPortable Dehumidifiers. These are smaller (usually less than3.5 kW), simpler versions of commercial dehumidifiers. They areself-contained and easily movable. They are designed to be used inlocaliz

22、ed areas, such as basements or other high-moisture areas.As shown in Figure 3, a single fan draws humid room air throughthe cold coil, removing moisture that either drains into the waterreceptacle or passes through the cabinet into some other means of dis-posal. The cooled air passes through the con

23、denser, reheating the air.Portable dehumidifiers ordinarily maintain satisfactory humiditylevels in an enclosed space when the airflow rate and unit placementmove the entire air volume of the space through the dehumidifieronce an hour.Design and Construction. Portable dehumidifiers use hermeticmotor

24、-compressors; the refrigerant condenser is usually con-ventional finned tube. Refrigerant flow is usually controlled by acapillary tube, although some high-capacity dehumidifiers use anexpansion valve. A propeller fan moves air through the unit at typ-ical airflows of 70 to 120 L/s.The refrigerated

25、surface (evaporator) is usually a bare-tube coil,although finned-tube coils can be used if they are spaced to allowrapid runoff of water droplets. Vertically disposed bare-tube coilstend to collect smaller drops of water, promote quicker runoff, andresult in less condensate reevaporation compared to

26、 finned-tube orhorizontally arranged bare-tube coils. Continuous bare-tube coils,wound in a flat circular spiral (sometimes with two coil layers) andmounted with the flat dimension of the coil in the vertical plane, area good design compromise because they have most of the advan-tages of the vertica

27、l bare-tube coil.Evaporators are protected against corrosion by finishes such aswaxing, painting, or anodizing (on aluminum). Waxing reduces thewetting effect that promotes condensate formation; however, testson waxed versus nonwaxed evaporator surfaces show negligibleloss of capacity. Thin paint fi

28、lms do not have an appreciable effecton capacity.Removable water receptacles, provided with most dehumidifi-ers, hold 7.5 to 11 L and are usually made of plastic to withstandcorrosion. Easy removal and handling without spillage are impor-tant. Most dehumidifiers also provide either a means of attach

29、ing aflexible hose to the water receptacle or a fitting provided speciallyfor that purpose, allowing direct gravity drainage to another meansof disposal external to the cabinet.An adjustable humidistat (30 to 80%) automatically cycles theunit to maintain a preselected relative humidity. The humidist

30、at mayalso provide a detent setting for continuous operation. Some modelsalso include a sensing and switching device that automatically turnsthe unit off when the water receptacle is full.Dehumidifiers are designed to provide optimum performance atstandard rating conditions of 27C db room temperatur

31、e and 60%rh. When the room is less than 18C db and 60% rh, the evaporatormay freeze. This effect is especially noticeable on units with a cap-illary tube.Some dehumidifiers are equipped with defrost controls that cyclethe compressor off under frosting conditions. This control is gener-ally a bimetal

32、 thermostat attached to the evaporator tubing, allowingdehumidification to continue at a reduced rate when frosting condi-tions exist. The humidistat can sometimes be adjusted to a higherrelative humidity setting, which reduces the number and duration ofrunning cycles and allows satisfactory operati

33、on at low-loadconditions. Often, especially in the late fall and early spring, sup-plemental heat must be provided from other sources to maintainabove-frosting temperatures in the space.Fig. 1 Dehumidification Process PointsFig. 2 Psychrometric Diagram of Typical Dehumidification ProcessFig. 3 Typic

34、al Portable DehumidifierMechanical Dehumidifiers and Related Components 25.3Capacity and Performance Rating. Portable dehumidifiers areavailable with moisture removal capacities of 5 to 30 L per 24 h, andare operable from ordinary household electrical outlets (115 or230 V, single-phase, 60 Hz). Inpu

35、t varies from 200 to 800 W,depending on the output capacity rating.AHAM Standard DH-1 establishes a uniform procedure fordetermining the rated capacity of dehumidifiers under specified testconditions and also establishes other recommended performancecharacteristics. An industry certification program

36、 sponsored byAHAM covers the great majority of portable dehumidifiers and cer-tifies dehumidification capacity.The U.S. Environmental Protection Agency (EPA) qualifies de-humidifiers to carry its ENERGY STARlabel if they remove thesame amount of moisture as similarly sized standard units, but use at

37、least 10% less energy. The EPAs ENERGY STAR Web site pro-vides additional information on qualifying products (EPA 2012).Whole-House Dehumidifiers. Whole-house dehumidifiers havehigher moisture removal capacity than portable dehumidifiers.Blowers in whole-house dehumidifiers are typically more powerf

38、ulbecause they draw air through the unit at higher external static pres-sures as compared to portable dehumidifiers. The design allows forconnection of ductwork to the units inlet and outlet. Moist air is typ-ically drawn from either the return plenum of the HVAC system duc-twork (Figure 4) or from

39、a centrally located register. The air isdehumidified and then discharged into the supply ductwork of theHVAC system for distribution. To prevent reevaporation of moisturefrom the HVAC system evaporator coil, the dry air enters the systemdownstream of the coil.Whole-house dehumidifiers typically inte

40、grate controls to allowoperation of the HVAC system blower while dehumidifying the air;this allows the dry air to be distributed throughout the home. Somedehumidifiers are equipped with controls to bring in outdoor air,which can then be run through the dehumidifier before being dis-tributed to the r

41、est of the home. Sensors are typically located insidethe dehumidifier to measure the temperature and relative humidityof air passing through the unit as opposed to the surrounding air.Controls on some models may provide for installation of a remotehumidistat or humidity sensor. Drain tubing must typ

42、ically beinstalled on the dehumidifier and routed to the nearest floor drain.Most models include an internal condensate overflow switch orprovide for the installation of a field-installed overflow switch thatwill turn the unit off if it overflows.Units are typically installed so as to not be exposed

43、 to the ele-ments and should not be installed in locations that can experiencefreezing temperatures. Whole-house dehumidifiers are typicallylocated near the HVAC system for convenient access to the supplyand return plenums and to drains. Systems may be installed in base-ments, closets, crawlspaces,

44、or attics, so the dehumidifier cabinetsare typically insulated. When units are installed over a finished area,drain pans are commonly installed under the unit.Codes. Domestic dehumidifiers are designed to meet the safetyrequirements of UL Standard 474, Canadian Electrical Code, andASHRAE Standard 15

45、. UL-listed and CSA-approved equipmenthave a label or data plate indicating approval. UL also publishes theElectrical Appliance and Utilization Equipment Directory, whichcovers this type of appliance.General-Purpose DehumidifiersBasic components of general-purpose dehumidifiers are shownin Figure 5.

46、 An air filter is required to protect the evaporator. Dehu-midifying coils, because of their depth and thoroughly wetted sur-faces, are excellent dust collectors and not as easily cleanable asmuch thinner air-conditioning evaporator coils. However, the largeamount of condensate has a self-cleaning e

47、ffect. A bypass damper atthe evaporator coil allows airflow adjustments for the evaporatorwithout decreasing airflow for the reheat coil. Dehumidifying andreheat coils may operate at different airflows.The compressor may be isolated from the airstream or located init. Locating the compressor in the

48、airstream may make service moredifficult, but this arrangement allows heat lost through the compres-sor casing to be provided to the conditioned space while reducingthe size of the enclosure. During the cooling season, this compressorlocation reduces the units sensible cooling capacity.Code-required

49、 outdoor air may be introduced between the evap-orator and reheat coil. The amount of outdoor air should be con-trolled to not adversely affect the refrigeration systems operation.Preheating outdoor air may be required in colder climates.Computerized controls can sense return air temperature and rel-ative humidity. Remote wall-mounted sensors are also available.More sophisticated controls are desirable to regulate dew-point tem-perature and maintain the desired relative humidity in the space.General Considerations. Before considering installation of anytype of dehumidification e