1、AIR-CONDITIONING SYSTEM DESIGN MANUALSECOND EDITIONAIR-CONDITIONING SYSTEM DESIGN MANUALSECOND EDITIONWalter T. Grondzik, EditorProduct code: 90338 4/13Errata noted in the list dated 01/04/2010 have been corrected.UFAD O therefore, currentpractice recommends raising thermostat set points compared to
2、 traditionaloverhead systems. The optimal ventilation strategy controls the supply out-lets to limit the mixing of supply air with room air to just below the breath-ing height of the occupied space. The warmer and more polluted airtransported by the thermal plumes is above the occupied space, out of
3、 thebreathing zone. The air that the occupant breathes will have a lower concen-tration of contaminants compared to conventional, uniformly mixed sys-tems.Many factors, including the ceiling height, diffuser characteristics,number of diffusers, supply air temperature, and outlet airflow rate, affect
4、the cooling load and ventilation efficiency of UFAD systems. Swirl-typeand displacement-type diffusers have been shown to successfully create lowair velocity in the occupied zone, whereas improperly applied linear diffus-ers created the highest velocity in the occupied zone, potentially disturbingth
5、ermal stratification and posing a potential draft risk. Additionally, manyfloor diffusers add an element of personal control within the reach of theoccupant, because users can adjust the amount of air delivered by the dif-fuser. Refer to the UFAD Guide (ASHRAE 2013) for additional informationon diff
6、user types and characteristics.UFAD O the suction could break andallow the panel to fall from your hand.After removing cut panels installed at walls and around columns, reinstallthem exactly where they came from. Interchanging panels that were pre-cisely cut for perimeter locations can cause interio
7、r panels to be tight orloose in the floor and also cause the floor to be out of square.It is good practice to remove panels only where immediate access isrequired and reinstall them as your work progresses. When a number ofpanels must be removed, do not take out more than six adjacent panels atonce
8、OR remove only every other panel in a row. Figure 2.1 Suction cup handle.Courtesy Tate Access Floors, Inc.2 Maintenance and Cleaning9The first panel taken out must be pulled with a lifter. Adjacent panels canbe removed with the lifter or by reaching underneath them and pushingupward. Kneeling on the
9、 floor while removing and replacing panels mayavoid back strain. 2.2.3 Removing Floor PanelsIn a corner lock system, back the screws out so they disengage the pedestalheads (the screws can remain in the panel to prevent misplacement). Place thelifter near an edge of the panel and push on the handle
10、to form suction. Lift theedge of the panel (as if opening a door) until its edge is above the adjacentpanel, then remove it completely by hand. Lay the panel on the floor andrelease the suction by squeezing the release bar directly above the suction cups.Do not slide panels across other panels, beca
11、use this can damage tile finishes.Follow these special precautions:Do not use screwdrivers, pliers, or other objects to pry or lift panels.Do not disturb the level adjustment of the support pedestals and position ofthe stringers (lateral member used to connect the pedestals to each other)while floor
12、 panels are out of the floor system.When reinstalling panels with trim, use a carpet shoehorn to ensure the lifeof the trim.Always make a final check to see that panels are correctly in place, level,and secure.2.2.4 Reinstalling Floor PanelsAll panels should be reinstalled with the aid of a panel li
13、fter to avoidinjury. To reinstall a panel using a lifter, attach the lifter near an edge and lowerthat side into place. In a corner lock positioning system, maneuver the panel asnecessary to align its corner lock holes with the holes in the heads. Fasten thecorner lock screws so their heads are full
14、y seated. Use no more than 30 to 35inlb (3.39 to 3.96 Nm) of torque. Do not attempt to reposition a panel on theunderstructure by dropping or kicking it into place.2.2.5 Stringer SystemsA stringer grid can become racked (out of square) (Figure 2.2) when agroup of panels is removed and the stringers
15、are bumped, pulled, walked on, orincorrectly reinstalled when underfloor work is performed. These abuses willcause panels to be visibly out of square after they are put back into the grid.Correcting a large out-of-square condition must be performed by a professionalinstaller. Be aware that 2 ft (0.6
16、 m) stringer systems are more susceptible toracking than 4 ft (1.2 m) stringer systems. Anyone performing underfloor workshould adhere to the following instructions to prevent stringer racking:Remove panels only where immediate access is needed, and reinstall themas your work progresses. The surroun
17、ding panels in the floor help to main-tain grid squareness while underfloor work is performed.UFAD O interchanging them can cause floors tobecome out of square, and also cause interior panels to become too tight ortoo loose.Do not overtorque the stringer screws with your driver; use no more than30 i
18、nlb (3.39 Nm) of torque.Do not attempt to resquare the stringer grid by hitting stringers with a ham-mer or other objects; this can result in forming bumps on the stringer tops,causing panels to rock.Figure 2.2 Plan view of racked access floor installation.Courtesy Tate Access Floors, Inc.2 Maintena
19、nce and Cleaning112.2.6 Adjustments to the Floor SystemThe floor owner or owners representative should see a demonstration ofthe pedestal height adjustment and leveling mechanism by a representative ofthe floor provider before making adjustments to the system. Periodicallyinspect the installed floor
20、 and make adjustments.2.2.7 Rocking PanelsCheck to ensure that pedestal head gaskets are positioned correctly and/orstringers are securely fastened. Also ensure that no carpet pile or dirt preventspanels from seating correctly. Adjust the pedestal leveling nut after confirmingthat the pedestal is si
21、tting flat on the subfloor. For persistent problems with tiledpanels, try rotating the panels by 90 degrees.2.2.8 Tipping at Perimeter PanelsTipping generally occurs when the perimeter panel is improperly sup-ported. Adjust the leveling nut on the pedestal support, raising or lowering thepedestal he
22、ad until the panel sits firmly on the pedestal head or stringer.2.2.9 Cleaning Floor FinishesAlways contact or refer to manufacturer-recommended methods for clean-ing floors, because certain finishes may be susceptible to stain or discolor-ations. Because there is a nearly infinite number of differe
23、nt floor finishes, it isbest to contact the manufacturer than to list them all.Avoid using a broom or dry mop for cleaning, because these will onlypush dirt and debris around the floor and cause them to settle in another area orin the small seams between panels that sometimes form over time as panel
24、s set-tle. It is recommended to first attempt to clean any panel surfaces first by vacu-uming to clean any large debris. Also be cautious of cleaning panels with verywet mops, because water may leak between tiles and damage or destroy dataand power wiring or HVAC equipment beneath.2.3 UFAD System Ma
25、intenanceUFAD systems generally require minimal system maintenance. This isgenerally covered by regular system cleaning, comparable to that needed forany air distribution system. This section outlines special considerations formaintaining a UFAD system.2.3.1 Plenum Cleaning and TimelineAfter the sub
26、floor is cleaned of dirt and debris from construction, furtheraccumulation will be minimal and can be removed with a shop vacuum.Because the cavity is enclosed, significant buildup may take several years todevelop. Determine when cleaning is necessary by occasionally inspecting thesubfloor. The requ
27、ired frequency of cleaning is estimated at two to three yearsbased on the rate of buildup observed. HEPA-filtered vacuums are recom-mended for cleaning to prevent dust and dirt from being released into the roomair.UFAD O with underfloor air plenums, these are often unneces-sary because of pretreated
28、 supply air pressurized plenums. Refer to equipmentspecifications for additional requirements regarding regular maintenancerequirements. Grilles and air diffuser baskets may be removed and cleanedusing a soft cloth soaked with warm water and a mild cleaner, and dried with asimilar cloth.2.3.4 Other
29、Systems and EquipmentUFAD plenums are almost always used to provide raised access floor ben-efits. For this reason, O&M access to other than UFAD systems and equipment,such as electrical and cabling, must be ensured as well. UFAD Control andOptimization3133.1 Controllability of SystemsThe control an
30、d optimization of thermal comfort in the occupied zone andthe amount of thermal stratification during cooling operation is crucial to systemdesign, sizing, energy-efficient operation, and comfort performance of UFADsystems. This section presents recommended control strategies for effective sys-tem o
31、peration specific to UFAD systems. Depending on the space requirements, the controllability of set points andallowable ranges may differ from zone to zone. In some, the control of parame-ters and values is primarily for occupant comfort. In others, more restrictive val-ues are required to protect pr
32、operty and processes. This chapter focuses oncontrol for occupant comfort and proper pressurization of the building duringboth occupied and unoccupied conditions.3.2 Thermal MassIt is important to consider the thermal mass of the supply air plenum whenchanging set points. The materials of the supply
33、 air plenum will absorb energyover time. It is recommended that changes to supply air temperature be made inincrements of 1F to 2F (0.5C to 1.0C) at a time. Results from changes maytake up to several hours or days, so sufficient time should be allowed to obtainthe results desired. Making changes lar
34、ger than this may cause overshooting ofresults and create other occupant comfort issues.3.3 Trend AnalysisOnce the UFAD system is commissioned, it should perform consistentlyunless altered or subjected to physical degradation. Current building automationsystems have the capability to trend operation
35、al data and optionally use faultdetection and diagnostic analysis. These tools should be used by the operators toproactively identify or anticipate impending problems and failures. 3.4 Common UFAD Control PrinciplesBecause of the number of different UFAD system configurations, this sec-tion does not
36、 cover all possible control scenarios, but is intended to introducesome of the basic control strategies that are frequently used in UFAD systems.UFAD O&M GUIDE143.4.1 Pressure Reset of Plenum Supply AirAs stated previously, the majority of supply air diffusers used for UFADapplications are pressure
37、dependent (refer to Figure 3.1 for a typical static pres-sure versus air quantity diffuser curve). In other words, given a specific staticpressure, the diffuser will deliver a specific quantity of air. Depending on thesystem design, control of air volume may be adjusted to match the space load atany
38、 given time by controlling the plenum pressure with respect to the space. Thismay be important should the load be intermittent or variable. Modulating fanspeed or discharge air dampers to maintain a predetermined supply air staticpressure is a common control strategy for this scenario, especially du
39、ring off-peak load conditions. Typically, this is done by resetting the supply air plenum pressure based onreturn air or space temperature (see Figure 3.2 for an example of supply air pres-sure reset schedule). The return air temperature or space temperature is used asan indication of the current lo
40、ad in the space. As the temperature rises, it indi-cates that there is more heat load in the space, requiring more supply air to satisfythe load. This can be especially important if manual diffusers are used in thespace or if there is more leakage through closed diffusers or other openings intothat
41、space than anticipated. Therefore, the cooling load present is much lower orhigher than the current capacity being delivered.Note that careful evaluation is needed to determine the best applicable strat-egy for controlling air volume to the space, especially if a common plenum isused to supply air t
42、o areas with different load profiles or schedules. For example,if the supply air pressure is reset down because of overcooling of an open officefloor plan, occupied conference rooms or office spaces served by the same ple-num may not be able to maintain space temperature because of the lower airflow
43、rates. In this case, it may be necessary to provide more diffusers in these zones tomaintain adequate cooling with lower-than-optimum design pressures. It is alsopossible to use zone booster fans, plenum dividers, or other means to maintainFigure 3.1 Typical pressure-dependent diffuser operation.3 U
44、FAD Control and Optimization15the proper volume of air to these spaces when system pressure has been resetdown.Diffuser selection should take into account different plenum pressures. Forexample, it would not be recommended that diffusers of different operatingpressures (e.g., type “A” full flow at 0
45、.03 in. of water 7.5 Pa and type “B” fullflow at 0.05 in. of water 12.5 Pa) be used with the same plenum without care-ful evaluation.3.4.2 Temperature Reset of Supply AirIn some designs or climate zones, it may be permissible to use a supply airtemperature reset schedule based on the load profile of
46、 the space (seeFigure 3.3).Much like supply pressure reset, the air temperature can be changed whilethe air volume (i.e., plenum pressure) remains constant, to better match the cur-rent load for a given zone as indicated by return air or space temperature. This istypically done by resetting the leav
47、ing-air coil temperature or modulating returnair bypass dampers or reheat coils.Figure 3.2 Typical supply air pressure reset schedule.UFAD O&M GUIDE16Temperature reset sometimes works better when minor adjustments aremade on a seasonal basis (e.g., a supply air temperature of 65F 18.3C whenoutdoor a
48、ir temperatures are above 50F 10C, and a supply air temperature of68F 20C when outdoor air temperatures are below 50F 10C). The same caution as for pressure reset should be considered. It may be pos-sible to satisfy the load for one space with temperature reset but negatively affectanother space ser
49、ved by the same plenum. It may be necessary to use plenumdividers, diffuser counts, or booster fans for these situations.In addition, if used aggressively, temperature reset could use more energyand make space temperature swings more dramatic. Note that energy will beabsorbed and released by the plenum materials (e.g., concrete slab), which coulddelay or even exacerbate the space conditions.3.4.3 Combined Temperature/Pressure ResetThis strategy may be used concurrently for faster zone reaction to load vari-ation. However, the same concerns as expressed in Sec
