ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 5-2012 IN-ROOM TERMINAL SYSTEMS.pdf

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1、5.1CHAPTER 5IN-ROOM TERMINAL SYSTEMSSystem Characteristics. 5.1System Components and Configurations . 5.4Secondary-Water Distribution . 5.5Piping Arrangements . 5.5Fan-Coil Unit and Unit Ventilator Systems . 5.6Variable-Refrigerant-Flow (VRF) units. 5.7Chilled-Beam Systems 5.7Radiant-Panel Heating S

2、ystems. 5.9Radiant-Floor Heating Systems. 5.9Induction Unit Systems 5.10Supplemental Heating Units 5.10Primary-Air Systems 5.10Performance Under Varying Load . 5.11Changeover Temperature . 5.12Two-Pipe Systems with Central Ventilation . 5.12Four-Pipe Systems 5.15Automatic Controls and Building Manag

3、ement Systems . 5.15Maintenance Management Systems and Building System Commissioning. 5.16ERY early in the design process, the HVAC design engineerVmust analyze and ultimately select appropriate systems, as dis-cussed in Chapter 1. Next, production of heating and cooling isselected as decentralized

4、(see Chapter 2) or centralized (see Chapter3). Finally, distribution of heating and cooling to the end-use spacecan be done by an all-air system (see Chapter 4), or a variety of all-water or air-water systems and local terminals, as discussed in thischapter.One option is use of in-room terminal syst

5、ems to provide heatingand/or cooling to individual zones. Terminal units include consoles,fan-coils, blower coils, unit ventilators, chilled beams, and radiantpanels. Terminal systems add heat energy or absorb the heat in theconditioned space served. The medium that transfers the heat eitherfrom the

6、 space to the outdoors or from a heat source to the condi-tioned spaces may be the same as used with nonterminal systems.Typical uses of in-room terminal unit systems include hotels/motels, apartments and multifamily dwellings, classrooms, andhealth care facilities. In older office buildings, in-roo

7、m terminalunits were commonly used for perimeter rooms, combined withcentral air handlers that served the interior spaces. Systems of thistype are making a comeback with the introduction of variable-refrigerant-flow (VRF) equipment, combined with dedicated out-door air systems (DOAS). Historical pre

8、servation projects often usein-room terminal units to minimize the space problems due to run-ning ductwork in historical structures.SYSTEM CHARACTERISTICSTerminal-unit systems can be designed to provide completesensible and latent cooling and heating to an end-use space; how-ever, most terminal syst

9、ems are best used with a central ventilationsystem providing pretreated air to the space. Heat can be providedby hot water, steam, or an electric heating coil. Cooling can beprovided by chilled-water or direct-expansion (DX) coils. Heatpumps (discussed in Chapter 2) can be used, either with a pipedw

10、ater loop (water-source) or air cooled. In-room terminals usuallycondition a single space, but some (e.g., a large fan-coil unit) mayserve several spaces. In-room terminal systems can allow individ-ual space control of heating or cooling during intermediate sea-sons; satisfying the heating and cooli

11、ng needs of various rooms ona single system. A thermostat for each terminal unit provides indi-vidual zone temperature control.A terminal unit used with central ventilation provides the cool-ing or heating necessary to handle only the sensible heat gain or losscaused by the building envelope and occ

12、upancy. Ventilation, orprimary, air is delivered by a separate ducted system, either to theterminal unit or ducted directly to the space, and should be pre-treated to handle the total latent load of the ventilation air, occu-pancy, and the space, as well as the sensible load of the ventilationair.Te

13、rminal units without central ventilation require additionalcoil capacity to heat or cool and dehumidify the ventilation airrequired for the end space. Terminal units are commonly small, withminimal coil rows; therefore, providing this additional capacity isoften difficult. Care must be taken to mini

14、mize the risk of frozencoils in the winter, and to have enough cooling capacity to not onlycool, but also dehumidify ventilation air in the summer. Terminalunits have very small fans, so ventilation air must be provided to theunit from a central fan-powered source or supplied from a nearbyopening in

15、 the building skin, thereby limiting the location of termi-nal units to the exterior wall of the building.Although a single in-room terminal unit can be applied to a sin-gle room of a large building, this chapter covers applying multiplein-room terminal units to form a complete air-conditioning syst

16、emfor a building.AdvantagesAdvantages of all in-room terminal unit systems include the fol-lowing: The delivery system for the space heating and cooling needs (pip-ing versus duct systems) requires less building space (a smallercentral fan room, or none, and little duct space)System has all the bene

17、fits of a central water chilling and heatingplant, but allows local terminals to be shut off in unused areasIndividual room temperature control allows each thermostat to beadjusted for a different temperatureMinimal cross contamination of recirculated airBecause this system can heat with low-tempera

18、ture water, it isparticularly suitable for use with solar or low-temperature/high-efficiency boilers or with heat recovery equipmentFailure of a single in-room unit affects only the one room, allow-ing other spaces to continue to operateFacilities personnel can remove and replace an in-room terminal

19、unit in hours, allowing them to install a spare and have the roomback in service quickly; units are comparatively inexpensive,small, and light, so the owner has the option of stocking spareunits on the premisesMaintenance procedures generally can be done by nonlicensedHVAC personnel, allowing in-hou

20、se crews to complete the tasksControls for each unit are very simpleCentral control systems can be incorporated to control unit oper-ation and space temperatures during unoccupied hoursThe preparation of this chapter is assigned to TC 9.1, Large Building Air-Conditioning Systems.5.2 2012 ASHRAE Hand

21、bookHVAC Systems and Equipment (SI)In-room terminal unit systems with central ventilation air havethese additional advantages:The central air-handling apparatus used for central ventilation airis smaller than that of an all-air system because less air must beconditioned at that location.Space can be

22、 heated without operating the primary air system,using just the in-room terminal units. Nighttime primary fan oper-ation is avoided in an unoccupied building. Emergency power forheating, if required, is much lower than for most all-air systems.Dehumidification, filtration, and humidification of vent

23、ilation airare performed in a central location remote from conditionedspaces.They allow using central heat recovery devices such as heatwheels.Ventilation air is positively supplied and can accommodate con-stant recommended outdoor air quantities, regardless of the tem-perature control of the room.U

24、se of central ventilation air with terminal units in some climatescan prevent the negative pressurization problems that occur whenoccupants turn off in-room units.DisadvantagesFor many buildings, in-room terminals are limited to perimeterspace; separate systems are required for other areas.In-room t

25、erminal unit fans with very little if any ductwork can benoisy. Many manufacturers have addressed this and providedunits that are acceptable in many situations.Because of the individual space control, more controls are neededthan for many all-air systems.The system is not appropriate for spaces with

26、 high exhaustrequirements (e.g., research laboratories) unless supplementaryventilation air is provided.Central dehumidification eliminates condensation on the secondary-water heat transfer surface (see the section on Secondary-WaterDistribution) under maximum design latent load, but abnormalmoistur

27、e sources (e.g., open windows, cooking, people congre-gating, a failed primary-air system) can cause annoying or dam-aging condensation. Therefore, a condensate pan must beprovided as for other systems.Primary-air supply usually is constant with no provision for shut-off. This is a disadvantage in r

28、esidential applications, where ten-ants or hotel room guests may prefer to turn off the airconditioning, or where management may desire to do so to reduceoperating expense; however, this can help to stabilize pressuriza-tion of a building when exhaust fans are centrally controlled or inconstant oper

29、ation.Low chilled-water temperature and/or deep chilled-water coilsare needed at the central ventilation air unit to control spacehumidity adequately. Low chilled-water temperatures can resultin excessive condensation occurring at terminal units, if chilled-water valves are not used to shut off wate

30、r flow through the ter-minal unit when the terminal unit fan is off.Low primary-air temperatures can require heavily insulatedducts; however, using neutral air temperatures minimizes thisrequirement and prevents overcooling of some spaces.In-room terminal units without central ventilation air may re

31、sultin greater infiltration levels due to numerous penetrations of theexterior of the building, which must be sealed adequately to pre-vent air infiltration and water intrusion. This may be accentuatedin winter conditions, when wind pressures and stack effectsbecome more significant.Adding necessary

32、 humidity in the winter is difficult.Maintenance must be done within the occupied space, which maydisrupt space use.Heating and Cooling CalculationsBasic calculations for airflow, temperatures, relative humidity,loads, and psychrometrics are covered in Chapters 1, 17, and 18 ofthe 2009 ASHRAE Handbo

33、okFundamentals. Caution must beused in determining peak load conditions for each space served by aterminal unit. Rather than depending on guidelines for typical light-ing, ventilation, infiltration, equipment, and occupancy levels, thedesigner should try to get realistic values based on the particul

34、arowners use plans for the facility. If the client has an existing or sim-ilar facility, visiting it to understand the actual occupancy hours, con-centration of equipment, and occupancy should help avoid unrealisticassumptions. Incorporating effects of planned energy-saving fea-tures (e.g., daylight

35、ing; high-efficiency/low-heat-producing light-ing; shading apparatus for privacy, glare, or solar radiant control; fanand outdoor air modulation; full-building energy management con-trol strategies) can prevent oversizing of terminal units and resultingpotential loss of humidity control. Determining

36、 areas where the nor-mal base load will be a small percentage of the concentrated usageload and understanding the usage schedule for the area can allowequipment selection and control strategies to maximize energy sav-ings and still provide excellent comfort control in both extremes.For example, in a

37、 zone with a terminal unit sized for 2 kW (com-mon for offices), a simple change of four 100 W incandescent lightbulbs to compact fluorescents could reduce the spaces heat load byabout 18% of the units capacity. If this is the consistent peak loadof the space for future years, the terminal unit is o

38、versized. Unlessa central ventilation pretreatment system handles the entire latentload of the space and the outdoor air, humidity control will be lostin the space.Integrated building design (IBD) techniques should be used toensure the building envelope provides adequate energy efficiency,airtightne

39、ss, and moisture penetration control to allow terminalunits to control the indoor environmental conditions without needfor excessive moisture control in each space. Close cooperation ofall parties during design is necessary to create an overall buildingdesign that minimizes the required mechanical s

40、ystems energyconsumption while achieving good indoor conditions. For details onIBD, see Chapter 58 of the 2011 ASHRAE HandbookHVACApplications.Computer programs generally can model primary ventilationsystems as well as secondary in-room terminal systems. Most, how-ever, do not allow the user to assi

41、gn the room latent load as part ofthe primary ventilation system capacity requirement. Therefore, thedesigner needs to either manually determine final capacity require-ments for both the primary and in-room units, or use overrides andmanual inputs to redistribute the loads within the computer progra

42、mafter initial sensible and latent loads as well as block and peak con-ditions have been determined.The design refrigeration load is determined by considering theentire portion or block of the building served by the air-and-watersystem at the same time. Because the load on the secondary-watersystem

43、depends on the simultaneous demand of all spaces, the sumof the individual room or zone peaks is not considered.Space HeatingSome in-room terminal units provide only heating to the endspace. Equipment such as cabinet or unit heaters, radiant panels,radiant floors, and finned-tube radiators are desig

44、ned for heatingonly. Extreme care must be used with these systems if they areincorporated into a two-pipe changeover piping distribution system,or any other system in which secondary water being piped is notconsistently over 38C. The heating coils in these units are notdesigned to handle condensatio

45、n, and there is no drain pipe in theunit. If cold water is provided to these units, dripping condensationfrom units, valves, or piping may damage building finishes or satu-rate the insulation, leading to mold growth. Ball valves tied into theautomatic temperature control (ATC) system and/or aquastat

46、sIn-Room Terminal Systems 5.3should be provided to prevent water at temperatures below space airdew point from reaching heating-only terminal units.Central (Primary-Air) Ventilation SystemsGenerally, the supply air volume from the central apparatus isconstant and is called primary or ventilation air

47、 to distinguish itfrom recirculated room air or secondary air. The quantity of primaryair supplied to each space is determined by the amount of outdoorair required by codes and other applicable guidelines for ventilation.If in-room terminal units are heating-only units, then the primary-air system m

48、ust also provide the required sensible cooling capacityat maximum room cooling load. The air may be from outdoors, ormay be mixed outdoor and return air. During the cooling season, airis dehumidified sufficiently in the central conditioning unit to main-tain required humidity conditions and to preve

49、nt condensation onthe in-room terminal unit cooling coil. (Both outdoor air and spacelatent loads should be handled by the central unit.) Centrally sup-plied air can be supplied at a low enough dew point to absorb mois-ture generated in the space, but as a minimum should be supplied ata condition so that the room terminal unit has to remove only thespace-generated latent load (this is only appropriate with unit ven-tilators with the capability to handle the latent space loads). In win-ter, moisture can be added centrally to limit dryness.As the primary air is dehumidified, it is al