ASHRAE HVAC APPLICATIONS SI CH 47-2015 DESIGN AND APPLICATION OF CONTROLS.pdf

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1、47.1CHAPTER 47DESIGN AND APPLICATION OF CONTROLSSystem Types 47.1Heating Systems. 47.1Cooling Systems. 47.4Air Systems. 47.8Special Applications. 47.18Design Considerations and Principles. 47.19Control Principles for Energy Conservation. 47.20UTOMATIC control of HVAC systems and equipment usuallyA i

2、ncludes control of temperature, humidity, pressure, and flowrates of air and water. Automatic controls can sequence equipmentoperation to meet load requirements and to provide safe equipmentoperation using direct digital control (DDC), electronic, electrical,mechanical, and/or pneumatic devices. Aut

3、omatic controls are onlyfully effective when applied to well-designed mechanical systems;they cannot compensate for misapplied systems, excessive under- oroversizing, or highly nonlinear processes.This chapter addresses control of typical HVAC systems, designof controls for system coordination and f

4、or energy conservation, andcontrol system commissioning. Chapter 7 of the 2013 ASHRAEHandbookFundamentals covers details of component hardwareand the basics of control.1. SYSTEM TYPESA building automation system (BAS) with direct digital, elec-tronic, or pneumatic controls has several physical contr

5、ol loops, witheach loop including a controlled variable (e.g., temperature), con-trolled device (e.g., actuator), and the process to be controlled (e.g.,heating system). BASs with DDC controllers can share sensor val-ues with several control loops or have multiple control loops selec-tively activate

6、 an actuator.BASs with DDC controllers allow information such as systemstatus or alarms to be collected in central controllers and sharedbetween HVAC systems, enabling advanced, energy-saving, system-level applications through a common communication protocol.ASHRAE Guideline 13 and Standard 135 have

7、 more detailed dis-cussions of networking and interoperability.2. HEATING SYSTEMSHeating systems include boilers, fired by either fuel combustionor electric resistance, direct flame-to-air furnaces, and electric resis-tance air heaters. Load affects the required rate of heat input to aheating system

8、. The rate is controlled by cycling a fixed-intensityenergy source on and off, or by modulating the intensity of the heat-ing process. Flame cycling and modulation can be handled by theboiler control package, or the BAS can send commands to the boilercontrols. The control designer decides under what

9、 circumstances toturn boilers on and off in sequence and, for hot-water boilers, at whattemperature set point to maintain the boiler supply water.Hot-Water and Steam BoilersHot-water distribution control includes temperature control athot-water boilers or the converter, reset of heating water temper

10、a-ture, and control of pumps and distribution systems. Other controlsto be considered include (1) minimum water flow through boilers,(2) protecting boilers from temperature shock and condensation onthe heat exchanger, and (3) coil low-temperature detection. If multi-ple or alternative heating source

11、s (e.g., condenser heat recovery,solar storage) are used, the control strategy must also include a wayto sequence hot-water sources or select the most economical source.Figure 1 shows a system for load control of a fossil-fuel-firedboiler. Boiler safety controls, usually factory installed with thebo

12、iler, include flame-failure, high-temperature, and other cutouts.Field-installed operating controls must allow safety controls to func-tion in all modes of operation. Intermittent burner firing usually con-trols capacity, although burner modulation is common in largersystems. In most cases, the boil

13、er is controlled to maintain a constantwater temperature, although an outdoor air thermostat or other con-trol strategies can reset the temperature if the boiler is not used fordomestic water heating. A typical outdoor air reset schedule isshown in Figure 1. With DDC devices, reset can be controlled

14、 fromzone demand, which can improve energy performance and ensure allzones are satisfied. To minimize condensation of flue gases andboiler damage, water temperature should not be reset below that rec-ommended by the manufacturer, typically 60C entering water tem-perature, or condensation may occur a

15、nd lead to corrosion-relatedfailure. Condensing boilers are specifically designed to allow fluegases to condense, and should operate at lower water temperatures toharness latent energy in the flue gas. Aggressive reset of hot-watertemperatures improves the efficiency of condensing boilers, be-cause

16、efficiency is a strong function of boiler entering water tem-perature. Systems with sufficiently high pump operating costs canuse variable-speed pump drives to reduce secondary pumpingcapacity to match the load and conserve energy. ASME StandardCSD-1-2012 requires a manually operated remote shutdown

17、 switchlocated just outside the boiler room door for boilers with fuel inputratings less than 3663 kW.Hot-water heat exchangers or steam-to-water converters aresometimes used instead of boilers as hot-water generators. Convert-ers typically do not include a control package; therefore, the engi-neer

18、must design the control scheme. The schematic in Figure 2 canbe used with either low-pressure steam or boiler water from 93 to180C. The supply water temperature sensor controls two modulat-ing two-way valves in a 1/3 and 2/3 arrangement in a steam or high-temperature hot-water supply line. An outdoo

19、r temperature sensor(or zone demand for a BAS) can be used to reset the supply watertemperature downward as load decreases to improve the controlla-bility of heating valves at low load and to reduce piping losses. AThe preparation of this chapter is assigned to TC 1.4, Control Theory andApplication.

20、 Fig. 1 Boiler Control47.2 2015 ASHRAE HandbookHVAC Applications (SI)flow or differential pressure switch interlock should close the two-way valve when the hot-water pump is not operating. Ensure thatthe flow switch operates as expected at minimum flow rate onvariable-flow systems. With a BAS, feedb

21、ack from zone heatingvalves can be used to control starting and stopping of the hot-waterpumps. When shutting down a steam converter or high-temperaturehot-water system, close the steam valves and allow the water to cir-culate long enough to remove residual heat in the converter andprevent the press

22、ure relief valve from opening.Hot-Water Distribution SystemsHot water is distributed using variable flow (primarily two-wayvalves at coils) or constant flow (three-way valves at coils). Anexample constant flow system is shown in Figure 3. Variable-flowsystems are similar to the chilled-water distrib

23、ution systems shownin Figures 10 and 11. Some boilers require constant flow or very highminimum flow rates. They typically are piped using a primary/secondary system (see Figure 11). These boilers are usually requiredby their listing to have flow switches to enable the boiler only whenflow is proven

24、. Boilers that require small (or zero) minimum flowrates are usually piped in a primary-only configuration with a bypassto maintain minimum flow (see Figure 10). A flow meter in the boilercircuit is usually installed to control the bypass valve. The bypass canalso be controlled to maintain minimum b

25、oiler entering water tem-peratures for noncondensing boilers.Heating CoilsHeating coils that are not subject to freezing can be controlled bysimple two- or three-way modulating valves (Figure 4). Steam-distributing coils are required to ensure proper steam coil control.(For information on air-side c

26、oils, see the section on Air Systems.)The modulating valve is controlled by coil discharge air temperatureor by space temperature, depending on the HVAC system. In coldregions, valves are set to open to allow heating if control power fails.In many systems, the heating discharge air controller is res

27、et basedon the outdoor air temperature, zone damper positions, return airtemperature, or some other load proxy.Heating coils in central air-handling units preheat, reheat, orheat, depending on the climate and the amount of minimum outdoorair needed. They can also provide morning warm-up on systemswi

28、th limited zone heating capacity.The equipment heating coil that first receives the outdoor airintake, even if mixed with indoor air, must have protection againstfreezing in cold climates, unless (1) the minimum outdoor air quan-tity is small enough to keep the mixed air temperature above freez-ing

29、in all expected operating conditions and (2) enough mixingoccurs to prevent stratification. Even when the average mixed-airtemperature is above freezing, inadequate air mixing may allowfreezing air to impinge on small areas of the coil, causing localizedfreezing. This blocks flow and, without a heat

30、 source, the rest of thecoil and equipment downstream is at risk. Preheating coils that heat100% outdoor air always need (1) protection against freezing and(2) constant water or steam flow in cold climates.Steam preheat coils should have two-position valves and vacuumbreakers to prevent condensate b

31、uild-up in the coil. The valveshould be fully open when outdoor (or mixed) air temperature isbelow freezing. This causes unacceptably high coil discharge tem-peratures at times, necessitating face-and-bypass dampers for finaltemperature control (Figure 5). The bypass damper should be sizedto provide

32、 the same pressure drop at full bypass airflow as the com-bination of face damper and coil does at full airflow. When the out-door air temperature is safely above freezing (roughly 1.7C), thebypass damper is full open to the coil face and the coil valve can bemodulated to improve controllability.Hot

33、-water coils must maintain a minimum water velocity in thetubes (on the order of 0.9 m/s) to avoid stratification and ensureproper heat transfer by maintaining turbulent flow. A two-positionFig. 2 Steam-to-Water Heat Exchanger ControlFig. 3 Load and Zone Control in Constant Flow SystemFig. 4 Control

34、 of Hot-Water CoilsFig. 5 Preheat with Face-and-Bypass DampersDesign and Application of Controls 47.3valve combined with face-and-bypass dampers (Figure 5) or a coilpump can be used. There are many coil pump piping schemes; themost common are shown in Figures 6 and 7. In each scheme, thecontrol valv

35、e modulates to maintain the desired coil air dischargetemperature and the pump maintains the minimum tube watervelocity needed when the outdoor air is below freezing. Pumpedcoils can still freeze in very cold regions, so additional low-temperature protection measures such as freezestats, glycol-base

36、dfluids, and default-to-open valves should be used.A low-temperature detector (commonly called a freezestat), is along, refrigerant-filled capillary tube used as a low-temperaturesensing switch. If any short section of the tube is exposed to a lowtemperature (typically 3.3C), it can provide an alarm

37、 or a hard-wired interlock to shut the outdoor damper and open the returndamper, or shut down the fan.Figure 6 shows the conventional primary/secondary (or second-ary/tertiary) arrangement where the coil pump and the pumpsfeeding the coil are hydraulically independent. It results in con-stant flow t

38、hrough the coil and in either variable flow through theprimary loop, if a two-way valve is used, or in constant flowthrough the primary loop, if a three-way valve is used (showndashed in the figure).In Figure 7A, the coil pump is in series with the primarypumps; this results in variable primary flow

39、, which can affect flowthrough parallel coils that do not have pumps when the three-wayvalve moves, unless the primary system has variable-speed pumpsand proper pressure control. The pump is decoupled when thethree-way valve is closed to the system. The three-way valve mustbe oriented with the commo

40、n port connected to the coil soflowthrough is not affected by the valve position.Figure 7B shows the coil pump piped in parallel with the primarypumps. This design has the advantage that hot-water flow can beachieved through the coil if the coil pump fails. This design resultsin coil flow that varie

41、s from the pump design flow rate (when thecontrol valve is closed) up through the sum of the pump flow rateplus the primary system flow rate (when the valve is wide open).Unlike the options in the previous two figures, the primary pumpmust be sized for the pressure drop through the coil at this high

42、 flowrate. Similarly, the coil pump must be sized for that pressure dropand not only for the flow it supplies. Flow through the primary cir-cuit may be variable, if a two-way valve is used, or constant, if athree-way valve is used.Some systems may use a glycol solution in combination with anyof thes

43、e methods; however, glycol affects control valve sizing (seeChapter 47 of the 2012 ASHRAE HandbookHVAC Systems andEquipment) and requires additional maintenance and careful han-dling.Steam Coils. Modulating steam coils are controlled in much thesame way as water coils. Control valve size and charact

44、eristics areimportant to achieve proper control (see Chapter 47 of the 2012ASHRAE HandbookHVAC Systems and Equipment). Because theentering steam is hotter than the entering temperature of most watercoils, a steam coil typically responds more rapidly and is smallerthan a comparable water coil. In low

45、-temperature applications, two-position control should be used, as discussed previously. For largecoils, control valves should be in a 1/3 and 2/3 arrangement.Electric heating coils (duct heaters) are controlled in either two-position or modulating mode. Two-position operation uses powerrelays with

46、contacts sized to handle the amperage of the heatingcoil. Step controllers provide cam-operated sequencing control ofup to 10 stages of electric heat. Each stage may require a contactor,depending on the step controller contact rating. Timed two-positioncontrol requires a timer and contactors. The ti

47、mer can be electrome-chanical, but it is usually electronic and provides a time base of 1 to5 min. Thermostat demand determines the percentage of on-time.Because rapid cycling of mechanical or mercury contactors cancause maintenance problems, solid-state controllers are preferred.These devices may m

48、ake cycling so rapid that control appears pro-portional; therefore, face-and-bypass dampers are not used. Use ofelectric heating coils is restricted in some areas by energy standards;check code compliance before using this application. A system witha solid-state controller and safeties is shown in F

49、igure 8.Current in individual elements of electric duct heaters is normallylimited to a maximum safe value established by the NationalFig. 6 Coil Pump Piped Primary/SecondaryFig. 7 Pumped Hot-Water Coil Variations: (A) Series and (B) ParallelFig. 8 Electric Heat: Solid-State Controller47.4 2015 ASHRAE HandbookHVAC Applications (SI)Electrical Code(NFPA Standard 70) or local codes. An electricheater must have a minimum airflow switch and two high-tempera-ture limit sensors: one with manual reset and one with automaticreset (Figure 9). The automatic reset high-limit thermostat normally