1、Fundamentals of Water System DesignSecond EditionMark Hegberg Richard A. HegbergA Course Book forSelf-Directed or Group LearningI-PInch-PoundIncludes Skill Development Exercises for PDH, CEU, or LU CreditsSDL_cover_I-P.indd 1 10/22/2015 1:52:53 PMFundamentals of Water System DesignSecond EditionMark
2、 HegbergRichard A. HegbergAtlantaFundamentals of Water System Design (I-P), Second EditionA Course Book for Self-Directed or Group LearningISBN 978-1-936504-66-4 (paperback)ISBN 978-1-939200-04-4 (PDF)SDL Number: 98020 1996, 2000, 2015 ASHRAEAll rights reserved.ASHRAE is a registered trademark in th
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10、 modern application of hot-water systems may have started in the1870s in the United States, and the technology has been refined ever since. Ofthe many benefits that water plays as a heat transfer medium for heating orcooling, fundamentally important is the ability to transport a lot of energy inthe
11、small area of a pipe (especially when compared to the equivalent size of anair duct). That benefit allowed for the development of central systems, elimi-nating the necessity of a local heating production system for each heated zone.The advancement allowed us to exercise individuality in our own buil
12、ding zonecomfort control and lead to widespread application of hydronic systems inHVAC. From the early days of gravity circulation hot-water heating systems tothe most complex of district energy chilled-water systems that are used today,the fundamentals presented here can be employed to successfully
13、 design themodern hydronic system.AcknowledgmentsASHRAE TC 6.1 and its members have played a key role in collecting,researching, and communicating the details and experiences for the successfulapplication of hydronic systems over the years. TC 6.1 is an ad-hoc collectionof individuals interested in
14、hydronic system technology who take the time tovolunteer and attend meetings and then communicate what they have learned tothe other 53,000 members of ASHRAE.Key in the educational organizing efforts of TC 6.1 in hydronic systemsand ASHRAE, a few individuals stand out when this text and the general
15、con-cept of the ASHRAEs professional development series (be they Self DirectedLearning manuals or the face-to-face version of the Professional DevelopmentSeminar) are considered. At the apex of this is William (Bill) J. Coad, formerASHRAE President and longtime member of several ASHRAE committees.Wh
16、en the professional development series was first developed, Bill Coad, J.Barry Graham, and Gerry Williams developed the first two classes on Air Sys-tem Design and Energy Efficient HVAC. Bills significant involvement in lead-ing many ASHRAE activities left little time to develop the Water Systemsman
17、ual, which lead to Dick Hegberg taking up the charge. All of these volun-teer member contributors are owed a deep debt of gratitude for their devotionto helpingASHRAE develop the initial active training efforts that have evolvedinto the many practical training programs that are available today.ASHRA
18、E has, from its very inception as ASHVE, represented a uniquecooperative collaboration between all aspects of the HVAC industry. Design-ers, owners, operators, academicians, and manufacturers have all met anddebated the merits of all things HVACa75 understand the difference between closed and open s
19、ystems;a75 know the components of a hydronic system;a75 understand heating versus cooling source devices;a75 understand how systems meet part-load conditions;a75 identify temperature and pressure ranges for low-, medium-, and high-temperature water systems;a75 know what sensible, latent, and total h
20、eat loads are and how they affectdesign water flow;a75 identify examples of heating and cooling load devices; anda75 know how load diversity suggests a reduction in the total cooling capacityrequired.InstructionsRead Chapter 1, and answer all of the questions at the end.Introductory ConceptsWater sy
21、stem design depends on the designers ability to evaluate the spaceloads, occupancy patterns, and indoor environment requirements. This chapterexamines the actual process of water system design and provides informationon how to evaluate space loads. It also provides strategies and formulas formasteri
22、ng the key requirements for water systems.Water systems that convey heat to or from a conditioned space or processwith hot or chilled water are frequently called hydronic systems. In general,these systems employ centrifugal pumps to force water flow from a heatingor a cooling source to the condition
23、ed space or load by means of various pip-ing, pumping, control, and terminal arrangements (ASHRAE 2012).2 Chapter 1 Water System Design ConceptsGiven the design requirements, it is the designers task to evaluate thespace loads resulting from building construction, weather distribution, occu-pancy pa
24、tterns, indoor environment requirements, and other internal loads todetermine the total load (Figure 1-1) subject to the local building codes. Theloads include transmission, solar radiation, infiltration, ventilation air, peo-ple, lights, power, appliances, and materials in and out (Sauer and Howell
25、2013).The historical weather distribution for the project location is important, anda means to control the systems at part-load conditions to maintain comfort con-ditions for the occupants must be studied for proper design. The designer mustweigh the cost of the source utilities available and also t
26、he efficiencies of boil-ers and chillers or other hydronic sources to determine the most efficient sys-tem design (Figure 1-2). The system must be able to operate between part-loadand full-load conditions. In many cases, the hydronic system is a support sys-tem providing the heating or cooling mediu
27、m for heat transfer equipment in anair distribution system. This course is intended to acquaint the student with thevarious hydronic principles and practices available for consideration in a proj-ects design concept.Figure 1-3 shows the basic components of a hydronic system that thedesigner must def
28、ine for an HVAC system, namely, a source of heating or cool-ing, a distribution system, and the load components. There are different classi-fications of hydronic systems; the most common types are summarized below.As you familiarize yourself with the various systems, remember that differentfactors m
29、ay come into play for each.Water systems may be closed or open types. The fundamental differencebetween them is the interface of the water with a compressible gas (such asair) or an elastic surface (such as a diaphragm). A closed water system isFigure 1-1 Source/load.Fundamentals of Water System Des
30、ign I-P 3defined as one with no more than one point of interface with a compressiblegas (air) or surface (Figure 1-4). This definition is fundamental to under-standing the hydraulic dynamics of these systems (to be discussed in the sec-tion Expansion Chamber).Figure 1-2 Sourcedistributionload.Figure
31、 1-3 Sourcedistributionpart-load.4 Chapter 1 Water System Design ConceptsAn open system has more than one such interface. For example, a coolingtower has at least two points of interface: the tower basin and the dischargepipe or nozzles entering the tower.Comparing Figure 1-4 and Figure 1-5, the dif
32、ferences between the hydrau-lics of the systems become evident as one analyzes the two systems. However,one major difference is that certain hydraulic characteristics of open systemsdo not occur in closed systems. For example, in a closed system,flow cannot be motivated by static head difference,pum
33、ps do not provide static lift, andthe entire piping system is always filled with water.These factors affect the installation and operating costs of the system overits service life.Closed water systems are classified by operating temperature:Low-temperature water (LTW) system. This hydronic heating s
34、ystemoperates within the pressure and temperature limits of the ASME Boilerand Pressure Vessel Code for low-pressure boilers (Figure 1-6). The maxi-mum allowable working pressure for low-pressure boilers is 160 psig, witha maximum temperature of 250F. The usual maximum working pressurefor LTW boiler
35、 systems is 30 psig; however, boilers specifically designed,tested, and stamped for higher pressures are frequently used. Steam-to-water and water-to-water heat exchangers are also used for heating LTW.Figure 1-4 Hydronic system fundamentals (closed system).Fundamentals of Water System Design I-P 5M
36、edium-temperature water (MTW) system. This hydronic heating sys-tem operates at temperatures between 250F and 350F, with pressures notexceeding 160 psig (Figure 1-7). The design supply water temperature isapproximately 250F to 325F, with a pressure rating of 150 psig for boil-ers and equipment.High-
37、temperature water (HTW) system. This hydronic heating systemoperates at temperatures over 350F, with pressures not exceeding 300 psig.The maximum design supply water temperature is about 400F, with a pres-sure rating for boilers and equipment of 300 psig (Figure 1-7). The pressure/temperature rating
38、 of each component should be checked for compliancewith the systems design versus the manufacturers rating.Figure 1-5 Cooling tower (open system).6 Chapter 1 Water System Design ConceptsFigure 1-6 Low-temperature water systemdirect or reverse return.Figure 1-7 Medium- or high-temperature water syste
39、m. Fundamentals of Water System Design I-P 7Chilled-water (CHW) system. A hydronic cooling system normally oper-ates with a design supply water temperature of 40F to 55F (usually 44For 45F) with a pressure rating of 120 psig. Figure 1-8 shows a small- tomedium-sized system with constant-speed pumpin
40、g using three-way valvesto ensure constant flow in the chiller source and balancing valves on eachload for flow measurement and adjustment. Larger systems may employtwo-way control valves and different chiller piping and pumping arrange-ments to reduce pumping power.Condenser water (CW) system. Open
41、 water systems are typically used inrefrigeration CW systems as once-through or cooling tower systems. Fig-ure 1-9 shows a water-cooled condenser using city, well, or river water. Thereturn is run higher than the condenser so that the condenser is always fullof water. Water flow through the condense
42、r is modulated by a control valvein the supply line. This is usually actuated by a condenser head pressure tomaintain a constant condensing temperature with load variations.Figure 1-10 shows two cooling tower applications to protect against lowoutdoor temperature conditions. Water flows to the pump
43、from the tower basin,and the level should be above the top of the pump casing for positive prime,and piping pressure drop should be minimized.Figure 1-8 Chilled-water systemdirect return piping.8 Chapter 1 Water System Design ConceptsFigure 1-9 Condenser open water system (once through).Figure 1-10
44、Condenser cooling tower system inside reservoir or heated sump.Fundamentals of Water System Design I-P 9Antifreeze or brine solutions may be used for applications (process appli-cations) that require temperatures below 40F or for coil freeze protection.Well water systems can use supply temperatures
45、of 60F or higher.In addition, there are other forms of system classification that tend to blurthe lines between system operating temperatures, pipe layout styles, and/or areference to the operated system style.Four-pipe dual temperature water systems are one such example of thisblur. This older exam
46、ple of system design combines the heating and coolingsystem, circulating hot and/or chilled water through separate supply andreturn pipes to common terminal coils. Figure 1-11 shows a very simplifiedpiping diagram for this type of system, which would not normally be pipedthis way. This system operat
47、es within the pressure and temperature limits ofLTW systems, with usual winter design supply water temperatures of about100F to 150F and summer supply water temperatures of 40F to 45F. Areading of ANSI/ASHRAE/IES Standard 90.1 (ASHRAE 2013) can suggestthat this type of system (especially as shown) i
48、s not allowed due to energyperformance issues associated with potential mixing of hot or cold condi-tioned water. Of course, given enough thought and control logic, this issuemay be overcome.A more common application is to have a two-pipe dual-temperature sys-tem where a common piping system serves
49、common loads with both hot andchilled water supplied seasonally to the system. In either case, some of theissues a designer must deal with start to show themselves so that the systemoperates properly. Think through the components and issues like thedesigner!Figure 1-11 Distribution orientation. 10 Chapter 1 Water System Design ConceptsThe design should consider source protection to prevent temperatureshocks to the chiller or boiler on cycle changeover. The designer might con-sider timing mechanisms, temperature mechanisms, or combinations of both s
