ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 13-2012 HYDRONIC HEATING AND COOLING.pdf

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1、13.1CHAPTER 13HYDRONIC HEATING AND COOLINGTemperature Classifications. 13.1CLOSED WATER SYSTEMS 13.1Method of Design. 13.2Thermal Components. 13.2Hydraulic Components 13.6Piping Circuits . 13.11Capacity Control of Load System 13.13Low-Temperature Heating Systems . 13.16Chilled-Water Systems . 13.17D

2、ual-Temperature Systems. 13.19Other Design Considerations. 13.20Other Design Procedures. 13.22Antifreeze Solutions 13.23ATER systems that convey heat to or from a conditionedWspace or process with hot or chilled water are frequentlycalled hydronic systems. Water flows through piping that connects ab

3、oiler, water heater, or chiller to suitable terminal heat transfer unitslocated at the space or process.Water systems can be classified by (1) operating temperature,(2) flow generation, (3) pressurization, (4) piping arrangement, and(5) pumping arrangement.Classified by flow generation, hydronic hea

4、ting systems may be(1) gravity systems, which use the difference in density betweenthe supply and return water columns of a circuit or system to circu-late water; or (2) forced systems, in which a pump, usually drivenby an electric motor, maintains flow. Gravity systems are seldomused today and are

5、therefore not discussed in this chapter. See theASHVE Heating Ventilating Air Conditioning Guide issued before1957 for information on gravity systems.Water systems can be either once-through or recirculating sys-tems. This chapter describes forced recirculating systems.Successful water system design

6、 depends on awareness of themany complex interrelationships between various elements. In apractical sense, no component can be selected without consideringits effect on the other elements. For example, design water temper-ature and flow rates are interrelated, as are the system layout andpump select

7、ion. The type and control of heat exchangers used affectthe flow rate and pump selection, and the pump selection and distri-bution affect the controllability. The designer must thus work backand forth between tentative points and their effects until a satisfac-tory integrated design has been reached

8、. Because of these relation-ships, rules of thumb usually do not lead to a satisfactory design.PrinciplesEffective and economical water system design is affected bycomplex relationships between the various system components. Thedesign water temperature, flow rate, piping layout, pump selection,termi

9、nal unit selection, and control method are all interrelated. Sys-tem size and complexity determine the importance of these relation-ships to the total system operating success. In the United States,present hydronic heating system design practice originated in resi-dential heating applications, where

10、 a temperature drop t of 11 Kwas used to determine flow rate. However, almost universal use ofhydronic systems for both heating and cooling of large buildingsand building complexes has rendered this simplified approach ob-solete.TEMPERATURE CLASSIFICATIONSWater systems can be classified by operating

11、 temperature as fol-lows.Low-temperature water (LTW) systems operate within thepressure and temperature limits of the ASME Boiler and PressureVessel Code for low-pressure boilers. The maximum allowableworking pressure for low-pressure boilers is 1100 kPa (gage), witha maximum temperature of 120C. Th

12、e usual maximum workingpressure for boilers for LTW systems is 200 kPa, although boilersspecifically designed, tested, and stamped for higher pressures arefrequently used. Steam-to-water or water-to-water heat exchangersare also used for heating low-temperature water. Low-temperaturewater systems ar

13、e used in buildings ranging from small, singledwellings to very large and complex structures.Medium-temperature water (MTW) systems operate between120 and 175C, with pressures not exceeding 1100 kPa. The usualdesign supply temperature is approximately 120 to 160C, with ausual pressure rating of 1 MP

14、a for boilers and equipment.High-temperature water (HTW) systems operate at tempera-tures over 175C and usual pressures of about 2 MPa. The maxi-mum design supply water temperature is usually about 200C, witha pressure rating for boilers and equipment of about 2 MPa. Thepressure-temperature rating o

15、f each component must be checkedagainst the systems design characteristics.Chilled-water (CW) systems for cooling normally operate witha design supply water temperature of 4 to 13C (usually 7C), andat a pressure of up to 830 kPa. Antifreeze or brine solutions may beused for applications (usually pro

16、cess applications) that requiretemperatures below 4C or for coil freeze protection. Well-watersystems can use supply temperatures of 15C or higher.Dual-temperature water (DTW) systems combine heating andcooling, and circulate hot and/or chilled water through common pip-ing and terminal heat transfer

17、 apparatus. These systems operatewithin the pressure and temperature limits of LTW systems, withusual winter design supply water temperatures of about 38 to 65Cand summer supply water temperatures of 4 to 7C.Terminal heat transfer units include convectors, cast-iron radia-tors, baseboard and commerc

18、ial finned-tube units, fan-coil units,unit heaters, unit ventilators, central station air-handling units, radi-ant panels, and snow-melting panels. A large storage tank may beincluded in the system to store energy to use when heat inputdevices such as the boiler or a solar energy collector are not s

19、upply-ing energy.This chapter covers the principles and procedures for designingand selecting piping and components for low-temperature water,chilled water, and dual-temperature water systems. See Chapter 14for information on medium- and high-temperature water systems.CLOSED WATER SYSTEMSBecause mos

20、t hot- and chilled-water systems are closed, thischapter addresses only closed systems. The fundamental differencebetween a closed and an open water system is the interface of thewater with a compressible gas (such as air) or an elastic surfaceThe preparation of this chapter is assigned to TC 6.1, H

21、ydronic and SteamEquipment and Systems.13.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)(such as a diaphragm). A closed water system is defined as onewith no more than one point of interface with a compressible gas orsurface, and that will not create system flow by changes in elevation.This d

22、efinition is fundamental to understanding the hydraulicdynamics of these systems. Earlier literature referred to a systemwith an open or vented expansion tank as an “open” system, but thisis actually a closed system; the atmospheric interface of the tanksimply establishes the system pressure.An open

23、 system, on the other hand, has more than one suchinterface. For example, a cooling tower system has at least twopoints of interface: the tower basin and the discharge pipe or nozzlesentering the tower. One major difference in hydraulics betweenopen and closed systems is that some hydraulic characte

24、ristics ofopen systems cannot occur in closed systems. For example, in con-trast to the hydraulics of an open system, in a closed system (1) flowcannot be motivated by static pressure differences, (2) pumps do notprovide static lift, and (3) the entire piping system is always filledwith water.Figure

25、 1 shows the fundamental components of a closed hydronicsystem. Actual systems generally have additional components suchas valves, vents, regulators, etc., but these are not essential to thebasic principles underlying the system.These fundamental components areLoadsSourceExpansion chamberPumpDistrib

26、ution systemTheoretically, a hydronic system could operate with only these fivecomponents.The components are subdivided into two groups: thermal andhydraulic. Thermal components consist of the load, source, andexpansion chamber. Hydraulic components consist of the distributionsystem, pump, and expan

27、sion chamber. The expansion chamber is theonly component that serves both a thermal and a hydraulic function.METHOD OF DESIGNThis section outlines general steps a designer may follow to com-plete system design. The methodology is not a rigid framework, butrather a flexible outline that should be ada

28、pted by the designer tosuit current needs. The general order as shown is approximatelychronological, but it is important to note that succeeding steps oftenaffect preceding steps, so a fundamental reading of this entire chap-ter is required to fully understand the design process.1. Determine system

29、and zone loads. Loads are covered in Chap-ters 14 to 19 of the 2009 ASHRAE HandbookFundamentals.Several load calculation procedures have been developed, withvarying degrees of calculation accuracy. The load determinesthe flow of the hydronic system, which ultimately affects thesystems heat transfer

30、ability and energy performance. Design-ers should apply the latest computerized calculation methods foroptimal system design. Load calculation should also detail thefacilitys loading profile facility to enhance the hydronic systemcontrol strategy.2. Select comfort heat transfer devices. This often m

31、eans a coil-or water-to-air heat exchanger (terminal). Coil selection andoperation has the single largest influence on hydronic systemdesign. Coils implement the design criteria of flow, temperaturedrop, and control ability. Coil pressure loss and location affectspipe design and sizing, control devi

32、ces, and pump selection. Fordetails on coils, see Chapters 23 and 27.3. Select system distribution style(s). Based on the load and itslocation, different piping styles may be appropriate for a givendesign. Styles may be comingled in a successful hydronic sys-tem design to optimize building performan

33、ce. Schematicallylay out the system to establish a preliminary design.4. Size branch piping system. Based on the selection of the coil,its controlling devices, style of installation, and location,branch piping is sized to provide required flow, and pressureloss is calculated.5. Calculate distributio

34、n piping pressure loss. Although the cri-teria for pipe selection in branch and distribution system pipingmay be similar, understanding the relationship and effect of dis-tribution system pressure loss is important in establishing thatall terminals get the required flow for the required heat transfe

35、r.6. Lay out piping system and size pipes. After preliminary cal-culations of target friction loss for the pipes, sketch the system.After the piping system is laid out and the calculations of actualdesign pressure loss are complete, note the losses on the draw-ings for the commissioning process.7. S

36、elect pump specialties. Any devices required for operation ormeasurement are identified, so their pressure loss can be deter-mined and accounted for in pump selection.8. Select air management methodology. All hydronic systemsentrain air in the circulated fluid. Managing the collection of thatair as

37、it leaves the working fluid is essential to management ofsystem pressure and the safe operation of system components.9. Select pump (hydraulic components). Unless a system is verysmall (e.g., a residential hot-water heating system), the pump isselected to fit the system. A significant portion of ene

38、rgy use ina hydronic system is transporting the fluid through the distribu-tion system. Proper pump selection limits this energy use,whereas improper selection leads to energy inefficiency andpoor distribution and heat transfer.10. Determine installation details, iterate design. Tuning thedesign to

39、increase performance and cost effectiveness is animportant last step. Documenting installation details is alsoimportant, because this communication is necessary for well-built designs and properly operated systems.THERMAL COMPONENTSLoadsThe load is the device that causes heat to flow out of or into

40、thesystem to or from the space or process; it is the independent variableto which the remainder of the system must respond. Outward heatflow characterizes a heating system, and inward heat flow charac-terizes a cooling system. The quantity of heating or cooling is cal-culated by one of the following

41、 means.Sensible Heating or Cooling. The rate of heat entering or leav-ing an airstream is expressed as follows:q = Qaacpt (1)whereq = heat transfer rate to or from air, WQa= airflow rate, Lsa= density of air, kgm3cp= specific heat of air, kJ(kgK)Fig. 1 Fundamental Components of Hydronic SystemHydron

42、ic Heating and Cooling 13.3t = temperature increase or decrease of air, KFor standard air with a density of 1.20 kgm3and a specific heatof 1.0 kJ(kgK), Equation (1) becomesq = 1.20Qat (2)The heat exchanger or coil must then transfer this heat from or tothe water. The rate of sensible heat transfer t

43、o or from the heated orcooled medium in a specific heat exchanger is a function of the heattransfer surface area; the mean temperature difference between thewater and the medium; and the overall heat transfer coefficient,which itself is a function of the fluid velocities, properties of themedium, ge

44、ometry of the heat transfer surfaces, and other factors.The rate of heat transfer may be expressed byq = UA(LMTD) (3)whereq=heat transfer rate through heat exchanger, WU=overall coefficient of heat transfer, W(m2K)A=heat transfer surface area, m2LMTD = logarithmic mean temperature difference, heated

45、 or cooled medium to water, KCooling and Dehumidification. The rate of heat removal fromthe cooled medium when both sensible cooling and dehumidifica-tion are present is expressed byqt= 1000wh (4)whereqt= total heat transfer rate from cooled medium, Ww=mass flow rate of cooled medium, kgsh=enthalpy

46、difference between entering and leaving conditions of cooled medium, kJkgExpressed for an air-cooling coil, this equation becomesqt= Qaah (5)which, for standard air with a density of 1.20 kgm3, reduces toqt= 1.20Qah (6)Heat Transferred to or from Water. The rate of heat transfer toor from the water

47、is a function of the flow rate, specific heat, andtemperature rise or drop of the water as it passes through the heatexchanger. The heat transferred to or from the water is expressed byqw= 1000 cp t (7)whereqw= heat transfer rate to or from water, W= mass flow rate of water, kgscp= specific heat of

48、water, kJ(kgK)t=water temperature increase or decrease across unit, KWith water systems, it is common to express the flow rate as vol-umetric flow, in which case Equation (7) becomesqw= wcpQwt (8)whereQw= water flow rate, Lsw= density of water, kgm3For standard conditions in which the density is 100

49、0 kgm3andthe specific heat is 4.18 kJ(kgK), Equation (8) becomesqw= 4180Qwt (9)Equation (8) or (9) can be used to express the heat transferacross a single load or source device, or any quantity of suchdevices connected across a piping system. In the design or diag-nosis of a system, the load side may be balanced with the sourceside using these equations.Heat-Carrying Capacity of Piping. Equations (8) and (9) arealso used to express the heat-carrying capacity of the piping or dis-tribution system or any portion thereof. The existing tempera