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

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1、13.1CHAPTER 13HYDRONIC HEATING AND COOLINGTemperature Classifications. 13.1CLOSED WATER SYSTEMS 13.2Method 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 20Fwas used to determine flow rate. Besides producing satisfactoryoperation and economy in small systems, this t enabled simple cal-culations because 1 gpm conveys 10,000 Btuh. However, almostuniversal use of hydronic systems for both heating and cooling oflarge buildings an

11、d building complexes has rendered this simplifiedapproach obsolete.TEMPERATURE CLASSIFICATIONSWater systems can be classified by operating temperature as fol-lows.Low-temperature water (LTW) systems operate within thepressure and temperature limits of the ASME Boiler and PressureVessel Code for low-

12、pressure boilers. The maximum allowableworking pressure for low-pressure boilers is 160 psig, with a maxi-mum temperature of 250F. The usual maximum working pressurefor boilers for LTW systems is 30 psi, although boilers specificallydesigned, tested, and stamped for higher pressures are frequentlyus

13、ed. Steam-to-water or water-to-water heat exchangers are alsoused for heating low-temperature water. Low-temperature watersystems are used in buildings ranging from small, single dwellingsto very large and complex structures.Medium-temperature water (MTW) systems operate between250 and 350F, with pr

14、essures not exceeding 160 psi. The usualdesign supply temperature is approximately 250 to 325F, with ausual pressure rating of 150 psi for boilers and equipment.High-temperature water (HTW) systems operate at tempera-tures over 350F and usual pressures of about 300 psi. The maxi-mum design supply wa

15、ter temperature is usually about 400F, witha pressure rating for boilers and equipment of about 300 psi. Thepressure-temperature rating of each component must be checkedagainst the systems design characteristics.Chilled-water (CW) systems for cooling normally operate witha design supply water temper

16、ature of 40 to 55F (usually 44 or45F), and at a pressure of up to 120 psi. Antifreeze or brine solu-tions may be used for applications (usually process applications)that require temperatures below 40F or for coil freeze protection.Well-water systems can use supply temperatures of 60F or higher.Dual-

17、temperature water (DTW) systems combine heating andcooling, and circulate hot and/or chilled water through common pip-ing and terminal heat transfer apparatus. These systems operatewithin the pressure and temperature limits of LTW systems, withusual winter design supply water temperatures of about 1

18、00 to150F and summer supply water temperatures of 40 to 45F.Terminal heat transfer units include convectors, cast-iron radia-tors, baseboard and commercial finned-tube units, fan-coil units,unit heaters, unit ventilators, central station air-handling units, radi-ant panels, and snow-melting panels.

19、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 supply-ing energy.This chapter covers the principles and procedures for designingand selecting piping and components for low-temperature water,chille

20、d water, and dual-temperature water systems. See Chapter 14for information on medium- and high-temperature water systems.The preparation of this chapter is assigned to TC 6.1, Hydronic and SteamEquipment and Systems.13.2 2012 ASHRAE HandbookHVAC Systems and Equipment CLOSED WATER SYSTEMSBecause most

21、 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 surface(such as a diaphragm). A closed water system is defined a

22、s 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 definition is fundamental to understanding the hydraulicdynamics of these systems. Earlier literature referred to a systemwith an open or vented expansio

23、n tank as an “open” system, but thisis actually a closed system; the atmospheric interface of the tanksimply establishes the system pressure.An open 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 an

24、d the discharge pipe or nozzlesentering the tower. One major difference in hydraulics betweenopen and closed systems is that some hydraulic characteristics 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

25、be motivated by static head differences, (2) pumps do notprovide static lift, and (3) the entire piping system is always filledwith water.Figure 1 shows the fundamental components of a closed hydronicsystem. Actual systems generally have additional components suchas valves, vents, regulators, etc.,

26、but these are not essential to thebasic principles underlying the system.These fundamental components areLoadsSourceExpansion chamberPumpDistribution systemTheoretically, a hydronic system could operate with only these fivecomponents.The components are subdivided into two groups: thermal andhydrauli

27、c. Thermal components consist of the load, source, andexpansion chamber. Hydraulic components consist of the distributionsystem, pump, and expansion chamber. The expansion chamber is theonly component that serves both a thermal and a hydraulic function.METHOD OF DESIGNThis section outlines general s

28、teps a designer may follow to com-plete system design. The methodology is not a rigid framework, butrather a flexible outline that should be adapted by the designer tosuit current needs. The general order as shown is approximatelychronological, but it is important to note that succeeding steps often

29、affect preceding steps, so a fundamental reading of this entire chap-ter is required to fully understand the design process.1. Determine system and zone loads. Loads are covered in Chap-ters 14 to 19 of the 2009 ASHRAE HandbookFundamentals.Several load calculation procedures have been developed, wit

30、hvarying degrees of calculation accuracy. The load determinesthe flow of the hydronic system, which ultimately affects thesystems heat transfer ability and energy performance. Design-ers should apply the latest computerized calculation methods foroptimal system design. Load calculation should also d

31、etail thefacilitys loading profile facility to enhance the hydronic systemcontrol strategy.2. Select comfort heat transfer devices. This often means a coil-or water-to-air heat exchanger (terminal). Coil selection andoperation has the single largest influence on hydronic systemdesign. Coils implemen

32、t the design criteria of flow, temperaturedrop, and control ability. Coil head loss and location affectspipe design and sizing, control devices, 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

33、 styles may be appropriate for a givendesign. Styles may be comingled in a successful hydronic sys-tem design to optimize building performance. Schematicallylay out the system to establish a preliminary design.4. Size branch piping system. Based on the selection of the coil,its controlling devices,

34、style of installation, and location,branch piping is sized to provide required flow, and head loss iscalculated.5. Calculate distribution piping head loss. Although the criteriafor pipe selection in branch and distribution system piping maybe similar, understanding the relationship and effect of dis

35、tribu-tion system head loss is important in establishing that all termi-nals get the required flow for the required heat transfer.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

36、the calculations of actualdesign head loss are complete, note the losses on the drawingsfor the commissioning process.7. Select pump specialties. Any devices required for operation ormeasurement are identified, so their head loss can be deter-mined and accounted for in pump selection.8. Select air m

37、anagement methodology. All hydronic systemsentrain air in the circulated fluid. Managing the collection of thatair as 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 verysma

38、ll (e.g., a residential hot-water heating system), the pump isselected to fit the system. A significant portion of energy 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 ineffi

39、ciency andpoor distribution and heat transfer.10. Determine installation details, iterate design. Tuning thedesign to increase performance and cost effectiveness is animportant last step. Documenting installation details is alsoimportant, because this communication is necessary for well-built design

40、s and properly operated systems.THERMAL COMPONENTSLoadsThe load is the device that causes heat to flow out of or into 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 inwar

41、d heat flow charac-terizes a cooling system. The quantity of heating or cooling is cal-culated by one of the following means.Sensible Heating or Cooling. The rate of heat entering or leav-ing an airstream is expressed as follows:Fig. 1 Fundamental Components of Hydronic SystemHydronic Heating and Co

42、oling 13.3q = 60Qaacpt (1)whereq = heat transfer rate to or from air, BtuhQa= airflow rate, cfma= density of air, lbft3cp= specific heat of air, Btulb Ft = temperature increase or decrease of air, FFor standard air with a density of 0.075 lbft3and a specific heatof 0.24 BtulbF, Equation (1) becomesq

43、 = 1.1Qat (2)The heat exchanger or coil must then transfer this heat from or tothe water. The rate of sensible heat transfer to 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 med

44、ium; and the overall heat transfer coefficient,which itself is a function of the fluid velocities, properties of themedium, geometry 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, BtuhU=o

45、verall coefficient of heat transfer, Btuhft2FA=heat transfer surface area, ft2LMTD = logarithmic mean temperature difference, heated or cooled medium to water, FCooling and Dehumidification. The rate of heat removal fromthe cooled medium when both sensible cooling and dehumidifica-tion are present i

46、s expressed byqt= wh (4)whereqt= total heat transfer rate from cooled medium, Btuhw=mass flow rate of cooled medium, lbhh=enthalpy difference between entering and leaving conditions of cooled medium, BtulbExpressed for an air-cooling coil, this equation becomesqt= 60Qaah (5)which, for standard air w

47、ith a density of 0.075 lbft3, reduces toqt= 4.5Qah (6)Heat Transferred to or from Water. The rate of heat transfer toor from the water 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 w

48、ater is expressed byqw= cpt (7)whereqw= heat transfer rate to or from water, Btuh= mass flow rate of water, lbhcp= specific heat of water, Btulb Ft=water temperature increase or decrease across unit, FWith water systems, it is common to express the flow rate as vol-umetric flow, in which case Equati

49、on (7) becomesqw= 8.02wcpQwt (8)whereQw= water flow rate, gpmw= density of water, lbft3For standard conditions in which the density is 62.4 lbft3and thespecific heat is 1 Btulb F, Equation (8) becomesqw= 500Qwt (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. Equati