1、36.1CHAPTER 36HYDRONIC HEAT-DISTRIBUTING UNITS AND RADIATORSDescription 36.1Ratings of Heat-Distributing Units 36.2Design . 36.3Applications. 36.5ADIATORS, convectors, and baseboard and finned-tube unitsRare heat-distributing devices used in low-temperature andsteam water-heating systems. They suppl
2、y heat through a combina-tion of radiation and convection and maintain the desired air tem-perature and/or mean radiant temperature in a space without fans.Figures 1 and 2 show sections of typical heat-distributing units. Inlow-temperature systems, radiant panels are also used. Units areinherently s
3、elf-adjusting in the sense that heat output is based ontemperature differentials; cold spaces receive more heat and warmerspaces receive less heat. DESCRIPTIONRadiatorsThe term radiator, though generally confined to sectional cast-iron column, large-tube, or small-tube units, also includes flat-pane
4、ltypes and fabricated steel sectional types. Small-tube radiators, witha length of only 1.75 in. per section, occupy less space than columnand large-tube units and are particularly suited to installation inrecesses (see Table 1). Column, wall-type, and large-tube radiatorsare no longer manufactured,
5、 although many of these units are still inuse. Refer to Tables 2, 3, and 4 in Chapter 28 of the 1988 ASHRAEHandbookEquipment, Byrley (1978), or Hydronics Institute (1989)for principal dimensions and average ratings of these units.The following are the most common types of radiators:Sectional radiato
6、rs are fabricated from welded sheet metal sec-tions (generally two, three, or four tubes wide), and resemble free-standing cast-iron radiators.Panel radiators consist of fabricated flat panels (generally one,two, or three deep), with or without an exposed extended fin surfaceattached to the rear for
7、 increased output. These radiators are mostcommon in Europe.Tubular steel radiators consist of supply and return headerswith interconnecting parallel steel tubes in a wide variety of lengthsand heights. They may be specially shaped to coincide with thebuilding structure. Some are used to heat bathro
8、om towel racks.Specialty radiators are fabricated of welded steel or extrudedaluminum and are designed for installation in ceiling grids or floor-mounting. Various unconventional shapes are available.Pipe CoilsPipe coils have largely been replaced by finned tubes. See Table5 in Chapter 28 of the 198
9、8 ASHRAE HandbookEquipment for theheat emission of such pipe coils.ConvectorsA convector is a heat-distributing unit that operates with gravity-circulated air (natural convection). It has a heating element with alarge amount of secondary surface and contains two or more tubeswith headers at both end
10、s. The heating element is surrounded by anenclosure with an air inlet below and an air outlet above the heatingelement.Convectors are made in a variety of depths, sizes, and lengths andin enclosure or cabinet types. The heating elements are available infabricated ferrous and nonferrous metals. The a
11、ir enters the enclo-sure below the heating element, is heated in passing through the ele-ment, and leaves the enclosure through the outlet grille located abovethe heating element. Factory-assembled units comprising a heatingelement and an enclosure have been widely used. These may be free-standing,
12、wall-hung, or recessed and may have outlet grilles or lou-vers and arched inlets or inlet grilles or louvers, as desired.Baseboard UnitsBaseboard (or baseboard radiation) units are designed for instal-lation along the bottom of walls in place of the conventional base-board. They may be made of cast
13、iron, with a substantial portion ofthe front face directly exposed to the room, or with a finned-tube ele-ment in a sheet metal enclosure. They use gravity-circulated room air.Baseboard heat-distributing units are divided into three types:radiant, radiant convector, and finned tube. The radiant unit
14、, whichis made of aluminum, has no openings for air to pass over the wallside of the unit. Most of this units heat output is by radiation.The radiant-convector baseboard is made of cast iron or steel.The units have air openings at the top and bottom to allow circula-tion of room air over the wall si
15、de of the unit, which has extendedsurface to provide increased heat output. A large portion of the heatemitted is transferred by convection.The finned-tube baseboard has a finned-tube heating elementconcealed by a long, low sheet metal enclosure or cover. A majorportion of the heat is transferred to
16、 the room by convection. The out-put varies over a wide range, depending on the physical dimensionsand the materials used. A unit with a high relative output per unitlength compared to overall heat loss (which would result in a con-centration of the heating element over a relatively small area) shou
17、ldbe avoided. Optimum comfort for room occupants is obtained whenunits are installed along as much of the exposed wall as possible.Finned-Tube UnitsFinned-tube (or fin-tube) units are fabricated from metallic tub-ing, with metallic fins bonded to the tube. They operate with grav-ity-circulated room
18、air. Finned-tube elements are available inseveral tube sizes, in either steel or copper (1 to 2 in. nominal steelor 3/4 to 1 1/4 in. nominal copper) with various fin sizes, spacings,and materials. The resistance to the flow of steam or water is thesame as that through standard distribution piping of
19、 equal size andtype.Finned-tube elements installed in occupied spaces generally havecovers or enclosures in a variety of designs. When human contact isunlikely, they are sometimes installed bare or provided with anexpanded metal grille for minimum protection.The preparation of this chapter is assign
20、ed to TC 6.1, Hydronic and SteamEquipment and Systems.36.2 2012 ASHRAE HandbookHVAC Systems and Equipment A cover has a portion of the front skirt made of solid material.The cover can be mounted with clearance between the wall and thecover, and without completely enclosing the rear of the finned-tub
21、eelement. A cover may have a top, front, or inclined outlet. Anenclosure is a shield of solid material that completely encloses boththe front and rear of the finned-tube element. The enclosure mayhave an integral back or may be installed tightly against the wall sothat the wall forms the back, and i
22、t may have a top, front, orinclined outlet. Heat EmissionThese heat-distributing units emit heat by a combination of radi-ation to the surfaces and occupants in the space and convection tothe air in the space.Chapter 4 of the 2009 ASHRAE HandbookFundamentals cov-ers the heat transfer processes and t
23、he factors that influence them.Those units with a large portion of their heated surface exposed tothe space (i.e., radiator and cast-iron baseboard) emit more heat byradiation than do units with completely or partially concealed heat-ing surfaces (i.e., convector, finned-tube, and finned-tube base-b
24、oard). Also, finned-tube elements constructed of steel emit alarger portion of heat by radiation than do finned-tube elementsconstructed of nonferrous materials.The heat output ratings of heat-distributing units are expressed inBtu/h, MBh (1000 Btu/h), or in square feet equivalent direct radia-tion
25、(EDR). By definition, 240 Btu/h = 1 ft2EDR with 1 psig steam.RATINGS OF HEAT-DISTRIBUTING UNITSFor convectors, baseboard units, and finned-tube units, an allow-ance for heating effect may be added to the test capacity (the heatextracted from the steam or water under standard test conditions).This he
26、ating effect reflects the ability of the unit to direct its heatoutput to the occupied zone of a room. The application of a heatingeffect factor implies that some units use less steam or hot water thanothers to produce an equal comfort effect in a room.RadiatorsCurrent methods for rating radiators w
27、ere established by the U.S.National Bureau of Standards publication, Simplified PracticesRecommendation R174-65, Cast-Iron Radiators, which has beenwithdrawn (Table 1).ConvectorsThe generally accepted method of testing and rating ferrous andnonferrous convectors in the United States was given in Com
28、mer-cial Standard CS 140-47, Testing and Rating Convectors (Dept. ofFig. 1 Terminal UnitsFig. 1 Terminal UnitsFig. 2 Typical RadiatorsFig. 2 Typical RadiatorsHydronic Heat-Distributing Units and Radiators 36.3Commerce 1947), but it has been withdrawn. This standard con-tained details covering constr
29、uction and instrumentation of the testbooth or room and procedures for determining steam and waterratings.Under the provisions of Commercial Standard CS 140-47, therating of a top outlet convector was established at a value not inexcess of the test capacity. For convectors with other types of enclo-
30、sures or cabinets, a percentage that varies up to a maximum of 15%(depending on the height and type of enclosure or cabinet) wasadded for heating effect (Brabbee 1926; Willard et al. 1929). Theaddition made for heating effect must be shown in the manufac-turers literature.The testing and rating proc
31、edure set forth by CommercialStandard CS 140-47 does not apply to finned-tube or baseboardradiation.Baseboard UnitsThe generally accepted method of testing and rating base-boards in the United States is covered in the Testing and RatingStandard for Baseboard Radiation (Hydronics Institute 1990a).Thi
32、s standard contains details covering construction and instru-mentation of the test booth or room, procedures for determiningsteam and hot-water ratings, and licensing provisions for obtain-ing approval of these ratings. Baseboard ratings include an allow-ance for heating effect of 15% in addition to
33、 the test capacity. Theaddition made for heating effect must be shown in the manufac-turers literature.Finned-Tube UnitsThe generally accepted method of testing and rating finned-tube units in the United States is covered in the Testing and RatingStandard for Finned-Tube (Commercial ) Radiation (Hyd
34、ronicsInstitute 1990b). This standard contains details covering construc-tion and instrumentation of the test booth or room, procedures fordetermining steam and water ratings, and licensing provisions forobtaining approval of these ratings.The rating of a finned-tube unit in an enclosure that has a
35、top out-let is established at a value not in excess of the test capacity. Forfinned-tube units with other types of enclosures or covers, a percent-age is added for heating effect that varies up to a maximum of 15%,depending on the height and type of enclosure or cover. The additionmade for heating e
36、ffect must be shown in the manufacturers litera-ture (Pierce 1963).Other Heat-Distributing UnitsUnique radiators and radiators from other countries generally aretested and rated for heat emission in accordance with prevailingstandards. These other testing and rating methods have basically thesame pr
37、ocedures as the Hydronics Institute standards, which areused in the United States. See Chapter 6 for information on thedesign and sizing of radiant panels.Corrections for Nonstandard Conditions The heating capacity of a radiator, convector, baseboard, finned-tube heat-distributing unit, or radiant p
38、anel is a power function ofthe temperature difference between the air in the room and the heat-ing medium in the unit, shown asq = c(ts ta)n(1)whereq = heating capacity, Btu/hc = constant determined by testts= average temperature of heating medium, F. For hot water, the arithmetic average of the ent
39、ering and leaving water temperatures is used.ta= room air temperature, F. Air temperature 60 in. above the floor is generally used for radiators, whereas entering air temperature is used for convectors, baseboard units, and finned-tube units.n = exponent that equals 1.3 for cast-iron radiators, 1.4
40、for baseboard radiation, 1.5 for convectors, 1.0 for ceiling heating and floor cooling panels, and 1.1 for floor heating and ceiling cooling panels. For finned-tube units, n varies with air and heating medium temperatures. Correction factors to convert heating capacities at standard rating condition
41、s to heating capacities at other conditions are given in Table 2.Equation (1) may also be used to calculate heating capacity at non-standard conditions.DESIGNEffect of Water VelocityDesigning for high temperature drops through the system (asmuch as 60 to 80F in low-temperature systems and as much as
42、200F in high-temperature systems) can result in low water veloci-ties in the finned-tube or baseboard element. Applying very shortruns designed for conventional temperature drops (i.e., 20F) canalso result in low velocities.Table 1 Small-Tube Cast-Iron RadiatorsNumber of Tubes perSectionSection Dime
43、nsionsCatalog Ratingper Section,aAHeight,in.bBWidth, in.CSpacing,in.cDLeg Height,in.bft2Btu/h Min. Max.3 1.6 384 25 3.25 3.50 1.75 2.5041.6 384 19 4.44 4.81 1.75 2.501.8 432 22 4.44 4.81 1.75 2.502.0 480 25 4.44 4.81 1.75 2.5052.1 504 22 5.63 6.31 1.75 2.502.4 576 25 5.63 6.31 1.75 2.5062.3 552 19 6
44、.81 8 1.75 2.503.0 720 25 6.81 8 1.75 2.503.7 888 32 6.81 8 1.75 2.50aRatings based on steam at 215F and air at 70F. They apply only to installed radiatorsexposed in a normal manner, not to radiators installed behind enclosures, behind grilles,or under shelves. For Btu/h ratings at other temperature
45、s, multiply table values by factorsfound in Table 2.bOverall height and leg height, as produced by some manufacturers, are 1 in. greaterthan shown in Columns A and D. Radiators may be furnished without legs. Wheregreater than standard leg heights are required, leg height should be 4.5 in.cLength equ
46、als number of sections multiplied by 1.75 in.36.4 2012 ASHRAE HandbookHVAC Systems and Equipment Figure 3 shows the effect of water velocity on the heat output oftypical sizes of finned-tube elements. The figure is based on workdone by Harris (1957) and Pierce (1963) and tests at the HydronicsInstit
47、ute. The velocity correction factor FvisFv= (V/3.0)0.04(2)where V = water velocity, fps.Heat output varies little over the range from 0.5 to 3 fps, whereFvranges from 0.93 to 1.00. The factor drops rapidly below 0.5 fpsbecause flow changes from turbulent to laminar at around 0.1 fps.Such a low veloc
48、ity should be avoided because the output is difficultto predict accurately when designing a system. In addition, thecurve is so steep in this region that small changes in actual flow havea significant effect on output. Not only does the heat transfer ratechange, but the temperature drop and, therefo
49、re, the average watertemperature change (assuming a constant inlet temperature).The designer should check water velocity throughout the systemand select finned-tube or baseboard elements on the basis of veloc-ity as well as average temperature. Manufacturers of finned-tubeand baseboard elements offer a variety of tube sizes, ranging from0.5 in. copper tubes for small baseboard elements to 2 in. for largeTable 2 Correction Factors c for Various Types of Heating UnitsSteam Pressure(Approx.)Steam or Water Temp., FCast-Iron Radiator Convector Finned-Tube BaseboardRoom Temp., F I
