Document 6Bq3VgyQzLwD0YR3pzqzzwLZd

612 CHAPTER 34 ? " r 1965 Guide And;Data Book "Hie total refrigeration load, qt, of a cooling and dehumidifying coil (or air washer) per lb of dry air is indicated on Fig. 13 and consists of the following components: 1. The censible heat, removed from the dry air and moisture in cooling from entering temperature, - to leaving tempera ture, t%. .' 2. The latent heat, q,, removed to condense the moisture at the dew-point temperature, ti, of the entering air. 3. The heat of tubeociinq, removed from the condensate in cooling it from the condensing temperature, tt, to the leaving condensate temperature, f*. ' Items 1, 2, and 3 may be related,by -- $ = ? + ?. + q* (5) If only the total heat .value is desired,,it.may be computed by qt (A* -- A*)--- (toi--- Wj)A (6) where ,..At and At .~ enthalpy at points l and 2, respectively. tpi'and trj -- humidity, ratio' at points 1 and 2` respectively. A.*'-- enthalpy of saturated'liquid at the final tem perature, f*. If .a breakdown into latent and sensible heat components is desired,' the followihg'relations may be used: v ' The latent heat may be found from I qi - (W r- uOA/*!. (7) where .. A/*enthalpy at the condensing temperature,' ti! The sensible heat may be shown to be -- A*> -- ( -- w*)A* ^ \ (? + (ta, -;w,)(C-'C), ,(8) where - j .A* " .enthalpy.at the condensihg temperature, ^ . AJ" enthalpy, of saturated liquid`at condensing' tempera: ture, <*:'i The'last;term.in Equation 8 is the heat of'subcoding the condensate from the condensing temperature,' U, `to its.final temperature, U. Then,. - : ' i. 9m - .(Wl - fc*). ` (?) ' The finAl condensate'temperature, 4, leaving the-system is subject to substantialvariations,depending upon1the method of;coil installation, as affected by cbil'face 'orientation,' aii- flow direction,-air duct insulation, etc.'In practice, frequently ft is the same as the leaving wet-bulb temperature.-Within the normal air-conditioning range;-.precise-values-of fc'are'hot necessary since heat of the sub^oooled condensate'represents about0:5 to l.5 percent of the total refrigeration load:- : t All values for solving the forgoing equations may be found on the ASHRAE Fsychbometsic Crabt '(Chart 'l)- ahd Tables 2and 3 of Chapter 3. vu<...V|. .i Example 7: Air enters a coil at 90 F dry-bulb. 75 F wetrbulb- it le&vtsiat^fil'FJdry^hulb;'. 58'; F,' wei-oulbj'lea'vmg"water ia ivcpimed to leave at a temperature.of 54 F which is between the leaving air dew point and coiTstiiface temperature. Find the total, .latent* and sensible pooling loads on the coil. . Solution: Fromthe ASHRAE Pstcheomethic Chart, find: the following:, " ; - ><([ ..... :A*.38.42Btuper.Ib<rf.diy air., ; ,i ..-:A*!--25.10, Btu per Ib.of dry air.. . t'<r, !, ' tt = 69 F 'dew point of entering air: ' *'. tci 0.01525 lb per lb of dry air. 'u* -- 0.00960 lb per lb of dry air. . - . * '< FVom.Table 3, find:', 1' - ! Ai -- 37.07;Btu per lb.-. .;A-e - 22.08 Bta-per lb;. '.Af* 1054,27 Btu' per lb. 1091.34 Btu'per lb. '' - `,f 1 * ., The total heat'from Equation 6 ia ' :' q,= (38.42 -25.Toj'~ (0.01525:- 0i00960) X 22B8 - '` ~ 13.32;-(0.00565 X 22.08) V : =,T3-32-'--0.12 "13.20'Btu per Ib tJry air, The latentheat fromEquation 7 is..t ?". 0.00565! X -1054J27-,* 5.96 Btu per lb of,dry air. . ' The tensibleheatbj difference,' ia ' 9*'+"`fi'*-1 9#'"'13;20 -- 5.96 =' 7.24 Btu per lB'bf dry air. Orthe sensible heat'mey becomputed fromEquation 8u.'*- 25.10)* - *(Oj6o565'x lwi&f' + 0.00565 (37.07 - 22.08)1 -- . -13.32 - 6.16 -F0.00565'X 14.99 rv-.- . -> 6.16 +.0.08 TM 7^4 Btu per-lb.of dry: air.4 ` .,-The sulxobling of. the condensate as apart ofthe sensible heat1is: indicated by the last term of the equation^ 0.08 Btu pwf.lb of d^y.airl' .' j -.10!iii1*! l-iviti" CHAPTER 35 AIR-HEATING COILS Uses for Coils; Coil Construction and Design: Steam Coils, Wafer Coils, Flow Arrangement, App/icdfioni; Coi/ Selection: CoS Ratings; Heat Transfer; Heating-Coil Performance: Overall Heat Transfer Coefficient* COItS described in this chapter are used for healing 'air of steel or various copper alloys is used' in applications where under forced convection. The total coil surface may con corrosive forces might-attack-the coils from either inride or sist of a oinglm cod section or a number of individual'ooil'seo; outride- The most common-combination is aluminum fins on- tjfipn assembled into a bank. The application of .the' coils, de^ copper tubes. scribed in this chapter is limited to comfort heating and.air lightweight extended-surface heating coils most commonly tinning employing steam or hot water (250 F maximum) have tubes with outride diameters of }, ,: |. } and 1 in.; and as the hosting medium. have fina spaced from 3 per inch up to 14 per inch. The tube sparing varies from approximately 1 to 3 in. on centers,`de USES FOR COILS Heating coils are used for tempering, prehearing, reheating,' pending on tube diameter, fin dimensions, and' other con siderations. or booster Wring. -The coils serve air-hearing purposes only, Steam Coils but the apparatus assembly may include means for humidifi cation and air cleaning. Steam or hot water are the usual heat ing media, although other media are used in special cases,- eg.; glycols or the discharge gas from a refrigerating system. For proper performance of steam heating coils, condensate and air or other noncondensables must be rapidly eliminated and the .steam.must be uniformly distributed !to the indi vidual tubes. Noncondensable 'gases; such as carbon dioxide, COIL CONSTRUCTION AND DESIGN remaining in a!coil, cause chemical corrosion and result in early coil failure. . The cods described in this chapter are eztended-extrface Uniform steam distribution is accomplished by different cods. The heat transfer surface consists.of (1). the'primary methods such as:> surface and (2) the secondary.surface. . The primary surface is the surface of the round tubes.br pipes, which are arranged in a repetitive pattern with respect to the air flow. ... The secondary surface (fins) consists of thin metal plates or a spiral ribbon uniformly spaced or wound along the length of the primary surface, and in intimate contact with it for good heat transfer from primary to secondary surface. , , The, bond between secondary and primary surface .is-a most important factor in the performance of extended-surface 1. Individual orifices in the tubes. *. y:. 2. Distributing plates in the steam headers. 3. Special perforated small diameter inner steam distributing tubes extending into the larger diameter tubes of the primary surface. Coils of the perforated inner tube type are constructed with different arrangements such as: r o. Supply and return on one end,' with the miming steam used to heat the leaving condensate.- . b. Supply and return on opposite ends. c. Supply and return on one end and a supply on the opposite cods..This bond must be maintained permanently to assure continuation of rated performan The heat transfer bond may be achieved in numerous ways among.which bonding tiia fma to the tubes mechanically .is employed, most frequently. The bonding is generally ac complished by grpwdtng the tubes into the tube holes in the fins to obtain a permanent mechanical bond. The tube holes are frequently provided with-a formed fin collar, which pro vides the area of thermal contact and may.serve as a m^ans of spacmg.the fins uniformly along the length of the tubes., i . _ The fins of-spiral or ribbon-type fin cods are tension-wound onto the tubes. In addition, an alloy with;low melting point, such as solder, may be used to provide a metallic;bond .be tween fins and tube. Some types of spiral fins are knuriwd into a.shallow groove on the exterior of the tube. Sometimes the . fius are formed out of .the mntrin1 of the tube itself. s The. fin designs most frequently used for heating coils are fiat plate fins, plate fins,of special shape, and spiral or ribbon fins, ad shown in Fig. 1 of .Chapter 34.. i - Coils''arranged as' described in Item 3a are sometimes referred to as non-freeze type. Particular (fare in piping,' controls,`arid, installation is neces sary to protect tiie coils from freere-up due to incomplete draining of condensate. '' ': Properly designed' and selected rieam distribution" tube coils distribute toe steam throughout the entire length of all primary tubes, even when the leaving air temperature is con trolled by modulating the'steam'supply through a' steain- metering valve. Thus more uniform leaving air temperatures are produced over the entire, length^and'face of'^the'coil than would result when using a stogle-tube:coiL When the entering air temperature is 32 F or lower, uniform leaving'air temperature is the first prerequirite' for protecting the condensate from freesing. For positive 'freeze' protection; the modulating valve must be so controlled, that the inside heat transfer surface temperature is always above''the free ing temperature. : . . .> f Copper, and;aluminum are the materials.most commonly used in the. fabrication of extended-surface endn Tubing " -*"4' "TC Water Coils " .^ r**::- , The performance,of water coils for.beating dependsrOn the ftlimiriation of air from the system and the proper distribution 613