Document YDzBedjje1579nE8RERm3q6e8

488 CHAPTER 25 W_____________ 1948 . Guide -Exposed to freezing temperatures in winter if the apparatus is used on winter humidifying duty. Access doors should be provided for servicing filters, humidifying nozzles, and fan bearings and for cleaning the. coils.' With certain designs of coils when used for deh'umidifying, eliminators must be used beyond the coil to catch any. water which may be blown into the air stream. It is customary to; include :these eliminators when; the air velocity exceeds about 450 fpm with the individual fins arid about 600 fpm for the continuous flat fin type. Where a number ofcoil.sections are stacked one upon another, and* where the velocities are low, so that eliminators need not be used, occasional trouble results wheri : writer splashes down from one coil.to the next and blows out'into'the air stream. Iri such cases drip troughs' as shown in Fig. 14 are used--to collect this water and conduct it to the condensate pan. -- Soriietiihes finned surface coils on summer cooling and dehumidifyirig F?g. 14.. Con. Arranged with Drip Trough Fig. 15. Recirculating,Spray System for Cleaning Coils j duty are provided with water sprays. These sprays are of two types. In the first type a set of spray nozzles is arranged for intermittent cleanings These sprays are not operative when the systeiri is in use and no' recircu lating pump-is provided. " . The secorid arrangement requires a collecting tank and a recirculating pump. The water is in circulation whenever the apparatus is in operation, and assists in keeping the coil clean and in absorbing odors'. Fig. 15 illustrates such an airangement. Wherever air by-passes are used around a coil on summer duty for control purposes, it is advantageous to direct only return air through the by-pass rather than a mixture of return and outside air. The casing should be arranged accordingly. To maintain the air quantity handled by the fari reasonably constant, and to assure the required design quantity of by-passed air when the by-pass damper is open, cooling coil, banks are frequently furnished with both face and by-pass dampers as shown in Fig. 13. . Although both heating and cooling coils are made of sufficient strength- to take up expansion and contraction arising within themselves, care should be taken to avoid.imposing strains from the piping, on to the coii connections. (See Chapter 23.) Radiators,'Convectors, Coils - ; ' ____________ 489 HEAT TRANSFER.AND AIR FLOW RESISTANCE The transfer of. heat between the heating, or cooling inedium arid. ,the air stream is influenced by several variables: 1. The temperature difference.' - 2. The design and surface'arrangement of the coil. i :3.-The velocity and character of. the air stream;. 4. The velocity and character of the medium in the tubes.. :. . . - The driving force is usually taken as the logarithmic-mean teiiperature difference for heating or cooling without dehumidification. For combined cooling and dehumidificatiop-, .the- lqgarithnjic,difference does not apply strictly , and such problems should be handled as described in Chapter 7. With volatile refrigerants there is often-an appreciable pressure, drqp and corresponding change in evaporating temperature through the refrigerant circuit. The problem is further complicated by'the fact that the refriger ant is evapo'ratirig in part of the circuit'and superheating in-the remain der. In spite of this, heat transfer and ratings for coils using volatile refrigerants are usually based on a refrigerant temperature corresponding to the average pressure in the coil. : ' The design and surface arrangement of the coil include such items as materials, type, thickness, height and spacing of the fins, and the ratio of this surface to that of the tube, the use of the staggered or.in-line tube arrangement,,and provisions .to increase the air turbulence'such as the use of corrugated as against flat fins. Staggered tubes increase the total, heat transfer as against the in-line.arrangement and corrugated fins may be more effective' than flat. This design' and surface arrangement has a large effect on the air film heat transfer resistance. The velocity of the air'usually considered is the coil face velocity.. This bears a varied, relation to the actual velocity over.the surface, depending upon the individual coil design. As long as a fixed design of coil is under consideratipri face velocities;may be used, but they may be unsatisfactory in comparirig differed.txlesigris,. as'it is the actual, surface velocity thdt. is' significant: . The air volume is often based on standard: air at 70 F and a barometric pressure of 29.92 in. Hg. The use of air volume in'coil rating., information, may.' be' misleading; . The :'significdfU\-skum''is;niqss< velocity in pounds per. {minute) (square foot offace area) and not. cubic feet per minute, because for a fixed volume the corresponding weight may vary, widely, depending upon the ternperature and barometric pressure. At the same mass air velocity, varying performance .can be. obtained deperiding upon the turbulence of the. air. flow into the coil and\upon the; uniformity of distribution of air over the coil face. : The latter is very im-: portant iri obtaining reliable test ratings and iri realizing rated perform-; ance in practical installations. The resistance through the coils will assist; in distributing the. air properly, but where the inlet duct connections are brought in at sharp angles to the coil face, the effect is: frequently, bad. and there may even be reverse air currents through the-coils. This reduces the capacity, but can be avoided by proper layout or by the use of directing baffles. Heat transfer depends also upon the velocity, of the medium in thetubes and upon its character, whether flowing water, condensing steam or, evaporating volatile refrigerant.: Heat transfer rates expressed as.Btuper (square foot of internal surface) (degree logarithmic mean -effectivetemperature difference between the fluid and tube wall) are, for example: