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614 CHAPTER 35 1965 Guide And Data Book of the water to the individual tube circuits. Air elimination in the heating medium is at right angles. In coils with more than system piping is described in Chapter 10 of the 1964 Guide And Data Book. one row of tubes in the direction of air flow, the heating medium in the tubes may be circuited in various ways, as Comfort heating systems employing hot water usually- ' illustrated in Fig. 8 of Chapter 34. require not more than one or two rows of tubes in direction Cross-flow is common in steam heating coils. The steam of air flow, in order to produce the desired heating capacity.. temperature within the tubes remains substantially uniform; To produce the most efficient capacity without excessive:.- and the mean temperature difference is basically the water pressure drop through the coil, various circuit arrange ments are used. whatever the direction of flow relative to the air. Parallel-flow and counter-flow arrangements are common - A singte-tube-serpentanfi circuit arrangement can be.used on in water coils. Counter-flow is the preferred arrangement'to small Rise booster heaters requiring small .water quantities up; obtain the highest possible mean temperature difference. The to a mmrimnm of approximately 4 or 5 gpm. With this mean temperature potential determines the heat-transfer driv arrangement, a single-tube handies the entire water quantity, ing force of the coiL The greater the mean temperature dif provided the tube'is circuited in sucha manner that it makes ' ference, tiie greater is the capacity of the coil. a number of pasra across the air stream. The most common circuiting arrangement is often referred Applications to as single-row serpentine orstandard .circuiting. With this arrangement all;tubes in each coil row are.supplied with an equal amount of water through a manifold, commonly called the^aril header. When the water volume-used with this ar rangement is small, so that the wateriflow in' the tubes.is laminar,1 some mechanical means,-such ,as mechanical tvrbu^ lalors installed in each of the tube circuits, is sometimes pro vided to produce turbulent water flow. The air flow in heating coils is vertical or horizontal. The latter arrangement is more common. For steam heating, the coils may be installed with the' tubes in vertical or horizontal position. Horizontal coil-tubes should be pitched or inclined toward the return connection to provide for condensate drainage. Water heating coils gener ally have horizontal tubes to avoid air and water pockets. When the leaving air temperature is controlled by modu lation of the steam supply to the coil, ,steam distributing tube type coils are used to provide the optimum in uniform leaving air temperatures when controlling the coil by modulating the steam supply. (See Section, Steam Coils.) The diameters'of the primary-surface tubes of these special coils are larger than those of comparable single-tube steam coils, and commonly, are } to 1 in. on the outside. The larger the tube diameter, the more uniform will be the temperature distribution. This also offers marimnm protection against freezing, even under modulated operating conditions, when the sir entering the coil is 32 orlower' and when the coil is properly applied and in stalled. Correctly designed steam-distributing tube type coils' limit the leaving cur temperature stratification to a maximum of 5 to 6 F deg over the entire length of the coil, even when the steam supply is modulated to a small fraction of the full load ZINGtC tube SERPENTINE -.STANDARD SERFCNTINC . 4-CIRCUITS ' MON*STANDARD OR . HALF SERPENTINE 3-CIRCUITS capacity. As an added precaution, with both steam and water heating coils, the outdoor-air inlet dampers usually close Fig. 1 .. . Water Grcuit Arrangements automatically when the fan is stopped. This special control arrangement minimizes the danger of freezing, and prevents drafts-caused by drifting of cold outdoor air into the system during off periods. - ,,Special, circuiting (serpentine)., arrangements accomplish Heating coils are designed to allow for expansion and coq- the same' effect as mechanical turbulators .when the water ' traction resulting from the temperature'ranges-within which volume supplied to. the. coil is small. Serpentines reduce .the they operate. Care must be taken to prevent imposing strains uumbr of circuits, thus increasing the'w&ter velocity, in each from the piping on the coil connections. This can usually be circuit arid creating turbulent flow. " accomplished by providing expansion loops, expansion or fig. 1 .illustrates, commonly .used water-circuit, arrange ments!^., '..V;j ' ... - swing joints, or flexible connections. (See Chapter 8 of the 1964 Guide And Data Book.) Water, coils are, usually designed to be self-venting. .This is'accomplished by supplying the water.,to the coil in such a It is good practice to support banked coils individually in an angle iron- frame or a similar supporting structure. With way that the .water-flows,upward.in'the'coil, forcing,air out this arrangement) the lowermost coil is not required to sup throughthe return .water connection-. This also assures, that port the weight of the- coils stacked above. This design also the coil is alwayB completely'filled .with, water, regardless of the water volume supplied. . facilitates the removal of individual coils in a multiple coil bank for repair or replacement. " Methods for controlling water coils to produce uniform - Low-pressure steam systems and coils controlled by modulat leaving air temperatures are discussed in Chapter 13. ing the steam supply, or both, should be provided with a vacuum breaker, or drained through a vacuum return system to Flow Arrangement . =7 : In heat transfer processes, the relative directions of flow of the fluids influpnpft the performance of the: heat-transfer surface. In air heating coils with only one row of tubes' in"the direction of air flow, the relative direction of the'air'flow to assure proper condensate drainage. -It is considered good practice to provide a dosed vacuum breaker, where one is . required, connected to the condensate return line through a check valve. This breaks the vacuum by equalizing the pres sure, yet minimizes the possibility of air being bled into the system. Also, coils supplied with low-pressure steam or con- Air-Heating Coils 615 trolled by modulating the steam supply, should not be trapped directly to overhead return lines. Condensate can be lifted to limits of the usual practice and to indicate the influence of the variables involved in coil selection. - overhead returns only, if sufficient pressure is available-to overcome the condensate head and any return line pressure that may exist. If, under these conditions, overhead returns Coil Ratings Steam and hot water coils are usually rated within these are necessary, the condensate should be pumped to the higher limits: elevation. COIL SELECTION The following factors should be considered in coil selection: 1. The required duty or capacity considering the other system components. t 2. Temperature of the air entering the coil. 3. Available beating media. - 4! Space and dimensional limitations. 5. Air quantity. -6. Permissible resistances for both the air and beating media. 7.'Characteristics of individual'coil designs. 8. Individual installation requirements, such as the'type of control to be used. Q. Coil face velocity. 1. Air Face Velocity.......... 300-1500 fpm, based on air at standard density of 0.075 lb per cu ft. 2. Entering Air Temperature.......... Minus 20 F to 80 F for steam coils; 32 F to 80 F for hot water coils. 3. Steam Pressures..........2 to 200 pszg at the coil steam sup ply connection (pressure drop through the steam'control valve must be considered). 4. Hot Water Temperatures..........120 F to 250 F. 5. Water. Velocities..........1 to 8 fps. Individual installations vary widely, but the fallowing values can be used as a guide: The most common air face velocities used are between 500 and 600 fpm. Delivered air temperatures vary from.about 72 F for ventilation only, to about 150 F for complete heating. Steam pressures vary from 2 to 10 psig,_with 5 prig most The duties required may be determined from information in Chapters 24,25, and 26. See also Chapter 1 of the 1964 Guide - And Data Book. There may be a choice of heating media, as well as operating temperatures, depending upon Whether the installation is new or being modified. Space limitations are dictated by the requirements of individual cases.' The air quantity is influenced by a number of considerations. The air hawHIeri may be limited by the use of installed ventilating ducts for air distribution, or it may be determined by re quirements for satisfactory air distribution or ventilation. The resistance through the air circuit influences the fan horsepower and speed. This resistance may be limited to allow the use of a given size of fan motor, or to keep the operating expense low. It may R-hn be limited because of sound-level requirements. Air resistance can always be kept low by designing for low air face velocities. The permissible voter resistance of a hot water coil may be dictated by the available pump head from a given rise of pump and pump motor. This can usually be controlled within reasonable limits by careful selection of the coil header size and the number of tube circuits. And finally, the performance of a heating coil depends upon the correct choice of the original equipment, and proper application and maintenance.' For proper steam coil performance, election of the correct type and size of stream trap is of the utmost importance. Coil ratings are based on uniform face velocity. .Therefore, nonunifonn air flow through the coil will affect performance. Nonuniform air flow may be caused by air entrance at odd angles) or by inadvertent blocking of a portion of the .coil face. To obtain rated performance, air quantity in the field must correspond with design quantity, and must always; be maintained. A common cause in the reduction in air quantity is the fouling of the filters. -. common. Water temperatures for comfort heating are com monly .between 180 and 200 F with water velocities between 4 and 6 fps. ~ Water quantity is usually based on about 20 F temperature drop through the coil. Air resistance is usually limited to from } to | in. of water for commercial buildings, and to about 1 in. of water for industrial buildings. The selection of heating coils is relatively simple, because it involves dry-bulb temperatures and sensible heat only, without .the complication of simultaneous latent heat loads, as in cooling coils. Heating coils are usually selected from charts or tables giving final air temperatures at-various-air velocities, entering air temperatures, and steam or water temperatures. ` The selection of hot water heating coils is slightly more complicated, because of the added water velocity variable and the fact'that the water temperature decreases as it flows through the coil circuits. Therefore,- in selecting hot water heating coils, the water velocity and the mean temperature difference (between the hot water flowing in the tubes and the air passing over the fins) must be taken into consideration. Most coil manufacturers have their own methods of pro ducing performance rating tables from a suitable number of coil-performance tests. A method of testing coils is given in ASHRAB Standard 33-64, Method af Testing for Rating Forced-Circulation Air-Cooling and Air-Heating Coils. A basic method of rating to provide a fundamental means for estab lishing thermal performance of dehunudifying coils by exten sion of test data, as determined from laboratory.tests on proto types, to other operating conditions, coil sizes, and row depths of a particular surface design and arrangement, is'given in Air-Conditioning and Refrigeration Institute Standard 410-62 far Forced-Circulation Air-Cooling and Air-Heating Coils.. When it is necessary to clean coils, a common method is to wash them with water. They can sometimes.be brushed and HEAT TRANSFER cleaned with a'vacuum cleaner. In bad oases of neglect,, rape- The transfer of heat between'the heating medium and the pally in restaurants where grease and dirt have accumulated, air stream is influenced by several variables: it is sometimes necessary to remove the coils and wash off the . -1. The temperature difference between the heating medium accumulation with steam, compressed air and water, or hot water and a suitable detergent. The best practice, however, is to keep the filters serviced; and to inspect and wash the coils at regular intervals. The proper selection of coils requires an understanding of the special requirements of each installation, and should be based on an economic analysis of the plant design as a whole. While no general rule can be established for the selection of heating coils, it is posable, nevertheless, to point out the and the air stream. 2. The design and surface characteristics of the coil. 3. The velocity and character of the air stream. 4. The velocity and character of the heating medium Sowing in the tubes. 5. fluid flow arrangement: counter-flow, par&Uei-flow, or cross-flow. The heat transfer driving force is usually expressed as the logarithmic mean temperature difference between the beating medium and the air.