Document XO6eMb6peDRO0N5qjQ2dobyOg

616 CHAPTER 35 1965 Guide And Data Book Hie design and surface characteristicsof the coil include fiii material, type, thickness, height, and spacing,:as:well asthe area ratio of the external heat transfer surface to that.of the internal tube surface. Also, provisions for increasing air tur bulence by means of specially-shaped fins may: be advanta geous- *: The coil air velocity is determined by dividing the air vol ume (based on standard air) by the actual net face area of the coil finned surface. .(Standard air, is substantially, equivalent to dry air at 70 F and 29.92 in- Hg barometric pressure.) !t* At the!same mas'air velocity,*varying performance.can be obtained,' depending upon the turbulence of the air flow across the coil surface-and the uniformity of distribution .of the air over the coil face. The latter is important in- obtaining reliable test ratings, and in realising rated performance; in actual installations. High air flow resistance ..through. the Coil assists in.distributing the air uniformly oyer the cop-for optimum performance. Incorrect inlet duct connections (at sharp, angle to 'coil,face) cause reverse air currentsthrough portions of the coil, reducing capacity. Good duct'approach and`use of baffles or vanes improves coil performance. ., Heat transfer capacity information given in manufacturers' catalogs is usually based on actual laboratory tests. To simplify coil, selection, data are presented in the form of tables or charts. HEATING COIL PERFORMANCE. ' Heating ocril performance depends'upon: ' 1. Overall heat transfer coefficient from the fluid flowing inside the tubes of the c?il to the air flowing over the cdl surface.'; 2. The logarithmic mean temperature difference'between the fluid flowing inside the tubes of the cofl and the air flowing over the coil surface.; 3. The heat transfer surface area or physical dimensions of the ccfiL . , , . 4. fluid flow arrangement The sensible beat transfer capacity of a coil.can be;expressed by the following basic equation: V'S-U.XfAtjxAxtf V."' where *' i . ' q' * total amount of heat transferred by the coil, Btu per . `(hour)"(square foot of cofl face area). , - U.** overall heat ,transfer coefficient of the 'cofl, per (hour) (square foot of external coil surface) (Fahren- . hmt degree temperature difference. between the fluid within the.cofl.and the air Sowing over the coil). , ' AC the logarithmic meani temperature difference between the fluid flowing made the cofl tubes and the air pyfog ' oyer the coil, Fahrenheit degrees. .......... ( A -- external `surface area of the cofl, square feet per (square ! foot of cofl face area) (row.of cofl depth). -. . , /-[ N - number of coil tube rowB in direction of air flow. Overall Heat Transfer- Coefficients . . The rate of heat flow-is usually expressed'by heat transfer coefficients. .The overall heat transfer coefficient for a specific coil design results from the combined effects of three heat transfer eoeffioente: ; > :* * . ' ' 1.' The film coefficient of heat transfer between the air and the external surface of the coil; usually given- in Btu per (hour) (square foot external surface) (Fahrenheit degree mean ten^ perature difference). 2. The film coefficient of*heat transfer between the internal surface of the coil and the fluid flowing inside the coil tubes,* usually given in Btu per (hour) (square foot internal surface) . (Fahrenheit degree mean temperature difference). 3. The conductivity of the coil surface, consisting of tube walls and fins, usually given in Btu per (hour) (squarefoot of surface) (Fahrenheit degree per inch). }( These three coefficients acting in series result roan overall heat transfer coefficient in accordance with, the .baric laws given in Chapters 4 and 24. The overall-heat transfer coeffi cient for a finned-tube coil for the sensible transfer*of heat (beating or cooling) can be expressed by the.following simpli fied basic equation: - - ' '1 B "`L 1 -- --- Ut K k fun ' (2> where * ' ' *: * r* , -. Ui " overall heat transfer coefficient, Btu per.(hour) (square * foot .external surface) (Fahrenheit degree, mean tem perature difference between air.and fluid within the cofl , - tubes). , , * hi * film, coefficient of heat transfer between the internal eurfaoe of the coil and-the fluid flowing within',the cofl , tubes,. Btu per (hour) (square foot internal surface) ,. (Fahrenheit degree mean temperature difference-be- ' tween the.surface end the average fluid temperature). B ".ratio between,external and internal heat transfer surface ' . arras,.This ratio is introduced into the equation in order i,. .to place the internal beat transfer fluid coefficients on .......... the basis of external surface. .. 1 --.Ju - heat transfer film coefficient .between, the air and the ; [external surface of the coil, Btu per (hour) (square foot , . .. external surface) (Fahrenheit degree mean temperature difference between* the air, and the external surface). k -- conductivity, of- ooil;tube material, Btu per* (hour) .. (square foot) (Fahrenheit degree per inch of tube wall v thickness).,.' i -.- .. -- thickness of tube wall, inches. , ... q -- fin efficiency,or effectiveness, to correct for the resistance to the flow ofheat.through the finw, percent.. 'When .the tube walls, of the coil are relatively thin' and made of materialshaving high conductivity, as is the case for all lightweight 6imed-tube4 heat transfer surfaces, 'the term L/k in' Eqdation 2 becomes'negligible: and is 'usually dis regarded/". ' .* *;* ` *.'' * For typical finned-tube coil designs, the ratio B. expressing the ratio bf thelidtal extenial'riirface to the internal surfiice; varies from approximately 10 to 35!' " Although:the:sehsibIe transfer of heat for finned-tube ooils can be expressfed by Equations 1 and 2, the performance of coi^'where dehuixudification takes plapei'cfumot be predicted oh ther basis of overall coefficients. The perfdrihance 'of de- bumidifying rir cobhng coils is discussed in Chapter 34. CHAPTER 36 AIR CLEANERS Supply Air Cleaners: Rating, Test Methods, Types, Performance, Selection and Maintenance, Installation THIS chapter deals with the cleaning of supply air or re characteristics of airborne particulate matter and the wide circulated air for conditioning of building interiors where range of particle size which may be encountered. Cleaning effi in dust content seldom exceeds 4 grains per 1000 cu ft of air. ciency also is affected to some extent by the velocity of the air With certain exceptions, air cleaners discussed in this stream. Since the purpose of a filter is to remove as much of - chapter are not applicable to the cleaning of exhaust' gas ' the contamination as practical, it is obvious that the degree streams, principally because of these extreme differences'in :of air cleanliness required is'a major factor in influencing dust concentrations; supply air filters are used orice,' after ' choice of filter design. Removal of these particle size fractions which they must be laboriously cleaned or discarded. becomes progressively difficult as the particle rises decrease. The function of air cleaners is to remove the particulate - SUPPLY AIR CLEANERS matter from outdoor air for supply to building interiors, and Atmospheric dust is a complex mixture of matter made up of pmnVpg, dusts, mists, and fumes which are generally referred to ss aerosols. A sample of atmospheric dust gathered at any given point will generally contain minute particles of materials that are common to that locality together with other com- portents which may have had their origin at quite some dis tance but which have been dispersed by wind or air currents. The components will generally vary with factors such asthe geography of the locality in question, the season ed the year, the direction of the wind, and proximity of manufacturing plants. A sample of atmospheric dust will usually mnfarin minute quantities of soot and smoke; silica; clay; decayed animal and vegetable matter; organic materials in the form of lint and plant fibers; metallic fragments. It may also contain mold spores, bacteria, plant pollen and similar allergens which may motivate attacks on persons of allergic sensitivity. The particles in the atmosphere can range in rise from less than 0.01 micron up to things which are caught by an ordi nary fiy screen, such as lint, leaves, and insects. Almost all conceivable shapes and sires. are represented. This wide variety makes it impossible to design one type of cleaner that from the recirculated air stream within the building interior. Removal of particulate matter is required in order to mini mize the soiling of surfaces and equipment in contact with the air stream. Surface soiling includes a.variety of effects, some caused'mainly by the coarser particles and some by the finer ones. Some effects are, for example, gravity sedimentation of the larger particles; accumulations on heat exchanger riirfaces; localized smudging of walls or ceilings; generalized darkening of walls, ceilings and draperies; contamination in ' various industrial processes, such as electric equipment rooms, paint drying and clean rooms. Considerations of cost, both initial investment and mainte nance, space requirements, and resistance in conjunction with ' wide-ranging individual criteria as to required degree of air cleanliness, have resulted in a wide variety of commercial >-air cleanera. Comparisons between them can be made only * * from data obtained by standardized test methods; intelligent [use of the test data depends on an understanding of test pro cedures employed. The following discussion of current meth ods of rating and testing will aid in better understanding of the later description of air cleaners. will be best for all applications. Mechanical filters of the lowpressure-drop type can remove large particles effectively. Other iripohnniiral filters remove extremely fine particles, RATING OF AIR CLEANERS The two operating characteristics that riigtingnish the but may have higher pressure drop and co6t. Electronic,air cleaners also remove the fine particles of atmospheric dust, but have a relatively high first cost. -*- Different fields of application require different degrees of effectiveness. In industrial ventilation, it may only beneces- sary to remove the coarser dust particles from the air stream insofar asthe olpanlinawa of the structure the protection 'of the mechanical equipment is concerned. In other ingtimwaa, the application may involve the protection of an area inhere discoloration-is to be prevented. Unfortunately, the components of atmospheric dust are the worst offenders from the point of view of smudging and discoloring the interior of filtered spaces. Electronic air cleaners or high-efficiency dry filters are required for their removal. When ultimate 'efficiency is required for removing as many radioactive or other danger ous particles as possible, extremely high efficiency mch*nid filters should be employed. *.' The characteristics of aerosols which affect the perform various types of air cleaners are.efficiency and dustholding capacity. Efficiency in common air cleaning terminology-is .synonymous with, the term a/restance and measures the ability of the air cleaner to remove particulate matter from an air stream! Dust-holding capacity defines the amount of a particular type of dust that an air cleaner can hold and determines the operating life of the air cleaner which is fired mainly by some value of maximum tolerable resistance in particular circumstances. Complete rating of . air cleaners .then requires data on both efficiency and dust-holding ca pacity; the only exception is the ionizer-plate type electronic air cleanerf wherein performance ratings are based on effi ciency only, rince.resistance does not increase significantly .with dust load. With the ionizer-plate electronic air cleaner, some other criterion must be used to establish clean ing or mointonftnAo schedule. Usually th* mintgnftn<ft cycle ;for cleanera is set at sufficiently frequent intervals to prevent blowoff of accumulated dust from collector plates. ance of an air filter include particle rise, particle shape, spe *_ Test Methods cific gravity, concentration, etc. One of the most important of t these is particle size. Fig. 1 of Chapter 11 shows the rises and '* A number of procedures for evaluating air cleaner efficiency * have been proposed. Except for electronic air cleanera and to TC 9.4, Air Ctaa- -- those ffiilltteerriinngg devices of cimflar high efficiency, most of these 617