Document zQbeXNE0ygkEnJynqxMvwBZ0n

332 CHAPTER 23 1960 Guide titled ASHRAE Standard Method of Testing and Rating Forced-Ciraviation Air-Cooling and Air-Heating Coils, has been adopted (1958). DETERMINING REFRIGERATION LOAD The following determination of the refrigeration load shows a division of the true sensible and latent heat loss of the air, which is accurate within the limitations of the data. These divisions will not correspond to load determination obtained from approximate factors or constants. The total refrigeration load g, of a cooling and dehumidifying coil (ot air washer) is indicated on Fig. 13 and consists of the following components: Fig. 13 .... Psychrometrie Performance of Cooling Coil in Central System 1. The sensible heat q, removed from the dry air and moisture in cooling from entering temperature to leaving tempera ture It. 2. The latent heat q, removed to condense the moisture at the dew-point temperature It of the entering air. 3. The heat of tu&cooltng q. removed from the condensate in cooling it from the condensing temperature It to the leaving condensate temperature t. Items 1,2, and 3 may be related by ?='?. + ?* + (5) If only the total heat value is desired, it may be computed by 9i * (A -- hi) -- (tt>i -- i)hi (5) u>here ' hi and As -- enthalpy at points 1 and 2 respectively. t?i and wt ~ humidity ratio at points 1 and 2 respectively. * h*t enthalpy of saturated liquid at the final tem perature, tt. If a breakdown into latent and sensible heat components is desired, the following relations may be used: The latent heat may be found from where q, - (wi - v>t)hf,t (7) h/,< = enthalpy at the condensing temperature, It. The sensible heat may be shown to be 9. + 9* = (A - A*) -- (i - *ot)A,t where + (u>i -- wi) (hwt -- A*t) (8) A#t * enthalpy at the condensing temperature, U . A*t " enthalpy of saturated liquid at condensing tempera ture, <. The last term in Equation 8 is the heat of subcooling the condensate from the condensing temperature t to its temperature t,. Then, 9 fun - to*) (A*, - A.,) (9) All values for solving the foregoing equations may be found on the ASHRAE Psychbometric Chart and Tables 2 and 3 of Chapter 3. Example J: Air enters a coil at 90 F dry-bulb, 75 F wet-bulb; it leaves at 61 F dry-bulb, 58 F wet-bulb; leaving water is assumed to leave at a temperature between the leaving air dew point and coil surface temperature of 54 F. Find the total, la tent, and sensible cooling loads on the coil. Solution; From the ASHRAE Pstcbbohztbic Chart, find the following: Ai * 38.42 Btu per lb of dry air. A* * 25.10 Btu per lb of dry air. U m 69 F wet-bulb of entering air. Wi .0.01525 lb per lb of dry air. Wt *= 0.00960 lb per lb of dry air. From Table 3, find: kwt m 37.11 Btu per lb. A 22.12 Btu per lbhfH = 1054.27 Btu per lb. Af w 109134 Btu per lb. The total heat from Equation 6 is q, - (38.42 - 25.10) - (0.01525 - 0.00960) X 22.12 - 1332 - (0.00565 X 22.12) -- 1332 -- 0.12 13.20 Btu per lb dry air. The latent heat from Equation 7 is q, - 0.00565 X 1054.27 = 5.96 Btu per lb of dry air. The sensible heat by difference, is 9 + 9 TM q -- q = 13.20 -- 5.96 * 724 Btu per lb of dry air. Or the sensible heat may be computed from Equation 8 as q. + q, - (38.42 - 25.10) - (0.00565 X 109134) + 0.00565 (37.11 - 22.12) - 1332 - 6.16 + 0.00565 X 14.99 =* 13.32 -- 6.16 4- 0.08 " 7.24 Btu per lb of dry air. The subcooling of the condensate as a part of the sensible heat is indicated by the last term of the equation, 0.08 Btu per lb of dry air. CHAPTER 24 AIR CLEANING Atmospheric Ait Cleaners: Airborne Particulate Matter, Viscous-Impingement Fillers, Dry Air Filters, Electronic Air Cleaners, Air Filter Performance, Selection, Maintenance, Installation, Adsorption of Vapors; Industrial Air and Gas Cleaners: Degree of Cleaning, Selection, Types, Application, Inertial Separators, Scrubbers, Wet Collectors, Filters, Electrostatic Precipitators, Adsorbers, Absorbers, Combustion Devices AIR cleaning devices remove contaminants from an air x or gas stream. They are available in a wide range of designs to meet various air cleaning requirements. Degree of removal required, quantity and characteristics of the con taminant to be removed, and conditions of the air or gas stream will have a bearing on the device selected for a given application. Definitions and a discussion of contaminant char acteristics are given in Chapter 7, together with some consid eration of their origin. Air cleaning devices are divided in this chapter into two basic groups; Atmospheric Air Cleaners, described in Part I, and Industrial Air and Gas Cleaners, described in Part II. Atmospheric Air Cleaners are ordinarily used to remove particulates such as are found in outdoor air, and are em ployed in ventilation, air conditioning, and- heating systems where dust content seldom exceeds 4 grains per 1000 cu ft of air. Industrial Air and Gas Cleaners are ordinarily used for the heavier concentrations encountered in local exhaust ventila tion where the particulate content (loading) ranges from 100 to 20,000 grains per 1000 cu ft of air. Because of these heavier loadings, atmospheric air cleaners can seldom be used for the control of process aerosols in industrial applications. PART I--ATMOSPHERIC AIR CLEANERS Conventional air filters and electronic air cleaners are in stalled in air handling systems to remove dusts. These dusts constitute a mixture of particle sizes within the classification of temporary and permanent impurities listed in Fig. 1, Chap ter 7, including bacteria, pollens, house dusts, and similar al lergens which motivate attacks on persons of allergic sensi tivity >* Since the purpose of the filter or cleaner is to free the air of as much existing contamination as practicable, the degree of air cleanliness required should influence the choice of appara tus. Atmospheric dusts are mixtures of particles in all sizes. The removal of these particles and fractions becomes pro gressively difficult as the particle size decreases. Smoke parti cles are of major importance in many applications. Air clean ers will justify their cost through a reduction in housekeeping expense in tile ventilated space, by the protection of the equipment in the ventilation system itself, and by providing relatively dust-free air for critical manufacturing processes. Cleaning devices for atmospheric air are classified by the principle employed to collect dirt particles such as impinge ment on viscous coated media, filtration through porous me dia, or electronic air cleaning. In some cases a filter may dis play a combination of these principles. Each type of air cleaner has certain advantages. There are applications where it is desirable to pas3 air through a series of two or more dif ferent types to obtain optimum results. AIRBORNE PARTICULATE MATTER Suspensions of particulate matter in the air are called aero sols and consist of smokes, dusts, mists, and fumes. The characteristics of the aerosols which affect the performance of an air cleaner include particle size, concentration, shape, den sity, velocity, and surface characteristics. One of the most im portant of these is size. The rate of settling of particles varies approximately as the square of the diameter. Particles larger than 10 microns in diameter settle so rapidly that the concentration of such particles decreases rapidly as the distance from the source increases. Particles less than 1 micron in size settle so slowly that the ordinary convection currents prevent sustained downward motion, and consequently such particles remain in the air for long periods. If particles could be examined through a super microscope having a magnification of 250,000 diameters, a tobacco smoke particle of 0.1 micron would appear to be 1 in. in diameter, or approximately the size of a golf ball; a soft coal smoke particle 03 micron in diameter would appear like a baseball; a ragweed pollen grain of 20 microns in diameter would ap pear 16.5 ft in diameter, while the 50 micron particle (just visible to the naked eye and able to pass through a 270 mesh screen) would appear to be 50 ft in diameter. Consideration of this range in particle size from a golf ball to a sphere 50 ft in diameter will emphasize the difficulty of devising any single test to measure adequately the performance of air cleaning devices under all conditions of service. The particles in the atmosphere can range in size from less than 0.01 micron up to things which are caught by an ordi nary fly screen, such as lint, feathers, and insects. Almost all conceivable shapes and sizes are represented. The material is very commonly soot, ash, soil, lint, smoke, and fumes, but may include particles of almost any inorganic or organic material and even such living organisms as virus, bacteria,* and fungus spores. This wide variety makes it impossible to design one type of cleaner which will be best for all applications. Mechanical filters of the low-pressure type can remove large particles effectively. Other mechanical filters can remove extremely fine particles, but may be handicapped somewhat by highpressure drop and cost. Electronic air cleaners have the ability to give high effectiveness on normal atmospheric contamina tion, with a low operating-pressure drop. As a general rule, the removal of the coarser dust particles and lint from the ventilating air produces tangible results in / 333