Document KRyGxk4poJbgKq5LyYrnEn65K

CHAPTER 30 s$ir <2)iitrilution Standards for Satisfactory Conditions, Definitions, Mechanics of Air Distribution, Types of Supply and Return Openings, Outlet Locations, 'Return, and Exhaust Intakes, Specific j4p- . plications. Balancing the System CORRECT air distribution contributes as much or more to the success of a forced air heating, ventilating, cooling or air conditioning system as does any other single factor. The scope of the chapter is limited to the air distribution within the conditioned space. Reference is made to the distributing duct system only insofar as it affects the performance of the air distribution outlet. See Chapter 31 for information on air duct design. STANDARDS FOR SATISFACTORY CONDITIONS The air distribution problem consists of distributing air as a cooling, heating, drying, moistening or ventilating medium in a designated space within accepted limits of air motion, temperature variation, temperature fluctuation, direction, humidity and noise. . Reference should be made to Chapter 2, Physiological Principles, for the accepted standards on room temperature, humidity, air motion and direction. Material in Chapter 32, Sound Control, covers acceptable room noise levels arid noise generated by air outlets. Variations from accepted standard limits of each element may result in discomfort to the occupants. Neglecting noise, discomfort complaints usually arise from draftiness, or stuffiness., A draft may be defined as an air current which, due to its temperature, humidity, or motion, removes more heat from a body surface than is usually dissipated. Although stuffiness may be attributed to odors, the complaint of stuffiness usually results from a person feeling too warm. Outside of localized sensation, such as caused by a single down draft, draftiness and stuffiness may be considered functions of effective temperature, which of course takes in the factors'of air temperature, motion and humidity. Draftiness can be associated with too low an effective temperature, and stuffiness with too high an.effective temperature. Therefore, satisfactory comfort conditions are the result of minimizing the factors of temperature variation, tem perature fluctuation, gusts, air motion and noise. Definitions 1. Supply Opening or Outlet: Any opening through which air is delivered into a space which is being heated, or. cooled, or humidified, or dehumidified, or ventilated. 2. Exhaust Opening or Return Intake: Any opening through which air is removed from a space which is being heated, or cooledi or humidified, or dehumidified, or ventilated. 3. Outside Air Opening: Any opening used as an entry for air from outdoors. 4. Grille: A covering for,any opening and through which air passes. 5. Damper: A device, used to vary the volume'of air passing through a confined cross-section by varying the cross-sectional area. 6. Multiple Louver Damper: A damper having a number of adjustable blades. 7. Single Louver Damper: A damper having one adjustable blade. ' 8. Face: A grille with provision for attaching a damper. 9. Register: A face with a damper attached. 10. Flange: The portion (either integral or separate) of a grille, face, or register extending into the duct opening for the purpose of mounting. Air Distribution 549 11. Frame: The portion (either integral or separate) of a" grille. face, or'register extending around the duct opening for the purpose of mounting. 12. Margin: The margin of a grille, face, or register is one-half of the difference between the duct dimension and over-all dimension measured either horizontally or vertically. ' 13. Fret: The member separating the openings of a grille, face, or register. 14. Free Area: The total minimum area of the openings in the grille, face, or register through which air can pass. 15. Core A rea: The total plane area of the portion of a grille, face, or register bounded by a line tangent to the outer edges of the outer openings through which air can pass. 16. Mean Area: The total of the core and free areas divided by two. 17. Duct Area: The area of a cross-section of the duct based on the inside dimensions at the point where the grille, face or register is mounted. 18. Percentage Free Area: The ratio of the free area to the core area expressed in . percentage. 19. Aspect Ratio: The ratio of length of the core of a grille, face or register to the width. 20. Throw: The distance air will carry measured along the axis of an air stream from the supply opening to the position in the stream at which air motion reduces to 50 fpm. 21. Envelope: The outer boundary of an air stream. 22. Drop: The vertical distance the lower edge of the air stream drops between the time it leaves the outlet and reached the end of its throw (h in ft). 23. Rise: The converse of drop". . 24. Induction: The entrainment of room air by the air ejected from the outlet. 25. Primary Air: The air leaving an outlet (ft in cfm). 26. Secondary Air: The room air picked up by the primary air through induction (ft in cfm). 27. Total Air: The mixture of primary and secondary air (ft in cfm). 28. Induction Ratio: The total air divided by the primary air equals r, or ft/ft. 29. Outlet Velocity: The average air velocity emerging from the outlet (Vi in fpm) measured at the plane of the opening. 30. Terminal Velocity: The average air stream velocity at the end of the throw (Vt in fpm). 31. Horizontal Spread: The divergence of the air stream in the horizontal plane after it leaves the outlet (Degrees). 32. Vertical Spread: The divergence in the vertical plane (Degrees). 33. Temperature Differential: Temperature difference between primary and room ft - <as)- 34. Vane Ratio: The ratio of depth of vane to shortest opening width between two adjacent grille bars. MECHANICS OF AIR DISTRIBUTION In the mechanics of air distribution, two major problems are involved: (1) complete mixing of the primary air and room air outside of the zone of occupancy and (2) counteraction of the natural convection and radia tion effects within the room. Induction When air is discharged from an outlet into a free open space, the primary air stream entrains room air as it traverses the space. This entraining effect increases the cross-sectional area and reduces the velocity of the resulting air stream, Induction takes place with the conservation of linear momentum; this has been confirmed by tests which indicate that the momentum remains almost constant throughout the entire measureable length of the air stream. This relationship may be expressed by the Equation 1: Ml V, + M; V; - (Ml + Ml) V, (1)