General Selection Procedure
|1.||When the heat loss or heat gain in Btuh for each room is not specified, it should be calculated in accordance with the methods outlined in the current ASHRAE Guide.|
|2.||Determine the number of outlets needed per room. At least one outlet for each exposed wall in each room should be used for heating, except that only one outlet may be used in bedrooms which have less than 7,000 Btuh heat loss and are located over a heated space. Select at least one heat outlet for each 8,000 Btuh heat loss or 4,000 Btuh heat gain for a room, whichever is greater. The use of several small outlets, each handling moderate air volumes, in place of using fewer and larger outlets may be necessary for better air distribution, and will cut down dirt streaking.|
|3.||The Btuh delivery required for each room outlets is determined by dividing the total room heat loss or gain by the number of outlets.|
|4.||Refer to the Heat Conversion Chart to find the CFM that will deliver the required Btuh at the desired supply air temperature.|
|5.||Select from the performance data the type and size of diffuser that gives the desired air pattern (spread and throw), at the CFM.|
All tests used in compilation of performance data in this manual were conducted under isothermal conditions (supply air and room air temperatures being equal), therefore, the throw data will be slightly greater for heating and slightly less for cooling.
Glossary of Terms and Definitions
DAMPER. A device used to control the volume of air passing through a duct by varying the cross sectional area.
DIFFUSER. An outlet discharging supply air in a spreading pattern.
GRILLE. A louvered covering for an opening through which air passes.
RETURN. Any opening through which air is removed form a conditioned space.
CORE AREA. The total plane area of the portion of a grille, face or register bounded by a line tangent to the outer opening through which air can pass. The core area is less than the register size. Example, a 14 x 8 register may have a core that is 1" less than the listed size, therefore, the core area is 13 x 7 = 91 sq. inch.
FREE AREA. The actual measured perpendicular area between the fins of a grille or register.
EFFECTIVE AREA. The calculated area of an outlet based on the averaged measured velocity between the fins.
OUTLET VELOCITY. The average velocity of air emerging from the outlet measured in the plane of the outlet.
JET VELOCITY. The average measured velocity of air passing between the fins.
C.F.M. A measure of volume of air in cubic feet per minute.
F.P.M. A measure of air velocity in feet per minute.
TOTAL PRESSURE. The sum of the velocity and static pressures measured in inches of water.
STATIC PRESSURE. The outward force of air within a duct measured in inches of water.
VELOCITY PRESSURE. The forward moving force of air within a duct measured in inches of water.
PRESSURE LOSS. The term used in the register industry to indicate how much total pressure is required to move air through a register.
TERMINAL VELOCITY. The point at which the discharged air from an outlet decreases to a given speed, generally accepted as 50 feet per minute.
THROW. The distance measured in feet that the air stream travels from the outlet to the point of terminal velocity. The throw is measured horizontally from registers and ceiling diffusers, and vertically from perimeter diffusers.
RADIUS OF DIFFUSION. The horizontal distance in feet from the diffuser to a point where the terminal velocity of 50 feet per minute occurs.
SPREAD. The maximum total width in feet of the air pattern at the point of terminal velocity.
DROP. The vertical distance (in feet) between the base of the outlet and the bottom of the air stream at the end of the horizontal throw.
INDUCTION. The process of drawing room air into the projected air stream because of the velocity of the projected air stream (sometimes called aspiration).
WALL OR CEILING EFFECT. The tendency of an air stream moving along a wall or ceiling surface to remain in contact with that surface. This effect extends the throw and reduces the drop of the air steam.
Alternate Sizing Chart
Recommended Delivery Velocities
|Broadcasting studios||500 FPM|
|Residences||500 to 750 FPM|
|Apartments||500 to 750 FPM|
|Churches||500 to 750 FPM|
|Hotel Bedrooms||500 to 750 FPM|
|Legitmate Theaters||500 to 1000 FPM|
|Private offices, acoustically treated||500 to 1000 FPM|
|Motion Picture Theaters||1000 to 1250 FPM|
|Private offices, not treated||1000 to 1250 FPM|
|General offices||1250 to 1500 FPM|
|Stores, all floors||1500 FPM|
|Industrieal Buildings||2000 to 1500 FPM|
|TEMPERATURE OF AIR AT REGISTER|
|BTUH AT REGISTER|
|NOTE:||Heating Based on 70-deg. Return Air
Cooling Based on 80-deg. Return Air
BTUH values listed are sensible heat gain and loss
Total cooling BTUH = sensible + lalent.
Total cooling BTUH (Residential) = 1.3 times sensible.