Law Supply, Inc.
Web Page - General Information

Grilles, Registers & Diffusers
The performance data in this manual was acquired by elaborate test work so that heating and cooling system designers would have accurate data for registers and diffusers.

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.
It is usually standard practice to use the cooling conversion chart for year around air conditioning, unless the heating requirements are larger, as the larger CFM required for cooling can be dampened down (lower than fan speed, etc.) for heating, always make sure the pressure loss is within your blower capacity.

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

REGISTER. A grille which is equipped with a damper or control valve, and which directs air in a non-spreading jet.

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

With a straight edge connect the horizontal and vertical dimensions of the standard size grille.  Using the same pivot point on the center line, rotate the straight edge until desired dimensions are located.

Recommended Delivery Velocities

The sound caused by an air outlet is directly proportional to the velocity of the air passing through it.  Therefore, the velocities of the outlets selected for a certain application should fall in the range of the recommended velometer velocities listed.  Outlets of the proper size will not add any appreciable noise to the sound level already existing in the ductwork behind the outlet.
APPLICATION
RECOMMENDED
VELOMETER 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

Heat Conversion Chart


C 
 F 
  M		 HEATING COOLING
TEMPERATURE OF AIR AT REGISTER
120 140 160 65 60 55
BTUH AT REGISTER
50 2,700 3,780 4,860 810 1,080 1,350
75 4,050 5,670 7,290 1,215 1,620 2,025
100 5,400 7,560 9,720 1,620 2,160 2,700
125 6,750 9,450 12,150 2,025 2,700 3,375
150 8,100 11,340 14,580 2,430 3,240 4,050
175 9,450 13,230 17,010 2,835 3,780 4,725
200 10,800 15,120 19,440 3,240 4,320 5,400
225 12,250 17,010 21,870 3,645 4,860 6,075
250 13,500 18,900 24,300 4,050 5,400 6,750
275 14,850 20,790 26,730 4,455 5,940 7,425
300 16,200 22,680 29,160 4,860 6,480 8,100
350 18,900 26,460 34,020 5,670 7,560 9,450
400 21,600 30,240 38,880 6,480 8,640 10,800
450 24,300 34,020 43,740 7,290 9,720 12,150
500 27,000 37,800 48,600 8,100 10,800 13,500
550 29,700 41,580 53,460 8,910 11,880 14,850
600 32,700 45,360 58,340 9,720 12,960 16,200
700 37,800 52,920 68,040 11,340 15,120 18,900
800 43,200 60,480 77,760 12,960 17,280 21,600
900 48,600 68,040 87,480 14,580 19,440 24,300
1000 54,000 75,600 97,200 16,200 21,600 27,000
1100 59,400 83,160 106,920 17,820 23,760 29,700
1200 64,800 90,270 116,640 19,440 25,920 31,400
1300 70,200 98,280 126,300 21,060 28,080 35,100
1400 75,600 105,840 136,080 22,680 30,240 37,800
1600 86,400 120,960 155,520 25,920 34,560 43,200
1800 97,200 136,080 174,960 29,160 38,880 48,600
2000 108,000 151,200 194,400 32,400 43,200 54,000
2200 118,800 166,320 213,840 35,640 47,520 59,400
2400 129,600 181,440 233,280 38,880 51,840 64,800
2600 140,400 196,560 252,720 42,120 56,160 70,200
2800 151,200 211,680 272,160 45,360 60,480 75,600
3000 162,000 226,800 291,600 48,600 64,800 81,000
3200 172,800 241,920 311,040 51,840 69,120 86,400
3400 183,600 257,040 330,480 55,080 73,440 91,800
3600 194,400 272,160349 349,920 58,320 77,760 97,200
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.
Taken from American Metal Products Catalog.
Last updated 11/28/2001
This information is for reference only
Not responsible for typographical errors