Brochure

Engineering Data
76
Recommended Noise Criteria and Face Velocity Ranges are on page 75
Engineering Data
Keeping face velocity low
• Returns should be at 400-600 fpm maximum
• Filter Returns should be at 450 fpm maximum
*ACCA recommends 300 max for filter grilles and 500
max for non-filter grilles.
• The rule of thumb is 2 cfm per square inch of filter size.
See table below.
• Low velocity reduces noise, especially on stamped
face grilles (672/673); fixed-bar grilles can handle more
velocity without noise (94A/96AFB/RH45/RHF45/RCB).
• You really can’t oversize a single point return like you
can with a supply. The system will not be affected
adversely, only improved. *This does not apply to
multiple return locations where balancing is more critical
to pull in relevant amounts from each room.
• Static pressure is also reduced. Pressure works
against & reduces blower delivery volume (cfm)
• No one expects noise from a return.
Location
• Returns should be put in stagnant air locations that
need to be reconditioned.
- High for cooling mode (hot air rises)
- Low for heating mode (cold air falls)
- Both modes, choose a primary season
• Returns should not be near a supply register’s throw
range. If at all possible place the return at an opposite
corner of the room.
Room Air Movement
• Returns do NOT have much effect on a room’s air
movement, regardless of face velocity. They only grab
air about a duct diameter away from the face. Most of
the room air movement is done by the supplies.
Unlisted Sizes–Engineering Data
When a size is not listed there are a couple ways to do
an engineered estimate. Airflow principles permit you to
utilize existing sizes to determine sizes not shown.
Method 1: Use nearest nominal size table entry. If a
14x14 is not given, but a 20x10 is, since the these two
sizes have an approximate equal core area (196 and
200) the table entry for a 20x10 can be used to
approximate what the 14x14 grille would perform to.
Method 2: A more exact method would be to do
interpolation process between two listed sizes. If 14x14
is not given, but 18x10 and 20x10 are, then this
equation will get more exact 14x14 data. Y = Y1 + [{(X -
X1) * (Y2-Y1)} / (X2 - X1)] where:
Y = unknown CFM or throw that is being computed for
14x14
Y1 = CFM or throw of listed 18x10 (for ex 600 cfm)
Y2 = CFM or throw of listed 20x10 (for ex 640 cfm)
X = 196 in² (nominal area of 14x14)
X1 = 180 in² (nominal area of 18x10)
X2 = 200 in² (nominal area of 20x10)
Using equation above computes Y = 600 + [{(196 – 180)
* (640 – 600)} / (200 – 180)] =
600 + [{16 * 40} / 20] = 600 + 32 = 632 cfm for Y
Method 3: Sizes beyond the table (smaller or larger)
can have their CFM or Throw determined by using listed
sizes by the following:
CFM for larger sizes:
If looking for 24x6 or 24x12 cfm that is not listed, using
the listed 12x6 cfm and doubling it or quadrupling it will
give the answer for the 24x6 and 24x12, respectively.
CFM for smaller sizes:
If looking for a 6x6 cfm that is not listed, using the listed
12x6 cfm and halving it will give the answer for a 6x6.
Throw:
Double the size and CFM, multiply the throw by 1.5
Quadruple the size and CFM, multiply the throw by 2
Half the size and CFM, multiply the throw by .67
One quarter the size and CFM, multiply the throw by .5
*Pressure loss, face velocity and noise criteria will all
remain the same relative to the listed size used to
determine the larger or smaller sizes not shown.
Filter Size
Area
(in)
Ton (cfm)
Filter Size
Area
(in)
Ton (cfm)
12
12
144
n/a
20
20
400
2 (800)
12
20
240
1 (400)
20
25
500
2.5 (1000)
12
24
288
1.5 (600)
20
30
600
3 (1200)
12
30
360
1.5 (600)
20
36
720
3 (1200)
14
14
196
1 (400)
24
24
576
3 (1200)
14
20
280
1.5 (600)
24
30
720
3 (1200)
14
24
336
1.5 (600)
24
36
864
4 (1600)
14
30
420
2 (800)
25
25
625
3 (1200)
16
20
320
1.5 (600)
30
30
900
4 (1600)
16
24
384
2 (800)
30
36
1080
5 (2000)