Brochure

Using the Engineering Data
For most of the models & sizes we’ve done the
calculations for you.
CFM = volume of air flow in cubic feet per minute
Face Velocity = speed of air at the face of diffuser in feet
per minute (FPM)
Ak = net area in square feet. This is the lab measured
area across the face when air is mechanically forced
through the opening.
Free Area (if given) = daylight area (in²) of blade
openings. Free area is typically only required on natural
/ gravity movement of air, non-mechanically forced, as in
free area needed for combustion air requirements on
heating equipment. Use the Ak value (*144 to get to in²)
if the free area has not been calculated, but is needed
for a given size/model grille requiring free area for
combustion.
Equation of Airflow: CFM = Ak (ft²) x Face Velocity (fpm)
Example from 421 table above: 100 = .17 x 600 _
numbers are often rounded
Sizing a Supply
Determine the amount of CFM (air volume) needed for
each supply outlet. This should be done by room
heating and cooling load requirements from various
design manuals (ACCA Man J, ASHRAE Fundamentals
Hndbk) and then followed by the duct design and layout.
Face Velocity - H&C recommends sizing a supply outlet
in the range of 500 to 800 fpm face velocity (700 being a
common target). The upper end of this range will create
better mixing of room air and longer throws, which is
what the typical forced air system is intended to do.
However, the Pressure resistance and Noise must be
taken into consideration depending upon the application.
In some instances, greater face velocity is allowed
because the pressure and noise can be accommodated.
Pressure Loss (inches of w.c.) – the selection of the face
velocity must consider the associated pressure loss that
deals with each relative model. An increase in face
velocity creates more pressure resistance against the
blower’s delivery of air volume. The velocity ranges
given previously, in most cases, will have minor effect on
the blower’s overall performance given the entire duct
system losses that it will encounter.
Noise – an increase in face velocity will create more
noise. The tables below show NC design guidelines and
also face velocity ranges if NC values have not been
tabulated.
Sizing a Return
Air volume going back to the air handler (fan) must
equal what is supplied from the air handler. Therefore
the total CFM capacity of the return grilles must equal or
exceed the total CFM capacity of all the supply diffusers.
421
Face
Velocity
300 400 500 600 700 800
Pressure
Loss
.006 .010 .016 .022 .031 .040
CFM 50 70 85 100 120 135
Spread 4.5 5.0 6.5 7.5 9.0 10.0
4x10
Ak
.170
Throw 4.0 6.0 8.0 10.0 11.0 12.5
Terminal velocity of 50 fpm
821-defl A
Face Velocity
400
500
600
700
800
Pressure Loss
.010
.016
.022
.031
.040
CFM
420
525
625
730
835
24 x 8
Ak
1.045
Throw
17.0
21.0
25.0
29.0
33.0
Terminal velocity is 75 fpm
Recommended Noise Criteria and Face Velocity Ranges are on page 75
Engineering Data
75
Engineering Data