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 require-
ments from various design manuals (ACCA Man J, ASHRAE Funda-
mentals 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 will, in most cases, 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.
Engineering Data
Using the Engineering Data
For most of the models & sizes we’ve done the calculations for you.
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
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 (in
) 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 in
) 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 (ft
) 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 typica
l 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 range
s 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.
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.
Recommended Noise Criteria and Face Velocity Ranges are on page 75
Engineering Data
75
Engineering Data