Carrier 48MA - Manual (Page 44)

The procedure for determining required fan
speed (rpm) and shaft horsepower for a 48MA (or
SOME) unit is described below.
Since the various unit zones operate at different
air quantities and different external resistances, it
is necessary to identify the zone having the highest
static pressure requirement for the supply duct and
supply outlet.
Usually, the longest duct run to the last outlet,
with the greatest number of offsets and elbows, has
the greatest static pressure requirement. Assume
that a duct friction analysis has been made and the
cfm and static pressure are as follows:
FAN PERFORMANCE
Zone No CFM ESP (in wg) No Modules CFM/Module
1 720 6 1 720
2 1080 8 1 1080
3 1080 5 1 1080
4 2700 6 3 900
5 900 4 1 900
6 1080 4 1 1080
7 1440 5 2 720
Totals 9000 10
The total unit cfm is 9000. Zones 4 and 7 have 3
and 2 modules, respectively, “ ganged” together to
comply with the limitation of 1200 cfm per
module.
The cfm for zone 2 is 1080 with an ESP of
.8 in. wg. This module appears to possess the
highest friction loss. Therefore, the main fan static
pressure is established at .8 in. wg ESP.
Enter Tabled fan performance at 9000 cfm,
.8 in. wg ESP and read: 1070 rpm and 5.7 bhp.
The 028 indoor fan motor data shows the standard
lOhp motor with a maximum bhp of 11.5.
Therefore, 5.7 bhp for this selection is satisfactory.
Two pulley selections are available with the 028:
Pulley A, shipped mounted; Pulley B, shipped in
the blower compartment. Pulley A has a fixed
pitch and at 1095 rpm is close enough to the
required cfm and should be used. Pulley B at
1230 rpm allows selection of the unit at higher
cfm’s and static pressures. (See Table 1.)
Pulley Selection — In general, for start-up, the
pulley producing an rpm higher than required but
closest to the required speed should be used. If this
results in excess air being delivered, a locally
supplied pulley that produces lower air quantity
should be used. Two pulley selections are provided
with each motor and cover most application
requirements.
Balancing Dampers — As in any multizone
application, suitable balancing dampers should be
provided in each zone duct run. Normally, a
2-bladed damper is preferred to a single blade. For
the selection example, balancing dampers should
be adjusted (by the installer) to give the desired
airflow and static pressures in each module. Ready
access to balancing dampers is a major considera­
tion when designing a multizone system. In a T-bar
ceiling, this is not a problem. In a plastered ceiling,
an access door at the damper location is required.
Low Cfm, Long Run Zones — Small zones with
long runs can cause problems in any multizone
system. As the cfm decreases at a given duct
velocity, the friction loss per 100 ft increases
significantly. Also, seam and joint leakage in a
small duct of long length can prevent delivery of
required cfm at the outlet grille.
Long ducts with low cfm should be oversized to
give lower velocities, lower friction rates and
reduced leakage rates. Recognition of the charac­
teristics of small ducts and proper design of such
runs will avert potential problem areas.
In addition to the fan performance table, fan
curves (Fig. 32, 33, 34) have been included for
easier interpolation and fan selection at rpm’s and
static pressures not shown in the fan performance
table.
42