035-17311-000 REV A (1200)
28
Unitary Products Group
1.
Remove the dot plugs from the duct panel (for location of
the dot plugs see figure 11).
2.
Insert eight-inches of 1/4 inch metal tubing into the air-
flow on both sides of the indoor coil.
NOTE:The tubes must be inserted and held in a position per-
pendicular to the air flow so that velocity pressure will not
affect the static pressure readings.
3.
Use an inclined manometer to determine the pressure
drop across a dry evaporator coil. Since the moisture on
an evaporator coil can vary greatly, measuring the pres-
sure drop across a wet coil under field conditions could
be inaccurate. To assure a dry coil, the compressors
should be de-activated while the test is being run.
NOTE:De-energize the compressors before taking any test
measurements to assure a dry evaporator coil.
4.
The CFM through the unit with clean 2 inch filters can be
determined from the pressure drop indicated by the
manometer by referring to the curves in Figure 21.
5.
After readings have been obtained, remove the tubes
and replace the dot plugs. .
SUPPLY AIR DRIVE ADJUSTMENT
At unit start-up, the measured CFM may be higher or lower
than the required CFM. To achieve the required CFM, the
speed of the drive may have adjusted by changing the datum
diameter (DD) of the variable pitch motor sheave as
described below:
Use the following table and the DD calculated per the above
equation to adjust the motor variable pitch sheave.
EXAMPLE
A 10 ton unit was selected to deliver 4,000 CFM with a 5 HP
motor, but the unit is delivering 3,750 CFM. The variable
pitch motor sheave is set at 2 turns open.
Use the equation to determine the required DD for the new
motor sheave,
Use Table 29 to locate the DD nearest to 4.26 in. Close the
sheave to 1/2 turn open.
New BHP
New motor Amps
TABLE 28: INDOOR BLOWER SPECIFICATIONS
MODEL
MOTOR
MOTOR SHEAVE
BLOWER SHEAVE
BELT
HP
RPM
Eff.
SF
Frame
Datum Dia. (in.)
Bore (in.)
Model
Datum Dia. (in.)
Bore (in.)
Model
DM090
1-1/2
1725
80%
1.15
56
3.4 - 4.4
7/8
1VM50
9.0
1
AK94
A57
2
1725
80%
1.15
56
3.4 - 4.4
7/8
1VM50
7.5
1
AK79
A55
DM120
2
1725
80%
1.15
56
3.4 - 4.4
7/8
1VM50
8.5
1
AK89
A56
3
1725
80%
1.15
56
3.4 - 4.4
7/8
1VM50
7.0
1
AK74
A54
DM150
3
1725
80%
1.15
56
3.4 - 4.4
7/8
1VM50
7.0
1
AK74
A54
5
1725
87%
1.15
184T
4.3 - 5.3
1 1/8
1VP56
6.7
1
BK77
BX55
Failure to properly adjust the total system air quan-
tity can result in extensive blower damage.
Specified CFM
Measured CFM
-------------------------------------
Existing DD
•
New DD
=
Before making any blower speed changes review
the installation for any installation errors, leaks or
undesirable systems effects that can result in loss
of airflow.
Even small changes in blower speed can result in
substantial changes in static pressure and BHP.
BHP and AMP draw of the blower motor will
increase by the cube of the blower speed. Static
pressure will increase by the square of the blower
speed. Only qualified personnel should make
blower speed changes, strictly adhering to the fan
laws.
4,000 CFM
3,750 CFM
---------------------------
4.0 in.
•
4.26 in.
=
Speed increase
(
)
3
BHP at 3,750 CFM
•
=
Speed increase
(
)
3
Original BHP
•
=
New BHP
=
Speed increase
(
)
3
=
Amps at 3,750 CFM
•
Speed increase
(
)
=
3
Orignial Amps
•
New Amps
=
