Lennox BAC020SNM, Installation, Operating And Maintenance, Page: 62 / 137

VENTILATION: AIRFLOW BALANCING
BALTIC R410A-IOM-0708-E Page 61
2
nd
method when adjusting the pulley :
-Close the pulley fully and count the number of turns
from fully closed position. Using
table_2 determine the
motor pulley actual diameter.
-Record the fix fan pulley diameter.(DF)
-Determine the fan speed using the following formulae:
Where: rpm
MOTOR
:from the motor plate or table_3
D
M
: from table2
D
F
: from machine
Once the pulleys are adjusted and the belt checked and
tensioned, start the fan motor and record the Amps and
Voltage between the phases :
Using the measured data and
table_3
-Theoretical mechanical power at the fan shaft:
P
meca fan
= P
meca Motor
x

Transmission
P
meca fan
= P
elec
x

meca motor
x

Transmission
P
meca fan
= V x I x

3 x cos

x

meca motor
x

Transmission
This formula can be approximated in this way
P
meca fan
= V x I x 1.73 x 0.85 x 0.76 x 0.9
With the fan “rpm” and t
he mechanical power at the fan
shaft an operating point and the supplied airflow can be
estimated using the fan curves.
CHECKING AIRFLOW AND ESP
Using the fan curves on
page 25, 26, 27 , the airflow, the
total pressure available (P
TOT
) and the corresponding
dynamic pressure (Pd) can now be estimated, for a specific
operating point;
The next step consists in estimating the pressure losses
across the unit.
This can be achieved using the “dirty filter pressure sensor”
and the accessories pressure drop table: table_4
Also the pressure drop due to the duct inlet into the roof-top
unit can be taken as 20 to 30 Pa.

P
INT
=

P
coil
+

P
Inlet
+

P
Options
using the results from above, the external static pressure
(ESP) can then be estimated:
ESP = P
TOT
- Pd -

P
INT
Table_ 3 Motor information
Motor Size Nom. Speed
Cos


meca motor
0.75 kW 1400 rpm 0.77 0.70
1.1kW 1429 rpm 0.84 0.77
1.5kW 1428 rpm 0.82 0.79
2.2kW 1436 rpm 0.81 0.81
3.0kW 1437 rpm 0.81 0.83
4kW 1438 rpm 0.83 0.84
5.5kW 1447 rpm 0.83 0.86
7.5kW 1451 rpm 0.82
0.87