Lennox ROOFTOP FLEXY FXK 55, Installation, Operating And Maintenance Manual

Page: 48 / 106

VENTILATION : AIRFLOW BALANCING
FLEXYII_WSHP-IOM-1307-E - 46 -
EXAMPLE
The unit used for this example is a FGM170ND with standard supply and return airflow configuration. It is also fitted with an
economiser and an electric heater type H.
It is fitted with 2 ADH450 L fans which curve is shown on previous page and 2x 5.5 kW motors..
-
Motor rpm: 1465 rpm
- cos
ϕ
= 0.79
- Voltage = 400V
- Current = 9.00A (per fan)
P
mech fan
= V x I x
√
3 x cos
ϕ
x
η
mech motor
x
η
Transmission
= 400 x 9.1 x
√
3 x 0.79 x 0.89 x 0.9 = 4.0 kW
The unit is also fitted with 2 transmission kits 3.
- Fixed Fan pulley: 200mm
- Motor adjustable pulley type “8550” opened 4 turns from fully closed or measured distance between pulley end plates is
29.1mm: from table_1 it can be determined that each motor pulley has a diameter of
114.2 mm
rpm
FAN
= rpm
MOTOR
x D
M
/ D
F
= 1465 x 114.2 / 200 = 836 rpm
Using the fan curve, the operating point can
be located.
In order to facilitate the calculation, you
won’t make any mistake by considering
that the external static pressure available
is the one calculated with one fan
providing the half of the nominal flow (here
15000m3/h).
It can be determined that the fan is providing
approximately
15000 m3/h with a total
pressure P
TOT
= 630 Pa
The pressure losses in the unit are the sum of
all pressure drops across the different parts of
a unit:
- Coil and filter (measured) = 89 Pa
- Inlet into the unit = 50 Pa
- Options = 16 Pa for economiser and 15
Pa for electric heater H
∆
P = 89 + 16 + 15 +50 = 170 Pa
The dynamic pressure at 15000m
3
/h is given
at the bottom of the fan curve.
Pd = 81 Pa
The external static pressure available is
therefore
ESP = P
TOT
- Pd -
∆
P
INT
=630 - 91 - 170 = 369
Pa
836rpm
630Pa