Drive Package Selection Example - McQuay MDB075D Owner's Manual

Drive Package Selection Example

The following are the design requirements for MDB250D2 unit:
Model:
Supply Air Quantity
External Static Pressure
Step 1: From the blower curve (at 8000 CFM),
Standard operating system;
Total Static Pressure
Internal Static Pressure
External Static Pressure
External Static Pressure of 20.5 mmAq did not fulfill the design requirements.
Step 2: Therefore at 8000 CFM and 29 mmAq External static pressure,
Total Static Pressure
Step 3: From the blower curve, the design requirement calls for RPM about 850 RPM, whereas the unit can only
deliver RPM about 770 RPM under the same CFM. Therefore, it is necessary to resize the pulley size
From the table:
Motor pulley
Blower pulley
Motor RPM
In order to obtain 850 RPM, we calculate the new blower pulley as:
(while maintaining the motor pulley)
Db
The nearest pulley size will be a diameter of 160 mm
Recheck, with Db
Blower pulley
We thus need to change the blower pulley from 180 mm to 160 mm in order to obtain the higher
operating static pressure.
Step 4: When the pulley is changed, the V-belt length must be rechecked. We have for horizontal air throw
configuration:
V-belt length, L
We thus can use a belt with a length of 1600 mm.
where, C
Step 5: From the blower curve, we can also notice that the motor power input has increased. At the new
operating point, the power is approximately 2.7 kW.
By applying a safety factor of 1.2 to account for losses, we calculate that the motor power input
requirement should be = 2.7 x 1.2 = 3.24 kW
Thus, the existing motor is still sufficient to drive the blower with the smaller 160 mm pulley.
Summary:
i) Fan motor kW
ii) Blower pulley diameter
iii) V-belt size
MDB250D2
=
=
= 46.5
= 26
= 20.5
= 26 + 29 mmAq
= 55
= 95 mm
= 180 mm
= 1440
= 95 x (1440/850)
= 160.9 mm
= 160 mm
= 1440 x (95/160)
= 855 RPM
= 2C + 1.57 (Db + Dm)
= (2 x 580) + 1.57(95 + 160)
= 1560.35 mm
= distance between the centres of the two pulleys
Db = diameter of blower pulley
Dm = diameter of motor pulley
= 4.0
= 160
= 1600
8000 CFM
29 mmAq
mmAq
mmAq
mmAq
mmAq
kW
mm
mm
67