McQuay MDB075D User Manual

67
Drive Package Selection Example
The following are the design requirements for MDB250D2 unit:
Model: MDB250D2
Supply Air Quantity = 8000 CFM
External Static Pressure = 29 mmAq
Step 1:
From the blower curve (at 8000 CFM),
Standard operating system;
Total Static Pressure = 46.5 mmAq
Internal Static Pressure = 26 mmAq
External Static Pressure = 20.5 mmAq
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 = 26 + 29 mmAq
= 55 mmAq
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 = 95 mm
Blower pulley = 180 mm
Motor RPM = 1440
In order to obtain 850 RPM, we calculate the new blower pulley as:
(while maintaining the motor pulley)
Db = 95 x (1440/850)
= 160.9 mm
The nearest pulley size will be a diameter of 160 mm
Recheck, with Db = 160 mm
Blower pulley = 1440 x (95/160)
= 855 RPM
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 = 2C + 1.57 (Db + Dm)
= (2 x 580) + 1.57(95 + 160)
= 1560.35 mm
We thus can use a belt with a length of 1600 mm.
where, C = distance between the centres of the two pulleys
Db = diameter of blower pulley
Dm = diameter of motor pulley
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 = 4.0 kW
ii) Blower pulley diameter = 160 mm
iii) V-belt size = 1600 mm

McQuay MDB075D Specifications

General

Brand	McQuay
Model	MDB075D
Category	Air Conditioner
Language	English