McQuay MDB100B User Manual

35
Drive Package Selection Example
The following are the design requirements for MDB125B unit:
Model: MDB125B
Supply Air Quantity = 4500 CFM
External Static Pressure = 18 mmAq
Step 1:
From the blower curve (at 4500 CFM),
Standard operating system;
Total Static Pressure = 26.5 mmAq
Internal Static Pressure = 17.5 mmAq
External Static Pressure = 9.0 mmAq
External Static Pressure of 9.0 mmAq did not fulfill the design requirements.
Step 2:
Therefore at 4500 CFM and 18 mmAq External static pressure,
Total Static Pressure = 17.5 + 18 mmAq
= 35.5 mmAq
Step 3:
From the blower curve, the design requirement calls for RPM about 800, whereas the unit can only
deliver RPM about 690 under the same CFM. Therefore, it is necessary to resize the pulley sizes.
From the table:
Motor pulley = 4"
Blower pulley = 8"
Motor RPM = 1425
In order to obtain 800 RPM, we recalculate the new blower pulley as:
(while maintaining the motor pulley)
Db = 4" x (1425/800)
= 7.125"
The nearest pulley size will be a diameter of 7"
Recheck, with Db = 7"
Blower pulley = 1425 x (4/7)
= 814
We thus need to change the blower pulley from 8" to 7" 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 (319 x 0.03937")) + 1.57 (4" + 7")
= 42.4"
We thus can use a belt with a length of 43"
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 1.25 kW.
By applying a safety factor of 1.2 to account for losses, we calculate that the motor power input
requirement should be = 1.25 x 1.2 = 1.5 kW
Thus, the existing motor is still sufficient to drive the blower with the smaller 7" pulley.
Summary:
i) Fan motor kW = 1.5 kW
ii) Blower pulley diameter = 7"
iii) V-belt size = 43"

McQuay MDB100B Specifications

General

Brand	McQuay
Model	MDB100B
Category	Air Conditioner
Language	English