An air cooled chiller with 4 nos. condenser fans is to be installed in an indoor chiller plant with headroom of 5m. Air is driven from intake louvre at low level while hot air is exhausted from an air duct over all the condenser fans to exhaust louvre at high level in the room. All condenser fans start or stop running at the same time. It is provided that the external static pressure of a condenser fan is 150 Pa.
In most fan static pressure calculation for air duct, only one fan is involved, and it is not difficult to figure out whether the fan is strong enough to push air through the ductwork system. When it comes to multiple fans calculation in an air duct, I feel so confused. I tried to calculate it like this.
It is given that total exhaust volume of the air cooled chiller is 20 m3/s. I calculated the total pressure loss using the big air duct dimensions (e.g. 2.5m(L)x 2.3m(W)x6m with an elbow). The total velocity is total volume of air cooled chiller divided by the cross sectional area of the big duct. Then I get the velocity pressure generated by the group of fans. My friend told me that it is acceptable when total pressure loss along duct (friction loss and dynamic loss) is smaller than the sum of given external static pressure (150 Pa) and velocity pressure. But the 150 Pa is for one fan only! We are using the big duct to do the calculation. Isn’t it confusing?
Could anyone please elaborate how to calculate external static pressure for each fan when there are multiple fans in an air duct? Thank you!