How to calculate fan static pressure required for indoor air cooled chiller with exhaust air duct?

[PandaDada]

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!

 

[GT-EGR]

Treat the four fans as one big fan for your calculations. If you size that combined discharge air duct for the combined airflow to be below the available static pressure, than that same duct size will have a much lower status pressure when only one fan is running.

Just be sure to include all the losses outside of the chiller package - on both the intake and the exhaust side - so your condenser fans might also have to pull your intake air through the initial louvers, dampers, filters, and whatever else if you don’t have some other fan bring that airflow in. Then on the exhaust side you’ll have the ductwork, discharge hoods and anything else on the way out.

As for the velocity pressure you only have to account for that if you are changing it compared on the exiting velocity - the chiller is starting you off with that velocity pressure along with the static they state.

With fans in parallel, the airflow is combined, but each fan in theory if the same just provides its specific airflow at that same static pressure as all the other fans that are operating at the same time. Just lookup fans operating in parallel and there are a lot of fan manufacturers with some good short diagrams explaining this.

 

[PandaDada]

Thank you for your detailed elaboration. I just note that I also have to verify if the intake louvre size is good enough for air intake.

 

[BronYrAur]

It has been way too many years since I dealt in metric units, so I can't help with the specifics, but here are a few things to consider

In my experience, prop condenser fans cannot handle the static of an significant ductwork. Putting air-cooled units inside always presents a challenge.

Will you be able to service the fans with a duct connected?

If one fan fails, air will re-circulate backward through that fan and short-cycle.

You should have end switches on the louvers to prove open

You may need an intake fan and/or exhaust fan to assist the condenser fans.