Parallel operation of Centrifugal fans
Parallel operation of Centrifugal fans
(OP)
Guys,
In a biosafety lab exhaust system,we have 2 fans each sized for 100% air flow capacity and pressure connected in parallel.Is it possible to run both the fans together at reduced speed so that in the event of one of them failing the other can quickly run up to full speed and takeover?How will we determine the speed and power consumption for the fan?What is the best way to control the speed? Duct static pressure control?
My concern are:
1) At reduced speeds the fan may develop less static pressure and may not be sufficient to overcome the system resistance
2)Running both fans together will not be energy efficient.
I would like to hear from those have tried this.





RE: Parallel operation of Centrifugal fans
Running two fans just chews up more energy, even though they are both (theoretically) operating at half capacity. Use the fan laws to roughly determine your energy expenditures.
RE: Parallel operation of Centrifugal fans
RE: Parallel operation of Centrifugal fans
RE: Parallel operation of Centrifugal fans
RE: Parallel operation of Centrifugal fans
The parallel operation is a viable option and is a common design today for BSL-3 exhaust systems.
The last three BSL-3 systems I've commissioned all had the configuration of dual fans wyed off a common duct, re-converging into the same stack at an angle that promotes discharge out the stack and not backfeed to the other fan. Each fan is equipped with either one or two low-leakage isolation dampers that close upon fan shut down.
The two fans operate in parallel to maintain a common suction pressure in the duct. The stack's discharge cone is sized as per ACGIH (target about 3,000 fpm) based on the design exhaust volume from the space. If the space exhausts a known, constant volume flow, the plume velocity is maintained regardless of whether one or two fans are used to maintain duct static pressure.
If continuous, uninterrupted exhaust flow from the lab is NOT necessary, the same configuration, or a system with separate stacks, can be utilized with a single fan in operation and the other acting as standby. Then (as suggested above) periodic changes in lead/standby is recommendable to even ware on the fans.
As also suggested above, a strobic system could be used. If the exhausted volume from the space does not vary, however, the strobic system is not necessary.
I'd be interested to know the activity in the space and the designated biosafety level. How critical is maintaining continuous exhaust?
Regards, -CB
RE: Parallel operation of Centrifugal fans
With each fans generating only 50% airflow ,the fans must discharge through a common stack in order to have a stack exit velocity of about 3000 fpm.For the above reasons indvidual fan stacks are ruled out.A motorised bypass damper is provided at the fan intake plenum which would open and maintain constant stack velocity if the system flow goes down.
So Chasebean,do not lose heart since you are not in the minority any longer!You have been on target as usual,Keep it up!Forgot to mention that this is a BSL-3 lab and that explains the need to have the fans running 24 hrs!
RE: Parallel operation of Centrifugal fans
Best of luck, -CB
RE: Parallel operation of Centrifugal fans