Parallel operation of Centrifugal fans

[SAK9]

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.

 

[MarauderX]

Haven't tried it, and wouldn't. I have done many lab systems, and each time I have always been concerned about the exhaust plumes rising high enough to avoid hazardous conditions. I would keep the redundacy for the fans and cycle them so they each run every other day. It keeps both fans from being neglected, they both need serviced, and the fans/bearings/etc. will last longer and can still be changed sequentially.

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.

 

[ChasBean1]

Duct static pressure control is the best way to control fan speed. I've often seen two run in parallel as you mention for BSL-3 applications, because it is important to avoid even brief moments of positive pressurization of the space during lead/standby switching. If both fans share a common stack, the exit velocity (if it's important in this application) will not be affected, assuming exhaust flow from the space is controlled via terminal boxes.

 

[lilliput1]

Do not share the same stack. Otherwize you may radically affect the performance of the other fan. Usually (3) VFD fans is better than (2) because change is less drastic if one fails. Provide OA bypass to plenum. After fan VFD has turned down to minimum approximately 3000 FPM exit velocity, open up the OA bypass to mix w/ the lab exhaust air & keep the stack discharge velocity in the 3000 FPM range. Consider using strobic fans w/ plenum & OA bypass dampers. Size duct connection to plenum at max 1200 FM velocity to minimize system effect losses. Provide tapered nozzle at each fan stack to reduce loss to exit velocity only & not constrict entire stack. Make sre fan do not operate in surge region on any mode of fan operation. You may have to put automatically controlled valved recirculation duct from fan discharge to fan suction to avoid surge.

 

[fredt]

I am not sure how correctly fans are selected to suit the system resistance in this type of application. Assuming that each fan is correctly selected to match the required resistance, then connecting the two of the same in parallel, especially at reduced speed, will (perhaps counter-intuitively) produce a much reduced airflow and the fans will most likely be operating in a stalled condition. It is not a viable option.

 

[ChasBean1]

SAK9, it appears you got conflicting results and I'm in the minority. I agree with others that energy expenditure will be slightly higher. The safety of continuous BSL-3 containment would outweigh that cost, in my opinion.

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

 

[SAK9]

Many thanks for all the replies.Spoke to couple of guys who have done this before.As mentioned earlier each fan should be selected for 100% airflow.Only two fans are required.When the fans operate in parallel, the sytem curve meets the combined fan curve at a point higher than the intersection point for a single fan curve.This curve would not be too far off from the intersection point for a single fan operation.By measuring and using the duct static pressure,the fan speeds are lowered to bring the operating point to that of a single fan.The reduction in speed is not substantial probably 80% of what the single fan would operate at. Each fan would provide 50 % air flow at the system static pressure requirement at 100% flow.Therefore the fan operation is marginally inefficient but not too much.In the event of one fan going off line,the other fan would speed up to full speed to maintain the total design flow.So a breakdown does not affect the system.

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!

 

[ChasBean1]

Sounds good SAK9. My only comment is that controls could get tricky with duct static pressure controlling fan VFDs AND a plenum bypass damper also affecting duct static. If it's a constant volume that needs to be exhausted from the space (and BSL-3 typically is), I'd recommend sizing the discharge cone right and leaving the bypass damper out of the equation...

Best of luck, -CB

 

[lilliput1]

I agree w/ ChasBean1. Leave margin for future hood additions though. Put exhaust fan on normal/emergency power & some supply also for makeup.