Parallel Blower Operation 1

[ICleanYourAir]

I am designing a custom filtration system that will operate at 1,500 cfm. Static pressure is approximately 6" w.g.. The customer wants me to supply 2 blowers that will operate in parallel. The blowers will be controlled by VFD's. During normal operation, each blower will operate at reduced rpms to provide a total flowrate of 1500 cfm. If one of the blowers fails, the other will speed up, allowing the overall system to maintain a flowrate of 1,500 cfm.

I have never installed parallel blowers. Under normal operation, each blower will be required to move 750 cfm. I've calculated the pressure drop through the system at 750 cfm to be approximately 3" w.g.. I'm assuming I need to select a blower capable of operating at 750 cfm at 3" static, with the capability to ramp up to 1500 cfm at 6" static. Does this make sense?

 

[ICleanYourAir]

It hit me like a ton of bricks on my way home from the office that this is not the proper way to size the blowers. The total static pressure at 1500 cfm is 6" w.g.. Thus, each blower will be required to operate at 750 cfm @6" static for normal operation and 1500 cfm @6" static for the backup operation (at higher rpms).

 

[quark]

Just recheck your numbers. If you double the flow, frictional losses should increase by 4 times.

You should select your fans(or blowers?) at 750 cfm and 6" static pressure but the control scheme will be different. At full flow you will operate both the fans at full speed (unless if your static pressure is redundant). When you reduce the flow to 750 cfm, one fan starts operating at a higher flow than 750 cfm because of reduction in pressure losses. So you have to reduce its speed to get 750 cfm approximately(here you will loose some extra power compared to 750 cfm and 3" static).

A flow transmitter(preferably in conjunction with a pressure transmitter) in the common supply duct can control your fans.

Regards,

 

[friartuck]

You have a run/std-by situation basicaly

The simplest way is to operate each at 1500cfm and 6" wg. Then you simply run fan a monday, fan b tuesday, fan a again wednesday etc.

I work on the KISS principle. Keep it simple stupid (no offence intended)

I cannot see any advantage in operating two in parallell if you really dont have to.

However if you do, the you say that the system must cope if one fan fails, so the minimum each fan must be designed for is 1500@6" anyway

Friar Tuck of Sherwood

 

[lilliput1]

You may have to put a recirculation ductwork with automatic valve around each fan to prevent operation at the surge region when both are operating. When only one fan runs and also when the fan is off, the damper in the recirculation duct should be shut. When Check fan curves at each operating point. Use the fact that pressure drop is proportinal to the sguare of the CFM to estimate duct pressure drop at each case. Combine discharge to one stack if discharge velocity is critical. Provide automatic damper at each fan inlet to shut down when the corresponding fan is off. Avoid discharge damper to minimize loss. Use full size fan discharge atack + nozzle at end sized for 3000 FPM if it is critical to blow exhaust discharge away from intakes.

 

[SAK9]

This is possible and I have done this before .You need to select each blower at 1500 cfm@6" static pressure.When you operate both of them together,you may get something like 2500 cfm at 8" static.All you need to do is to bring the speed down to make it operate near 1500 cfm @6".Please note that the fans may operate at around 80~90% of their normal speed(not 50% as you may think).The situation would be much clearer when you plot the combined fan curve next to the single fan curve.A staic pressure transmitter may be used to control the speed of the fans.When any of the fans fail,the sensor reading would drop below the setpoint and it could be used to ramp the other fan to full speed

I too had laughed this off initially until some one convinced me it could be done

 

[MintJulep]

Selecting components to run well off of their best efficiency points for the majority of their opeating life doesn't sound like a great idea from a life-cycle cost standpoint.

Look into selecting components that you can run near peak efficiency when operating normally, and run faster if one fails.

In general, fans are pretty reliable. If you go direct drive you can eliminate belt failures. If you oversize the bearings and actually pay a little attention to temperatures and lube selection you can pretty much bring the chance of bearing failure to near zero.

Does this application really need the complication of a redundant system? You might well be better off focusing on high reliability combined with a preventive maintenance program.

 

[quark]

There may be few problems with 1500cfm fan at 6" pressure and that is why I didn't suggest it.

1. When the fan has to run at 750 cfm and 3" pressure, the extra cfm to be pumped in by a 1500cfm fan is around 300 cfm. (cfm2 = (3/6)[sup]1/2[/sup]1500, which comes out to be 1060 cfm). This is wastage of energy.

2. The fan may not run at BEP when the speed is reduced (though reduction in speed follows BEP, the pressure correction doewn't).

3. If the fan is a forward curved blade type then there are potential problems of the fan operating in the unstable region.

Regards,

 

[Istre]

I was going to post exactly what Lilliput said, but I'll just stress how important what he said about dampers is.

If you have a parallel system, using redundancy to protect the system, and you fail to prevent the makeup air from leaving the system via the non-working leg in a failure, you're going to be giving yourself and your employers a false sense of security and will get a nasty surprise in the event of a failure. Spring loaded dampers that the fans can pull open while operating sound like a good idea.

 

[ChasBean1]

ICleanYourAir,

If I could arbitrate, I side with friartuck's post and pieces of SAK9's.

Size each for 1,500 cfm, 6 inches TSP. Run the system as friartuck describes, unless guaranteed, uninterrupted flow is necessary, in which case, run both in parallel. Guaranteed flow applications (such as clean room, BSL2+ and higher, etc.) should generally supercede BEP issues.

The static pressure in the duct controlling the VFD will reduce speed of parallel operating fans to 50-80%, depending on friction loss assumptions, filter loading, etc.

You might want to consider a small packaged AC system with appropriate filter section for this application? Maybe Munters has something?

Best of luck,
-CB

 

[quark]

hmmm...then I will team up with Lilliput1 and Istre. Any other groups in the fray? Hey, I am just kidding(only in this post).

Observation: HVAC guys seems to be the most serious lot among all the eng-tips fora.

 

[show]

We use this concept in clean room exhaust systems and it does work. We have found that the actual rpm when operating the fans in parallel is in the range of 75-85% of the rpm of one fan operating. Of course, as noted, backdraft dampers or motorized dampers are required to avoid drawing air through the non-operating fan.

Although this concept assures continuous exhaust when a fan fails, it is difficult to guarantee the duct static will not drop. Our systems have a bump of 0.2-0.4" before the fan gets ramped to 100% (this is with normal operating ranges of 6"-10"). We compensate for this by making sure production tools have 30-second time delays on their exhaust failure logic.

Regards,
Rod S.

 

[ChasBean1]

Nothing personal, Quark!