Fan Not Responding to High Pressure in Duct

[RJB32482]

Hi,
If a industrial fan inlet is blocked by a damper, will the flow decrease even if it is controlled to increase when it sees a spike in static pressure in the duct before the damper? Our fan is controlled by static pressure. We see that it is not responding to an increase in pressure (it usually speeds up). If the damper after the pressure element but before the fan is not fully open could this cause this effect (fan slowing down)?

Thank You

 

[AEBuck]

If the partially-closed damper is sufficiently close to the fan inlet, such that it adversely alters the velocity profile entering the fan, then I'm tempted to say yes - the fan could speed up in response to a rise in inlet duct static pressure, but actually provide less flow. However, a little more info could be helpful. What controls damper position? Does damper position remained constant throughout one of these "cycles"? Type of damper ... parallel, opposed or other? Proximity of damper to fan inlet ... in wheel and or hydraulic duct diameters? Type of fan ... SWSI, DWDI, other? What causes pressure changes in the inlet duct? Out of curiosity, what's the purpose of the damper?

 

[imok2]

Unless there's something I'm missing it seems to me that as the static pressure goes up the fan should slow down example: say in an air conditioning system you have a set static pressure of 1.5" then all of the air supply dampers close. the static pressure will go up and the senser will dectect this and tell the fan "HAY FAN" you need to slow down and bring the static with you.

 

[quark]

imok2,

The pressure sensor is in the suction side of the fan.

RJB,

Positioning the sensor before the damper will not yield you the exact result you want, particularly if the damper is not activated with increased speed of the fan. This is because, at increased flowrates, considerable value of pressure is dropped across the partially closed damper and the DP across the fan may not vary much.

A description of the process and your objective will help us giving you better suggestions. Particularly, you should tell us how the pressure in the fan suction increases.

Regards,

 

[RJB32482]

Thanks for the responses.
Here is some more information on the process:
The fan is controlling the flow rate of air through a thermal oxidizer. The oxidizer has 3 canisters which contains heat transfer media. The air path goes through one canister, then through a combustion chamber, then out another canister. The cycles run every 2 minutes with one canister inlet damper closing and one opening. The same occurs with the outlets. The unused canister purges back into the air header. The static pressure sensor is located before the air goes through the inlet canister (measures flow). The fan is located roughly 20-30 feet after the outlet of the canisters. I don't have the fan info with me right now so I can't give what type of fan this is. There are three cycles where the dampers switch. We see that when the cycles switch from the 1st to the second, a pressure spike always occurs. This switch is where the pressure increase occurs and shuts down the oxidizer.

Right before the oxidizer shuts down, the fan amps drops and the fan starts to slow down in a stepwise fashion. The fan usually speeds up when static pressure increases in the header. This is why I asked the question in the first place.

Thanks

 

[imok2]

Quark: AH SO! :>)

 

[quark]

RJB,

I fear the process is still not clear. What I understood is that air flows from a common header, in which the pressure switch is installed, into the canisters and then to the combustion chamber and then to the duct to which the fan is connected. At any time the air passes through only one canister. Correct me if I am wrong.

Is the outlet damper opening first or the inlet damper?

 

[friartuck]

One note. If the fan is forward curved, it WILL speed up if loaded...it does not slow down...A peculiar characteristic of forward curved bladed fans (It almost self compensates by speeding up when the airflow is restricted)



Friar Tuck of Sherwood

 

[RJB32482]

outlet dampers open first