Constant pressure variable volume fan calculation

[Waramanga]

Hi All,

How do I calculate fan RPM required to maintain constant pressure and halve the volume? If I assume constant resistance to airflow in the system its just a proportional relationship ie half the volume is half fan RPM but the system resistance is going down by the square root of the flow.... ideas?

 

[willard3]

Find and read the fan laws.

 

[Waramanga]

They assume one variable is changing at a time. Volume proportional to speed, pressure to the square of the speed, I want to keep the pressure constant and change the volume. Which fan law covers that situation Willard?

 

[317069]

why do you want to calculate rpm?

 

[Waramanga]

I have a constant volume system on a 3 storey building that we are decommissioning floor by floor. Still need the pressure to feed each floors mixing boxes but 1/3 less volume. Change the pulley to suit etc etc..... any ideas??

 

[dbill74]

Don't just look at the fan laws, look up the fan curve of the fan in the system. To do what you are asking Waramanga, you have to shift the fan curve and the only way to do that is change the physical properties of the fan blades, either their size, shape or pitch.

Fan laws assume the blades of the fan are fixed, and for most fans out there this is an accurate assumption. So unless you are able to physically change the blades of the fan, what you are asking for isn't possible. When you use the fan laws, everything changes simultaneously; change the RPM you change the CFM, SP and HP.

Gotta love those laws of physics.

For your situation, I see 3 options;
1) Replace the fan each time you take a floor off the system.
2) Install a relief/bypass damper on the first 2 floors to be decommissioned.
3) Decommission the entire building at one time.

If by some chance you are using a vane-axial fan with variable pitch blades, it would be a matter of moving a static pressure sensor to ensure the terminal boxes have adequate upstream pressure.
But who am I kidding, if this is what you have, you'd have a variable volume system.

 

[MintJulep]

You can't.

As you have noted, flow and pressure are related. Impossible to change only one and nothing else and have the other remain the same.

 

[SAK9]

Waramanga: When you reduce air flow in a duct work system,system pressure drop also goes down as the duct work is oversized for the new flow.So recalculate the system pressure drop based on the new air flow and check on the fan fan curve to see how much you need to slow down the rpm.If your existing motor is not suitable for VSD, put a new motor in so that you can vary the speed with a VSD.

 

[Waramanga]

Yes it will have to be a VSD, just adjust until the flow is correct and ignore the static pressure.

 

[Drazen]

if you don't have fan curves for different rpm, you can use affinity laws to find pressure at your desired flow. than, you can only change fan rotor if you have sufficient documentation, it cannot be explained in a simple post. the other solution is to install additional balancing damper. of course, all under assumption that you have sufficient pressure.

 

[dbill74]

Wara, you can't "just ignore" static pressure. Your terminal boxes need a minimum upstream static pressure they need in order to operate properly and ensure you get the airflow they are calibrated for. If you simply slow down the supply fan, the static pressures in the duct will drop off per the affinity laws.

 

[chicopee]

You can approximate other fan curves by using the affinity laws as long as you have the original one provided by the manufacturer . Also you will not have constant resistance to varying airflow because resistance is a function of airflow or velocity at that flow.