Constant pressure variable volume fan calculation
Constant pressure variable volume fan calculation
(OP)
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?
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?





RE: Constant pressure variable volume fan calculation
RE: Constant pressure variable volume fan calculation
RE: Constant pressure variable volume fan calculation
RE: Constant pressure variable volume fan calculation
RE: Constant pressure variable volume fan calculation
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.
RE: Constant pressure variable volume fan calculation
As you have noted, flow and pressure are related. Impossible to change only one and nothing else and have the other remain the same.
RE: Constant pressure variable volume fan calculation
RE: Constant pressure variable volume fan calculation
RE: Constant pressure variable volume fan calculation
RE: Constant pressure variable volume fan calculation
RE: Constant pressure variable volume fan calculation