HVAC fans quadratic function

[davaleric]

I need to model an extract fan that will ramp down if the pressure in the area it is extracting from reaches -50PA. Would using a duct with a fan set on quadratic be able to do this ? Or is anyone aware of a better way ? Thanks

[drjfloyd]

I don't think a quadratic would do what you need. A custom fan curve I think would.

[davaleric]

Thanks for the prompt reply I will look into that. There are two extract fans and in test runs the extract does reduce, but the fans end up with different rates of extract .

[davaleric]

This is a very rough example and I would need to check with the smoke control engineers but would the following achieve what I need?

[drjfloyd]

You will have to try it and see.

[drjfloyd]

Wondering if I correctly understood your first post:

When you say "an extract fan that will ramp down if the pressure in the area it is extracting from reaches -50PA", do you mean:

-the motor controller for the fan reduces power input when the pressure in the building becomes very negative? In this case, I think the best way to handle it would be a custom curve as you could input a total fan curve that is some combination of the full power curve for low values of negative pressure and the low power curves for higher values of negative pressure.

or

-the fan flow is decreasing simply because there is reduced back pressure on the fan (i.e. fan power is the same but the fan has to work harder to overcome the negative pressure)? In this case a quadratic curve is probably OK.

I had intrepreted that statement as the first..

[davaleric]

It is the first case, pressure sensors will be in the corridor to ensure that negative pressure doesn’t make it difficult to open doors. At a reading of -50pa the fans will be instructed to reduce power to an acceptable level. I changed the previous script to increase the fan speed to a maximum of 4m3/s ( which is the value we want to test) but if -50pa is reached I’ve now set the fan to drop to 10 per cent. The flows achievable will vary depending on whether the stair supply vent is available or not ( if the stair door is open) There are two extract fans so trial and error I suppose until we get acceptable results . Thanks for your help with this