HVAC Fans

Again?

again? what do you mean?

Whenever a system has duct work centrifugal fans have to be used as the static pressure drop is considerable. When there is no duct work propellers or axial fan can be used. Axial flow fans are suitable for handling large volumes and can be considered where noise level considerations are not important. Mostly these type so fans are used in industrial ventilation systems.

I am not sure I understand the question.

I have never seen a contra-rotating fan in an HVAC system. I have heard of counter rotating props on high performance airplanes, the the benefits do not seem to overcome the problems and complexities of the hardware involved.

Can you explain what you're looking for?

I too am not sure what your situation is.

In an HVAC system you would not normally use an axial flow fan as they tend to be appreciably noisier than an centrifugal one for the same duty.

You would almost never us multistage axial flow fans as the ðp across a pair of identical fans would only give you 1.8 to 1.9 times that across one fan. However if you reverse the second stage motor direction and fit an opposite hand impeller you get between 2.5 * and 3 times for the same fan power as the second stage benefits from the opposite hand swirl entering its impeller and the kinetic energy of the swirl is converted into ðp. Same capital outlay just greater efficiency. I have used CRAF in industrial dryers when the flow rate is high and the pressure drop is on the low side for a centrif and they do work in quite well, but they are noisy.

Maybe in an emergency state application ? Eg. for the as quick as possible an air extraction from, or air supply to, a cite/object.