2x Centrifugal Fans in Series with different volume outputs

[sgel83]

This is probably a very fundamental question as HVAC Engineering is not my profession but I am working on hobby project and trying to figure out the consequences of the following scenario.

I have 2 units using common ductwork which results in having 2 Centrifugal fans in Series with different volume outputs.

FAN-1 (5.7 - 6.6 CMM) ----> FAN-2 (3.2 - 7.1 CMM)

FAN-2 will always be running but the volume of air it moves varies, and more often with less output than FAN-1.

I am trying to understand what are the consequences of having it mismatched, and should I try to run them in parallel or use software and electronically controlled valves, as well as adjust the fan speed to try to match the FAN-2.

If they stay in Series and FAN-1 and FAN-2 are both running what is the consequences of the mismatch. Would the expected life expectancy of either Fan be reduced due to pressure build up?

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[willard3]

Read Buffalo Forge "Fan Engineering" on fans in series and fans in parallel

 

[sgel83]

@willard3

Thanks, I think I found the book but couldn't find anything specific on parallel and series fans.

https://archive.org/details/engineershandboo00buff/page/568/mode/2up

Although, perhaps I am just not finding the relevant section.

I have though before posting read a number of articles and references as such:

http://docshare01.docshare.tips/files/3692/36927047.pdf

What I do find is that that ideally they should be 2 identical fans in series and this would help create more pressure but the same volume. I couldn't find anything that really discussed mismatched systems and not so much the effects on the air but on the longevity of the fan itself.

 

[EnergyProfessional]

both fans have the same airflow. they just operate at different points on the fan curve. How much flow, depends on pressure drop. Pressuredrop of a system (i.e. duct system) depends on flow. As a rule of thumb there is a square relationship, but several devices, like coils and dampers, make this more complex. it is a dynamic system and flow will "settle" on where a balance.

since both of your fans have some overlap in possible flow, that balance should be somewhere there. but look at the fancures and consider what is connected to the fans.

 

[sgel83]

Thank you.

So, netting this down I am inferring from both answers and what I have read is that I shouldn't be too concerned about of this affecting the longevity and integrity of the fan itself but just make sure I am achieving the volume and pressure I am looking for.

 

[GT-EGR]

Is there a particular reason why you want these fans in series instead of parallel?

You give fan volume limits but what you really need to see is how their curves line up. What airflow and pressure did you need when only one is on, and what airflow and pressure did you need when they both run. If that combined airflow is in an unstable range of operation you will have issues both with the longevity of the fan and also getting a reliable amount of airflow.

 

[sgel83]

@GT-EGR

Series was an easier setup in regards to duct work, but after a bit of research and comments here I think parallel is the way to go.

The setup is integrating an aircon/heat pump to an already existing fresh air ventilation system with a rotary heat exchanger. I have control of the outputs of both systems and by using electronic dampers and having the fans in parallel will give me the most flexibility.

E.g.

Spring/Fall seasons will not require the Aircon nor the Heat Pump.
Humidity and CO2 Sensors will provide input to logically determine how much fresh air is needed at a given time.

Having them in series I think in this case would just be a bad idea and limit the flexibility and make calculations much more difficult. I am certainly open to counter advice, as again this is not my forte.