2 supply air fans in series

[232gulfstream]

I realize 2 fans in series can overcome more static pressure while fans in parallel would increase the flow rate however, what would the final flow rate be for 2 fans in series at different flow rates?

 

[EnergyProfessional]

Manufacturers have fan curves and selection software. That will tell you what is needed.

In series, you would add the curve based on the pressure numbers. However, this being compressible fluid, it isn't as simple as for pumps in series. So the manufacturer software accounts for it, or the manufacturer can advise on what to do.

 

[willard3]

Get the Buffalo Forge "Fan Engineering" handbook where this is covered in detail.

Not a simple problem.

 

[LittleInch]

"what would the final flow rate be for 2 fans in series at different flow rates?"

What does this mean? Do you mean each fan has a different flow rate? Makes no sense.

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[232gulfstream]

The system is 100% OA. The OA comes through an ERV unit and is 450 cfm. Then I have a supplemental cooling unit ducted to the outlet of the ERV. I am having cooling capacity issues and curious if I went to 2 ton supplemental unit but kept the ERV in place what would my resulting flow rate be. Would the 2-ton units flow be decreased by the restriction of the ERV units fan?

 

[EnergyProfessional]

You are doing it backwards. You have a design airflow for OA. This is based on code/ASHRAE or whatever requirement you have. then you size your system and get the pressure drop. then fans. If you start out with fans, you end up with too much or little OA.

An ERV also has a design flow range regardless of fan size.

 

[232gulfstream]

I think everyone is reading too much into it. Let me make it simple:

Lets put a 400 cfm inline fan in series with a 800 cfm fan that share the same size duct sized for 800 cfm (say for sake of conversation its a 12x12 duct. What is the resulting flow rate?

 

[EnergyProfessional]

There is no such thing as an 800 cfm fan. Fans have a pressure/flow curve. To add more, you can vary fan speed. Also review system curve, pressure drop of a duct or device is not static, but depends on flow rate. It all will settle on a stable equilibrium...

If 2 fans (or pumps) are in series, obviously the flow through each is the same.

And again, for OA, you start with the flowrate you need, and then design the ducts, and select equipment based on that. We don't just randomly ventilate buildings (at least I hope not) based on what equipment just happens to be there. You would either get too little (unhealthy and illegal) or too much (wasteful, humidity issues) OA.

 

[GT-EGR]

You can eliminate this fan in series issue by providing your 2 ton unit with a return air path from the space. Otherwise your two ton unit is stuck at 450 CFM which is probably not enough. So it will get the makeup air supplied into the return duct, and then however much more it needs to operate it will pull from the space.

 

[LittleInch]

Does all the air come through the ERV?



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[232gulfstream]

Yes all air comes through ERV. It is a 100% OA system with supplemental cooling since the ERV doesn't have the cooling capacity. Right now I don't have enough cooling capacity and the thought would be to increase the size of the supplemental unit that is downstream of the ERV. This presents a problem though now since the new unit will be double the flow of the ERV hence my question in this post. Thanks.

 

[BronYrAur]

Is your cooling unit DX or chilled water? If DX, you probably have too little flow. The fans in series are giving you less than the 800 CFM that the 2 ton system wants. What cooling issues are you experiencing?

If your cooling unit is chilled water, you should be able to slow down the 2 ton system fan to 400 CFM.

I like GT-EGR's idea of cutting a return into the 2-ton unit so that it can cool all 800 cfm, with 450 coming from the OA unit.

 

[232gulfstream]

The existing system is DX. I figured the 2-ton unit would be starved by the ERV and we wouldn't have enough cfm/ton and could possibly freeze the supplemental units cooling coil.

 

[LittleInch]

You're making the mistake of confusing rated flow with some sort of magical fixed flow.

Your 450cfm ERV unit will become your choke point by the sound of it, but non of the fans or bits of equipment have some magic fixed flow. They all have a pressure drop or gain to flow curve and without that data anything else is mere speculation.

If you create a pressure drop double or treble what the ERV is rated at you might get 800 CFM through it, or maybe it will take 5 times the pressure drop to get 800 - we have no idea and I don't think you do either?

Lets say your two fans are rated 800 and 400 both with a pressure drop of say 10 inches WG. Put the 2 side by side and providing your system doesn't react badly, you will probably get ~1150-1200. IN line you won't get 20 inches WG or 800 CFM. You'll get something like maybe 600 CFM at a total of 14 " WG.

It's all in the detail of matching air pressure and flow versus system resistance. When it all balances out, then you get a total flow. Not before.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.