Fan Performance and Pressure Distribution in Parallel Centrifugal Fans

[Mech_guy_al]

I have two different capacity centrifugal Induced Draft fans.I attach them in parallel.They both pull air through the same duct and deliver the exhaust to the same stack.
Is the pressure at the inlet and outlet of both fans same?
What is the fan curve for such a combination?

 

[russ_watters]

I have two different capacity centrifugal Induced Draft fans.I attach them in parallel.They both pull air through the same duct and deliver the exhaust to the same stack.
Is the pressure at the inlet and outlet of both fans same?
What is the fan curve for such a combination?

Yes, the pressures are the same.

Each fan has it's own curve, who's shape is unique for the fan type and conditions.

This is a very risky setup. I've done it, but would generally recommend against it.

 

[jrmichler]

This is a very risky setup.

He speaks wisdom. You need a very good understanding of fan curves to even attempt a parallel installation.

I've done it successfully with centrifugal pumps, after spending considerable time up front engineering out the problems.

 

[Mech_guy_al]

Thank you for the reply

.

This is a very risky setup. I've done it, but would generally recommend against it.

Would it be possible for you to share a detailed report of your system setup. It would help me better understand the intricacies of how fans are setup in parallel.

 

[russ_watters]

Would it be possible for you to share a detailed report of your system setup. It would help me better understand the intricacies of how fans are setup in parallel.

No, I don't have such a report. It's all just raw engineering. All the client saw was the final design (which I also can't share), not the engineering that went into it.

However, if you tell us more about what you are trying to do and why, we might be able to provide more specific advice.

...though the basic answer is probably going to be: "you should not do this yourself; you should hire an engineer." And the fact that you apparently want to attempt this yourself after being strongly advised (twice) that it is risky is not a good sign...

 

[Mech_guy_al]

No, I don't have such a report. It's all just raw engineering. All the client saw was the final design (which I also can't share), not the engineering that went into it.

However, if you tell us more about what you are trying to do and why, we might be able to provide more specific advice.

...though the basic answer is probably going to be: "you should not do this yourself; you should hire an engineer." And the fact that you apparently want to attempt this yourself after being strongly advised (twice) that it is risky is not a good sign...

I am not attempting it on my own.There are experts who are handling this problem. Since they are extremely busy (and not very helpful) I thought trying out some calculations on my own would help increase my knowledge
So the exact problem is this. In a fume extraction system the capacity of the originally installed ID fan isn't enough to capture all the fumes. So a second ID fan is being installed to increase the overall capacirc of the system. The original fan had a capacity of 65000 m3/hr and the new one has a capacity of 55000 m3/hr. Blades of both are backward curved and the original fan runs at 1500 rpm.

 

[russ_watters]

I am not attempting it on my own. There are experts who are handling this problem.

Ok, great. If this project is indeed being undertaken with due dilligence, there should be a report detailing the problem and investigation into solving it. It should look something like this:

1. Start with an investigation of the process needing to be exhausted. Determine the exhaust quantity required and required size and shape of extraction system (hood?). Compare requirements with what is actually installed.

2. Measure the airflow and pressure. Compare with the requirements.

3. Evaluate the fan performance and capacity. If the ductwork system is adequate but the fan not, calculate new fan performance requirements. Compare with the fan's capabilities.

4. Size/select/recommend fan replacement or new parallel fan.

After a direction has been decided-on, design the replacement.

If such a report exists, get it and read it. If it doesn't exist, that could be cause for concern.

Since they are extremely busy (and not very helpful)

If you are in a position where you should know the details, I'd suggest finding a way to get those details from them.

I thought trying out some calculations on my own would help increase my knowledge.

Unfortunately, most real-world engineering problems are not primarily a matter of doing calculations, they are about interpretation of information, and that's not a simple/linear process but an experience-based algorithmic problem solving process. The calculations only support that and while important, are a relatively small and simple part of the effort.

For example, if you have the existing airflow and system pressure drop and you know the new airflow you want, calculating the new pressure is a simple matter of multiplying the existing pressure by the square of the airflow difference ratio.

From that you can try calculating new fan performance, but the fans don't really exactly follow the theoretical laws, so it is typically done with selection software from or by the fan vendor. And the fan vendor would never recommend mismatched parallel fan operation, so you have to be capable of understanding the software and its output well enough to do some trial and error to come up with two different but sort of linked fan selections.

So you see, it isn't a "take this data and plug it into this equation to get the answer" process.

 

[ChemAir]

So the exact problem is this. In a fume extraction system the capacity of the originally installed ID fan isn't enough to capture all the fumes. So a second ID fan is being installed to increase the overall capacirc of the system. The original fan had a capacity of 65000 m3/hr and the new one has a capacity of 55000 m3/hr. Blades of both are backward curved and the original fan runs at 1500 rpm

You don't mention the static pressures of the fans at those airflows. In addition to the capacities being different, their pressures/curves will not match.

A couple of thoughts here:
1) Two *not* identical fans will require additional engineering for the duct arrangement to result in system pressures that make the airflows additive. This is not normally done. It will probably be more costly to configure the fans/duct to make two dissimilar fans work together, and it still probably won't work that well when perfectly adjusted. If someone moves a damper or a little material builds up, whatever capacity gain you made could go away.

2) Attempting to increase the airflow in a fume extraction system may require additional ducting/hoods/pickups as @russ_watters already mentioned. If you intend to increase your airflow by a factor of 2, the duct needs to be able to flow that at the pressures the fans can move. Velocities at pickups and in the duct branches need to be considered.

This is a case where the entire system has to be evaluated if you want to get the best result from an engineered study. Realize you may be trying to make an entire collection system twice as large, not just increase fan capacity.

 

[Mech_guy_al]

Thank you for the answers.However, there are things I am still confused about.
How exactly do you configure two fans.I mean, If the fans have two different static pressures then what is the common pressure that will be generated across the fans.
How then , is the total volume of the airflow calculated (from the fan curves?).
Is the system resistance curve same for both the fans?

 

[russ_watters]

How exactly do you configure two fans.I mean, If the fans have two different static pressures then what is the common pressure that will be generated across the fans.

A system can only one static pressure profile at a time. As said before, the static pressures generated by the fans will/must be the same.

How then , is the total volume of the airflow calculated (from the fan curves?).

You have to match the two fan curves against the system curve.

Is the system resistance curve same for both the fans?

The system resistance curve is for the system, not the fans. A system (without control devices) only has one system resistance curve.

 

[jrmichler]

There is no short simple easy explanation for how to do this. @russ_watters has fully explained how to do it, but you need more background to properly understand his explanations.

Get a copy of the ASHRAE HANDBOOK OF FUNDAMENTALS and study the chapter on ducts. Then get a copy of the ASHRAE HANDBOOK - HVAC SYSTEMS AND EQUIPMENT and study the chapter about fans. The ASHRAE Handbook series is the definitive resource for these types of problems. It would be worthwhile to get a copy and keep for future reference.