AHU Commissioning

[viskir]

Hi,

We are executing the Hospital project.All AHUs are having the plug fan with different frequencies for each unit like from 50 - 90 Hz.

The designed airflows can be achieved on Auto mode by with VFD on the respective frequency settings.But in the hand mode (50 Hz) conditions how we can able achieve the airflows for which AHU is working in 80 Hz (auto mode )

Please clarify on the technical issues.

 

[BronYrAur]

I am a little confused by the wording of your question, but I think you are asking how you can achieve the same airflow at 50Hz in Hand that you get at 80Hz in Auto. You cant! Your fans are spinning much faster at 80Hz. Maybe I am misunderstanding your question.

 

[viskir]

Yes Mr. BronYrAur. your understood is correct. Thank you for your reply. So what will be the solution?

 

[LittleInch]

Normally the VFD will have a facility to change manually the frequency of the output.

Find out from the VFD manual how to do that (RTFM). You might need to plug in a laptop or have supervisor or engineer privileges or a key switch.

Only you know your make, model etc of the VFD or what the panel looks like.
We don't.
You didn't tell us.
We can't guess.



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

 

[viskir]

My question is if VFD fail, how i can achieve the designed airflow in hand mode.

 

[LittleInch]

Simple

Buy a new VFD or use a spare one.

If your incoming supply is 50htz and you bypass the VFD, 50 htz is what you get.

To get 80 htz, Use a VFD.

Or find a generator and run it at 8/5 of it's normal RPM to creat 80 htz. But you might blow up your fans as lots of other things change when you do that including voltage/htz and all sorts of strange electrical things.

There is no other way.

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

 

[viskir]

Thank you for your reply Mr.LittleInch.

 

[BronYrAur]

Now I understand your issue. I have never liked systems that increase the speed beyond the Hz offer by the electric utility for this reason. In my opinion, motors should be selected to perform at design conditions using 50Hz (60Hz in my case). The VFD should take the speed down from there, not up. If it is critical, you should recommend that they have a redundant VFD ready to go.

 

[EnergyProfessional]

With the over-speeding of motors i would be more concerned about bearing failure. in addition those often are small inefficient high-speed fans larger low-speed fans tend to be more efficient everything else being equal).
in bypass, you will only have 50/60Hz that comes from the grid. but for emergency that should be enough in most applications. Better than nothing.

Note some bypasses are part of the VFD and require the VFD power supply to work. If the VFD failure is its power supply, you also lose the bypass. Ask me how I found that out..... If you want redundancy, you need 2 VFDs, or even 2 fans, or some external bypass not relying on the VFD for power.

 

[viskir]

Thank you for your reply Mr. BronYrAur & Mr. EnergyProfessional.

 

[Artisi]

What is standard motor speed on your fans, 4 pole or 2 pole?



It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[viskir]

Mr. Artisi, Please find the uploaded documents for your reference.