Fan Backup Power Supply Requirements

[SepiaTones]

For a new boiler installation, we are looking at keeping the FD and ID fans running in case of a power failure. This would be at low load, but high enough to keep a draft and prevent an explosion. Let's say 25% of full load, or 25% speed (these are VFD).

The question I have is how much power the fans will draw. I am getting conflicting information:
[ul]
[li]The affinity laws are giving me what I consider unrealistically low numbers. For a 850 connected HP motor, designed to draw 700 HP at full load, I'm getting 10 HP at 25% speed.[/li]
[li]The affinity law I'm referring to is that the HP ratio the cube of the speed ratio. But I have seen and heard enough to make me suspicious that the exponent is generally not 3 except in rare instances, but somewhere between 2 and 3.[/li]
[li]The affinity laws assume a constant motor efficiency, fan efficiency and VFD inverter efficiency, all of which actually decrease as you get farther away from the operating point.[/li]
[li]It also assumes that pressure drops are all proportional to v^2, which is not always the case. For a baghouse for example, which this system has, the exponent is 1.2.[/li]
[/ul]

For all of these reasons I am reluctant to put my faith in the affinity laws.

Another reason I don't trust the results (10 HP for a FLA (full load amps) of 700HP) is that I think this is less than NLA (no load amps). For NLA, I've seen numbers as high as 30% FLA, although this may be applicable mainly to smaller motors.

One other thing, I have the fan curve, but only at 100% speed. I've made curves at lower speeds myself using the affinity laws, including down to 25% and below. The curves don't look too bad until you get below about 50%, then the just don't look like any other VFD curves I've seen before. Most of the other curves I've seen have been for a less than 50% speed reduction. I've always suspected that the affinity laws were similarly limited to modest speed changes.

So, can anybody tell me how much power I would need to keep these fans running at these low speeds? Thanks.

 

[PersonalProfile]

I have had only somewhat limited exposure to boilers [and the bulk of that exposure has been PF], and despite that the hazard that you have identified seems valid to me in principle; I do not believe I have come across this previously.
It may be worth evaluating why this is [this boiler may be gas fired etc cf. PF the boilers that I have been exposed to have: auxiliary power and I just was not aware of it; or this hazard had not been identified with the boilers that I have been exposed to].

Obviously I am not familiar with your installation, though hopefully the following is of some assistance:
1. I calculated the same active [mechanical] power value as you - assuming the cited drawn HP is active [mechanical power] power. The apparent power will be larger than this as it includes: reactive power [magnetic field], and efficiency [as you correctly say, the: VSD, motor, and “pump” may be relatively low].
2. I propose: the affinity laws do hold for the "pump" itself, and if you are concerned about the validity of the exponent, only the OEM's specifications could provide a more “precise” result.
3. Refer above items: 1, and 2.
4. Again without knowing your situation, the 1.2 factor is apparently a fixed constant multiplier [of flowrate / velocity], however I propose in principle the head loss across the bag filters would vary proportionally to the flowrate / velocity squared [I actually know this as a fact from observation for at least some PF bag filter units] i.e. bag filter hl= k*V^2, where k includes 1.2, and any other relevant factors.
You point is valid, as there may be “fixed” losses such as the physical height of the discharge etc.
If you able to physically inspect your situation [or you have access to drawings etc] I propose you could relatively quickly establish a model that is: relatively accurate, though simple; through considering Bernoulli’s equation, and hl = k [you can back calculate this using the demand power – assuming you know the flow rate, or velocity] V^2. I sometimes find it more convenient to express this equation as hl=kQ^2.
5. I propose the affinity laws accurately predict the active [mechanical] power of the "pump" itself, though the no load current you refer to includes the driver’s reactive power component [in addition to losses etc]. Refer item 1.

6. I am not confident, I understood the matter, however part of this assessment must also include verifying that the installation is suitable for this operation, e.g. the VSD can stably actually turn down that amount, the “pump” motor will not overheat, there are no “NSPHA” issues, there are no anti runbacks that will now not lift due to insufficient speed, or bearing skidding due to insufficient load etc.

There may be other options [that have their own issues], including: leaving the “pumps” run at full capacity [if there is sufficient power, or consider putting in more power, in lieu of modifying the “pumps”], flywheels, or auxiliary smaller “pumps” [either another motor that comes online, or a stand alone other: motor, and “pump” etc] etc.

Regards,
Lyle