Reduce the power of gas burner by lowering the motor rpm

[FiFtHeLeMeNt]

Hey guys !
We have a big industrial boiler (installed to provide heating for 80 apartments) which comes with a natural gas burner.
The burner has a 4KW 3000RPM motor which provides the air flow to the burner, is it possible to replace the motor with a 1500RPM motor to reduce the heat output of the burner ? (The air damper of burner is fully closed now and power can not be further reduced by reducing air flow)
In order to test the theory we actually installed a 3 phase motor drive on the burner and reduced the rpm, but we have issues getting the initial flame going when we reduce the rpm significantly and the control relay goes to lock out mode after ignition stage. (UV sensor doesnt detect the flame and locks the burner)
We figured by reducing the RPM, we need to open the air damper a bit more to provide more air flow, but it doesnt seem to work.
The burner has a high voltage spark ignition system.
I would appreciate your help and guidance.

 

[oz-plumber]

You're getting into high end technical gas works here.
For what you want to do, you will be changing a lot of parts to make the boiler work as you wish.
Have you got any FGA readings from the flue after you changed the fan motor.

It would be best to go back the the manufacturer of the boiler and see if they can help you.

 

[Gasmk1]

i would have thought a package burner that size would be fully controlled and would and have auto dampers to control high and low fires. unless its really old

 

[Excite]

It's always best to get advice from the manufacturers first.

 

[Last Plumber]

There's a lot more to it than swapping motors. You need to speak to the Burner and Boiler manufacturers regarding this, in my opinion. I wouldn't modify anything like that without written consent and technical advice from both.

 

[John.g]

One would be well advised to get the burner/boiler people in to advise on derating. First off anyway, all gas boilers (and i also think domestic gas boilers) have to establish a minimum airflow as a starting interlock, on the fairly large (50,000Kw) boilers, both oil and gas, that I maintained for over 30 years, this was set at 25% of the max airflow, this was measured by a airflow meter on these boilers or in smaller boilers by a airflow pressure switch. IF you have reduced the fan speed to 1500 rpm you will then only get 50% airflow with damper full open so would need a large opening of the damper to meet the 25% minimum for the presently installed burner. I am surprised the ignition sequence got as far as opening the gas valve so IMO all the more reason to get the whole system checked over.

 

[bacon_sandwich]

Sounds like you are trying to tune the burner by reducing the burner pressure and have hit a limit with the air damper.
The manufacturer of the burner will have the KW range that the burner can manage, you may need new jets but i personally would not try to get the right combination of jet, pressure and damper setting without a manufacturer guiding me. Whats wrong with it running at its normal output , is it cycling too much ?

 

[FiFtHeLeMeNt]

Thank you everyone for your replies !
We actually did contact the manufacturer but as expected they said they recommend us to buy a new burner and do not support such modification. installing a smaller burner also requires some modifications to the boiler and they even talked about some cement work ! so it would be quite costly to replace the burner.
@bacon_sandwich you are right, it cycles too much even with damper almost closed.
There is something that I do not understand quite right, I would appreciate if someone can clarify this for me, theoretically speaking doesn't a 3000 rpm motor with closed damper create the same air flow and pressure as a 1500 rpm motor with open damper in burner chamber ?
I am asking this because I tried to reduce the rpm (using motor driver) and opening the damper a little bit to compensate for lost rpm, but it didnt seem to work and ignition could not create the flame.

 

[John.g]

If your damper isn't very badly worn or doesn't have a minimum fixed setting then it should be able to control the air demand from practically 0% to 100% by throttling the air intake which reduces its air density (weight/cu meter) and was/is used almost exclusively for boiler control until the advent of variable speed drive where the air is unthrottled but controlled by the fan speed.
This damper is often made up of a number of louvers, if so, its quite possible that one of these is defective, you should be able to inspect the damper easily enough.

So to answer your question above, a fan speed of 1500 RPM with a fully open damper will produce 50% of the "flow" of a 3000 RPM fan also with a fully open damper. If that damper is airtight then ignition cannot occur when it is closed and I would investigate this first and maybe check for any corrosion/holes in any trunking between the damper and the burner.

Did the boiler modulate before this damper problem and what was its turn down?.

 

[FiFtHeLeMeNt]

Hello everyone,
I just wanted to give you an update in case someone had the same issue in the future.
We managed to run the burner by reducing the RPM from 3000 to 1500 successfully !
It is a little bit tricky to get the right combination but it is definitely possible.