Furnace philosophy air to oil ratio

[thakkarmayank24]

hi all
can any body describe me the logic behind the Air to oil ratio control for reheating furnace.
How to maintain the ratio.
As to how many loops will be required and how the temperature can be controlled incorporating the ratio
(ratio is 1:12, (1 oil flow:12 air flow).

i am having motorised digital valves for air and oil flow control.

 

[Ron Beaufort]

triple-post alert ...

 

[Lancie1]

In general, the air-to-fuel ratio in furnaces is fixed with a set screw. Once set during start-up and prove-out, the ratio stays constant, unless the type of fuel is changed.

What changes is the amount of opening of a throttle valve (fuel-air valve). That is controlled with a valve motor and a temperature controller.

 

[thakkarmayank24]

Hi
1.Yes it is right that once ratio is set ,then it is fixed until the Oil is changed.
2.But how we will control the opening and closing of Digital valves for maintaing set temperature taking consideration the ratio as well.
as in howmany controllers we will require,

Details
For Zone 1
1.Motorised Air control valve.these are Digital output valves.
2.motorised Oil control valve.this is Digital output valve
Let us aassume 1:12 as ratio
i.e 1=Oil then Air flow =12.Oil

Can i control using two step controllers
1.1st controller will control Digital air valve as per the SP and PV
SetpointSP=set temperture of zone1
Process valuePV=actual zone temperatue.

2.2nd controller will control Digital OIL motorised valve.
Setpoint=AIR FLOW/12 considering the ratio.
Process value PV=Oil flow.

by using these two controller ,will i be able to control the temperature of the zone properly takin ratio in to consideration..

Kindly reply.

 

[Lancie1]

Normally, the air and fuel valves are mechanically linked so that (once ratio is set), they open proportionally from one PLC PID control output.

If your valves are not mechanically linked, then you will need to do as you said. I would use only ONE PID control block, set up for oil, and then multiply this output by 12 to control the air. This insures that the air valve is ALWAYS 12 times the oil valve. Using two controllers will not guarantee the ratio.

EDIT: The above ASSUMES that your oil and air valves are same size and that the FLOW RATES are equivalent. If not, you will have to figure out the correct multiplier to get the 12/1 ratio for the valves that you do use.

EDIT2: During lighting of the flame (burner start), you have to release temperature control (bypass PLC control) so that the flame relay can control the valves for ignition purposes. Once ignited, control is returned to the temperature controller.

 

[thakkarmayank24]

Furnace temp control-Digital valves-ratio control

thanks for the reply..

Normally, the air and fuel valves are mechanically linked so that (once ratio is set), they open proportionally.

If your valves are not mechanically linked, then you will need to do as you said. I would use only ONE PID control block, set up for oil, and then multiply this output by 12 to control the air. This insures that the air valve is ALWAYS 12 times the oil valve. Using two controllers will not guarantee the ratio.

ANS:
My valves are not mechanacally linked.
did u mean i will use only one PID and this PID will control the Oil valve.
Then u will multiply the oil flow by 12 and compare with air flow and will operate the air valve.
this is what u mean .Please elaborate.


EDIT: The above ASSUMES that your oil and air valves are same size and that the FLOW RATES are equivalent. If not, you will have to figure out the correct multiplier to get the 12/1 ratio for the valves that you do use.

ANS:
Flow rates are not equal
Oil and air valve are motorised valves.air flow is in Nm3/hr and Oil flow is in kg/hr.

 

[Lancie1]

Too bad. You will have to calculate the ratio of your air valve to oil valve, based on valve flow ratings, then try it to see if you need to make any fine adjustment. Usually a burner expert makes the final ratio adjustment based on flame color. In any case, I still think you need to use only ONE control output based on temperature.

EDIT: I should warn that burner control is not a good thing to be doing for the first time. People can get hurt, buildings can be burned down. You may need some help from an experienced person to set this up.

Did you mean [that] I will use only one PID and this PID will control the Oil valve?

YES

Then will multiply the oil flow by 12 and compare with air flow and will operate the air valve. Is this what you mean?

Yes, except the multiplier will probably not be 12. Most likely, if the air valve has been selected properly (12 times flow rate of oil), then the multiplier will turn out to be close to 1, mabe 0.95, or 1.05, or something like that.

Please elaborate.

Sorry, for safety reasons, and due to a good possiblility of being misunderstood, I respectfully decline to elaborate any farther.