Process control and fuel mix

realolman

what I'm looking for is a thought on what the effect may be on controlling a process temperature if the burner is set rich (2% Oxygen)... or set leaner (8% oxygen). I would expect the 2% flame to be hotter than the 8%

What effect might each of those settings have on the same controller's ability to control the process?

If the controller controlled a burner that was set @ 8% , what characteristics might that same system exhibit by setting the burner to 2% Oxygen.

I don't know how to put it any clearer than that. :)

realolman

We have

a 14 MBtu gas burner in a heat exchanger that heats oil to about 375° which is pumped to a process. The process extracts about 40° heat from the oil, and then it is returned to the heat exchanger.

The heat exchanger is controlled by a pneumatic controller which modulates a wafer valve that varies the combustion air supplied from a blower to the burner.

Through tubing, the modulated combustion air varies a regulator which regulates the gas pressure to the burner maintaining the ratio of fuel mix as the air is modulated.

We have not been able to control the process temperature correctly. This has been going on for some time, and I and others have been messing with the pneumatic controller, and it sometimes seems that the problem is solved... but then it returns. The system seems to drift too hot, even though the wafer valve is throttled back as the process temperature reaches and passes set point. We can increase the throughput of the process, which removes more heat from the oil, which is expected... to a point.


It still seems to drift slowly hotter, causing us to increase throughput until we reach a point where things stabilize, but then because the process is not perfectly consistent, we will often lose heat because the heat exchanger cannot recover. It seems we are controlling the heat exchanger with the process throughput, rather than the controller...


After monitoring this system for a day and a half, I am convinced that the controls are working properly. The throttling range can be adjusted and seems to me to react appropriately. I am thinking that the burner is set too rich.

It seems to me that somewhere along the line ( this is old stuff ) we were told by the manufacturer to set the burner at 7 or 9% Oxygen... I can't remember. It has been set at 2% for the last several years by a service company that I believe know their stuff about burners quite well, but are unaware of our process "load".

So... to make a long story short ( although I know it's too late for that ) my question is this: What would be the effect on controlling the process temperature of setting this burner at 2% Oxygen instead of 7% ?

mark_120

On the issue of temp control RTD ? or thermocouple?
Is the sensing device is direct imersed or in a thermowell
if in a thermowell are you using heatsink compound.
In most cases a direct inersed thermo device is the best way to go. it has the quickest response to the change in temp.
it seems oil is not a real harmfull agent to any thermo device that is made of stainless steel.
RTDs are the best in my book they respond very quickly given they have only 100 ohms of range as with thermocouples 500-900 ohms.
as for 2% o2 it is not that bad it has a little to do with how much draft and chamber pressure with in the unit.
if the unit is running all the time it means your getting the most out of your fuel
on off hot cold not good you will have bad condensing issues and combustion chamber scaling from the acids in condensate.
a little more info on your controller would help to target a possible control fix as well.

realolman

These are probes

I think they may be filled with mercury. I'm not sure. They expand and contract, and the controller is strictly mechanical. Through linkage, it transfers the expansion of the probes to a pneumatic valve that allows 15 psi air to either be held , or leak off through an orifice... the rate depending on the position of the linkage. As it approaches set point, the pressure drops.

This varying air pressure controls an airmotor which positions a wafer valve, which modulates the combustion air to the burner. The combustion air modulates a gas regulator which varies the gas pressure to the burner , maintaining the fuel ratio.


Maybe I shouldn't have described the controller so much, because my question is:

Assuming the controller works O.K., How would setting the burner to 2% Oxygen affect the control of the process temperature, as compared to setting it at, say, 8%?

Would the process temperature cool down at a slower rate with the burner set at 2% than at 8%?

mark_121

alternate solution

The use of a PID with RTD and a I/P would capture the temp issue. Check to see if there is a control company that might help you out. Or go to OMEGA they have all the stuff and they offer help as well.
as for the O2 issue a call to the manufacture of your equipment or burner they have the data you need. If this unit is a Johnny Cash "One Piece At A Time" good luck.
I have set some burners up at low O2 levels but they have FGR recirculation. IE low Nox situation.