Furnace gas air ratio control cross limiting

pauly

Member
Join Date
May 2002
Location
South Wales,U.k
Posts
244
Does any body have experience in gas air ratio control for a combustion system? I have seen a few descriptions of ratio control with cross limiting and wondered how it was applied in the control system. One example I have seen mentions scaling the fuel and air flows so that at the correct mix the ratio between gas and air is 1:1. This doesn't help if I need to change the ratio.
I am measuring the air flow compensated for pressure and temperature and measuring the gas flow compensating for pressure, and I intended to employ a P.I.D controller for the air flow control (output is to a flow control valve) with a set point from the thermal demand, and employ a P.I.D controller for the gas flow with its' set point derived from the air flow divided by the ratio required.
I would welcome any feed back from any one with any experience with this especially as I am not sure how the cross limiting function is employed.
Practical examples would be very welcome. I am using a Controllogix PLC
Thanks in advance
 
Here you go...if this helps.

http://www.asminternational.org/documents/10192/1917838/htp00402p021.pdf/02ef5802-ead8-4c6e-9006-8fc8af8d13dc

Most of the systems I deal with are "Excess Air" controlled....proportional gas and fixed air flow.

It sounds like you want to use "Ratio Control"...
http://www.lesman.com/train/webinars/Webinar-Slides-Air-Fuel-Ratio-Control-101-2014-12.pdf



By they way...
Are you incorporating combustion control AND Flame Safety (limits for flame ignition and control of gas valve and ignition) in the PLC?
In the US, NFPA and FM only allow Flame Safety in the PLC by using APPROVED PLC modules for combustion safety.
I usually piggy back a flame safety device (ie...Eclipse Veriflame 5603) to the PLC...ie.. I send all limits that are feeding the flame safety device to the PLC for monitoring and also have an additional relay in the limit circuit, so the PLC can have some control of combustion system.
Limits are Furnace High Limit, high/low gas pressure, and proof of combustion air flow at a minimum.
Also all the Flame Safety outputs (gas valve sol, ignition sequence, flame on and alarm output for flame safety failure) are monitored by the plc.
 
Last edited:
There's quite a few Google hits for boiler cross limiting logic.

These three app notes for the old Moore/Siemens 352/353/Procidia controllers has the SAMA style control logic. The signal greater than/ signal less than function blocks create the limits.

https://cache.industry.siemens.com/dl/files/486/51435486/att_85342/v1/Ad353-103r3.pdf

https://cache.industry.siemens.com/dl/files/536/51436536/att_89839/v1/AD353-132_r2.pdf

https://cache.industry.siemens.com/dl/files/624/51436624/att_75679/v1/AD353-131r2.pdf

The last two have similar doc numbers but have different content.

Typical diagrams:
194_appli2_fig3.gif


crosslim-remote-big.jpg
 
Last edited:
Thanks for the reply RClem

Great example, just a couple of queries:
Am I correct in saying that the top connection to the PID functions are the measured variable feed back?
And the BZ_Ratio_X_G signal is the required gas air ratio?
Why is the ratio applied to the gas flow feedback?
 
Great example, just a couple of queries:
Am I correct in saying that the top connection to the PID functions are the measured variable feed back?
And the BZ_Ratio_X_G signal is the required gas air ratio?
Why is the ratio applied to the gas flow feedback?

The top connection on all three PIDs is the feedback, either the Process Variable from the thermocouples or the scaled signal from the air and gas transmitters. The furnace burners (12 in this zone) are setup to operate at a 10:1 ratio, the BZ Ratio_X_G is a multiplier to adjust from the 10:1 ratio. We typically adjust ± 10- 15% (.85 to 1.15). We only need to trim the gas, since the both air and gas PIDs use the same demand % and cross limiting keeps us within 5% deviation.
 
Air gas ratio

Can I assume that you would set up the flow meter for the gas so that at maximum flow 20 mA (for example) would be to 10% of the maximum flow of the air?
Thanks again for taking the time to answer
 
The initial ratio is set up at high fire, air and gas at maximum flow and each burner is trimmed and balanced to the others, so the 10:1 ratio is essentially designed in. Air and gas inputs are scaled 0-100% from the 8"w.c. DP transmitters with a 4-20mA output, and follow the demand which is also 0-100%. Our flows are 0-400 KCFH Air and 0-40 KCFH Gas.
 
Last edited:
Here is an example of a Lead/Lag with double cross limiting on an HC900.

This is from an HC900 tech note and you have edited it?
Do you have the HC900 Tech note from using Pulse Fired burners?
I have been searching for it everywhere.
 
Our combustion contractor designed and programmed the furnace so I don't know the source, I only maintain it. I've made some modifications over the years but the lead/lag/dcl logic is unchanged since it's commissioning.
 

Similar Topics

Hi; For a loaf making plant where natural gas and LPG (alternatively) were used in burner to cook the loaf, the customer was facing loaf...
Replies
21
Views
8,758
Hello all. I am going to integrate a small industrial gas furnace/reactor. It currently has combustion blower, tube type burners, air/gas mixing...
Replies
0
Views
1,515
This morning we had a problem with our Fluid Bed Furnace, It started with the flames going off and only to realize the temperatures on the...
Replies
1
Views
1,455
Good day all. I have been tasked with automating a vacuum furnace and unfortunately, I have no experience with vacuum furnaces. Would anyone...
Replies
7
Views
3,587
body Attached are a few pictures of AVS Reclaim Furnace and control cabinet The I/O configuration Slot # 2 Analog Input 4 ch IC693ALG220f...
Replies
6
Views
2,114
Back
Top Bottom