Re: Heater outlet temperature control

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Posted by Mike McCarty on June 13, 2000 at 12:44:25:

In Reply to: Re: Heater outlet temperature control posted by Anonymous2 on May 29, 2000 at 16:38:54:

Like Tod, I also like the TC-FC set up. Again, the big
advantage of this is that it automatically compensates for
furnace burner changes. If, as you say, you don't want to neglect
FG header pressure or composition changes, then I'd implement a
TC - compensated FC cascade, or TC - fuel gas Q cascade (using flow).
The PC cascade would not help you on composition changes.

Also, if a furnace has a firing limit (maybe due to permit
constraints), this limit will be based on fuel flow rates. So it
is more straightforward to limit this in the DCS system when your
temperature controller is set up as a TC-FC cascade.

The big advantage of a TC-PC cascade is that most furnace safety
shutdown systems are tied to FG burner pressure. Therefore, it
is more straightforward to limit the range of the allowable
burner pressure moves in the DCS system when your temperature
controller is set up as a TC-PC cascade.

I don't know about your set up with a circulating FO flow, but it
sounds like TC-PC is indeed the best cascade for your system. The
flow we're talking about in the TC-FC cascade is the fuel flow
that is being burned. If this flow is not measured directly, then
a FC cascade is probably not a good idea.

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Re: Heater outlet temperature control Rafael Gonzalez-Martin 10:10:41 7/03/00 (0)

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Comments:

: Like Tod, I also like the TC-FC set up.  Again, the big 
: advantage of this is that it automatically compensates for 
: furnace burner changes.  If, as you say, you don't want to neglect 
: FG header pressure or composition changes, then I'd implement a 
: TC - compensated FC cascade, or TC - fuel gas Q cascade (using flow).
: The PC cascade would not help you on composition changes.

: Also, if a furnace has a firing limit (maybe due to permit 
: constraints), this limit will be based on fuel flow rates.  So it 
: is more straightforward to limit this in the DCS system when your 
: temperature controller is set up as a TC-FC cascade.   

: The big advantage of a TC-PC cascade is that most furnace safety 
: shutdown systems are tied to FG burner pressure.  Therefore, it 
: is more straightforward to limit the range of the allowable 
: burner pressure moves in the DCS system when your temperature 
: controller is set up as a TC-PC cascade.    

: I don't know about your set up with a circulating FO flow, but it
: sounds like TC-PC is indeed the best cascade for your system.  The 
: flow we're talking about in the TC-FC cascade is the fuel flow 
: that is being burned.  If this flow is not measured directly, then 
: a FC cascade is probably not a good idea.

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