Draft of a fired heater

I’m a chemical engineer learning about fired heaters. I’m learning about draft and I picked this out of a textbook-

For example - Drafts:

• Just above the burners = $-0.3\text{in H}_2\text{O}$
• At the bridgewall = $-0.05\text{in H}_2\text{O}$
• At the breeching = $-0.6\text{in H}_2\text{O}$
• Above the stack damper = close to zero.

Why is the draft at the breeching more negative than the draft just above the burned and at the bridgewall ? Isn’t the draft when moving up the furnace be less negative ?

The draft will decrease as you move up the radiant section as the elevation is reducing with very little pressure drop. The convection section on the other hand will have significant pressure drop for the flue gas flow with little elevation change to move through it. The negative pressure above the convection section exit must be at a higher negative pressure than the inlet due to this pressure drop.

A more helpful way to think about this might be to envision a 35 foot tall radiant section operating at 1800F. The base draft is -0.40 In. W.C. and with the elevation change a rough decrease in draft expected is 0.10 In. W.C. for every 10 feet in elevation change so lets say the draft at the top of the radiant section or bridgewall is now -0.05 In. W.C. (-0.40 minus 0.10/10 feet * 35 feet). Just above this starts the convection section with several rows of close tolerance tubes. At the operating conditions the flue gas must all flow through these spaces which will increase its velocity and cause several changes in direction to the flow. The convection section is only 10' deep here but contains 20 tube banks which causes a differential pressure of 0.30 In. W.C. from inlet to exit. Here we know what the pressure is just below the tubes (-0.05 In. W.C.) and we know the final stack pressure on the downstream side of the convection tubes must be enough to overcome the loss in draft across the tubes. The loss in draft across the tubes is the elevation change plus the flowing pressure drop or 0.40 In. W.C. (0.30 + 0.10/10 feet *10 feet). The downstream side of the convection section or breeching section must then be -0.35 In. W.C. (-0.05 + 0.40 = -X).

I do take issue with the stack damper listing. The draft at the end or top of the stack is close to zero however the draft at the stack damper is very unlikely to be close to zero unless the damper is at the top of the stack which is essentially never done. The damper adds pressure drop to the exiting flue gas, the elevation and flue gas temperature is where the draft is created. If the breeching section is at -0.35 In. W.C. and the stack damper is fully open the stack must be around 35 feet (0.10 In W.C./10 feet * 35 feet). If the damper were to be closed down to say only 25% open the downstream side of the damper would still be at -0.35 In W.C. however the upstream side would now be lower due to the pressure drop added by the damper ( a rough estimate might be for the upstream damper draft to now be -0.2 In W.C. with the damper 25% closed).