Firstly why heater is operated at slightly negative pressure ??
This is to ensure that Hot flue gases inside the heater does not leak in to the surrounding atmosphere. there by making it possible for operator to work around.
Heater casing can not be 100% air tight.
Hence bcoz of draft inside the heater ambient air flows in to heater ( reffered as tramph air) and not the flue gases to atmosphere.
Direct fired heaters can and are designed and manufactured for either positive of negative draft. The more common is the negative draft. The reason is that it is much more difficult to seal the casing and pipe portals for positive pressure. Hot flue gas leaks could cause structural failures as well as problems for personnel who must work around heater. A negative draft throughout the heater is assured by maintaining a negative pressure at the bridgewall, which is defined as the point just below the shield section of the convection. If the pressure at this point is below atmospheric pressure, then you can be generally assured that the pressure is negative throughout the setting. For details on how this is calculated, see Process/Heater Stack Draft Analysis at
jehar (Mechanical) May 11, 2003
Direct fired heaters can and are designed and manufactured for either positive of negative draft.
...........
Heaters with positive draft??
Kindly give more info about such heaters.....
means services in which these are used... how the heater casing sealing is achieved...such heaters comes under API560 ??.......etc.
Buoyancy; Hot gasses are lighter (having lower density) the entering colder gasses. Hot gasses will go up through the stack creating a draft (slight negative pressure). There will be some pressure drop due to friction losses, obstacles like convection section pipes and stack dampers.
Stacks are designed for a flue gas flow and additional air leakage through the heater structure (tramp air). Basically the stack has two parameters height and diameter. More height gives more buoyancy and draft & larger diameter gives smaller frictional losses and better draft.
Auxiliary fans (or blowers) can be used to create this draft as well. An induced draft fan will draw the flue gas out of the heater and create a draft.
dvd111: imagine the atmosphere and the chimney as two arms of a U tube. At the bottom of the atmospheric side we have atmospheric pressure, anywhere along the warmer arm the pressure will be lower-than-atmospheric because this arm contains hot gas with a density lower than air's. Movement from the "heavy" side to the lighter one will start, and continue as long as the warmer side is "lighter" than the atmospheric. Keeping needed high temperatures on the chimney by suitable insulation (guniting) should help in creating and maintaining the draft. Corrosion by acid deposition (dew points) at lower temperatures is also a factor.
The flow rate wil also depend on the height of the arms (i.e., the chimney) and the friction drop through the system (fired heater and chimney).
Height and diameter of chimneys are also governed by local ground pollution laws, whereby the exit velocity of the gases is determined to comply with these requirements.
I hope having succeeded in clarifying the point. :-D
You have got most of the answers, just one more addition; if you are firing fuel that makes soot, then you will have soot coming out of the casing and it is nusiance and health hazard; the place will have soot all over. Sometimes you may like to run the convection section slightly positive for a very short time if you are doing efficiency checks while measuring ex O2, that way you do not get inteference from air leaking into the convection section if your O2 probe is at the stack.
Most, refinery heaters are designed with natural draft (almost all negative pressure in radiant and convection), if these heater have positive pressure in any part of the heater, the flue gas will leak through heater case, then you will not able to control the excess air (then efficiency). Natural draft is due to different density inside flue gas (hot gas has low density) as stated by SAYEDM.
GOOD LUCK....NOROS
The original question is a pretty good one that I have wondered about also. My previous experience has been that air register adjustments at the burners, and stack damper adjustments must work together to achieve reasonable excess air and draft objectives, however, in recent years, control engineers have been putting closed loop control on all our heaters stack O2's by manipulating the damper. On almost all of our heaters we monitor stack O2 and have actuator on damper, but rely on manual adjustments at the burners. This control has led to more frequent situations of slight measured positive pressure as the operators get lazier in making the manual furnace adjustments.
Has anyone else seen this trend, and can it lead to any mechanical damage to the heater or other hazards? Note- most of these dampers seem to be designed with a diameter smaller than the stack to prevent totally bottling up the heater in the event of control failure (we may also be using travel stops in some cases).
sshep, raises a very good point. Dampers are in stacks to control draft, not O2(air flow). The damper, which as mentioned, is useually not a full shut off type so as to prevent total closure, is there to prevent excess draft and is to be manipulated to reduce but not eliminate a negative draft throughout the furnace. The air registers at burner are used to control O2(air flow). It is obvious, that if you close the damper some, you reduce the overall air flow, so you get a lower O2, but it must be operated only to facilitate a better draft condition.
Jack Hardie
The use of analysers for optimization of heaters is not uncommon, the control system used is a function of economic factors.
Damper control is done on forced draft heaters where the outlet damper on the induced flow fan is controlled together with the inlet air damper on the forced draft fan. This is done on both dampers simultaneously to avoid upsetting the combustion chamber pressure because of flow changes.
Losses of energy are measured by the oxygen content in the flue gases. Although temperature is the main item in enthalpy content of the flue gases, composition (=O2%) can be controlled and therefore heat losses minimized. It cannot be driven to zero because of the dangers posed by explosions, and the effects of incomplete combustion; the optimum value is determined by fuel type, heater characteristics and process heat loads. Because of the load relationship, the setpoint of the O2 controller must be programmed as a function of the load as developed from a series of tests. Air leaks and imbalanced burners reduce efficiency at all O2 levels, therefore, the heater needs to be monitored frequently on these items to assure that the programmed setpoint, stays, in fact, an optimum.
Alternatively, the concentration of CO could be used to manipulate the fuel-air ratio in the same way, especially on boilers. It is said that its optimum setpoint is independent of load, and is adjusted to minimize heat losses while avoiding smoke (soot) formation.
The main reason for my question is, in our visbreaker heater the screen (FOX1A) shows that we have a good draft but when I checked the convection section on the heater side I noticed that we do have a positive draft (hot gas coming out ).
We realized that we do have a damper problem and we should wait in order to get spare parts which are not available on site. Since one month ago we still operate with low draft in the firing box and positive draft in the convection section.
So based on your feed back, I can conclude that there is a little harm by operating with a positive draft in the upper section.
Please confirm. Thank you all for your assistance and help
