Positive Heater Draft
Positive Heater Draft
(OP)
Hello there,
What is the effect of having Positive Draft in the convection section?
What is the effect of having Positive Draft in the convection section?
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS Come Join Us!Are you an
Engineering professional? Join Eng-Tips Forums!
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail. Posting GuidelinesJobs |
|
RE: Positive Heater Draft
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.
RE: Positive Heater Draft
How pressure in the natural draft furnace remains negative? What is the principle?
RE: Positive Heater Draft
Jack Hardie
RE: Positive Heater Draft
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.
RE: Positive Heater Draft
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.
RE: Positive Heater 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.
RE: Positive Heater Draft
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.
RE: Positive Heater Draft
GOOD LUCK....NOROS
RE: Positive Heater Draft
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).
RE: Positive Heater Draft
Jack Hardie
RE: Positive Heater Draft
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.
RE: Positive Heater Draft
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
Best Regards,
Falcon03