Fired heaters come in numerous configurations and designs. These configurations include systems that are forced draft, induced draft or a combination of both, but many are natural draft. The natural draft designs are what we will concentrate on in this section.
Regardless of the mechanics involved, the stacks purpose is the same, to safely disperse the products of combustion into the atmosphere. For environmental reasons, many stacks are required to discharge at a particular height. But in most cases they are designed only to meet the needs of the furnace or furnaces they are designed for. In the case of the natural draft furnace, the stack serves another purpose, that of assuring that the furnace stays below atmospheric pressure throughout the setting.
For most stack designs, a gas velocity at the exit of about 15 to 25 ft/sec is sufficient to discharge the gasses into the atmosphere at a rate that will assure they disperse properly. Additionally, most natural stack are designed for 125% of the design flue gas flow to assure that if the furnace is operated above the design point that it will still operate safely.
We can use the sketch of a typical horizontal cabin heater to look at the important features affecting the heater draft.
 | Natural Draft In this sketch, the area marked "A" is the height available, for the differences in the density of the ambient air and the flue gas, to create the draft required. Normally the draft required is that which will result in a slightly negative pressure at point "B". It should be noted that, for most heaters, the draft at point "C" required by the burners to induce combustion air is not considered in setting the stack height. The burners are normally sized to use only the draft in the firebox. |
Net loss across convection:
The calculation of this pressure loss was covered in detail on page 4 of this section. This loss is discounted by the draft gain across the convection section.
Pressure loss across stack entry:
This pressure loss can normally be considered as a sudden entry since the area of the outlet gas plenum in the heater is usually much greater than the area of the inlet to the transition. A sudden entry pressure loss can be approximated by the following equation.
Where,
| Dp = Pressure drop, inH2O |
| Vh = Velocity head at inlet area, inH2O |
Pressure loss across stack transition:
This pressure loss can normally be considered as a gradual contraction since the area of the inlet and the outlet are usually close in area. A gradual contraction pressure loss can be approximated by the following equation.
Where,
| Dp = Pressure drop, inH2O |
| Vh = Velocity head at outlet area, inH2O |
| Ca = Coefficient based on included angle |
And the coefficient can be described as,
| Included Angle | Ca |
| 30 | 0.02 |
| 45 | 0.04 |
| 60 | 0.07 |
Pressure loss across stack damper:
This pressure loss is normally accounted for by rule of thumb. This may be 0.5 or 0.25 velocity head. We will use 0.25.
Where,
| Dp = Pressure drop, inH2O |
| Vh = Average velocity head of stack, inH2O |
Stack friction loss:
For the stack friction loss, we can use the following equation.
Where,
| Dp = Pressure drop, inH2O |
| Vg = Average velocity of stack, ft/sec |
| rg = Density of flue gas, lb/ft3 |
| Ds = Stack diameter, ft |
| Ls = Stack length, ft |
Stack draft gain:
The draft gain will be taken based on the height, "A" on above sketch.
Where,
| Gd = Draft Gain, inH2O |
| rg = Density of flue gas, lb/ft3 |
| ra = Density of ambient air, lb/ft3 |
| A = Height of gas path, ft |
Pressure loss across stack exit:
This pressure loss, since it normally exits to atmosphere, can be considered as a sudden exit. A sudden exit pressure loss can be approximated by the following equation.
Where,
| Dp = Pressure drop, inH2O |
| Vh = Velocity head at inlet area, inH2O |
Velocity head of gas:
Using these formulas, we can now determine the height that A is needed to be to provide the draft required. It should be noted that many heaters are not this simple, in that they may have multiple stacks, collection ducts, etc.
