Chimney Sizing PDF
Chimney Sizing PDF
Chimney Sizing PDF
Project Name:
Location:
Type Appliance: Boiler
Hot Water Heater
Incinerator
Type Fuel: Natural Gas
LP Gas
#2 Oil
#6 Oil
Wood/Coal
Waste (Type_______)
Appliance Input: BTU
Hp
Lbs/hr
Breeching Description/Notes:
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Theoretical Draft
Chimney design involves balancing forces, which tend to produce flow (friction). The
force producing the flow in gravity or natural draft chimneys is termed theoretical draft,
defined as the static pressure resulting from the differences in densities between a
stagnant column of hot flue gases and an equal column of ambient air.
Dt = .2554 B H 1 _ 1
To Tm
Where: Dt = theoretical draft, inches of water
B = barometric pressure, inches of mercury
H = effective height of chimney, feet
To = outside temperature, (F + 460)
Tm = mean chimney temperature, (F + 460)
For more simplified calculation, Table 3 can be used to determine theoretical draft
assuming the density of chimney gas is the same as that of air, the barometric pressure
is at sea level and the ambient temperature is 60F.
ALTITUDE PRESSURE
(Feet) (Inches of Mercury)
Sea Level 29.92
2000 27.8
4000 25.8
6000 24.0
8000 22.3
10,000 20.6
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Theoretical Draft Per Foot of Chimney Height
Table 2
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Mass Flow of Combustion Products
Mass flow in a chimney or venting system may differ from that in the appliance
depending on the type of draft control, or number of appliances operating in a multiple
system. The use of mass flow is preferable (rather than cubic feet) because it remains
constant in any continuous portion of the system regardless of changes in temperature
or pressure. For the chimney gases resulting from any combustion process, mass flow
w, in pounds per hour, can be expressed as:
W=IM
Table 4 lists mass flow input ratio values for various fuels and appliances. If a BTU input
rating is not given for the appliance, Table 5 lists conversion factors for other appliance
ratings.
Mass flow within incinerator chimneys must account for the probable heating value of
the waste, plus its moisture content, plus the use of additional fuel to initiate or sustain
combustion. Where constant burner operation accompanies the combustion of waste,
the additional quantity of products due to the additional fuel should be considered in the
design process. For incinerator chimneys, mass flow can be calculated using a slightly
different formula.
Table 6 gives the values for # combustion products based on the type of waste being
burned.
In order to determine the losses in the chimney, the velocity must first be calculated.
Velocity can be computed using the following equation.
V= W
Pmx19.635xd2
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If the volume of flow in the chimney is required, possibly for fan selection, the following
formula can be used.
Q=AV
Conversion Factors
Table 5
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Mass Flow for Incinerator Chimneys
Table 6
Combustion Products
Type of Waste BTU/lb Waste CFM/lb Waste @1400F lb/hr per lb Waste
Type 0 8500 10.74 13.76
Type 1 6500 8.40 10.80
Type 2 4300 5.94 7.68
Type 3 2500 4.92 6.25
Type 4 1000 4.14 5.33
System Losses
Flow losses due to friction may be estimated by means of several methods using
formulas for flow in pipes or ducts. These include the equivalent length method and the
loss coefficient or velocity head method. Primary emphasis will be placed in this
treatment on the loss coefficient method, because in chimney systems, fittings usually
cause the greater portion of system pressure drop, and conservative loss coefficients
(which are practically independent of piping size) provide an adequate basis for system
designs.
Using the velocity head method for resistance losses, a fixed numerical coefficient
(independent of velocity) or k factor is assigned to every turn in the flow circuit, and to
piping as well. Table 7 offers design values for the resistance loss coefficient for various
fittings.
Once the loss coefficients for the system have been evaluated, the system losses can
be calculated using the following formula.
kp
p = mV2
5.2 (2g)
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Resistance Loss Coefficients
(Velocity Heads, Dimensionless)
Table 7
Suggested Design
Component Estimated Span/Notes
Value
Inlet-acceleration
Gas vent with draft
1.5 1.0 3.0
hood
Barometric regulator 0.5 0.0 0.5
Also dependent on blocking damper
Direct connection 0.0
position
Round elbow, 90 0.75 0.5 1.5
Round elbow, 45 0.3 -
Tee or 90 breeching 1.25 1.0 4.0
Y breeching 0.75 0.5 1.5
Cap top
Open straight 0.0 -
Low resistance (UL) 0.5 0.0 1.5
Other - 1.5 4.5
Spark Screen 0.5 -
Converging exit cone (d1/d2) 4-1 System designed using d1
Tapered reducer
1- (d2/d1) 4 System designed using d2
(d1 to d2)
0.4 L, ft
Piping d, in Numerical coefficient from 0.2 0.5
See Figure 1 for friction factors
0.4
F, FRICTION FACTOR
VELOCITY
0.3
10
20
0.2 40
80
4 5 6 7 8 10 12 18 24 36 48 60
DIAMETER, INCHES
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Density vs. Temperature
Table 8
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Masonry Chimney Liner Dimensions with Circular
Equivalents
Table 9
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Balancing The System
Equipment or appliances can be placed in three broad categories: Negative Pressure
appliances which require a negative pressure at the outlet to induce combustion air flow
into the combustion zone, Atmospheric appliances which require a neutral pressure at
the outlet without need for chimney draft, for example, draft hood type of gas appliances
in which the combustion process is isolated from chimney flow variations, and Forced
Draft appliances which operate at above atmospheric pressure and have sufficient
pressure to force the products of combustion through the appliance resulting in either a
slight positive pressure or a zero pressure at the outlet.
Pressure Equation
Appliance Type (Loss = Draft Notes
Requirements)
Do is the amount of
Negative Pressure p = Dt - Do
negative pressure needed
Atmospheric p = Dt Neutral pressure at outlet
Do is the amount of positive
pressure at the outlet due
Forced Draft p = Dt + Do
to the force draft system (if
any)
The values previously calculated for the system should be evaluated using these
equations. In all three cases, if the system losses exceed the draft requirements, the
system will not properly exhaust the combustion products. At this point, using an
interactive process, the stack and breeching diameters should be increased (or
decreased) until the system balances. Another solution for an undersized system would
be the inclusion of a draft inducer in the chimney system. For a system with a draft
inducer, the static pressure supplied by the inducer should be added to the draft
requirements in the pressure equation.
In reverse, a draft inducer can be selected based on the additional draft required to
balance the pressure equation. Knowing the volume of flow in the system and the static
pressure required, the fan selection for the inducer is simplified.
Multiple Systems
The most common configuration is the individual vent, stack, or chimney, where one
continuous system carries the products from appliance to terminus. Other configurations
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include the combined vent serving a pair of appliances, the manifold serving several,
and branched system with two or more lateral manifolds connected to a common
vertical system.
For the configurations where one system is used to vent several appliances, the sizing
procedure involves the summation of losses through out the various branches of the
system. The system should be divided into sections based on the number of appliances
venting into that portion of the system. The velocity and loss coefficient are calculated
separately for each branch. Using these values, the losses in each section can be
computed.
A minimum mean frontal inlet velocity of 0.8 fps should control smoking adequately, in
conjunction with a chimney gas temperature at least 300 to 500F above ambient. For a
reasonably conservative design, a frontal inlet velocity of 1.0 fps and a temperature of
350F can be used.
To determine the volume of air entering the fireplace at 70F, the following equation can
be used.
Qf = Vc x Af
Where: Qf = volume of air entering the fireplace @ 70F, cu. ft./min.
Vc = capture velocity, fpm
Af = fireplace frontal area, sq. ft.
The volume of flue gas in the chimney can be calculated using the following equation:
Q c = Qf
DCF
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Where: Qc = volume of gas entering the chimney at the average chimney
temperature, cu. ft./min.
Qf = volume of air entering fireplace @70F, cu. ft./min.
DCF = density correction factor (see Table 10)
Vc = Qc
Ac
In most applications, the area of the chimney will fall between 1/10th and 1/12th of the
fireplace frontal area.
Once the velocitys computed, the system losses can be calculated following the
procedure previously described. For a fireplace application, there are additional loss
coefficients, which need to be considered. These values can be found in Table 11.
The procedures for calculating theoretical draft and balancing the system follow the
previous sections. Draft inducers can also be used on fireplace chimneys to supplement
the theoretical draft if needed to help prevent smoking.
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Density Correction Factor
Table 10
Loss k factor
Loss to initiate flow 1.0
Inlet loss
Cone type fireplace 0.5
Masonry damper throat = 2 x flue area 1.0
Masonry damper throat = flue area 2.5
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