Specifications

39
- transition from rebox to ue
In addition there are “external” factors inuencing the
resistance:
- internal building pressure
- external building pressure
- wind patterns
When trying to determine the total resistance in a
chimney system, resistance factors, or k-values,
are used to ease calculations. K-values are
dimensionless.
Flue pipe resistance
As in any pipe material, the roughness of the
surface inuences the ow resistance - the rougher
the surface, the more resistance. Roughness of a
pipe surface is normally expressed as a Reynold’s
number, but for all practical purposes these can be
converted into k-values that are easier to work with.
ASHRAE has set a range of friction factors that
chimney and stack manufacturers use for the
calculation of k-values for different types of chimneys
and vents:
Vent type Friction factor, F
Class A chimney 0.22
Gas Vent (B-vent) 0.25
Single wall steel 0.32
Refractory liners 0.34
Clay tile liners 0.38-0.42
Corrugated liners 0.50
The k-value of a chimney can be determined with this
formula:
Example: An 8” single steel wall system with a total
length of 25’ has a k-value of:
K
L
= .32 x 25 / 8 = 1.0
Resistance inside rebox
Resistance can be found in the transition between
the rebox and the chimney. The transition can be
considered a converging tapered entry. Allow for a
k-value of min. 2, if the inlet is smooth – more if it’s
rough.
Stoves and replace inserts
Stoves are different from open replaces due to the
fact that they have a small inlet for the combustion
air. However, the worst-case scenario occurs when
the door is open, so this is the situation the sizing
should be based on.
Again, use the airow formula with a recommended
frontal inlet velocity of 36-48 FPM.
This design method is also used for a replace insert
The ow is determined by using the replace opening
with the doors opened.
Residential wood-red pizza ovens
This type of appliance is both an oven and a rebox.
The door opening serves as an air inlet as well as the
ue outlet.
Temperatures can reach 500-600°F, but the ue gas
temperature is much lower due to the design of the
smoke chamber. The smoke chamber is just a small
hood that leads into the ue.
For design purposes it is recommended to use a
frontal inlet velocity of 36-48 FPM. The formula for
calculating the airow is:
Estimating Flow Resistance
The total ow resistance in a vent system that moves
air is normally referred to as “total static pressure
loss”. The formula used to determine the static
pressure loss in a system is:
The are several factors creating resistance in
replace chimney system:
- the ue
- ue components and ttings
Q
300
= A x B x V
inlet
x 1.6
144
A = height of door opening
B = width of door opening
P
s
= .003 x d
m
x V
pipe
2
x Σk
= .003 x d
m
x (Q
t
/ A
pipe
)
2
x Σk
d
m
= gas density (lb/ft
3
)
V
pipe
= system gas velocity at mean condition (ft/s).
Can also be expressed as ue gas volume / ue
area)
A
pipe
= area of ue (square inches)
Σk = sum of all resistance factors
K
L
= F x L / D
i
L = Length of entire chimney system (ft)
D
i
= Diameter of chimney (inches)
F = Friction Factor
Q
300
= A
inlet
x V
inlet
x 1.6
144