Northline Express EXHAUSTO RS 12, Руководство по применению

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Flue Pipe resistance.
As in any pipe material, the roughness of the pipe
influences the flow resistance. The rougher the pipe
surface, the more resistance. The roughness of a pipe
is normally expressed as a Reynolds number, but for all
practical purposes these can be converted into k-
values that are easier to work with.
Chimney and stack manufacturers as well as
ASHRAE have set a range of friction factors (F) to use
for the calculation of k-values for different types of
chimneys and vents:
Vent Type Friction Factor, F
Pressure tight stacks 0.22
Gas Vent (B-Vent) 0.25
Single wall steel pipe 0.32
Poured liners 0.34
Clay tile liners 0.38
Corrugated liners 0.50
The k-value of a vent can be determined with this
formula:
k
L
= F x L
d
i
L = Length of entire flue system, feet
d
i
= Diameter of flue, inches
Example: A 8" single steel wall flue system of a
total length of 25', has a k-value of:
k
L
= .32 x 25 = 1.0
8
Firebox resistance
Some resistance can be found in the transition
between the firebox and the flue. The resistance could
be compared to a converging tapered entry into the
chimney. Allow for a k-value of min. 2, if the inlet is
smooth - more if it's rough.
Component resistance.
Chimney and stack manufacturers as well as
ASHRAE have set up a range of k-values for different
types of components in chimney systems. These can
be found in Appendix C.
Other Resistance.
In addition to the resistance from the flow, resis-
tance from negative building pressure, external build-
ing pressures etc. must also be taken into consider-
ation. This could actually be measured by using a
pressure gauge in the fireplace chimney before starting
a fire. It is not unusual to see a negative pressure of 0.1-
0.3"WC for problem fireplaces.
Estimating Natural Draft
The theoretical draft of a gravity chimney or vent is
the difference in weight (mass) between a given col-
umn of warm flue gas and an equal column of cold
ambient air. The theoretical draft can be derived from
the following formula:
D
t
= .2554 x B x H x (1/T
o
- 1/T
m
)
D
t
= Theoritical draft, in. WC
B = Local barometric pressure, in Hg
H = Height of the chimney, feet.
T
o
= Ambient Tempe 460, °R
T
m
= Mean flue gas tempe 460, °R
The formula indicates that the draft increases with
the height. However, as the mean chimney gas tem-
perature decreases when the height incrreases, add-
ing height to a chimney is only effective to a certain
height.
With a barometric pressure of 29.92 inHg, a 25 feet
chimney, 70°F ambient temperature and a mean flue
gas temperature of 250°F,
D
t
= .2554 x 29.92 x 25 x (1/530-1/710)
= 191.04 x (.002-.0014) = 0.115 inWC
The formula also shows that realistically a chimney
without draft does not exist.
Estimating available draft.
To find the available draft (D
a
) for a chimney sys-
tem, the static pressure loss is deducted from the
theoretical draft:
D
a
= -D
t
+ P
s
Notice that "draft" is always negative, while static
pressure is always positive.
Example:
Theoretical draft, D
t
- .138 inWC
Static pressure loss, P
s
+ .100 inWC
Available draft, D
a
- .038 inWC
EXHAUSTO 23
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