Freestanding fireplaces
This type of fireplace requires a high frontal inlet
velocity in order to capture the products of combustion.
For design purposes it is recommended to use a
velocity of 84-96 FPM. This is in line with the code
requirements for kitchen hoods, where a capacity of
100 CFM per square foot is recommended.
Stoves and fireplace inserts
Stoves are different from open fireplaces due to the
fact that they just have a small air inlet for the combus-
tion air. However, the worst case scenario is when the
doors are open, so this is the situation the sizing should
be based on.
Again the formula:
Q
300
= A
inlet
x V
inlet
x 1.6
144
should be used, and the frontal inlet velocity recom-
mended is 36-48 FPM.
The design method for a fireplace insert is similar to
the one used for a stove.
The flow is determined by using the fireplace open-
ing with the doors opened as this represents a worst
case scenario.
Wood-fired Pizza Ovens
A new type of bread/pizza oven that is wood fired.
The uniqueness is the design of the oven which is
both an oven and a firebox. The door opening serves
as an air inlet as well as a the flue outlet.
Even though the temperature inside the oven can
reach 500°F, the flue 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 flue.
For design purposes, it is recommended that a
frontal inlet velocity of 84-96 FPM is used.
The formula for calculating the flue gas volume is:
Q
300
= A x B x V
inlet
x 1.6
144
A
= Height of door opening
B
= Width of door opening
Estimating Flow Resistance
The total flow 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:
EXHAUSTO
22
P
s
= .015 x d
m
x V
pipe
2
x
Σ
k
or
P
s
= .015 x d
m
x (Q
t
/ A
pipe
)
2
x
Σ
k
d
m
= gas density, lb/ft3
V
pipe
= system gas velocity (flue gas volume/flue area) at mean
condition, Ft/s
A
pipe
= area of flue, square inches
Σ
k
= Sum of all resistance factors
There are several factors creating resistance in a
fireplace/chimney systems:
The flue.
Flue components
The transition from firebox to flue.
In addition there are "external" factors influencing
the resistance:
Internal building pressure.
External building pressure.
Wind pattern.
When trying to determining the total resistance in a
chimney system, it eases the work to use resistance
factors, or k-values. Any part of a chimney system or
duct can be assigned a k-value, and the higher the
value, the more resistance.
K-values are dimensionless and are used in most
other industries dealing with air-flow and vents, like
f.inst. the ventilation industry.
There are only a few differences between a ventila-
tion system and a chimney system:
1.
A chimney system has natural draft to remove
the flue gases, as long as there is a vertical
chimney or vent. A ventilation system does not
have any natural draft, so mechanical draft
fans are always required.
2.
A chimney system deals with high temperature air
with products of combustion. A venti lation sys-
tem removes room temperature air with dust par-
ticles.
3.
Chimney flues are build of stainless steel pipe,
sheet metal, steel liner, tile liner, or poured/cast-in-
place liners. Most ventilation systems use prefab-
ricated vents of sheet metal or plastic.
In other words: A chimney system removes air with
a higher temperature and a lower density than a
ventilation system. Further the ventilation system is
often built in a material with less flow resistance.
Summary of Contents for EXHAUSTO RS 12
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