Johnson Controls YORK YPC Series, Installation Manual

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JOHNSON CONTROLS
60
SECTION 19 – CHIMNEY DESIGN AND DRAFT THEORY
FORM 155.17-N1
ISSUE DATE: 4/1/2013
CHIMNEY DESIGN THEORY
It is not the intent of this section to ad-
dress the fine de tails of proper chimney
design - this must be ad dressed on an
application-specific basis by an ex pe ri-
enced de sign er knowledgeable in chim-
ney systems, draft control and local code
requirements. This section is prepared
only to identify certain considerations
which may bear upon final chimney sys -
tem configuration, and to provide a brief
description of two commonly employed
methods of draft control.
For the sake of discussion, it is nec es sary to de fine the
following terms:
Available Draft (Da)
The draft required at the outlet exhaust flange of the
ParaFlow
TM
high-temperature generator.
Theoretical Draft (Dt)
The natural draft or “chim ney effect” produced by the
difference in densities of hot exhaust gas relative to
cooler ambient air.
Pressure Drop (dP)
Frictional losses in the chimney which act against theo-
retical draft.
The theoretical draft needed to overcome chimney fric-
tion al losses is described as follows:
Dt = dP + Da equation 1
In the case of a ParaFlow
TM
Chiller-Heater, (Da) is neg-
a tive, resulting in Equation 1 becoming:
Dt = dP - Da equation 2
Proper chimney design balances the theoretical draft
(Dt) against the pressure drop (dP) of the chimney
sys tem in order to provide the required available draft
(Da) at the out let of the unit under all operating con-
ditions. Because the difference between summer and
winter ambient conditions can result in (Dt) variations
of 50% or greater, some method of draft control is usu-
ally re quired in order to maintain a steady draft.
Theoretical draft is a function of mean or net chim-
ney gas tem per a ture (always a tem per a ture lower than
unit outlet tem per a ture), ambient temperature, ambient
baro met ric pres sure and chimney height. It should be
noted that this value can vary significantly from sum-
mer am bi ent con di tions (when Dt is smallest) to win-
ter am bi ent con di tions (when Dt is greatest). Be cause
ParaFlow
TM
chiller-heaters are typ i cal ly required to op-
erate at “high fire” throughout the summer months, it is
im por tant to de sign the chimney system for summer
ambient de sign conditions to avoid undersizing.
When designing the chimney, it is recommended to de-
sign it for a (Da) of 0 (zero) inches of water column.
This will prevent the chimney from becoming pres sur-
ized at any point along the flue gas path.
If the stack-chimney pressure is ever
above zero (0) inches of water column,
there is a chance that flue gases could leak
into the equipment room.
WIND AND WEATH ER
Windy conditions will tend to in crease the draft in the
chimney as the wind helps to re move the com bus tion
products leaving the chimney at a much faster rate.
However, down draft may occur, caus ing a tem po rary
positive pressure in the chimney sys tem. The stack
may be designed to prevent not only wind, but rain
and snow from entering the stack. A flue cap should
be installed.
INADEQUATE SYSTEM INSTALLATION
If the diameters of the chimney system are too restric-
tive, the combustion products and flue gases may not
be allowed to leave the system. On the other hand, if
the flue passages are too large, the chimney is never
given a chance to completely warm due to the large
surface area of the flue. This sit u a tion may cause poor
draft and flue gas condensation.
If the flue gases are allowed to condensate, sulfur
dioxide and nitric oxide, along with other nitrogen
compounds, will combine with water vapor in the air
causing corrosion within the flue pipe and/or chiller
firebox. To allow the chimney system to heat up faster,
insulation should be installed on all exposed flue pip-
ing. Insulation is also a good safe ty measure (often re-
quired by code) as the breeching and flue pipes will
heat to temperatures in excess of 400°F.
Chimney design and draft control are topics best han-
dled by experienced engineers. Johnson Controls
strongly recommends that the installer engage the ser-
vices of a competent en gi neer to design the chim ney
and provide the type of draft control best suited for
each application.