BENSON 30 - Air Distribution System

Doc No. 20-45-159 Page 10 of 60
It is necessary to provide an automatic means to safely inhibit
heater(s) operation should mechanical air supply fail for any
reason
The table to follow provides specic data for each heater
model as -
Mechanical Ventilation Rate
for Plant Room or Enclosure
Model M³/h
30 139
40 206
60 274
85 409
120 567
135 717
180 822
205 987
235 1131
300 1410
375 1830
1.12 Air distribution system
All materials used within the construction of the delivery and
return air ducts must not represent a re hazard and should
be made from thermally inert materials. The selection of
materials must take account of the environment into which
the heater and its air delivery system is expected to work, it
must also take account of the stresses and loadings placed
upon it during its normal working life. Where interjoist spaces
are used to route ducting these must be lined with re
resistant insulation material.
In installations where forced recirculation is a feature, a full
and unobstructed return air path to the heater(s) must be
provided, with return air grilles connected by ducting directly
to the return air inlet on the heater.
The limit for recirculation should not be greater than 85%
re-circulated air to 15% fresh air.
Where the heater is installed within a compartment or plant
room the return air and discharge air arrangement must be
such that the air circulation fan does not interfere with the
operation of the ue.
The return air intake and warm air outlet should therefore be
fully ducted to and from the heater, respectively, within the
compartment or plant room.
If the inlet air is ducted to the outside, then the lowest edge of
the inlet air duct must be at least 500mm above the outside
oor or ground level, it must also be tted with an access
point(s) to allow for cleaning and servicing to occur.
The openings in the structure of the plant room, through
which the ductwork passes must be of re resistant material
and constructed to prevent the likelihood of any re from
spreading.
In ducted applications the ductwork must be designed so as
to give a static pressure within the limits stated in section 15 of
this manual. It should be noted that if the static pressure is too
high, nuisance shut-down will occur when the heater goes out
on the overheat limit thermostat, if the static pressure is too
low, then damage can be caused to the fan motor.
Warm air outlets on ducted applications must be such that
they cannot be closed or become blocked, which again would
lead to an increase in static pressure and nuisance shut-down.
The outlets must not be sited so that warm air can be
discharged onto combustible materials, if necessary, guard
rails should be used to ensure that eected areas are kept
clear.
Return air intakes must not be located so that potentially
harmful or hazardous contaminated air can be drawn into the
system.
1.13 Flue system.
Warning
It is essential that the products of combustion are ued to
the outside of the building. Each heater must have its own
separate ue, with a ue diameter of not less than is detailed
in section 15 within this manual.
The minimum vertical length of ue must not be less than
3m. The ue should rise vertically, and the number of bends
should be kept to a minimum.
Flue pipes below a height of 2m should be guarded against
the possibility of being accidentally touched when hot by
personnel
Note:
It is strongly advised that BS 5854; 1980, and BS 5440; parts 1