Types of Direct-Fired Gas Heaters
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Constant Volume System
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The constant volume 100% direct-fired make-up air system heaters
provide a constant rate of supply air to the space. The gas flame
in the unit heats the fresh make-up air directly, eliminating energy
losses associated with other types of heating. For most applications,
introducing slightly more make-up air into the space than what is exhausted
produces the best results. A slightly positive building pressure assures the
best performance. Some applications, such as the commercial kitchens, require
more air exhausted than introduced to control over pressurization of the kitchen.
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Two-Speed System
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The two-speed, direct-fired make-up air system functions the same
as constant volume, however provides two different air-delivery rates
by using either an 1800/900 RPM motor or 1800/1200 RPM motor. Two-speed
units offer flexibility to buildings with two, fixed-exhaust loads or where
an increase of airflow is desired.
Units for buildings with two fixed-exhaust loads should have CFM ratios
that match the available two-speed motor RPM ratios. The low speed is
interlocked with the first exhaust fan, while high speed operates when both
exhaust fans are running, allowing one unit to handle two different exhaust loads.
The example above illustrates the use of a two speed unit for transit or
automotive type facility. Low speed is used for general ventilation and
heating. However, if the co-detector senses high limits of carbon monoxide,
the two speed unit goes to high speed and the second exhaust fan turns on to
ventilate the space. There are many uses for the two-speed system, however
another use is for paint booths designed for painting and curing of paint.
High speed use in paint mode and low speed is used in curing mode to dry the
painted surface.
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Variable Air Volume System
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The variable air volume (VAV) system allows variable amounts of make-up
air through one heater. This system allows a heater to turn down to as low as
25% of its designed CFM by use of a variable frequency drive (VFD) or airfoil
blade designed discharge reduction damper. One unit can handle varying exhaust
loads by changing the CFM out put of the unit. This system is ideal for spaces
that have multiple exhaust loads and recirculated air is not an option.
The use of the VFD in the system offers the highest level of operating
efficiency of any of the direct fired systems, using only the required electrical
energy to provide the CFM needed. The VFD can be setup to operate in many
different ways to change the CFM required to the space. A potentiometer can
be used to manually position output CFM, multiple set points can be set in the
VFD to vary output CFM, or a building pressure control device can be used to
vary output CFM by sensing pressure in the space.
The use of the discharge reduction damper in this system offers a low
cost up front investment with the same operation benefits of the VFD. By
closing the reduction damper, the output CFM is reduced. This design has the
ability to be setup as a two position CFM rate unit or a varying CFM unit based
off pressure in the space.
Both VAV systems have been designed with additional safety features and
controls to insure safe burner operation.
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80/20 System
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80/20 systems can be a variable make-up air unit, a total-building heat
unit, or a combination of both. By re-circulating air, this system provides
the most efficient building heat system.
One unit can handle multiple and varying exhaust loads as a variable,
make-up air unit using the 80/20 System. This system offers constant discharge
airflow but varies the amount of outside air introduced to the building. With
the use of the 403M or the 404M space-temperature control system, the unit can
be a complete building-heating system for either fixed or variable exhaust loads.
The 80/20 System supplies at least 20% outside air to the building and mixes
with re-circulated room air at a maximum of 80%, without the return air
re-circulated through the burner. A building-pressure switch controls the
modulating-burner bypass and return-air dampers introducing more outside air
as the buildings exhaust load changes. The varying exhaust-air loads
automatically and continually adjust the amount of make-up air introduced
into the system.
All of our direct fired systems are provided with terminals to integrate
operation with the building exhaust system.
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80/20 System Features
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The 80/20 System is simple to operate and maintain, with both heating
and make-up air combined into one unit. With its ability to control static
pressure, it automatically manages outside air percentages and either
space or discharge temperature, eliminating the need to interlock
exhaust fans with separate make-up air units. The 80/20 System automatically
adjusts to the building's pressure changes to bring in outside air, up to
100%. No flue through the roof means no leaks or flue maintenance. Finally, due
to its high combustion efficiency, 80/20 System is economical to operate.
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