Handbook of air conditioning and refrigeration / Shan K

8.6 CHAPTER EIGHT
Horizontal Gas-Fired Furnace
A typical horizontal gas-fired furnace in a rooftop packaged unit is shown in Fig. 8.2. It mainly
consists of multiple gas burners, a heat exchanger, a combustion blower, and an ignition device. The
supply fan used to force air to flow through the gas-fired furnace is the same supply fan used to
force air through the filters and DX coil in the rooftop unit.
A power burner is often used in a horizontal gas-fired furnace. This type of burner provides
better combustion and higher efficiency than atmospheric burners. A power draft centrifugal blower
may be added to extract the combustion products and discharge them to a vent or chimney. The gas
supply to the burner is controlled by a pressure regulator and a gas valve for the purpose of controlling
the firing rate. In a premix power burner, gas and primary air are mixed first; then the mixture
is forced to mix with secondary air in the combustion zone. A power burner usually has a higher gas
pressure than the atmospheric burner used in residences.
The heat exchanger usually has a tubular two-pass arrangement, typically with 16-gauge
(1.5-mm-thickness) stainless steel for primary surfaces and 18-gauge (1.2-mm-thickness) stainless
steel for secondary surfaces, as shown in Fig. 8.2. The primary surface is the heat-transfer surface
of the combustion chamber. The secondary surface is the surface of the tubes through which the
flue gas flows after the combustion chamber. A cone-shaped flame is injected into a tubular or
drum-shaped combustion chamber. A centrifugal blower is used to provide secondary air for forced
combustion. Another small centrifugal blower may be used to induce the flue gas at the exit of the
heat exchanger, the power vent, to maintain a negative pressure at the heat exchanger section so as
to prevent the mixing of any leaked flue gas with the heated air. The mixture of outdoor and recirculating
air is forced by the circulating blower and is heated when it flows over the primary and
secondary surfaces.
Furnace Performance Factors
The performance of a gas-fired furnace is usually measured by the following parameters:
● Thermal efficiency Et, in percent, is the ratio of the energy output of the fluid (air or water) to the
fuel input energy. Input and output energy should be expressed in the same units. The value of Et can be calculated as
E t �
FIGURE 8.2 Horizontal gas-fired furnace in a rooftop packaged unit.
100 (fluid energy output)
fuel energy input
(8.1)