Handbook of air conditioning and refrigeration / Shan K

Gas and Oil Burners
When natural gas is used as the fuel in boilers, as in gas-fired furnaces, atmospheric and power gas
burners are usually used. Oil burners used in boilers differ from gas burners in that oil requires atomization
and vaporization prior to combustion. Combustion air is supplied by either natural drafts
or forced drafts. Ignition is usually provided by an electric spark or a standing pilot using gas or oil.
Oil burners used in residences are usually pressure atomizing-gun burners. This type of burner
either directly atomizes oil at 100 to 300 psig (690 to 2069 kPa�g) high pressure or uses combustion
air to atomize the oil. Combustion air is supplied by a blower, and electric spark ignition is
generally adopted. Oil burners for boilers in commercial buildings inject oil and atomize it into fine
sprays. Burners also force combustion air to mix with atomized oil and ignite the mixture, often
with an electric spark. Complete combustion can be sustained with as little as 20 percent excess air.
Sometimes a boiler can be designed to use either gas or oil. A combination of a ring-type gas
burner with an oil burner at the center is often used. Combination gas-oil burners are used more
often than single-fuel burners in large boilers.
Condensing and Noncondensing Boilers
Boiler Efficiency
HEATING SYSTEMS, FURNACES, AND BOILERS 8.13
In condensing boilers, the water vapor in the flue gas is condensed and drained by means of a heat
exchanger, as in gas furnaces. Thus, the latent heat of condensation can be recovered. In hot water
boilers, the cooling medium is usually the return water from the conditioned space. The lower the
temperature of the return water, the higher the amount of heat recovered. If the dew point of the flue
gas is 130°F (54°C), return water or service water at a temperature below 125°F (51.7°C) may be
used as the condensing cooling medium. Corrosion in the heat exchanger and flue gas passage
caused by the condensate should be avoided. Noncondensing boilers have no way to condense the
water vapor contained in the flue gas.
Boilers can be assessed by two efficiency values. The combustion efficiency E c, in percent, is the
ratio of heat output from the hot water or steam Q out to the heat content rate of the fuel consumed
Q fuel, that is,
Ec � (8.4)
Both Qout and Qfuel are expressed in Btu/h (kW). Annual fuel utilization efficiency (AFUE) is also
an efficiency value used for hot water boilers. For noncondensing boilers, Ec varies from 80 to 85
percent. For condensing boilers, Ec varies from 85 to 90 percent.
ASHRAE/IESNA Standard 90.1-1999 mandates that the minimum efficiency requirements of
gas- and oil-fired hot water boilers are as follows:
100Qout Qfuel Equipment type Size, Btu/h Rating condition Minimum efficiency
Efficiency as of
10/29/2001
Gas-fired boilers � 300,000 Hot water 80% AFUE 80% AFUE
� 300,000 and
� 2,500,000
Maximum capacity 80% Ec 75% Et � 2,500,000 Hot water 80% Ec 80% Ec Oil-fired boilers � 300,000 80% AFUE 80% AFUE
� 300,000 and
� 2,500,000
Maximum capacity 83% Ec 78% Et � 2,500,000 Hot water 83% Ec 83% Ec (Continued)