Chapter 6 - Combustion Air Criteria Flashcards
Why are two sources of air needed for combustion?
There is not enough pressure/energy in the fuel to draw all the combustion air into the burner as primary air
Define primary air
combustion air that enters the combustion process upstream of the gas burner ports
Why is it better to operate with high amounts of primary air
High primary air rates result in short compact flame kernels that have small dead zones.
These are called hard flames
Higher primary air leads to better combustion
What are drawbacks of operating with high amounts of primary air
More susceptible to flashback, high burner temperatures and resonance
Benefits of hard flames and high primary aeration
little or no flame impingement, ease of control, higher appliance efficiencies
What are hard flames
Flames with short compact flame kernels that have small dead zones. They have high primary aeration
What is the downside to high primary aeration rates?
more susceptible to flashback, high burner temps and resonance
What makes it possible to operate with high primary aeration
Low port loading makes this possible - is a hard flame with short flame kernels and small dead zones
How to increase the primary aeration rate
reduce port loading open the air shutter reduce the gas input rate reduce backpressure inside the burner reduce backpressure in the flue (opposite actions would decrease the primary aeration rate)
What is secondary air?
combustion air that enters the combustion process downstream of the gas burner ports
Define secondary air penetration
The act of getting air into each and every port where it is needed
Importance of secondary air penetration
Provides a clean burning system
Provides just enough air to maintain desired efficiency and provide clean combustion
Proper amount assures that “transport losses” do not become excessive
Define Transport Losses
Thermal losses that result when a gas appliance is operating with too much excess air.
Excess air wastes thermal energy by carrying it out through the flue and vent
How to increase secondary air?
eliminate flue restrictions
increase flue area
open inlet to burner box (air orifice)
How to improve secondary air penetration?
increase burner port spacing
install baffling to direct the secondary air
decrease port loading
eliminate flue restrictions
What indicates inadequate secondary air and/or poor secondary air penetration
High CO flame impingement floating flames recirculation carbon deposits
Define fuel/air ratio (F/A)
dimensionless ratio that describes the quality of a combustible gaseous fuel mixture
Can be by volume or weight
volume/weight of fuel over volume/weight of air used
Define air/fuel ratio (A/F)
dimensionless ratio that describes the quality of a combustible gaseous fuel mixture
Can be by volume or weight
volume/weight of air over volume/weight of fuel used
Define stoichiometric mixture ratio
theoretical air/fuel ratios that are predicted by the combustion chemistry equations
Define TAR
“Theoretical Air Requirements”
expressed in cubic ft/hr is synonymous with the stoichiometric air requirements of a gas burner or combustion system
Excess air
amount of combustion air provided in abundance to the stoichiometric mixture ratio requirements
Rich mixtures
air/fuel mixtures that have less air as compared to the stoichiometric mixture
Rich - too much fuel
Lean mixtures
air/fuel mixtures that have more air as compared to the stoichiometric mixture
Lean - too much air
Rich mixture combustion products
CO, CO2, N2, H2, CH4, H2O, Aldehydes
