Chapter 7 - Combustion Stability Flashcards
What turndown ratio and port loading will a stable combustion system have?
It will have 6:1 turndown at least and be able to operate at port loadings of 10,000
What is turndown ratio
ratio of the maximum gas flow rate to minimum gas flow rate
Define port velocity
average velocity of the fuel/gas mix as it flows through and leaves the gas burner ports (ft/s)
What is port velocity a function of (what affects it?)
total port area
gas input rate
port loading
fuel/air ratio
Define flame speed
speed of a burning flame front as it moves through a combustible gas/air mixture (ft/s)
What affects flame speed?
varies with mixture temperature
fuel/air ratio
how well the fuel and air are mixed
what is combustion stability
Describe by what it's not: It's not flashback lifting shutdown pop candling
what is flashback
flashback happens when the flame front moves through the burner ports and establishes itself at the exhaust face of the orifice.
happens when the flame speed in the combustible air/gas mix exceeds the port velocity of the air/gas mix exiting the burner ports
what is flashback an indicator of
lack of burner port stability
what is lifting
when 3 or more adjacent flame kernels on a gas burner lift and move away from the normal flame stabilization point
port velocity of air/gas mix is higher than the flame speed of the gas
What is lifting an indicator of
lack of gas burner port stability
what is shutdown pop
slight explosion or “pop” that occurs during the shutdown transient on some gas burners or combustion systems
Instantaneous ignition of all the air/gas mixture that remains in the gas burner tube
What causes shutdown pop
the flow of the air/gas mixture through the burner ports decreases as a result of the closing of the main gas valve
What is candling
small flame that burns right at the exit of the orifice immediately after shutdown
Is undesirable but not destructive to the appliance or humans
If the mixture velocity is greater than the flame speed which way will the flame front move?
In the direction of the mixture velocity
towards the fire
if the mixture velocity is less than the flame speed which way will the flame front move?
in the direction opposite the mixture velocity
towards the gas
What are the 5 stabilization positions that occur frequently?
Lifting soft normal hard flashback
What is port loading
parameter: gas input rate/total burner port area
[BTU/hr]/[in^2] = [BTU/hr in^2]
What can port loading be used for
Estimating gas burner port sizes and number of ports
resolving field problems
understand if burners are over or under loaded
comparing burners to each other
general indication of port stability
what is the dead zone
distance between the burner port and the bottom of the flame kernel (air gap)
how does the dead zone compare to the port loading
proportional to the port loading
smaller dead zone - smaller port loading
How does the dead zone affect the quality of burn
More dead zone indicates less primary aeration than with a smaller dead zone. Greater chance for impingement and dirty combustions
Explain L/d ratio
L is the length of the burner port
d is the diameter of the gas burner port
L/d ratio spans from 1 to 10
Ideal L/d ratio
Large L/d Ratios (6 and above) indicate stable burner ports
2 and less indicate potential lack of stability
