Chapter 7 - Combustion Stability

Question 1

What turndown ratio and port loading will a stable combustion system have?

Answer 1

It will have 6:1 turndown at least and be able to operate at port loadings of 10,000

Question 2

What is turndown ratio

Answer 2

ratio of the maximum gas flow rate to minimum gas flow rate

Question 3

Define port velocity

Answer 3

average velocity of the fuel/gas mix as it flows through and leaves the gas burner ports (ft/s)

Question 4

What is port velocity a function of (what affects it?)

Answer 4

total port area
gas input rate
port loading
fuel/air ratio

Question 5

Define flame speed

Answer 5

speed of a burning flame front as it moves through a combustible gas/air mixture (ft/s)

Question 6

What affects flame speed?

Answer 6

varies with mixture temperature
fuel/air ratio
how well the fuel and air are mixed

Question 7

what is combustion stability

Answer 7

Describe by what it's not: It's not
flashback
lifting
shutdown pop
candling

Question 8

what is flashback

Answer 8

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

Question 9

what is flashback an indicator of

Answer 9

lack of burner port stability

Question 10

what is lifting

Answer 10

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

Question 11

What is lifting an indicator of

Answer 11

lack of gas burner port stability

Question 12

what is shutdown pop

Answer 12

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

Question 13

What causes shutdown pop

Answer 13

the flow of the air/gas mixture through the burner ports decreases as a result of the closing of the main gas valve

Question 14

What is candling

Answer 14

small flame that burns right at the exit of the orifice immediately after shutdown
Is undesirable but not destructive to the appliance or humans

Question 15

If the mixture velocity is greater than the flame speed which way will the flame front move?

Answer 15

In the direction of the mixture velocity

towards the fire

Question 16

if the mixture velocity is less than the flame speed which way will the flame front move?

Answer 16

in the direction opposite the mixture velocity

towards the gas

Question 17

What are the 5 stabilization positions that occur frequently?

Answer 17

Lifting
soft
normal
hard
flashback

Question 18

What is port loading

Answer 18

parameter: gas input rate/total burner port area

[BTU/hr]/[in^2] = [BTU/hr in^2]

Question 19

What can port loading be used for

Answer 19

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

Question 20

what is the dead zone

Answer 20

distance between the burner port and the bottom of the flame kernel (air gap)

Question 21

how does the dead zone compare to the port loading

Answer 21

proportional to the port loading

smaller dead zone - smaller port loading

Question 22

How does the dead zone affect the quality of burn

Answer 22

More dead zone indicates less primary aeration than with a smaller dead zone. Greater chance for impingement and dirty combustions

Question 23

Explain L/d ratio

Answer 23

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

Question 24

Ideal L/d ratio

Answer 24

Large L/d Ratios (6 and above) indicate stable burner ports

2 and less indicate potential lack of stability

Question 25

What do large L/d ratios indicate

Answer 25

gas burner will be resistant to flashback and shutdown pop

has a higher pressure drop across the gas burner ports

Question 26

What do low L/d ratios indicate

Answer 26

poor resistance to flashback and shutdown pop

Question 27

Design benefits of large L/d ratios

Answer 27

Can operate at lower port loadings
Can operate at higher primary aeration rates
Cleaner combustions
Higher turndown ratios
Increased resistance to flashback, shutdown pop and resonance
Reduced chance of candling
Increased efficiency

Question 28

What is quench diameter

Answer 28

largest diameter burner ports can have and still resist flashback

Question 29

What is quench diameter a function of

Answer 29

the gas type being used

Question 30

How is quench diameter affected by flame speed

Answer 30

Fuels with faster flame speeds will have smaller quench diameters