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CGE Certification > Chapter 6 - Combustion Air Criteria > Flashcards

Chapter 6 - Combustion Air Criteria Flashcards

1
Q

Why are two sources of air needed for combustion?

A

There is not enough pressure/energy in the fuel to draw all the combustion air into the burner as primary air

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2
Q

Define primary air

A

combustion air that enters the combustion process upstream of the gas burner ports

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3
Q

Why is it better to operate with high amounts of primary air

A

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

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4
Q

What are drawbacks of operating with high amounts of primary air

A

More susceptible to flashback, high burner temperatures and resonance

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5
Q

Benefits of hard flames and high primary aeration

A

little or no flame impingement, ease of control, higher appliance efficiencies

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6
Q

What are hard flames

A

Flames with short compact flame kernels that have small dead zones. They have high primary aeration

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7
Q

What is the downside to high primary aeration rates?

A

more susceptible to flashback, high burner temps and resonance

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8
Q

What makes it possible to operate with high primary aeration

A

Low port loading makes this possible - is a hard flame with short flame kernels and small dead zones

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9
Q

How to increase the primary aeration rate

A 
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)
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10
Q

What is secondary air?

A

combustion air that enters the combustion process downstream of the gas burner ports

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11
Q

Define secondary air penetration

A

The act of getting air into each and every port where it is needed

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12
Q

Importance of secondary air penetration

A

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

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13
Q

Define Transport Losses

A

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

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14
Q

How to increase secondary air?

A

eliminate flue restrictions
increase flue area
open inlet to burner box (air orifice)

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15
Q

How to improve secondary air penetration?

A

increase burner port spacing
install baffling to direct the secondary air
decrease port loading
eliminate flue restrictions

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16
Q

What indicates inadequate secondary air and/or poor secondary air penetration

A 
High CO
flame impingement 
floating flames
recirculation
carbon deposits
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17
Q

Define fuel/air ratio (F/A)

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

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18
Q

Define air/fuel ratio (A/F)

A

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

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19
Q

Define stoichiometric mixture ratio

A

theoretical air/fuel ratios that are predicted by the combustion chemistry equations

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20
Q

Define TAR

A

“Theoretical Air Requirements”

expressed in cubic ft/hr is synonymous with the stoichiometric air requirements of a gas burner or combustion system

21
Q

Excess air

A

amount of combustion air provided in abundance to the stoichiometric mixture ratio requirements

22
Q

Rich mixtures

A

air/fuel mixtures that have less air as compared to the stoichiometric mixture
Rich - too much fuel

23
Q

Lean mixtures

A

air/fuel mixtures that have more air as compared to the stoichiometric mixture
Lean - too much air

24
Q

Rich mixture combustion products

A

CO, CO2, N2, H2, CH4, H2O, Aldehydes

25
Q

Lean mixture combustion products

A

CO2, O2, N2, H2O

No CO produced

26
Q

What is the equivalence ratio

A

fuel/oxidizer ratio divided by the Stoichiometric fuel/oxidizer ratio
Gives an indication if the mixture is rich or lean
Less than one - lean
More than one - rich

27
Q

Rule of Thumb #1

For every _____BTUs of NG burned, ____cubic feet/foot of air will be used

A

For every 100 BTUs of NG burned, one cubic foot of air will be used

28
Q

Rule of Thumb #2

Propane gas requires [more/less] air per BTU burned than Natural gas

A

Propane gas requires more air per BTU burned than Natural gas
aeration rate at a given CO2 value will always be higher for LP

29
Q

Three ways to determine aeration rates

A

Sight
Smell
Measuring CO2

30
Q

What is the most accurate way to determine aeration rates

A

By measuring CO2
Best through measuring vent gas
Higher CO2 lower the excess air
Lower than 3 CO2 is suspect - dilution in sample

31
Q

What is aeration rate

A

Is the percentage of air used to burn gas. The rate at which air enters the burning process??

32
Q

What causes carbon deposits

A

insufficient amounts of combustion air

33
Q

What causes dirty combustion

A

impingement
lack of sufficient combustion air
lifting

34
Q

What causes flashback

A

excess amounts of combustion air
damaged burner ports
poor choice of port loading

35
Q

what causes floating flame

A

insufficient amounts of secondary air

36
Q

what causes low thermal efficiency

A

too much secondary air (increases transport losses through the flue)

37
Q

what causes re-circulation

A

insufficient secondary air
re-circulation can indicate poor secondary air penetration
Can leave rusty-colored deposits on the burners

38
Q

What causes yellow tipping

A

Lack of adequate combustion air
At the tip of the primary cone - inadequate primary aeration (Propane gas)
At the tip of the secondary cone - inadequate secondary air (Natural gas)

39
Q

What causes yellow tipping at the tip of the primary cone

A

Inadequate Primary aeration

happens most on Propane- not usually an issue

40
Q

What causes yellow tipping at the tip of the secondary cone

A

Inadequate Secondary aeration

happens most on NG - can be a problem resulting in poor combustions and carbon deposit

41
Q

which fuel type is most prone to yellow tipping and why

A

LP because it requires more air to burn than NG

42
Q

What’s the solution to yellow tipping?

A

Provide more combustion air

43
Q

What two functions do gas orifices have?

A

Control the amount of gas entering a burner

Draw in more primary air

44
Q

What three aspects of gas orifices will affect their performance?

A

Machined correctly with no burrs
Back pressure in burner is normal and not excessive
Pressure regulator accurately controls the inlet pressure

45
Q

What is aspiration?

A

the velocity of the gas entering the orifice causing air to be pulled in with it. aka primary air injection

46
Q

How much of the primary air can be pulled in during aspiration?

A

40-60% of the stoichiometric air

47
Q

At the optimum amount of primary air, what is the exit velocity of the gas leaving the orifice?

A

it approaches 140ft/s

48
Q

What is one major problem with gas orifices?

A

Spillage - when the gas orifice is not lined up properly with the burner and the gas exiting the orifice spills outside the burner

CGE Certification (18 decks)

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