Chapter F - Combustion Process

Question 1

What three elements are required for combustion to occur?

Answer 1

Fuel - usually natural gas, or LP gases.
Oxygen - obtained from the air surrounding a burner.
Heat - enough to bring the fuel to ignition point.

Question 2

What percentage of air is oxygen, and what percentage is nitrogen?

Answer 2

Air is 20% oxygen, and 80% nitrogen.

Question 3

What are the three types of combustion?

Answer 3

Perfect combustion (aka stoichiometric combustion), complete combustion, and incomplete combustion?

Question 4

What is the perfect combustion formula for natural gas?

Answer 4

CH4 + 202 + 8N2 –> C02 + 2H20 + 8N2 + Heat (1, 2, 8, 1, 2, 8)

Question 5

What is the perfect combustion formula for propane?

Answer 5

C3H8 + 502 +20N2 –> 3C02 + 4H20 + 20N2 + Heat (1, 5,20 –> 3, 4, 20)

Question 6

What are the two life threatening products of incomplete combustion?

Answer 6

Carbon monoxide, and aldehydes.

Question 7

Name three possible sources of the initial heat required for combustion.

Answer 7

1) Pilot flame. 2) Spark igniter 3) Hot surface igniter (glow coil)

Question 8

How much primary air, and how much secondary air is in combustion air?

Answer 8

1/3 primary air, and 2/3 secondary air.

Question 9

What part of combustion air is mixed with the fuel gas before ignition?

Answer 9

Primary air.

Question 10

Secondary air is supplied:

Answer 10

around the flames.

Question 11

What is excess air?

Answer 11

Air supplied to the combustion zone in excess of the required air for complete combustion. It’s supplied to ensure that complete combustion has occured.

Question 12

Why must you keep excess air to a minimum?

Answer 12

Because it lowers the efficiency of the appliance by absorbing heat.

Question 13

What is the purpose of dilution air?

Answer 13

Dilution air is admitted into a venting system at the draft control device of the appliance. It’s used to cool the hot vent gases, and to control the draft influence on the combustion chamber.

Question 14

If the equipment has no draft control device, does it need dilution air?

Answer 14

No.

Question 15

For every 1000 btu’s there is how much air required for perfect combustion?

Answer 15

1000 Btu’s of input + 10 cu ft. of air = perfect combustion.

10:1 ratio.

Question 16

For every 1000 btu’s of input, how much excess air is required?

Answer 16

For every 1000 btu’s of input there’s 5 cu ft. of excess air supply.

Question 17

For every 1000 btu’s of input, how much dilution air is required?

Answer 17

For every 1000 btu’s of input there’s 15 cu ft. of dilution air.

Question 18

What is total air?

Answer 18

Total air is the total amount of air required to allow the appliance to operate safely and efficiently. It’s a total of combustion, excess, and dilution air.
1000 btu’s of input = 30 cu ft. of total air supply.

Question 19

How many cu ft. of primary air is required to mix with 1000 btu’s of input?

Answer 19

3.33 cu ft. of primary air. (1/3rd of combustion air.)

Question 20

How many cu ft. of secondary air is required to mix with 1000 btu’s of input?

Answer 20

6.66 cu ft. of secondary air. (2/3rd of combustion air.)

Question 21

Measuring flue gas products and using flue gas analysis equipment when setting up the burner system ensures:

Answer 21

Improved efficiency and reduction in dangerous levels of toxic gases.

Question 22

What are three types of flue gas analyzers?

Answer 22

Tube, liquid, and continuous sampling.

Question 23

What is the difference between the flue gas temperature and the ambient (room) temperature?

Answer 23

The net stack temperature.

Question 24

Under what conditions may carbon monoxide be found in the flue gas?

Answer 24

A lack of excess air (incomplete combustion), poor mixing on the burner, and chilling of the flame.

Question 25

Free hydrogen is a sign of incomplete combustion, but is not normally present unless the level of carbon monoxide reaches a level of at least?

Answer 25

2%.

Question 26

What type of flue gas analyzer uses a dry chemical which reacts with a particular component flue gas and changes color?

Answer 26

Detector tube types. (Ex. Gastec, Draeger).

Question 27

Name two liquid type flue gas analyzers:

Answer 27

Orsate, and Fyrite.

Question 28

What are three conditions that will reduce the efficiency of the heating equipment and increase the temperature of the flue gas?

Answer 28

Over firing of the burner, high draft, and dirty heat exchangers.