Chapter 13 Book 1 Combustion And Draft

Question 1

Natural draft

Answer 1

This occurs due to. Pressure diff between the hot glue gas and the cold I take gas. The only way to increase pressure diff is to increase stack height or increase flue gas by increasing the firing rate. Since increasing the stack is not a option the firing rate is pretty much how you can control it. Sometime there are damper in the stacks to help control it a bit

Question 2

Forced draft fan

Answer 2

Crests a positive press in the furnace and flue gas passes. Is much cheaper since it only handles cooler outside air and handles a smaller volume of air then the induced draft fan.

Question 3

Induced draft fan

Answer 3

The air needed for combustion is drawn up through the tubes and through the fan then out the stack. These are used less then forced draft fans. The pressure in the furnace is below atmosphere due to the fan drawing air instead of pushing it in.

Question 4

Balanced draft

Answer 4

Is a combo is ID and FD fans usually keeps a slight negative press in the furnace. When furnace press is maintains at a constant regardless of firing rate then balanced draft is used

Question 5

General knowledge

Answer 5

If outside make up air is not used a negative air pressure can occur and starve the burner. A min amount of air needs to be supplied on top of what is supplied for human comfort. This is important as incomplete combustion will lead to sooting, plugging of gas passages, poor heat transfer and possibly furnace explosion.

Two methods of measuring draft in a furnace is include draft gauge which is very accurate in measuring small pressure diffs and the U tube gauge which measure the furnace press against atmosphere and then takes the diff which is your draft number

Question 6

Coal

Answer 6

Is a solid fuel, components of coal that burn are carbon sulfur and hydrogen. The carbon and hydrogen combine to make a hydrocarbon while the sulfur is actually undesirable due to the fact it makes sulfur dioxide when burned which corrodes the stack. Any excess hydrogen that is not a hydrocarbon is considered fixed hydrogen.

Question 7

Fuel oils

Answer 7

Derived from petroleum products. They are refined by fractional distillation to separate then into groups using there boiling point, specific gravity, and viscosity. Fuel oils are made mostly of hydrocarbons. And are classed according to certain characteristics like relative density, viscosity, flash point, fire point. Grade 1 and two are fuel oils with grade 2 being most popular. Grade 5 is heavy crude. Advantages of fuel oil over coal are less storage space, more easy to control feed into furnace, less handling equipment needed, burns cleaner and more efficiently. Main disadvantage is the cost and availability compared to coal

Question 8

Natural gas

Answer 8

Main components in natural gas are 85 percent methane and 15 percent ethene it is very clean and has no spills no ash produced when burned but is more expensive then coal and may be more expensive the fuel oils

Question 9

Diff in complete and proper combustion

Answer 9

Complete combustion is when some of the gas burns as just CO and not CO2 proper and complete combustion is when the gas mixes properly first to make CO2 then burns completely