Boiler

Question 1

major functions of an economizer

Answer 1

✧ to reduce exhaust gas temperature
✧ to raise overall boiler thermal efficiency
✧ to reduce fuel consumed
✧ to work as heating surface or an evaporting surface

Question 2

3 types of superheater

Answer 2

✧ radiant superheater
✧ semi radiant superheater
✧ convective superheater

Question 3

radiant superheater

Answer 3

✧disposed in furnace wall; in configuration similar to water-cooled wall
✧ receives radiation from the furnace

Question 4

semi radiant superheater

Answer 4

✧ receives radiation from the furnace and convection heat from flue gas
✧ located @ upper portion of furnace

Question 5

convective superheater

Answer 5

✧ located @ horizontal pass right downstream on the furnace exit, where high temp gas pass.
✧ receives heat from flue gas. primarily by convection, secondarily by radiation

Question 6

types of water cooled wall

Answer 6

2 :
✧ plain tube
✧ membrane

Question 7

plain tube type of water cooled wall

Answer 7

consists of row of tubes in the inner side of furnace wall, by joining together drum(s) and headers

Question 8

membrane type of water cooled wall

Answer 8

panel by joining together finned tubes or plain tubes through flat steel bars

Question 9

membrane water cooled wall

Answer 9

✧ most boiler furnace have all water coole membrane wall
✧ this is to reduce the maintenance of furnace wall and reduce the temp of gas entering the convection bank, to the point where slag deposition and superheater corrosion can be controlled by sootblowing.

Question 10

classification of economizer

Answer 10

2 :
✧ by material (can be classified into
cast iron type & steel tube type)
✧ by outlet fluid state (can be classified into steaming type & non steaming type)

Question 11

most used

Answer 11

seamless steel tube

Question 12

cast iron type (advantage)

Answer 12

corrosion and erosion resistance

Question 13

cast iron type (disadvantages)

Answer 13

✧ thick in wall tubes, large in volume and weight
✧ water weeping and sweating are prone to occur due to large quantity of bends
✧ ash deposition and clogging easily occur between fins
✧ can only be non-steaming type, if otherwise, water hammer may occur and result in the damage of the economizer

Question 14

functions and configurations of superheater

Answer 14

• to further heat the saturated steam to desired temperature.

Question 15

function of water cooled wall

Answer 15

✧ to absorb heat energy from flame in furnace
✧ to protect boiler setting

Question 16

slagging (concept)

Answer 16

• the accumulation of fused ash deposits on the heat transfer surface

Question 17

slagging (location)

Answer 17

• mainly occur on furnace wall & high temperature convection heating surface; such as screen tube and leading tube of superheater.

Question 18

slagging (reason)

Answer 18

• ash particles entrained in flue gases, which are fused and/or partly fused, are cooled and resolidified when they impinge on furnace wall, water-cooled wall, or tubes.

Question 19

corrosion temperature

Answer 19

by the temperature of corrosion takes place, corrosion can be classified into 2 types:

✧ high temperature corrosion
✧ low temperature corrosion

Question 20

high temperature corrosion

Answer 20

mainly refers to the gas side corrosion of furnace water cooled wall tubes and the external corrosion of superheaters

Question 21

low temperature corrosion

Answer 21

mainly refers to the corrosion of cold end of air heater

Question 22

for low pressure boiler

Answer 22

refers to corrosion taking place on economizer

Question 23

measures of preventing and mitigating water cooled wall corrosion

Answer 23

6
✧ to remove sulfur directly from fuel
✧ to reduce the production of H2S (hydrogen sulfide) and SO3 (sulfur trioxide) and to lower their concentration as much as possible
✧ to reduce the tube metals temperature
✧ to improve the surrounding atmosphere
✧ to employ corrosion resistance material
✧ a stream of air is injected over the wall into the region where corrosion is apt to occur in order to form protective air screen.

Question 24

4 types of boiler

Answer 24

✧ natural circulation boiler
✧ forced circulation boiler
✧ once through boiler
✧ once through with part-load circulation boiler