Watertube and Firetube Boilers page 31 - The Practical Knowledge ...

Watertube and Firetube Boilers page 31
101 PRACTICAL KNOWLEDGE OF STEAM BOILERS THIRD EDITION
Question
How many types of tubes are there in a watertube boiler?
Answer

There are six basic types:
1. Downcomers: These tubes have large diameters of which main function is to circulate water from
the steam drum to mud drum or any other collecting bottom headers. The downcomers are normally
placed outside the boiler casing so that it cannot absorb heat. Fresh boiler feed water flows through
downcomers by gravitation and introducing heat to the downcomers will cause reversal of flow and
upset the boiler natural circulation.
2. Generating tubes: These tubes form a large bank of heater exchangers and located at the
convective bank of the boiler. The heat transfer occurs through convection. The tubes form largest
heating surface thus the highest proportion of steam is generated in the generating tubes.
3. Risers: return steam and water from the top waterwall headers to the steam drum.
4. Waterwall tubes: these tubes protect the boiler sidewall refractory from the direct heat of
combustion and generate a small amount of steam. There are many types of waterwall tubes
construction.
5. Superheater: consists of small-diameter tubes (OD is in the range of 32~51 mm and thickness in
the range of 3~7 mm), and receives saturated steam from the boiler proper and superheat the steam.
6. Screen tubes: protect the convective superheaters from direct radiant heat from the furnace. The
screen tubes consist of two to three staggered rows of tubes, which are usually connected from the
steam drum to the water drum.

Watertube and Firetube Boilers page 32
101 PRACTICAL KNOWLEDGE OF STEAM BOILERS THIRD EDITION
Question
What is a bent-tube boiler?
Answer
Normally, the bent-tube boiler has several drums (multi-drum). It is more flexible because it may enter the
drum radially. The bent-tubes allow free expansion and contraction. It is also a rapid steamer and response
to fluctuating load is small because of the relatively small volume for the generating capacity. The gas
baffles are arranged in many different patterns.
Question
What is the weakest area (in regards to construction) of a bent tube water-tube boiler?
Answer
All boiler pressure parts are built to last for a long time. All parts are carefully calculated and high factor of
safety was considered in the design.
The weakest however, in regards to construction, is the area where the tube is bent at the largest bending
radius. The tube thickness decreases at the outer tube surface but thicker at the inner as shown in the
example below.

Question
The firetube boilers are normally restricted to 250 psig. Elaborate the reasons.
Answer
Required thickness of a shell or a tube is a function of the radius. Therefore, as the tube or shell gets larger,
the thickness has to be increased to withstand the pressure. In order to contain high pressure, the shell has
to be very big and very thick, which may be heavier and more expensive. The mobility of a very large boiler
shell is also difficult.
The formula to calculate the required drum thickness is:
Where t = minimum required thickness
P = MAWP
S = maximum stress
t = ___PD___
2S + 2yP