Steam Pressure

Steam Pressure

Steam pressure also affects the furnace size and, of course, the boiler arrangement. If the pressure is low, then the latent heat of steam is high, and hence, the furnace and evaporator heating surface will be larger compared to the case where steam pressure is very high as in utility boilers. Figure 2.5a shows the distribution of energy as a function of steam pres- sure. Since the total sensible or liquid plus latent heat of steam is absorbed in the econo- mizer, evaporator tubes, and furnace, proper proportioning of duty is a must among these surfaces. Very high-pressure boilers may have simply a furnace followed by superheaters and an economizer as the latent heat is small (Figure 2.5b), while low-pressure boilers will have a furnace, a large evaporator (sometimes called a boiler bank), and an economizer (Figure 2.5c and d). To minimize furnace size, the evaporator surface is sometimes added in the furnace itself as water platens.

Figure 2.5b shows a steam generator for low- to medium-pressure steam generation (1500–5000 kPa). A large evaporator in the form of boiler bank tubes is used. The latent heat is absorbed in the furnace and in the bank tubes, which is a three-pass evaporator.

S team G

en er

125 psig, sat.

14 86 Heating

at or sa

650 psig, 750°F

20 64 16 Industrial

Sensible heat

dW n

Latent heat

a ste H

1500 psig, 880°F

32 46 22 Large industrial

Super heat

2620 psig, 1000°F/1000°F

:F (reheat) rs or P

26 23 35 16 Utility

(a)

ro

ce ss a

n dP

la n tE n g in

ee rs

S team G

Steam drum

Hanger tube

F.S.H

P.S.H–II low

Gas f

P.S.H–I

Air Heater

Air from

FD fan

(b)

General arrangement of high pressure and temperature oil & gas fired boiler

FIGURE 2.5

(a) Distribution of energy in boilers as a function of steam pressure. (b) High-pressure steam generator. (Courtesy of ISGEC Heavy Engineering Ltd, India.)

(Continued)

Steam drum

LHS

RHS

Design parameters:

03. Super heater outlet pressure 02. Design temperature 01. Design pressure

: 320 °C : 77.0 kg/cm (g)

team G

05. Total evaporation 04. Super heater outlet temperature : 500 °C Side wall

Main steam

: 67 kg/cm 2 (g)

: 25000 kg/hr Top HDR.

Brief description of boiler:

at or sa

Natural circulation, Bi–Drum, bottom supported,

LRSB C L of

membrane type, fluidised bed combustion boiler.

dW

man hole & C L of

peep hole

Water drum Eco.outlet HDR.

EW CL of bo iler

:F

or P

C L of over

C L fire air at

of over

L.H.S & R.H.S fire air at

ECO.coil Tube

ro

L.H.S & R.H.S

ce ss a

CL of spreader

man hole & L C of

CL of manhole

dP

peep hole

& peep holes

la n

Fu ture

Fu ture

ture Fu

View–E

evaporator

Bed evaporator

View–A

(c)

rs

S team G

Steam drum

S.S.H

Bank tubes

P.S.H

Air preheater–II

to windbox Hot air

Air preheater–I

Air from FD fan

Mud drum

Hot air from air heater

(d)

General arrangement of medium pressure and temperature oil & gas fired boiler

FIGURE 2.5 (Continued)

(c) Steam generator for medium-pressure steam. (Courtesy of Thermodyne Technologies, Chennai, India.) (d) Low-pressure steam generator with large evaporator. (Courtesy ISGEC Heavy Engineering Ltd, India.)

52 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

The liquid heat or the energy absorbed in the economizer is not much compared to high- pressure boilers, and hence, it is small.

Figure 2.5c shows a steam generator for a high-pressure boiler (11,000 kPa). The liquid heat (or the energy absorbed in the economizer) is large, and hence, we see a large econo- mizer. The latent heat is small, and hence, we have only the furnace to absorb the latent heat and a large superheater for the superheat. Since the design pressure is rather high, an external steam drum is used with high ligament efficiency in order to reduce its thickness. Locating the drum up above also helps in improving circulation through the furnace tubes.