Hot Casing Design

Whenever hot gases are contained in an internally refractory-lined (or insulated) duct, the casing temperature can fall below the acid dew point in cold weather. The hot gases can seep through the refractory cracks over a period of time and can cause acid condensation with its associated problems. Some engineers prefer what is called a hot casing whereby the duct casing is made hot (above the acid dew point) by externally insulating the duct (Figure 6.5). This lowers the heat losses also. However, if calculations are not properly done and a thicker-than-required external insulation is used, the duct temperature can be made significantly hot and can cause expansion problems. If the gas temperature varies along the gas path (say, a boiler), then higher thicknesses have to be applied externally as the gas temperature reduces in order to keep the casing temperature within a range of temperature.

Example 6.9

A waste heat boiler has a gas inlet temperature of 704°C and cools to 250°C. The casing has 125 mm of Greencast 22 refractory followed by 50 mm of castable block mix refractory. The casing temperature based on 27°C ambient and zero wind velocity is estimated as 78°C as shown later. Since this is below the sulfuric acid dew point temperature of 120°C, external insulation should be applied to raise the casing temperature. The results from a computer program are shown in Table 6.3 for various cases. Let the acid dew point be 130 °C. (For simplicity and to illustrate the point, gas temperature is assumed to be the hot face temperature.)

In case 1, the hot gases are contained in a refractory-lined casing. The hot layer of refractory is Castolite 22. K ranges from 0.245 to 0.324 W/m K from 200°C to 871°C. Refractory 2 is castable block mix, and K ranges from 0.082 to 0.121 W/m K from 93°C to 427°C. Mineral fiber insulation K ranges from 0.052 to 0.095 W/m K from 93°C to 427°C.

Hot casing arrangement.

Miscellaneous Boiler Calculations 323

TABLE 6.3

Summary of Refractory Calculations

Case 3 Case 4 Layer

Case 1

Case 2

Temp. In–Out

Temp. In–Out Temp. In–Out

250°C–196°C 250°C–176°C Refractory 2

Refractory 1

700°C–411°C

700°C–459°C

196°C–134°C 176°C–89°C Min fiber

411°C–78°C

459°C–204°C

204°C–72°C

134°C–38°C 89°C–42°C

0.150 Fiber thickness, mm

Heat loss, kW/m 2 0.640

0 12 38 12 Note: Refractory 1: Castolite 22. Refractory 2: Castable block mix.

TABLE 6.4

Heat Loss Equation

SI Units British Units

q = 5.67 × 10 −11 × ∈ (T s 4 − T a 4 ) + 0.00195(T s −T a ) 1.25 ×

Q = 0.1714 × 10 −8 × ∈ (T s 4 −T a 4 ) + 0.296(T s −T a ) 1.25 ×

{(V + 21)/21} 0.5 {(V + 69)/69) 0.5 q is the heat loss, kW/m 2 Btu/ft 2 h

T s is the hot face temperature, K

°R

T a is the cold face temperature, K

°R

V is the wind velocity, m/min

ft/min

∈ is the emissivity

It can be seen that when we have only the two layers of refractory inside the casing, the casing temperature drops to 78°C. This may cause water and acid condensation if some flue gases seep through the refractory. When we add 12 mm insulation outside the casing, the casing temperature increases to 204°C, and heat loss is kept low as the casing temperature of the insulation is only 72°C. Thus, casing corrosion concerns are minimized. As the gas cools, we note that we have to use a higher external insulation thickness to keep the casing hot. If we use say 12 mm thick insulation when the gases are at 250°C (case 4), the casing temperature drops to 89°C and below the acid dew point temperature. If we use a higher thickness of 38 mm (case 3), the casing temperature increases to 134°C while the heat loss is also minimized.

The other option of minimizing corrosion is to use membrane wall casing; in this case, the tube wall temperature is close to the saturation steam temperature and always above the dew point. External mineral fiber insulation of say 50–75 mm will be adequate. The choice is left to the boiler supplier. Membrane wall casing may be more expensive for some boiler suppliers.

Table 6.4 shows the heat loss equations in both SI and British units.

Hot Casing Design