The starting point for determining gas and steam temperature profiles and steam generation is the assumption of pinch and approach points, as discussed above. The values that are known are gas flow rate (Wg, gas temperature at HRSG inlet ( t g1), feed water temperature (tw1), temperature of steam leaving the superheater (ts2), and steam pressure (Ps). Assuming a reasonable pressure drop in the superheater, we can determine the saturation temperature ( t s) at the evaporator.
Once the pinch point is selected, we know the temperature of the gas leaving the evaporator ( t g3) and the approach point gives the temperature of the water leaving the economizer (tw2), since the saturation temperature is known. The heat loss (hl) ranges from 2% in small HRSGs to about 0.5% in large units. Methods of estimating heat losses are outlined elsewhere (2).
Considering the energy balance across the superheater and evaporator (Figure 2), the energy absorbed by the superheater and evaporator is given by: Q1.2 = WgCpg( tg1 - tg3) (hl) = Wsd[(hs2 - hw2) + (bd)(h f- h w2)] (1) Since tg 1 and tg3 are known, Q1 2 can
be computed and the design steam flow (Wsd) can be determined. The superheater duty is: Q1 = Wsd(h s2 - h v) = Wg Cp g ( tg l - tg2) (hl) (2)From above, the temperature of the gas leaving the superheater ( t g2) can be determined, since all the other data are known.
The economizer energy balance gives: Q3 = Wsd(hw2 - hw1 )(1 + bd) = WgCpg( tg3 - tg4 ) (hl) (3) The gas temperature leaving the
economizer ( t g4) can be obtained from this. Thus, the complete gas/steam profiles and steam generation rate for the design case can be determined by assuming the pinch and approach points. In addition, once the pinch and approach points are selected, the logmean temperature differences (DT) at the various surfaces are fixed. Since from basic heat-transfer principles surface area is given by S = QIUAT, the surface areas of all the components, such as the superheater, evaporator, and economizer, are fixed once U is computed. (To calculate U one should have such mechanical data as tube size, fin density, tube pitch, etc.) But if U is not known, US is, which indirectly fixes the surface areas.
Now, if we want to know how the HRSG behaves at different gas conditions, we have to perform off-design calculations and use the "surface areas" we have indirectly established. It may also be noted that we are using a pinch point of about 15-20°F, which results in a low AT in the evap orator and thus the need for large surface area. (The pinch point in a conventional steam generator could range from 150°F to 400°F, so the DT is much higher and the required surface
area much less.) This is why extended surfaces are a must in HRSGs.
Once the pinch point is selected, we know the temperature of the gas leaving the evaporator ( t g3) and the approach point gives the temperature of the water leaving the economizer (tw2), since the saturation temperature is known. The heat loss (hl) ranges from 2% in small HRSGs to about 0.5% in large units. Methods of estimating heat losses are outlined elsewhere (2).
Considering the energy balance across the superheater and evaporator (Figure 2), the energy absorbed by the superheater and evaporator is given by: Q1.2 = WgCpg( tg1 - tg3) (hl) = Wsd[(hs2 - hw2) + (bd)(h f- h w2)] (1) Since tg 1 and tg3 are known, Q1 2 can
be computed and the design steam flow (Wsd) can be determined. The superheater duty is: Q1 = Wsd(h s2 - h v) = Wg Cp g ( tg l - tg2) (hl) (2)From above, the temperature of the gas leaving the superheater ( t g2) can be determined, since all the other data are known.
The economizer energy balance gives: Q3 = Wsd(hw2 - hw1 )(1 + bd) = WgCpg( tg3 - tg4 ) (hl) (3) The gas temperature leaving the
economizer ( t g4) can be obtained from this. Thus, the complete gas/steam profiles and steam generation rate for the design case can be determined by assuming the pinch and approach points. In addition, once the pinch and approach points are selected, the logmean temperature differences (DT) at the various surfaces are fixed. Since from basic heat-transfer principles surface area is given by S = QIUAT, the surface areas of all the components, such as the superheater, evaporator, and economizer, are fixed once U is computed. (To calculate U one should have such mechanical data as tube size, fin density, tube pitch, etc.) But if U is not known, US is, which indirectly fixes the surface areas.
Now, if we want to know how the HRSG behaves at different gas conditions, we have to perform off-design calculations and use the "surface areas" we have indirectly established. It may also be noted that we are using a pinch point of about 15-20°F, which results in a low AT in the evap orator and thus the need for large surface area. (The pinch point in a conventional steam generator could range from 150°F to 400°F, so the DT is much higher and the required surface
area much less.) This is why extended surfaces are a must in HRSGs.