6.6.1   Desuperheating section

 

The bled-steam has a high degree of superheat which allows the feedwater at the exit of the desuperheating section to approach very near to, or to be slightly above, the saturation temperature equivalent of the pressure at inlet to the desuperheating section.

To prevent erosion due to wet steam flowing through the exit of the desuperheating section it is established practice to retain about 28°C of superheat at the end of the desuperheating section, so T2 is made equal to the saturation temperature equivalent to the pressure in the condensing section plus 28°C. Figure 3.51 (c) is based on this assumption.

The temperatures in the desuperheating section are used to evaluate the LMTD as shown in Fig 3.51 (d), so the only unknown in the equation to find the surface area is the heat transfer coefficient.

The methods of calculating the heat transfer coefficient for a particular tube bundle are given in detail in BEAMA 1968 [13] or ТЕМА 1978 [12]: for a typical desuperheater in the last stage HP heater its value is about 500-600 W/m2K.

The heat transfer coefficient increases with the increased velocity across the tube bundle. The higher the velocity, the greater the pressure drop across the desuperheating section, which in turn reduces the saturation temperature within the condensing section.

The heat exchanger designer has to reach a cost effective compromise between increased condensing surface area against decreased desuperheating surface area, the velocity being dependent on the pitch of the baffles in the desuperheating section for a given tube pattern. Once an acceptable compromise has been reached, a check can be made to ensure that the pitch chosen is equal to or less than that allowed by consideration of tube damage due to flow-induced vibration (as described earlier in this section).

The surface area can now be calculated. From the area, number of tubes and surface area/metre length of tube, the length of the desuperheating section is found. The baffle pitch may need slight readjustment to meet the overall length requirement.

 

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