6.6 Thermal design
Earlier in this section it was explained how the tem¬perature of the high pressure feedwater varies as it passes through an HP heater.
The heater temperature limits, combined with the physical size of tube bundle diameter, shell diameter, tube size, etc., are the factors which determine the heat transfer surface area of a heater.
Figure 3.51 (a) shows typical heat balance informa¬tion for the ultimate stages of feedheating for a 660 MW unit. Also shown, is the associated temperature profile for the ultimate stage of feedheating (Fig 3.51 (c)). The basic equation used to find the surface area (A) for each section is A = (qmh)/K(LMTD):
Figure 3.51 (a) and (b) illustrate how the heat balance information is used to find the enthalpy rise through each section in turn and the resultant tem¬perature profile.
From this information, the surface area for each section is calculated in the manner described below.