8.7 Thermal design
The determination of the surface area of a condensing type LP heater is similar to the method already outlined in Section 6 of this chapter, which refers to the thermal design of the HP heater condensing section.
Although the higher pressure LP heaters have some superheat in the bled-steam, it is ignored for the purpose of calculating the LMTD. Using the equation given in Section 6 of this chapter, the heat transfer surface is determined.
Baffle pitch is again determined by the need to distribute the steam throughout the heater and is usually close to the maximum allowable without risk of damage to the tubes due to flow-induced vibration.
The U-tube length is found from tube outer surface area/metre length, number of tubes and the surface needed.
The need to vent the heaters to prevent the heat transfer process being reduced in efficiency due to the presence of non-condensable gases has already been discussed for HP heaters. It is just as important to have the LP heater adequately vented: this may be more difficult in some cases than with HP heaters, as some LP heaters work under vacuum and the vents have to cope with any slight air inleakage which may occur local to the heater. For the heaters illustrated in Figs 3.60 and 3.61, the air is extracted from the centre of the nest for its full length to ensure that no pockets of air can be trapped between the baffles.