6.6.4 Other factors affecting thermal design
The total length of the heater U-tube is the sum of the lengths of the three sections. This length is taken as the mean effective length of a U-tube in determining the heater shell length.
The baffles within the heater shell, which form the compartments through which the steam or condensate flow, have windows so that the fluid can pass longitudinally. The area used is equal to, or greater than, that available for cross-flow.
From the references given to find the heat transfer coefficients , it will be seen that the theoretical values are modified to allow for leakage through the tube/baffle plate clearances, lack of true cross-flow and the stagnant areas in the corners of the compartments formed by the baffles.
The heat transfer coefficients obtained by using these references, assume that the condensing section within the heater is adequately vented to the condenser to avoid the accumulation of air or non-condensable gases within the section. The heat transfer coefficient used to determine surface area is significantly influenced by non-condensables which tend to form a gaseous muff around the condensate film on the outside diameter of the tube. The slowing of the diffusion of vapour into the condensate by the non-condensables, significantly affects the condensing section performance. This is a most important factor in heater performance, and an unexpectedly poor performance from a new heater can be usually traced to incorrect air venting. The usual vent rate is about 0.5% of the bled-steam flow to the heater.
Accumulation of air can also occur in the drain cooling section and cause pockets in the corners local to the baffle plates. This reduces the available heat transfer surface which again reduces heater performance.
For an overview of the complete thermal/mechanical design of HP heaters, reference should be made to the Heat Exchanger Design Handbook 1984 .
Should cycle economics dictate a low first-cost feed system, then HP heaters can be used with no drain cooling section and/or no desuperheating section. Where no desuperheating section is provided, a small section local to the steam inlet is baffled to help to absorb the superheat, but is not allowed for in the calculation of the surface area.
The omission of the drain cooling section means that the drains are discharged from the heater at saturation temperature and there is no need for a fixed water level in the heater shell to protect the drain cooling section from erosion. The design procedures to determine the surface area of each section is the same whatever combination of sections is used.
Vertical-attitude heaters differ slightly in the internal arrangement of components but use the same design principles for mechanical and thermal design.