8.2 Surface type low pressure heaters
Because LP heater extraction points are normally on the LP turbine cylinders, the superheat (even on the highest pressure LP heater) does not justify the provision of desuperheating sections within the heaters.
Drain cooling sections could be provided but the complication and cost of a drains level control system can seldom be justified. It is usual practice to have the LP heaters constructed as condensing-only heaters, and to provide a drain cooler upstream of the lowest pressure heater to recover some of the heat in the drains.
The drain cooler used is usually of the 'flashing' type. A flashing drain cooler is a separate heater placed in the feed train upstream of the lowest pressure LP heater. It is a small heater with no bled-steam supply but has a chamber at one end where the combined LP drains are flashed into steam and water. The steam is condensed on the heater tubes and the drains are discharged to the condenser via a loop seal.
To reduce the water burden of the steam flowing to the last row of LP turbine blades, a special purpose heater called the turbine moisture extraction condenser (TMEC) is provided. The TMEC is designed to extract the steam/water mixture from the water extraction belt situated immediately before the last row of LP turbine blades. Modern TMECs, if required, are provided in the form of a multi-compartment LP surface type heater. Each compartment is connected to a low pressure cylinder. This prevents interconnection of cylinders and consequent difficulties caused by recirculation of a water/steam mixture between cylinders.
As the construction of a TMEC is so similar to an LP heater, its detailed construction is not described.
The steam inlet size of an LP heater is large compared with the heater body size due to the rapid increase in the specific volume of the steam as the pressure decreases. On some of the lower pressure heaters, this results in multiple inlets with steam belts to keep the inlet velocity to the tube bundle to an acceptable value.
A current development in LP heater design, which has not yet reached service, is to place the two lowest pressure LP heaters in the condenser neck. Figure 3.59 shows this diagrammatically. The heaters are of the duplex type (i.e., LP1 and LP2 combined) with gravity drains into the condenser. The general construction is similar to other LP surface type heaters. The main advantage of this arrangement is the saving in turbine hall space. Small savings are also made due to elimination of the pressure drop associated with the bled-steam isolating and non-return valves. The pressure drop in the bled-steam pipework is also virtually eliminated.