Reheater in steam plant with layout

A reheater is a heat exchanger in the boiler that reheats steam coming from the HP turbine at constant intermediate pressure before it enters the LP turbine.

Layout

               1
  +----------+----->+------+     +------+
  |  Boiler  |      |  HP  |=====|  LP  |===> Generator
  |  + S/H   |  2   | turb |     | turb |
  | Reheater |<-----+------+  +->+------+
  |          |------------3---+     | 4
  +----------+                      v
       ^ 6                    +-----------+
       |      +------+   5    | Condenser |
       +------| Pump |<-------+-----------+
              +------+

Working

1. Feed pump (5-6) sends water to the boiler where it is heated, boiled and superheated (6-1).
2. Steam expands in the HP turbine (1-2) to about 20-25% of boiler pressure.
3. It returns to the reheater and is heated at constant pressure to almost the initial temperature (2-3).
4. Reheated steam expands in the LP turbine to condenser pressure (3-4); the condensate is pumped back (4-5).

How reheating increases output and efficiency

• Output: each kg of steam now does work $(h_1-h_2) + (h_3-h_4)$, which is larger than $(h_1 - h_{4'})$ without reheat, because constant-pressure lines diverge on the h-s chart. Steam consumption per kWh falls, so for the same boiler the plant output rises.
• Efficiency: heat in the reheater is added at a high mean temperature, so the cycle's mean temperature of heat addition rises (gain about 4-5%).
• Dryness: exhaust steam becomes much drier (about 0.9 or more), reducing blade erosion and wetness loss, so turbine internal efficiency improves.
• Allows higher boiler pressures without excess moisture at exhaust.

$$\eta = \frac{(h_1 - h_2) + (h_3 - h_4) - W_p}{(h_1 - h_6) + (h_3 - h_2)}$$

T-s diagram

  T
  ^                  1          3
  |                 /|         /|
  |                / |        / |
  |     f--------g'  |       /  |
  |    /             2------'   |
  |   /                         |
  |  6                          |
  |  5--------------------------4
  +--------------------------------> s
  5-6 pump, 6-f-g'-1 boiler (heat,
  boil, superheat), 1-2 HP turbine,
  2-3 reheater, 3-4 LP turbine,
  4-5 condenser