Steam plant components and reheating

Main components of a steam turbine power plant

A steam (thermal) power plant works on the Rankine cycle. Its main parts are:

                superheated steam (1)
  +---------------+ -------------> +---------+   +-----+
  | Boiler        |                | Steam   |===| Gen |
  | (furnace,     |                | turbine |   +-----+
  |  economiser,  |                +---------+
  |  superheater) |                     | exhaust (2)
  +---------------+                     v
     ^   ^   |                    +-----------+
 fuel| air|  v flue gas           | Condenser |<--cooling
     |   |  chimney               +-----------+   water
     |                                  | condensate (3)
     |  feed water (4)  +-----------+   |
     +------------------| Feed pump |<--+
                        +-----------+

• Boiler (steam generator): burns coal/oil/gas and converts feed water into high-pressure steam. Includes furnace, water walls, drum, superheater, reheater, economiser (heats feed water by flue gas) and air preheater.
• Steam turbine: expands steam and converts its heat energy into shaft work; drives the alternator (generator).
• Condenser: condenses exhaust steam under vacuum using cooling water; the low back pressure increases turbine work. Cooling water is cooled in a cooling tower or taken from a river.
• Feed pump (with condensate pump): returns the condensate to boiler pressure, usually through feed water heaters (regeneration).
• Fuel and ash handling, draught system (FD/ID fans), chimney, ESP/dust collector.
• Water treatment plant for make-up water.

How reheating increases efficiency

In a reheat cycle the steam leaving the boiler (state 1) expands in the HP turbine only up to an intermediate pressure (state 2). It is taken back to the reheater in the boiler and heated at constant pressure to nearly the original temperature (state 3). It then expands in the LP turbine to condenser pressure (state 4).

How reheating increases efficiency and output:

• Higher mean temperature of heat addition: heat added in reheating (2-3) is at a high temperature. If the reheat pressure is chosen properly (about 20-25% of boiler pressure), the average temperature at which heat is supplied rises, so $\eta$ rises by about 4-5%.
• More work per kg of steam: the expansion 3-4 lies to the right of the original expansion line, so the specific work output increases. The steam consumption (kg/kWh) falls and the plant output for the same boiler size increases.
• Drier exhaust steam: without reheat, expansion from high pressure ends in very wet steam (dryness below 0.85). Reheat gives exhaust dryness about 0.9 or more, which reduces blade erosion and friction loss, so the turbine internal efficiency improves.
• It makes very high boiler pressures usable, which is itself a means of raising efficiency.

Cycle efficiency with reheat:

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

Limitations: extra cost of reheater, piping and controls; useful only for large units (above about 100 MW) and high pressures.

Layout and T-s diagram of reheat cycle

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

  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