Question

In a cogeneration plant, the power load is 5.6 MW and the heating load is 1.163 MW. Steam is generated at 40 bar and 500°C and is expanded isentropically through a turbine to a condenser at 0.06 bar. The heating load is supplied by extracting steam from the turbine at 2 bar which condensed in the process heater to saturated liquid at 2 bar and then pumped back to the boiler. Compute (a) the steam generation capacity of the boiler in tonnes/h, (b) the heat input to the boiler in MW, and (c) the heat rejected to the condenser in MW.

Answer 1

From steam table at 40 bar 500°C

h₁ = 3445.3 kJ/kg

s₁ = 7.090 kJ/kg

If mass flow rate at ‘1’ of steam is w kg/s the

w (1257.5 – 508m) = 5600  ...(i)

w_m × 2187.1 = 1163 ...(ii)

From (i) & (ii) w = 4.668 kg/s = 16.805 Ton/h

∴ m = 0.11391 kg/kg of the generation

(a) Steam generation capacity of boiler = 16.805 t/h

(b) Heat input to the boiler

= W [(1 – m) (h₁ – h₅) + m (h₁ – h₇)] 

= 15.169 MW

(c) Heat rejection to the condenser 

= (1 – m) (h₃ – h₄) 

= 8.406 MW