The pressure drop before and after the furnace of a steam generator are 0.55 m a

Question

The pressure drop before and after the furnace of a steam generator are 0.55 m and 1.15 m water, respectively. The steam generator operates with both forced and induced draft fans. Calculate the total power consumed, in megawatt and horsepower, by forced- and induced draft fans if the fuel flow rate is 120 kg/sec with an air-to-fuel ratio of 20. Assume leakage of 1.2 % into the steam generator. Consider the air temperature is 300 K and the gases temperature is 460 K. The FD and ID fans efficiencies are 0.82 and 0.72 %

Explanation / Answer

Pressure drop between furnace of steam generator befor and after = 1.55 - 0.55 m of water

total power consumed = P

mass flow rate = 120 kg/sec air /fuel ratio = 20

leakage = 1.2 %

air temperaure = 300 k

Gases temperature = 460 K

EFFICIENCY OF FD FAN = 0.82

EFFICIENCY OF ID FAN = 0.72

Total combustion Air, Kg/sec = 2400 kg /sec
= 2400/(1.17 ) m3/s :Air density-1.17kg/m3
= 28.08 m3/s

Primary Air , 70% of total , m3/s = 0.7 x 28.08
= 19.6 m3/s

Take 20% margin on discharge capacity. So FD Fan flow is 1.2 x 19.6 = 13.75 m3/s

Head required = Draft loss across Air Heater
= 550 mmW
Take 15-20 % margin on head. So FD fan head should be about 632.5 mm of WC.

Therefore, FD fan rating is 19.6 m3/s of air at 632.5 mm WC static head.

Power requirements of FD Fan :
Fan efficiency as 82%
Power required for FD Fan, BHP = Flow x Head / ( Efficiency x 75.8 )
= 19.6 x 632.5 / ( 0.82 x 75.8 )
= 199 HP

= 15 MW

Total wet gases, Kg/sec = 120 kg /sec

Gas Density = 1.3265 Kg/Nm3
Therefore, gas flow in Nm3/hr = 79,228 / 1.3265
= 59227 Nm3/hr
= 16.6 Nm3/s

Gas flow at 150C in m3/s = 16.6 x (460)/300 = 25.45
ID Fan capacity taking 20% margin on flow = 25.45 x 1.2
= 30.5 m3/s

ID Fan static Head = Draft Loss in (Boiler
= 1150mm WC : Approximate
= 1150 mmWC

Taking 20% margin on head, ID Fan head = 1150 * 1.2 =1380 mm WC

Power requirements of ID Fan :
Let us assume Fan efficiency as 75%
Power required for ID Fan, BHP = Flow x Head / ( Efficiency x 75.8 )
= 30.5 x 1380 / ( 0.72 x 75.8 )
= 771 HP

P =58.458 MW

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