Answer:
net boiler heat = 301.94 kW
Explanation:
given data 
saturated steam = 6.0 bars
temperature = 18°C
flow rate = 115 m³/h = 0.03194 m³/s
heat use by boiler = 90 %
to find out
rate of heat does the boiler output
solution
we can say saturated steam is produce at 6 bar from liquid water 18°C
we know at 6 bar from steam table 
hg = 2756 kJ/kg
and
enthalpy of water at 18°C
hf = 75.64 kJ/kg
so heat required for 1 kg is 
=hg - hf
= 2680.36 kJ/kg 
and
from steam table specific volume of saturated steam at 6 bar is 0.315 m³/kg 
so here mass flow rate is 
mass flow rate = 
mass flow rate m = 0.10139 kg/s
so heat required is 
H = h × m  
here h is heat required and m is mass flow rate 
 H = 2680.36  × 0.10139
 H =  271.75 kJ/s = 271.75 kW
now 90 % of boiler heat is used for generate saturated stream 
so net boiler heat = 
net boiler heat = 
net boiler heat = 301.94 kW