What is the impact of heat loss on the exit temperature of steam in a turbine?

[rhyso88]

A steam turbine in a power plant accepts 4500 kg/hr of steam at 60 bar and 500°C and exhausts steam at 10 bar. Heat transfer to the surroundings (Tsurr = 300K) at a rate of 70 kW.

(a) What condition needs to be satisfied for the turbine to generate the maximum possible power?

I said process must be isentropic and reversible (ie ignore the heat loss as this is insignificant)

(b)Calculate the specific entropy of the steam leaving the turbine when the latter is generating the maximum possible power.

I said the entropy must reman constant(ie isentropic), and then used my steam table for superheated vapour at 10 bar = 1 Mpa and entropy 6.8802kJ/kgK, I get a final temperature of 239.35 degrees celcius (ie 512.5K)

BUT! the next questions says show the exit steam temp = 199.9 degrees C.

COULD SOMEONE please give me some hints at how to do it if you can't choose isentropic?...or mayb point out where I may be missing something?

Thanks guys

 

[Chestermiller]

Maybe the 70 kW rate of heat loss had to be taken into consideration.