Problem 3.6 Estimate the specific enthalpy of steam (kJ/kg) at 350°C and 100 bar relative to steam at 100°C

and 1 atm using: (a) The steam tables. (b) Table B.2 and assuming ideal gas behavior. What is

the physical significance of the difference between the values of H calculated by the two

methods?

Problem 3.7

A stream of water vapor flowing at a rate of 250 mol/h is brought from 600°C and 10 bar to

100°C and 1 atm.

(a) Estimate the required cooling rate (kW) three ways: (i) from the steam tables, (ii) using heat

capacity data in Table B.2, and (iii) using specific enthalpy data in Table B.8.

(b) Which of the answers in part (a) is most accurate, and why? (c) What is the physical

significance of the difference between the values calculated with methods (i) and (ii)?

Problem 3.8

A fuel gas containing 95 mole% methane and the balance ethane is burned completely with

25% excess air. The stack gas leaves the furnace at 900°C and is cooled to 450°C in a waste heat

boiler, a heat exchanger in which heat lost by cooling gases is used to produce steam from

liquid water for heating, power generation, or process applications.

(a) Taking as a basis of calculation 100 mol of the fuel gas fed to the furnace; calculate the

amount of heat (kJ) that must be transferred from the gas in the waste heat boiler to

accomplish the indicated cooling.

(b) How much saturated steam at 50 bar can be produced from boiler feed water at 40°C

for the same basis of calculation? (Assume all the heat transferred from the gas goes

into the steam production.)

(c) At what rate (kmol/s) must fuel gas be burned to produce 1250 kg steam per hour (an

amount required elsewhere in the plant) in the waste heat boiler? What is the

volumetric flow rate (m3

/s) of the gas leaving the boiler?​