Thermodynamics: Homework A Set 3 Jennifer West (2004)

Transcription

1 Thermodynamics: Homework A Set 3 Jennifer West (2004) Problem 1 In situations when only superheated steam is available a need for saturated steam arises. The required saturated steam can be obtained be desuperheating the superheated steam. This can be accomplished in an adiabatic desuperheater in which liquid water is sprayed into the superheated steam in such amounts that dry saturated steam leaves the superheater. The following data from such a superheater is available. Superheated steam at 320 C, 3 MPa enters the desuperheater at a flow rate of M kg/s. Liquid water enters the desuperheater at 40 C, 5 MPa. Dry saturated vapor leaves at 3 MPa. Determine the mass flow rate, in kg/s, of liquid water required. M = 0.25 kg/s Question 1 of 2: What are the correct assumptions for this problem? Check all that apply. The control volume operates at steady state conditions Q = 0 Change in Potential Energy ~ 0 (negligible) Change in Kinetic Energy ~ 0 (negligible) None of the above Question 2 of 2: What is the mass flow rate of liquid required? Used by permission. Page 1 of 7

2 Problem 2 Steam at 10 psia with a quality of x enters an adiabatic compressor at a rate of Mlbm/s. The steam leaves at 400 F, 100 psia. x = 0.90 (quality) M = 500 lbm/s What are the assumptions for the problem statement above? Check all that apply. Steady State Constant Temperature Process Q = 0 Change in PE and change in KE are negligible All of the above What is the specific enthalpy for the inlet stream? What is the horsepower required for the compressor to change the inlet stream conditions to the outlet stream conditions? Used by permission. Page 2 of 7

3 Problem 3 Water enters a heat exchanger at 180 F, 20 psia and leaves at 160 F, 19.8 psia. Air enters the heat exchanger at 60 F, 15 psia and leaves at 100 F, 14.7 psia. The mass flow rate of the water is Mlbm/min. Neglect heat losses to the surroundings from the system. Assume all flows are steady state. M = 40 lbm/min What is the specific enthalpy of the water entering at 180 o F, 20 psia? A Btu/lbm B Btu/lm C Btu/lbm D Btu/lbm E Btu/lbm What is the mass flow rate of air (lbm/min)? What is the heat transfer rate (Btu/min)? Used by permission. Page 3 of 7

4 Problem 4 Steam enters an adiabatic turbine at 1000 F, 700 psia and leaves at 800 F, 400 psia. The mass flow rate is M lbm/hr. M = 1000 lbm/h What is the specific enthalpy of the steam entering at 1000 F, 700 psia (Btu/lbm)? A Btu/lbm B Btu/lbm C Btu/lbm D Btu/lbm E Btu/lbm What is the amount of work produced (hp)? What is the ration (expressed as a decimal) of the inlet flow area to the outlet flow area to keep the exit velocity equal to the inlet velocity? _ Used by permission. Page 4 of 7

5 Problem 5: A steam turbine is used to drive a nitrogen compressor, as shown. The turbine is well insulated and delivers 42 kw to the compressor and the balance of its output to an electric generator. The steam enters the turbine at a rate of Ms kg/s at 4 MPa and 350 C. Steam exits the turbine at a pressure of 20 kpa and a quality of The N2 enters the N2 compressor at a rate of Mn kg/s at 100 kpa and 20 C. The N2 travels through the N2 compressor to an aftercooler; the N2 exits the aftercooler at 10 MPa and 35 C. Ms = 0.25 kg/s Mn = 0.04 kg/s What is the specific enthalpy of the team entering the turbine? A kj/hk B kj/kg C kj/kg D kj/kg E kj/kg What is the amount of available power to drive the generator (kw)? What is the rate of heat transfer (kj/s) from the nitrogen as is flows though the compressor and after cooler? Used by permission. Page 5 of 7

6 Problem 6 As illustrated in the adjacent figure, a steam turbine at steady state is operated at part load by throttling the steam to a lower pressure before it enters the turbine. Before throttling, the pressure and temperature are, respectively, 200 psi and 600 F. After throttling, the pressure is 120 psi. At the turbine exit, the steam is at 1 psi, and a quality of x. Heat transfer with the surroundings and all kinetic and potential energy effects can be ignored. Problem from Fundamentals of Engineering Thermodynamics, 5th Ed. by Michael J. Moran and Howard N. Shapiro. John Wiley & Sons, Inc X =.90 (quality) Question 1 of 2: What is the temperature (F) at the turbine inlet? A. 501 F B. 589 F C. 389 F D. 895 F E. None of the above Question 2 of 2: What is the power developed by the turbine, in Btu per lb of steam flowing? Used by permission. Page 6 of 7

7 Problem 7 Refrigerant-134a enters the flash chamber operating at steady state, illustrated in the adjacent figure, at 10 bar, 36 C, with a mass flow rate of M kg/h. Saturated liquid and saturated vapor exit as separate streams, each at pressure P = 4 bar. Heat transfer to the surroundings and kinetic and potential energy effects can be ignored. M = 482 kg/h Question 1 of 2: What is the specific enthalpy of stream 2? A kj/kg B kj/kg C. 482 kj/kg D. 0 kj/kg E. None of the above Question 2 of 2: What is the flow rate for stream #2 when P=4 bars? Used by permission. Page 7 of 7