Section: 1
Problem I. (a) Steam at a pressure of 0.14 bar has a dryness fraction of 0.92. Find:
(i) its specific enthalphy
(ii) its specific entropy
(iii) its specific volume.
(b) Explain and distinguish between the terms:
(i) isenthalpic process
(ii) isentropic process
In each case give an example to illustrate your answer.
Problem 2. Steam is expanded isentropically from a pressure of 40 bar and a temperature of 300°C to a pressure of 0.05 bar. Find the final dryness fraction by calculation using values from the property tables.
Problem 3. Dry saturated steam at a pressure of 20 bar is throttled to 0.5 bar and then expanded isentropically until it again becomes dry saturated. Use your h - s chart to find the pressure at the final state.
Problem 4. (i) Find the change in specific entropy when saturated water is boiled to produce dry saturated steam at 50 bar.
(ii) What does this change tell you about the reversibility of the boiling process?
Problem 5. How Much steam is produced when 1 Kg of water at 1650C and 8 bar is flashed into a Vessel at 2 Bar?
Problem 6. (a) Explain how air can get into and contaminate a steam supply.
(b) What effect does the presence of air, in steam, have on its quality and properties?
(c) Explain the operating principles of one type of air release trap.
(d) State. giving reasons, the most desirable position for an air release trap.
Problem 7. The partial pressure of saturated steam in a steam/air mixture is 1.40 bar. while the partial pressure of the air is 0.15 bar. Find:
a. the total pressure of the mixture
b. the temperature of the mixture
c. the air concentration
Problem 8. The overall heat transfer coefficient in a process heater is 2.15 kW m-2 K-1, and the temperature of the process fluid is essentially constant at 80°C. Find:
(a) the rate at which heat will be transferred by a square metre of heater surface from steam condensing at 1.2 bar
(b) How much dry saturated steam must be condensed in an hour to provide the above heat transfer?
Problem 9. A heat exchanger, supplied with steam at 2 bar, is designed to maintain a process fluid at 95°C on a continuous basis. Using the data given below, calculate the following:
(i) an overall heat transfer coefficient for the heat exchanger
(ii) the rate of heat transferred per unit surface area from the condensing steam to the process fluid
(iii) the quantity of dry saturated steam that must be condensed per hour to maintain the rate of heat transfer in (ii) above
(iv) the rate of heat transferred per unit surface area for an air/steam concentration ratio at the steam/condensate interface of 0.55 (you may use values taken from FIGURE 2, page 8 of lesson PS - I - 4). Comment on your answer.
DATA:
Heat transfer coefficients:
• condensate (hc) 16 kW m-2 K-1
• tube wall (hT) 210 kW m-2 K-1
• scale (hs) 3.5 kW m-2 K-1
• liquid boundary layer (hL) 5.5 kW m-2 K-1
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Section II
Problem 1. Superheated steam at a pressure of 40 bar and a temperature of 500°C is supplied to the turbine of a Rankine cycle. If the condenser pressure is 0.03 bar, find the thermal efficiency of the cycle. Neglect feed pump work.
Problem 2. If the isentropic efficiency of the steam turbine in Question 1 above is 0.82, find:
(a) the enthalpy of the steam discharged into the condenser and
(b) the thermal efficiency of the cycle.
Problem 3. A steam turbine operates with a condenser pressure of 0.04 bar and has a single open-type regenerative feed heater. How much steam, at a pressure of 2 bar and a dryness fraction of 0.98, must be bled from the turbine to produce 1 kg of saturated feedwater at 2 bar?
Problem 4. Steam at 30 bar and 500°C enters a pass-out turbine at a rate of 18000 kg h-1. The bulk of this steam is expanded isentropically to a condenser pressure of 0.03 bar, but 32% is bled from the turbine at a pressure of 3 bar for process heating. If the condensate from the process heater is directly pumped back to the boiler, find:
(a) the power output from the turbine
(b) the rate of process heating
(c) the efficiency of the plant.
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