MAE 407 Test 2

Consider a simple ideal Rankine cycle with fixed turbine inlet temperature and condenser pressure. What is the effect of increasing the boiler pressure on: (i) Pump work input (ii) Turbine work output (iii) Heat supplied (iv) Heat rejected (v) Cycle efficiency (vi) Moisture content at turbine exit

(i) increases (ii) increases (iii) increases (iv) decreases (v) increases (vi) increases

How do the following quantities change when a simple ideal Rankine cycle is modified with reheating? Assume the mass flow rate is maintained the same. (i) Pump work input (ii) Turbine work output (iii) Heat supplied (iv) Heat rejected (v) Moisture content at turbine exit

(i) remains the same (ii) increases (iii) increases (iv) increases (v) decreases

Consider a simple ideal Rankine cycle with fixed boiler and condenser pressures. What is the effect of super heating the steam to a higher temperature on: (i) Pump work input (ii) Turbine work output (iii) Heat supplied (iv) Heat rejected (v) Cycle efficiency (vi) Moisture content at turbine exit

(i) remains the same (ii) increases (iii) increases (iv) increases (v) increases (vi) decreases

How do actual vapor power cycles differ from idealized ones?

- Irreversibility: fluid friction, heat loss

Consider a simple ideal Rankine cycle and an ideal Rankine cycle with three reheat stages. Both cycles operate between the same pressure limits. The maximum temperature is 700C in the simple cycle and 450C in the reheat cycle. Which cycle do you think will have a higher thermal efficiency?

- Thermal efficiency = net work/heat input - Thermal efficiency decreases with increased heat input Thermal efficiency is higher for the ideal Rankine cycle.

How do the following quantities change when the simple ideal Rankine cycle is modified with regeneration? Assume the mass flow rate through the boiler is the same. (i) Turbine work output (ii) Heat supplied (iii) Heat rejected (iv) Moisture content at turbine exit

(i) decreases ? (ii) decreases ? (iii) decreases ? (iv) remains the same

Consider a simple ideal Rankine cycle with fixed turbine inlet conditions. What is the effect of lowering the condenser pressure on: (i) Pump work input (ii) Turbine work output (iii) Heat supplied (iv) Heat rejected (v) Cycle efficiency (vi) Moisture content at turbine exit

(i) increases (ii) increases (iii) increases (iv) decreases (v) increases (vi) increases

The entropy of steam increases in actual stem turbines as a result of irreversibilities. In an effort to control entropy increase, it is proposed to cool the steam in the turbine by running cooling water around the turbine casing. It is argued that this will reduce the entropy and the enthalpy of the steam at the turbine exit and thus increase work output. How would you evaluate this proposal?

I do not agree. Turbine work output depends on the enthalpy at boiler exit point. If the boiler is cooled by cool water there will be heat lost from boiler and thus enthalpy decreases. As a result turbine work output decreases.

How do open feedwater heaters differ from closed feedwater heaters?

In open feedwater heaters, steam mixes with cold feedwater. In the closed feedwater heaters, the extraction steam is kept superheated. there is no mixing.

Show the ideal Rankine cycle with three stages of reheating on a T-s diagram. Assume the turbine inlet temperature is the same for all stages. How does the cycle efficiency vary with the number of reheat stages?

The cycle efficiency increases with each stage of reheating. (As the working fluid passes through each stage of reheating, moisture from the steam is removed accordingly)

During a regeneration process, some steam is extracted from the turbine and used to heat the liquid water leaving the pump. This does not seem like a smart thing to do since the extracted steam could produce some more work in the turbine. How do you justify this action?

The extracted steam has little work potential left and most of its energy would be part of the heat rejection anyway. Therefore, by regeneration, we utilize a considerable amount of heat by sacrificing little work output.

Consider a simple ideal Rankine cycle and an ideal regenerative cycle with on open feedwater heater. The two cycles are very much alike, except the feedwater in the regenerative cycle is heated by extracting some steam just before it enters the turbine. How would you compare the efficiencies of these two cycles?

They both have the same efficiency because the extraction steam just creates a mini cycle that recirculates. The energy given to the feedwater heater is equal to the heat added in the boiler to the feedwater in the simple cycle to raise its temperature to the same boiler inlet condition.

Is it possible to maintain a pressure of 10 kPa in a condenser that is being cooled by river water entering at 20 C?

Yes because the saturation temperature of steam at 10 kPa is 45.81 C. The cooling water temperature is less.