Ch 10 Thermo concept questions

What are the four processes that make up the simple ideal rankine cycle?

1-2 Isentropic compression in a pump 2-3 constant pressure heat addition in a boiler 3-4 Isentropic expansion in a turbine 4-1 Constant pressure heat rejection in a condenser

Why is the carnot cycle not a realistic model for steam power plants?

1.) Limiting the heat transfer process to two phase systems for maintaining isothermal conditions severely limits the maximum temperature that can be used in the cycle. 2.) Turbine handles steam with high moisture content that causes erosion. 3.) It is not easy to control the condensation process so precisely as to end up with the desired quality at state. 4.) Designing a compressor that can handle two phases is not practical.

What is the difference between cogeneration and regeneration?

Cogeneration is the production of more than one useful from of energy from the same energy source. Regeneration is the transfer of heat from the working fluid at some stage to the working fluid at another stage.

In a combined gas- steam cycles, what is the energy source for the steam?

Energy is recovered from the exhaust gases from the turbine outlet of the gas turbine cycle and is supplied to the steam power cycle in a heat exchanger.

How is the utilization factor Eu for cogeneration plants defined? Could Eu be unity for a cogeneration plant that does not produce any power?

Eu = (Net work output+Process heat delivered)/(total heat input) yes it can be unity because all the steam produced or heat supplied is used in the process heater. So heat supplied is equal to heat delivered

Consider a simple ideal rankine cycle and an ideal regenerative rankine cycle with one 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 efficiences of these two cycles?

If some quantity of steam is extracted before entering into the turbine for heating the feed water. It increases the heat addition temp thus increasing efficiency. Bit the work output decreases as some steam is extracted before it enters into the turbine.

Consider a cogeneration plant for which the utilization factor is 1. Is the irreversibility associated with this cycle necessarily zero?

No, they should not be zero because we still have internal and external irreversibility's. such as, Throttling, friction, non-quasi-equilibrium expansion or compression

How do open feedwater heates differe from closed feedwater heaters?

Open feed waters are called as mixing chambers thus the pressure is constant. Closed feed waters do not mix thus they can have different pressures.

consider a simple ideal rankine cycle with fixed turbine inlet conditions. What is the effect of lowering the condensation pressures?

Pump work input: increased Turbine work output: increased Heat supplied: increased heat rejected: Decreased Cycle efficiency: increases Moisture content at turbine exit: Increases

Consider a simple ideal rankine cycle with fixed turbine inlet temperature and condenser pressure. What is the effect of increasing the boiler pressure on:

Pump work input: increases turbine work output: Increases Heat supplied: Increases Heat rejected: decreases cycle efficiency: increases Moisture content at turbine exit: Increases

how do the following quantities change when a simple ideal rankine cycle is modified with reheating? Assuming the mass flow is maintained?

Pump work input: remains the same turbine work output: Increases Heat supplied: Increases Heat rejected: increases Moisture content at turbine exit: decreases

Consider a simple ideal Rankine cycle with fixed boiler and condenser pressures. What is the effect of superheating the steam to a higher temperature on:

Pump work input: remains the same turbine work output: Increases Heat supplied: Increases Heat rejected: increases cycle efficiency: increases Moisture content at turbine exit: decreases

Why is the combined gas-steam cycle more efficient than either of the cycles operated alone?

Takes advantage of the desirable characteristics of the gas cycle at high temps and of the steam cycle at low temps. The high temperature exhaust gases from the turbine of gas cycle are used to heat the steam of steam power cycle thus using the heat energy of exhaust gases.

Why is excessive moisture in steam undesirable in steam turbines? What is the highest moisture content allowed?

The excessive moisture content in steam causes erosion on the turbine blades. This lowers the efficiency of the turbine. The dryness fraction should not be less than .88. so the max allowed moisture content is about 10-12%

During a regeneration process, some steam is extracted from the turbine and is 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 rejected anyways. Due to regeneration we utilize a considerable amount of heat by sacrificing little work input. so it is justified.

The entropy of steam increases in actual steam turbines as a result of irreversibility's. 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 the work output. How would you evaluate this proposal?

The following factors influence the turbine output with the given suggestion. 1.) Enthalpy at the turbine exit 2.) Heat losses in the turbine since heat is being carried away we are reducing the work output and efficiency.

Is there an optimal pressure for reheating the steam of a rankine cycle?

The optimum pressure is about 1/4 of the maximum cycle pressure. Reheating is used for reducing the mositrue content of the system during the final stages of the expansion process.

Cinsider a simple ideal rankine cycle and an ideal rankine cycle with 3 reheat stages. Both cycles operate between the same pressure limits. The max temp is 700c in the simple and 450c in the reheat. Which cycle do you think will have a higher thermal efficiency?

The simple should be bore efficient due to the increase in temperature.

How do actual vapor power cycles differ from idealized ones?

They differ due to irreversibility's in various components such as fluid friction and heat loss.

is it possible to maintain a pressure of 10 kpa in a condenser that is being cooled by river water entering at 20c.

Yes, Because the saturation temperature of steam at 10kpa is 45.81c. Since the cooling water is less than the saturation temperature it will work.

Compare the pressures at the inlet and the exit of the boiler for an a.) actual and b.) ideal cycle

a.) actual cycles will have a significant pressure drop in the boiler, and therefore the exit pressure will be somewhat lower compared to the inlet pressure of boiler. b.) In ideal case inlet and outlet pressure will be the same because there will be no pressure loss in the boiler.