undefined | unit 4 steam and gas nozzles

Unit-4

APPLIED THERMODYNAMICS

Q 1) Difference between steam turbine and steam engine?

A 1)

Steam Turbine	Steam Engine
1) Conversion of heat energy to mechanical work, there is no frictional loss.	1) High frictional loss for reciprocating parts.
2) Balance is good.	2) Balance is not so good.
3) Foundation is light weight.	3) Foundation is heavy weight.
4) It can run high speed.	4) It cannot run so much speed.
5) Lubrication is simple as there is no rubbing parts available.	5) Lubrication is not so simple for rubbing parts.
6) Power generation uniformly.	6) It does not generate power uniformly.
7) Steam consumption is less than the reciprocating steam engine.	7) It consumes more steam than steam turbine.
8) It is more compact and requires less attention.	8) Steam engine needs more attention.
9) It is suitable for large power plants.	9) It is not suitable for large power plants.
10) Steam turbine is more efficient than steam engine.	10) Steam engine is not so much efficient.

Q 2) Difference between impulse and reaction turbine?

A 2)

Impulse Turbine	Reaction Turbine
1) In impulse Turbine, only impulsive force strikes to the blades fixed to the rotor	1) In reaction turbine, vector sum of impulsive and reactive force strikes the blades fixed to the rotor.
2) Steam expands completely when it passes through the nozzles and its pressure remains constant.	2) Pressure can't expand fully. It partially expands when it pass through the nozzles and rest on the rotor blades.
3) Blades are symmetrical shape.	3) Blades are asymmetrical shape.
4) Since the velocity of steam is high, speed is high in impulse turbine.	4) But reaction turbine speed is much lower than impulse turbine because steam velocity is lower in reaction turbine as compared to impulse turbine.
5) For producing same power, the number of stages required is much less.	5) It requires more stages to develop same power.
6) The blade efficiency curve is high.	6) The blade efficiency curve is lower than impulse turbine.

Q 3) Discuss Main advantages with steam turbines?

A 3)

The economical generation of high temperature and pressure steam is required to reduce the pressure to utilize for various purposes. This will be done by steam turbine there by developing power to run pump etc.

A pump driven by turbine can be operated over a wide speed range utilizing the turbine governor system.

The use of steam turbine driver permits operation of the driven pump is independent of electric power or distribution system.

A turbine may be used as stand by drive system.

The steam turbine controls-governor system and over speed trip system are inherently spark proof.

Steam turbines are inherently self-limiting with respect to power developed. Special protective devices are not required to prevent damaging the turbine because of over load condition.

Q 4) What are the Advantages velocity compounded steam turbine?

A 4)

1.This arrangement needs small space.

2. This is very reliable and easy to operate.

3.Initial cost is low for this arrangement.

4. Since nozzle's steam is considerable, the turbine does not need to work in high pressure and turbines structure need not be very strong.

Q 5) What are the Disadvantages velocity compounded steam turbine?

A 5)

1.Friction loss is high due to high velocity of steam of nozzle.
2.Its efficiency is low because the ratio of blade velocity and steam velocity is not optimum.
3. First, row is developed maximum power in this system. Later rows are developed very small power rather than first row but fabrication and cost of materials will be same in all rows.

Q 6) Which has higher efficiency: Impulse or reaction turbine?

A 6) The answer to that depends completely on the operating conditions under which the two types of turbines are used.

For any given turbine type (i.e., impulse or reaction), the quantity that generally has the highest impact on efficiency (to first order) is the turbine ‘velocity ratio’, usually written as ‘U/C’ in the literature.

In the term ‘U/C’, U represents the turbine blade translational velocity (i.e., a local radius multiplied by a rotational speed). This obviously means that U varies continuously along the blade span. For most of these discussions, U is generally taken to be that at the blade mid-span (although you should certainly check this if you are using someone else’s data). C in the above expression represents an isentropic ‘spouting velocity’ that is obtained by expanding the turbine working fluid across the turbine inlet-to-outlet conditions.

For a typical full admission impulse turbine (and I mean one that is large enough to avoid the performance losses that are unique to ‘small’ turbines) the efficiency generally peaks at or around a U/C value of 0.45. When a reaction turbine is used at a U/C value of less than 0.45, its efficiency is generally lower than that of the impulse turbine.

As we allow the turbine U/C value to increase (either by increasing blade speed, or by changing the turbine conditions such that C decreases) the reaction turbine efficiency will generally exceed that of the impulse turbine at a U/C of somewhere between 0.45 and 0.50. At U/C of something like 0.55, the reaction turbine will (normally) have a noticeably higher efficiency.

So, as you can see, you can think of the impulse and reaction turbine types as being sort of ‘complimentary’ to each other. Each can perform with high efficiency, but the two types operate at their best at different sets of conditions.

Q 7) What is Mach number?

A 7) It is the ratio of speed of flow in a medium to the speed of sound in that medium

For Mach numbers ≤ 0.3 we consider the flow to be incompressible because the density variation is below 5% and for flows having Mach number ≥ 0.3 we consider the flow to be compressible because the density variation cannot be neglected now. For supersonic flows increase in velocity causes flow velocity to increase. And therefore for our case i.e. supersonic flows we will be doing all he calculations considering compressible flow only.

Q 8) Explain Normal Shock?

A 8) It is a completely irreversible process takes place in the Convergent divergent type of nozzles (or in venturi) at the divergent section. A sudden change in pressure, temperature, and flow velocity takes place while supersonic flow was taking place. After shock flow becomes subsonic and stays subsonic till end. Width of this shock is very less i.e. about 4 times the mean free path of the gas molecules.

Q 9) What is nozzle efficiency?

A 9) Nozzle efficiency

In nozzle, the wall friction is small in convergent portion as compared to divergent portion. The friction losses in the nozzle depend upon the type of material, size, and shape, properties of the fluid and flow conditions is nozzle.

The nozzle efficiency can be defined as the ratio of actual enthalpy drop to ideal (isentropic) enthalpy drop in nozzle.

Therefore
Nozzle efficiency, ηn = (Actual enthalpy drop) / (isentropic enthalpy drop)

ηn = (h1 – h2) / (h1 – h'2

The nozzle efficiency can be defined as the ratio of actual enthalpy drop to isentropic enthalpy drop.

Q 10) What is bleeding?

A 10) Bleeding

In regenerative heating, some steam is carried out from the turbine at certain points during its expansion. This team is used to the feed water in the feed water heater increasing its temperature and then supplying to the boiler is known as bleeding.Using this process, there is a slight increase in efficiency but there is also a decrease in the horsepower developed.

Sign Up