Steam Nozzles

We assume the steam nozzle to be a tube of varying diameter by means of which the energy of steam is converted into kinetic energy. The shape of the nozzle is designed to perform this conversion of energy with minimum losses. One may also define a nozzle as an opening through which steam is passed from a region of high pressure to that of lower pressure so as to derive additional speed. In heat power engineering it is mainly used as a means of accelerating the flow of steam passing through it, so its main purpose is producing a jet of steam to drive steam turbines.

The steam expanding, its speed and specific volume increase; condensation changes the degree of steam dryness. We know all these changes to affect the design of the nozzle. The weight of steam per second passing any nozzle section must be constant; hence, the nozzle diameter varies according to the speed and the specific volume.

At first the nozzle diameter narrows down to a smaller diameter to allow for these changes. On reaching this small diameter it will diverge to a larger one. We find the throat to be the smallest section of the nozzle.

A nozzle, which first converges to the throat and then diverges, is known to be a convergent-divergent nozzle: in this type the greatest diameter is at the exit. Steam flows from the chamber into the converging portion of the nozzle, past the throat, through the diverging portion and then exhausts as a jet. Some forms of nozzles end at the throat, and don’t have any diverging section; this type is known as a convergent nozzle and has its exit at the throat.

The steam enters the nozzle with a relatively low speed and a high initial pressure. As the steam expands, the speed increases and the heat energy of the steam is converted into kinetic energy.

As the steam loses its pressure in passing through the nozzle, it also loses its total heat. The change of total heat of the steam is equal to the increase of kinetic energy. Hence, the work done is equal to heat loss.

In practice there is friction produced between the steam and the sides of the nozzle; this friction causes a resistance to the flow, which is converted into heat. The heat formed dries the steam. All these must be taken into account in the design of the nozzle, as it makes a great difference to the results obtained.

• Transcribe the following words, practice reading them, give Russian equivalents:

Nozzle, assume, passage, kinetic, vary, function, guide, weight, specific, diameter, diverge, converge, exit, initial, jet

• Are the following sentences true or false? Correct the false sentences

1. The energy of steam passing through the nozzle is converted into potential energy.

2. The cross-section of the throat is very small.

3. In convergent nozzles the greatest diameter is at the exit end.

4. The initial speed of the steam entering the nozzle

5. The increase of the kinetic energy is equal to the total heat of the steam.

6. The friction between the flow of the steam and the sides of the nozzle can be ignored.

• Use the following paragraphs to answer the questions.

Paragraph 1

What is the function of a nozzle?

Paragraphs 2, 7

What factors influence the design of the nozzle?

Paragraph 4

What types of nozzles can you name?

Paragraphs 5, 6

What does the work done by steam equal to?