4th Class Power Engineering turbines, engines and cooling towers practice questions (4B).

Five free practice questions on turbines, engines and cooling towers: unit B7 of paper B (4B) of the 4th Class Power Engineer certificate exam, written to the SOPEEC syllabus. They are in the exam's format: four options, one correct answer. Answer each one before opening the explanation. No account, no email.

01The questions

1. Recall · Discuss the history of the steam engine and the expansive power of steam

   In a reciprocating steam engine, closing the admission valve part-way through the stroke and letting the trapped steam expand for the rest of it

   1. leaves the exhaust steam hotter for heating use
   2. raises the steam pressure on the piston as it moves
   3. raises the most power the engine can develop
   4. gets more work out of each kilogram of steam
   Show the answer

   D. Correct. After cut-off the trapped steam keeps pushing the piston while its pressure falls, so work is done by the steam's expansion with no more steam drawn from the boiler. This expansive use of steam is the idea a turbine carries further, expanding the steam stage by stage to its exhaust pressure.

   Why not the others

   A. The opposite: as the steam expands and does work, its pressure and temperature fall, so more of its energy leaves as work and less in the exhaust. Early cut-off is about getting more work from each kilogram of steam.

   B. As the trapped steam expands its volume grows and its pressure falls. The gain is that the falling pressure still does work on the piston, so more work comes from each kilogram of steam.

   C. Early cut-off lowers the average pressure on the piston, so the work per stroke and the most power the engine can give fall. Steam is admitted for longer when full power is needed; expansive working pays in steam used per unit of work, not in peak power.

2. Procedural · Describe the lubrication and sealing requirements for steam turbine shafts

   A condensing turbine-generator's start-up procedure reads: rotor on turning gear, admit sealing steam to the glands, then start the air ejectors to raise vacuum. Why must sealing steam reach the exhaust-end gland before vacuum is raised?

   1. To heat the rotor so it can be rolled off turning gear sooner
   2. To keep exhaust steam from blowing out into the turbine hall
   3. To give the condenser the steam it needs to form a vacuum
   4. To keep air from leaking into the condenser along the shaft
   Show the answer

   D. Correct. Once the condenser is below atmospheric pressure, the exhaust-end gland would leak inward. Sealing steam fed to that gland a little above atmospheric pressure fills the gap, so steam rather than air leaks in; air would blanket the condenser tubes and spoil the vacuum.

   Why not the others

   A. Gland steam warms the shaft only locally at the glands, which on a stationary rotor can distort it; warming the rotor is not its purpose. It is there to stop air leaking into the condenser along the shaft.

   B. That is the high-pressure end's problem, where the steam inside the casing is above atmospheric and leaks outward. The exhaust end of a condensing turbine runs below atmospheric, so its leakage is inward: air, which the sealing steam keeps out.

   C. The vacuum is drawn by the air ejectors and held by condensing the exhaust steam; the gland steam does not form it. Its job at the exhaust end is to stop air leaking in along the shaft.

3. Recall · Describe the construction and operation of natural draft cooling towers

   A large natural-draft cooling tower with a tall hyperbolic shell moves air through its fill without fans because

   1. the narrow throat of the shell speeds up and draws in the air
   2. the falling water drags the air down through the fill
   3. the warm moist air inside is lighter than the air outside
   4. wind blowing across the open top pulls air up the shell
   Show the answer

   C. Correct. Air warmed and moistened in the fill is less dense than the cooler, drier outside air, so the column inside the tall shell rises like flue gas in a chimney, and fresh air flows in at the base to replace it. The taller the shell and the warmer the air inside, the stronger the draft.

   Why not the others

   A. The hyperbolic shape gives the thin shell its strength, but a shape alone does not move air. The draft comes from the density difference between the warm, moist air inside and the cooler air outside.

   B. In a natural-draft tower the air flows up through the fill, against the falling water, so the water is not what moves it. The draft comes from the light, warm, moist air in the tall shell.

   D. Wind affects the tower's performance, but the draft does not depend on it; the tower draws air on a still day. It works like a chimney: the warm, moist air inside is lighter than the air outside.

4. Procedural · Identify the operational characteristics of gas turbines

   On a 33 °C afternoon, running at its turbine-inlet temperature limit, a hospital's cogeneration gas turbine cannot make the output it made at the same limit on a −12 °C morning. What is the main reason?

   1. Warm air is less dense and less mass flows through
   2. The warm fuel gas carries less heat per cubic metre
   3. The compressor's pressure ratio rises in warm air
   4. The turbine blades need more cooling on a hot day
   Show the answer

   A. Correct. At a given speed the compressor takes in a nearly fixed volume of air. Warm air is less dense, so less mass passes through the machine and, at the same turbine-inlet temperature limit, less work is produced, while the compressor needs more work per kilogram of warmer air. Output and efficiency both fall as the air warms.

   Why not the others

   B. The fuel control meters the fuel the turbine needs; warmer gas only means a slightly larger volume for the same energy. The limit is the mass of air the compressor can move, which falls as the air warms and thins.

   C. At the same speed the pressure ratio tends to fall in warm air, not rise. The output drops because the warmer, less dense air means less mass flow through the machine.

   D. The turbine-inlet temperature is held at the same limit morning and afternoon, so the blades face the same gas temperature. The loss comes from the lower air density: less mass flows through the machine.

5. Recall · Describe the working cycles of the 4-stroke and 2-stroke spark ignition engines

   In a small two-stroke spark-ignition engine with ports in the cylinder wall, the fresh charge enters and the exhaust leaves

   1. through valves opened every second revolution by a camshaft
   2. during separate intake and exhaust strokes of the piston
   3. near bottom dead centre, through ports the piston uncovers
   4. near top dead centre, just after the spark has fired
   Show the answer

   C. Correct. A two-stroke engine completes its cycle in one revolution, so there are no separate intake and exhaust strokes. Near the bottom of the power stroke the descending piston uncovers the exhaust port and then the transfer port, and the incoming charge helps push the burnt gas out (scavenging).

   Why not the others

   A. Cam-driven valves opening every second revolution are the four-stroke arrangement. A ported two-stroke engine has no valves for this: the piston uncovers the ports near bottom dead centre every revolution.

   B. Separate intake and exhaust strokes belong to the four-stroke cycle, which takes two revolutions. The two-stroke engine exchanges its gases near bottom dead centre, through ports the piston uncovers.

   D. Near top dead centre the charge is being compressed and fired, with the ports covered by the piston. Gas exchange happens near bottom dead centre, when the piston uncovers the ports.

02Answer key

03What unit B7 covers

Turning heat into shaft work: heat engines and prime movers and the history of the steam engine, steam turbine stages, glands, governing and start-up, condensers and natural- and mechanical-draft cooling towers, gas turbines and their cycles, and spark-ignition and diesel engines.

Part B of the SOPEEC 4th Class syllabus names this unit “B7 Prime Movers and Engines” and lists five objectives under it (objectives 28 to 32 of 52). SOPEEC does not publish how many exam questions come from each unit, and neither do the ABSA, Technical Safety BC, TSASK or TSSA pages we checked. The right-hand column is how many questions our bank holds on each objective, Standard and Advanced together, not a count from the exam.

04What the questions turn on

Nozzles make velocity; reaction stages drop pressure twice

Fixed nozzles turn the steam's pressure energy into velocity for the moving blades. In an impulse stage the moving blades see no pressure drop; when the pressure also falls across the moving blades, the stage is reaction.

Seal steam before vacuum, turning gear after stop

Sealing steam goes to the exhaust-end gland before the air ejectors raise vacuum, so air is not drawn in along the shaft. After shutdown the turning gear keeps a hot rotor turning so it cools evenly and does not bow. A rotor is run up through its critical speed without dwelling there.

Approach is cold water minus wet bulb

A cooling tower's range is hot water minus cold water; its approach is cold water minus the entering air's wet-bulb temperature, and a widening approach at the same load points to a fault. In the condenser, absolute pressure sets the saturation temperature that condensate subcooling is measured from.

Gas turbines breathe mass; four-strokes fire every other turn

Warm air is less dense, so a gas turbine at its temperature limit makes less power on a hot day. A four-stroke engine fires each cylinder once every two revolutions, and its camshaft turns at half crankshaft speed.

05The other units

Paper 4B has twelve units, B1 to B12. Each has its own page of free questions:

The 4th Class Power Engineering practice exam (4B) page describes the full question bank; paper A has its own 4A practice exam. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.