4th Class Power Engineering refrigeration practice questions (4B).

Five free practice questions on refrigeration: unit B9 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 · Describe the safety concerns of common refrigerants, such as R 717, CFC's and HCFC's

   R-22, an HCFC in safety group A1, is leaking in a machinery room that also houses a gas-fired unit heater. What is the main hazard of the refrigerant reaching the heater's flame?

   1. It ignites, and the flame can travel back along the vapour to the leak
   2. It breaks down into toxic, corrosive gases such as hydrogen chloride
   3. It reacts in the flame to form ozone, which irritates the lungs
   4. It smothers the burner flame, so raw fuel gas escapes into the room
   Show the answer

   B. Correct. Halocarbon refrigerants are stable at room temperature but break down in a flame or on a hot surface. A refrigerant carrying chlorine and fluorine, such as R-22, yields hydrogen chloride and hydrogen fluoride and can form phosgene, all toxic and corrosive; a sharp, acrid smell near a heater is a warning. Leaks near flames and hot work are stopped and the room ventilated.

   Why not the others

   A. Safety group A1 means no flame propagation: R-22 does not burn or carry a flame back to the leak. The danger in a flame is chemical breakdown into toxic, acidic gases, not fire.

   C. Ozone depletion by HCFCs happens high in the atmosphere, not in a burner flame, and R-22 in a flame produces acid gases such as hydrogen chloride and hydrogen fluoride, not ozone.

   D. A refrigerant leak would have to be very large to put out the flame, and the heater's flame safeguard is there to shut off the gas if it did. Long before that, refrigerant drawn into the flame is breaking down into toxic acid gases, which is the hazard to act on.

2. Procedural · Describe the layout of packaged refrigeration systems and the role of a refrigeration economizer

   A two-stage centrifugal chiller has an economizer between the condenser and the evaporator. Liquid from the condenser flashes in it at a pressure between condensing and evaporating, and the flash vapour is piped to the second-stage inlet. What does this do?

   1. It lowers the evaporator pressure so the chiller can make colder chilled water
   2. It separates oil from the liquid refrigerant and sends the oil back to the sump
   3. It strips air from the condenser liquid and passes the air to the purge unit
   4. It sends less flash gas to the evaporator and the flash vapour skips stage one
   Show the answer

   D. Correct. Flash gas does no cooling in the evaporator, so drawing it off in the economizer sends colder liquid with less vapour on to the evaporator, raising the refrigeration effect per kilogram. The flash vapour goes to the second stage, so it is compressed only from the intermediate pressure, which saves compressor work per kW of cooling.

   Why not the others

   A. The economizer does not set the evaporator pressure, which follows the chilled-water temperature and load. It improves efficiency by removing flash gas at an intermediate pressure.

   B. Oil is separated from the hot discharge gas by an oil separator in the discharge line, not by a flash vessel in the liquid line. The economizer handles flash gas, not oil.

   C. Air is removed by the purge unit, which draws from the top of the condenser where non-condensables collect. The economizer deals with refrigerant flash gas.

3. Procedural · Describe the different methods used to control the capacity of evaporators

   A dairy's glycol chiller has a hot-gas bypass valve that opens at light load to keep the evaporator working. Where is the hot gas injected, and why?

   1. At the evaporator inlet, adding a false load so the evaporator pressure stays up
   2. Into the condenser inlet, raising the head pressure so the expansion valve can feed
   3. Into the liquid line, warming the liquid so less of it flashes in the expansion valve
   4. Into the receiver, raising its pressure to push liquid out to the evaporator
   Show the answer

   A. Correct. At light load the evaporator pressure would fall toward a freeze or low-pressure trip and the compressor would short-cycle. Bypassing some discharge gas into the evaporator inlet, after the expansion valve, adds a false load that holds the evaporator pressure up, and because the gas passes through the evaporator the expansion valve keeps feeding liquid, so the suction gas does not overheat the compressor. (Some designs bypass to the suction line instead, which then needs a liquid-injection valve to cool the gas.)

   Why not the others

   B. Raising head pressure in cold weather is done with condenser fan or water control, or by flooding the condenser. Hot-gas bypass for light-load control goes to the evaporator side, where it holds the evaporator pressure up.

   C. Warming the liquid would make more of it flash in the valve, not less. Hot-gas bypass is injected at the evaporator inlet to hold the evaporator pressure at light load.

   D. Hot gas piped to the receiver is a cold-weather head-pressure arrangement that keeps the receiver pressure up when the condenser is held back; it does nothing for a lightly loaded evaporator. Light-load bypass goes to the low side, at the evaporator inlet, where it holds the evaporator pressure up.

4. Procedural · Describe the theory and operation of a lithium bromide absorption refrigeration system

   An arena owner asks whether a lithium bromide absorption chiller, fired by waste heat, could chill the rink's brine to −9 °C. Why can it not?

   1. Waste heat is too weak to drive it there, though a direct-fired unit could
   2. Its lithium bromide would boil out of the solution at that temperature
   3. Its evaporator pressure would rise above atmospheric at that temperature
   4. Its refrigerant is water, which freezes at 0 °C, well above −9 °C
   Show the answer

   D. Correct. In a lithium bromide machine water is the refrigerant, boiling in the evaporator under a deep vacuum (about 0.9 kPa absolute for 5 °C). Water freezes at 0 °C, so these machines make chilled water a few degrees above freezing, not brine at −9 °C. Sub-zero duty needs a vapour-compression plant or an ammonia-water absorption machine.

   Why not the others

   A. A direct-fired machine still uses water as its refrigerant, so it meets the same 0 °C limit. The heat source changes how the generator is fired, not what the evaporator can reach.

   B. Lithium bromide is a salt that does not boil off; only the water leaves the solution. The limit is the refrigerant, water, which freezes at 0 °C.

   C. Lower evaporating temperatures mean lower pressures, not higher, and the evaporator already runs under a deep vacuum. The limit is that the water refrigerant would freeze.

5. Procedural · Describe the properties of refrigerants relating to miscibility, leakage tendency, odour, moisture reaction, toxicity, and flammability

   A food plant's ammonia system uses a mineral compressor oil, and some oil gets past the oil separator into the low side every day. Where does the plant drain this oil from, and why there?

   1. From oil pots at the bottom of the low-side vessels, because the oil does not mix with ammonia and is heavier
   2. From the top of the low-pressure receiver, because the oil does not mix with ammonia and floats on it
   3. From the compressor crankcase only, because the oil dissolves in the ammonia and returns with the suction gas
   4. From the purge point on top of the condenser, because the oil collects there with the air
   Show the answer

   A. Correct. Mineral oil and liquid ammonia hardly mix, and the oil is denser than liquid ammonia, so oil that escapes the separator sinks and collects at the low points of receivers, surge drums and evaporators. Ammonia plants fit drain pots there, and the operator drains them on a routine under the plant's oil-draining procedure.

   Why not the others

   B. The oil does stay separate from the ammonia, but it is the heavier of the two, so it sinks rather than floats. Opening a connection at the top of the vessel would release ammonia vapour, not oil.

   C. That is how oil behaves in most halocarbon systems, where oil and refrigerant mix and the oil rides back to the compressor with the suction gas. Mineral oil and ammonia hardly mix, so in an ammonia plant the oil settles in the low side and has to be drained out.

   D. Air and other non-condensable gases collect at the top of the condenser and are purged there. Oil is a heavy liquid; it settles at the bottom of the low-side vessels, not with the gases at the top of the condenser.

02Answer key

03What unit B9 covers

Refrigeration from the cycle to the plant room: the vapour-compression cycle and how operating pressures and capacity are set, refrigerant classes, properties and hazards, direct and indirect systems, compressors, evaporators and condensers, controls and safety cut-outs, leak testing, charging, oil and purging, and ammonia-water and lithium bromide absorption systems.

Part B of the SOPEEC 4th Class syllabus names this unit “B9 Refrigeration” and lists four objectives under it (objectives 36 to 39 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

In the evaporator, a pressure is a temperature

Refrigerant boils at the saturation temperature for its pressure, so a suction pressure switch works as a temperature control. To set an evaporator, read the saturation pressure for the boiling temperature and subtract atmospheric for the gauge; compression ratios are worked in absolute pressure.

Capacity, tons and COP

A chiller's load is the chilled-water flow times c times the temperature drop; one ton of refrigeration is 3.517 kW. COP is that cooling divided by compressor input, and a falling COP in the log is an early sign of fouled tubes, air in the condenser or a short charge.

Ammonia's hazards and habits

Ammonia is toxic and flammable over a range of concentrations in air. It attacks copper, so industrial ammonia compressors are normally open-drive. Mineral oil does not mix with ammonia and is heavier, so it is drained from pots at the bottom of the low-side vessels.

Vacuum machines leak inward

A low-pressure chiller under vacuum draws air in at a leak and lets no refrigerant out, so it is raised above atmospheric before a leak search, and a rising purge run time is a leak warning. A lithium bromide machine uses water as its refrigerant, so it cannot make brine below 0 °C, and a sudden drop in cooling-water temperature risks crystallization.

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.