4th Class Power Engineering combustion, draft, feedwater and blowdown practice questions (4A).

Five free practice questions on combustion, draft, feedwater and blowdown: unit L of paper A (4A) 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 characteristics of solid, liquid, and gaseous fuels

   The higher (gross) heating value of a fuel is greater than its lower (net) heating value because the higher value includes

   1. the heat carried up the stack by the excess air that is supplied with the fuel
   2. the latent heat given up when water formed from the fuel's hydrogen condenses
   3. the heat released by burning the sulphur in the fuel to form sulphur dioxide
   4. the sensible heat that is still held in the ash when it leaves the furnace
   Show the answer

   B. Correct. Burning hydrogen forms water. If the products are cooled until that water vapour (and any fuel moisture) condenses, its latent heat is recovered and counted: that is the higher value. Flue gas from a non-condensing boiler leaves with the water still as vapour, so the lower value is what such a boiler can use.

   Why not the others

   A. Heat lost with excess air is a boiler loss that depends on how the burner is set, not a property of the fuel. Both heating values assume complete combustion; they differ by the latent heat of the water vapour in the products, most of it formed from the hydrogen.

   C. The heat from burning sulphur is counted in both values, since both assume every combustible element burns completely. The difference between them is the latent heat of the water vapour formed from the hydrogen.

   D. Ash is incombustible and its sensible heat is a boiler loss, not part of either heating value. The higher value exceeds the lower by the latent heat of the water vapour formed when the fuel's hydrogen burns.

2. Procedural · Describe gaseous fuel delivery systems

   In the natural-gas train of a hospital boiler, the main pressure regulator is installed upstream of the firing-rate (flow control) valve. What does this arrangement achieve?

   1. It raises the gas supply pressure up to the pressure the burner nozzles need
   2. It shuts off the gas tightly on a flame failure, before the control valve
   3. It steadies the control valve's inlet pressure, so flow follows its opening
   4. It measures the gas flow to the burner so the plant can keep its fuel records
   Show the answer

   C. Correct. Flow through a control valve depends on its opening and on the pressure difference across it. With the regulator holding the inlet pressure steady despite swings in the supply, each valve position gives a repeatable gas flow, so the fuel rate tracks the air damper.

   Why not the others

   A. A gas pressure regulator can only reduce pressure, holding a set outlet pressure below the supply; it cannot raise it. Its job here is to give the flow control valve a steady inlet pressure.

   B. Tight shut-off on a flame failure is the job of the automatic safety shut-off valves in the train, not of the regulator. The regulator holds a steady pressure so the firing-rate valve's position gives a predictable flow.

   D. Gas is measured by a gas meter, not by the regulator. The regulator holds a steady pressure at the control valve inlet so that the valve's position sets the flow.

3. Procedural · Relate differential pressure to the creation of draft

   A natural-draft chimney at a heating plant is 42 m high. Outside air has a density of 1.22 kg/m³ and the flue gas in the chimney 0.74 kg/m³. What theoretical draft does the chimney produce, to three significant figures?

   1. 198 Pa
   2. 305 Pa
   3. 503 Pa
   4. 808 Pa
   Show the answer

   A. Correct. Draft is the difference in weight between a column of outside air and an equal column of hot flue gas: Δp = gH(ρair − ρgas) = 9.81 × 42 × (1.22 − 0.74) = 9.81 × 42 × 0.48 = 198 Pa, about 20 mm of water. Friction in the chimney makes the actual draft less.

   Why not the others

   B. This uses the flue-gas density alone: 9.81 × 42 × 0.74 = 305 Pa, the pressure of the hot column, not the draft. Draft comes from the difference between the two columns: 9.81 × 42 × (1.22 − 0.74) = 198 Pa.

   C. This uses the outside-air density alone: 9.81 × 42 × 1.22 = 503 Pa. The hot gas column pushes back with 305 Pa of its own; the draft is the difference, 198 Pa.

   D. This adds the densities instead of subtracting them: 9.81 × 42 × (1.22 + 0.74) = 808 Pa. Draft is the difference between the outside-air and flue-gas columns: 9.81 × 42 × 0.48 = 198 Pa.

4. Recall · Describe methods of supplying feedwater to steam heating boilers

   Condensate from a hospital's low-pressure steam heating system drains to a boiler feed unit: a vented receiver with a pump that delivers to the boiler. What starts and stops the feed pump?

   1. A float switch in the receiver that senses the receiver level
   2. A pump control on the boiler that senses the boiler water level
   3. A pressure switch on the boiler that senses steam pressure
   4. A thermostat in the receiver that senses condensate temperature
   Show the answer

   B. Correct. A boiler feed unit feeds on demand from the boiler: a level-sensing pump control on the boiler starts the pump when the water falls and stops it at the normal level. The receiver is the reserve, topped up with make-up when it runs low.

   Why not the others

   A. That describes a condensate transfer (return) unit, whose receiver float runs the pump to send condensate back whenever it collects, whatever the boiler needs. A feed unit is controlled from the boiler's water level.

   C. Steam pressure says nothing about how much water is in the boiler; a pressure control governs the burner's firing, not the feed. The feed pump runs on the boiler's water level.

   D. Condensate temperature does not show whether the boiler needs water, so it cannot be the feed signal. A level-sensing control on the boiler starts and stops the pump.

5. Recall · Describe continuous blowdown, blowdown equipment, and blowdown procedures

   Where is the internal collecting pipe for continuous blowdown usually placed in a two-drum watertube boiler?

   1. Along the bottom of the mud drum, where sludge settles
   2. In the steam drum's steam space, above the water level
   3. In the steam drum, beside the feedwater inlet pipe
   4. In the steam drum, a little below the low water level
   Show the answer

   D. Correct. Steam leaves the water near the surface, so dissolved solids are most concentrated there. In most boilers a perforated pipe runs along the steam drum a short distance below the low water level, so it stays submerged at every level the boiler runs at, and draws that water off steadily through a metering valve.

   Why not the others

   A. Water at the bottom of the mud drum is not where dissolved solids are most concentrated, and a collector there would draw sludge as well. Dissolved solids build up in the steam-drum water near its surface, where steam leaves it, and that is where continuous blowdown draws from.

   B. A collector in the steam space would draw off steam, not boiler water, wasting heat and removing no solids. It must stay submerged, a little below the low water level.

   C. Water near the feed inlet is the freshest and least concentrated in the drum; drawing from there would waste feed and remove few solids. The collector goes where the concentration is high, below the water surface away from the incoming feed.

02Answer key

03What unit L covers

The systems that serve a boiler: combustion and excess air, fuel properties and refractory, coal, gas and oil delivery and firing, flue gas analysis, natural and mechanical draft, combustion air control, feedwater and condensate systems with single, two and three-element level control, blowoff and continuous blowdown, and fireside cleaning with sootblowers and shot.

The SOPEEC 4th Class syllabus names this unit “Boiler Systems” (ABSA numbers it A12) and lists six objectives under it. 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

Excess air is measured on theoretical

Excess air is actual air less theoretical air, divided by the theoretical air. The best setting is the least excess air that still burns the fuel out; cutting air until CO climbs sends fuel up the stack unburned.

Draft comes from column weights

Theoretical chimney draft is g times height times the difference between outside air and flue gas densities. Draft loss through tube banks rises with the square of gas flow, so a modest load increase needs much more fan pressure.

Swell fools single-element control

A sudden rise in steam demand drops drum pressure and makes the level swell while the boiler is losing water, so single-element control cuts the feed. Three-element control measures feedwater flow and matches it to steam flow.

Balance the water in and out

Feedwater must equal the steam made plus the blowdown, and make-up is that total less the condensate returned. When blowdown flashes in a tank, the fraction flashed is the drop in hf divided by hfg at the tank pressure.

05The other units

Paper 4A has twelve units, A to L. Each has its own page of free questions:

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