4th Class Power Engineering HVAC and air handling practice questions (4B).

Five free practice questions on HVAC and air handling: unit B10 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 operation of air handling units

   In most air-handling units the filter bank sits upstream of the heating and cooling coils. The main reason is to

   1. keep dust off the coil fins, where it cuts heat transfer and airflow
   2. keep the filter media warm and dry so it holds its rated efficiency
   3. let the coils catch any fibres that the filter media sheds into the air
   4. slow the air before the coils so condensate is not blown off the fins
   Show the answer

   A. Correct. Dust that reaches a coil sticks to the wet or oily fins, insulates them and narrows the passages, so capacity falls and the fan moves less air. Filtering first keeps the coils (and everything downstream) clean, and a clean coil is far cheaper than a coil cleaning.

   Why not the others

   B. Upstream of the coils the filters see the coldest, untempered air in winter, so this placement does not keep them warm. They are placed first to keep dust off the coil fins, where it would cut heat transfer and airflow.

   C. Coils are not meant to be filters: anything that lodges in their fins fouls them. The filter bank is placed first so dust never reaches the fins, where it would cut heat transfer and airflow.

   D. Air velocity through a coil is set by the coil's face area and the airflow, not by a filter upstream of it. The filters come first to keep dust off the coil fins, where it would insulate them and choke the airflow.

2. Recall · Describe residential and warm air types of humidifiers

   A flow-through (bypass) humidifier on a house's warm-air furnace trickles water over its evaporator pad, and the water the air does not pick up runs to a floor drain. Sending that water to drain rather than recirculating it

   1. flushes minerals away so they do not build up as scale on the pad
   2. keeps the furnace heat exchanger from overheating while the unit runs
   3. dumps surplus water whenever the humidistat calls for less moisture
   4. cools the supply air so that it can carry more moisture into the house
   Show the answer

   A. Correct. As water evaporates from the pad, the minerals it carried are left behind. A steady surplus flowing to drain carries them away, so the pad scales up more slowly and no reservoir of concentrated, stagnant water is kept. The pad still scales slowly and is replaced as the maker directs.

   Why not the others

   B. The humidifier's water never touches the furnace heat exchanger; it runs over a pad in the plenum or bypass duct. The surplus goes to drain to flush away the minerals left behind as water evaporates.

   C. The humidistat controls the solenoid valve, which stops the water entirely when no moisture is wanted. The surplus that runs to drain while the unit operates is there to flush away the minerals left behind on the pad.

   D. Cooler air can hold less moisture, not more, and the humidifier is not there to cool the air. The surplus water runs to drain to flush away the minerals left on the pad as water evaporates.

3. Recall · Describe the major types of air handling fans, their construction, and operation

   A vane-axial fan differs from a tube-axial fan by having fixed guide vanes beside the wheel inside its housing. The guide vanes

   1. stop air from flowing backward through the housing when the fan stops
   2. throttle the airflow to set the fan's output at part load
   3. turn the air's swirl into straight flow and useful static pressure
   4. let the blade pitch be adjusted while the fan keeps running
   Show the answer

   C. Correct. The rotating blades give the air a spin as well as forward motion. Fixed guide vanes straighten it, recovering that swirl energy as static pressure, so a vane-axial fan develops more pressure at a better efficiency than a tube-axial fan of the same size.

   Why not the others

   A. Preventing reverse flow is the job of a backdraft damper, whose blades close when the air stops. The fixed guide vanes are always open; they straighten the swirl the wheel gives the air and recover it as static pressure.

   B. Fixed vanes cannot throttle anything; flow control needs a movable element such as variable inlet vanes, adjustable blade pitch, a damper or a speed change. The fixed guide vanes straighten the swirling air and recover it as static pressure.

   D. Adjustable pitch is a feature of the wheel's own blades and their actuator, not of fixed guide vanes. The guide vanes straighten the swirling air leaving the wheel and turn that swirl into useful static pressure.

4. Procedural · Describe the various contaminants found in air

   A plant fits much finer pleated filters in an air-handling unit, but the paint-solvent odour from a neighbouring shop still comes in with the outdoor air. Why did the change not help?

   1. The odour is a vapour whose molecules pass through particle filters
   2. Finer filters load faster, and odour slips past them as they clog
   3. The solvent dissolves the filter media and is released downstream
   4. Odours pass through filters only when the relative humidity is high
   Show the answer

   A. Correct. Air contaminants fall into particles (dust, smoke, pollen, microorganisms), and gases and vapours (solvents, combustion gases, odours). Particle filters catch particles in their fibres (by straining, impaction and diffusion); gas and vapour molecules are far smaller and pass straight through. Gases are removed by adsorption, for example in activated carbon, or kept out by moving the intake.

   Why not the others

   B. A loading filter catches particles better, not worse, and no particle filter catches odour at any stage. The odour is a vapour whose molecules pass through particle media; it needs adsorption (activated carbon) or a relocated intake.

   C. Solvent vapour at odour levels does not dissolve filter media. The filters fail to help because the odour is a gas or vapour, whose molecules pass through any particle filter; adsorption media such as activated carbon is needed.

   D. Humidity does not decide whether a particle filter stops a vapour; it never does. Odours are gases or vapours whose molecules pass through particle media, so they need adsorption media such as activated carbon.

5. Recall · Identify the types of liners, dampers, and louvres used in air duct systems

   A fire damper is installed where a supply duct passes through a fire separation. In a fire it closes when

   1. the supply fan stops and the airflow no longer holds it open
   2. a smoke detector in the duct signals its motor actuator to close it
   3. heat melts its fusible link and a spring or gravity closes it
   4. the operator pulls its release cable at the fire alarm panel
   Show the answer

   C. Correct. A fire damper is held open by a fusible link that melts when hot gases reach it; the blades or curtain then close under spring or gravity force and keep fire from passing through the separation along the duct. A link that has melted must be replaced and the damper reset; it does not reopen by itself.

   Why not the others

   A. That describes a backdraft (gravity) damper, whose blades are held open by the airflow and fall shut when it stops. A fire damper closes when heat melts its fusible link.

   B. That describes a smoke damper, which is closed by an actuator on a signal from a smoke detector or the fire alarm system. A fire damper closes when heat melts its fusible link.

   D. Fire dampers are automatic; they do not wait for anyone to act. Heat melts the fusible link and a spring or gravity closes the damper.

02Answer key

03What unit B10 covers

Conditioning air for comfort and health: the categories of HVAC systems and air-handling units, humidity, dew point and dry- and wet-bulb temperatures, humidifiers, air flow and pressure, fans, fan curves and volume control, natural and mechanical ventilation, air contaminants and filters, duct systems, dampers and diffusers, and steam, hot-water and glycol coils.

Part B of the SOPEEC 4th Class syllabus names this unit “B10 Heating Ventilating and Air Conditioning” and lists six objectives under it (objectives 40 to 45 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

Dry bulb, wet bulb, dew point

For unsaturated air the dry bulb is highest, then the wet bulb, then the dew point. Relative humidity is the vapour pressure at the dew point over the saturation pressure at the dry bulb. Heating cold outdoor air leaves its moisture unchanged, so its relative humidity falls sharply.

Humidifier load is airflow times the humidity-ratio rise

The water a humidifier must add is the dry-air mass flow times the difference between the humidity ratio wanted and the one entering. Over-humidifying in a deep cold snap streams water down the windows, so the setting comes down while the cold lasts.

Total pressure is static plus velocity pressure

A pitot tube's velocity pressure is total minus static, and velocity follows from ρv²/2. On a fixed system flow follows fan speed, pressure the speed squared and power the speed cubed. A forward-curved fan's power keeps rising with flow, so running it with the filters out can trip the motor.

Filters catch particles; coils trap water

Particle filters do not stop vapours and odours; those need adsorption or a moved intake. A coil drained by gravity still holds water in its tubes and return bends, so coils that meet freezing air are blown out or filled with glycol.

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.