Air, Water and Drainage Services: Ventilation, Air Conditioning, Water Supply, Reuse, Storm Water, Sewage and Plumbing

Practice questions (20)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

1. In stack-driven natural ventilation, the airflow increases with

   1. a larger windward wall area, because the stack effect depends on wind pressure on the façade
   2. greater height between the low inlet and the high outlet and a larger indoor–outdoor temperature difference
   3. a smaller height between the openings combined with the outdoor temperature rising towards the indoor value
   4. placing the inlet and the outlet at the same level on one wall so that both see the same pressure
   Show answer

   Answer: B — greater height between the low inlet and the high outlet and a larger indoor–outdoor temperature difference

   The buoyancy pressure is proportional to the height between the openings and to the temperature difference, so flow rises with the square root of their product. If the outdoor air approaches the indoor temperature the driving force vanishes, and openings at one level give no stack pressure difference. Windward wall area helps wind-driven flow, not the stack effect.

2. A hall of 20 m × 12 m with a clear height of 3.6 m is occupied by 80 persons. The ventilation requirement is the larger of 5 air changes per hour and 10 L/s per person. The required ventilation rate, in L/s, is ____.

   Numerical answer — type the value.

   Show answer

   Answer: 1200

   V = 20 × 12 × 3.6 = 864 m³. By air changes: 5 × 864 = 4,320 m³/h = 4,320 ÷ 3,600 = 1.2 m³/s = 1,200 L/s. By occupancy: 80 × 10 = 800 L/s. The larger figure governs, so 1,200 L/s. Check: 864 × 5 ÷ 3.6 = 1,200.

3. Which of the following spaces should be kept at a negative pressure relative to its surroundings?

   1. A pharmaceutical clean room in which the product must be protected from dust
   2. An operating theatre in which the surgical site must be protected from contamination
   3. A hospital room for a patient with an airborne infectious disease
   4. A data-centre hall in which dust must be kept away from the equipment
   Show answer

   Answer: C — A hospital room for a patient with an airborne infectious disease

   Negative pressure makes air flow into the room and be exhausted, so contaminated air cannot leak out to corridors. The clean room, the operating theatre and the data-centre hall need the opposite: positive pressure that pushes air out so that dust and germs do not enter. The operating theatre is the tempting distractor, but it protects the patient from the surroundings and so is positive.

4. Which of the following heat gains to an air-conditioned room are purely sensible?

   1. Solar radiation transmitted through window glass
   2. Heat from lighting fixtures
   3. Heat and moisture released by the occupants
   4. Warm humid outdoor air brought in for ventilation
   Show answer

   Answer: A — Solar radiation transmitted through window glass; B — Heat from lighting fixtures

   Solar gain through glass and heat from lighting raise only the air temperature, so they are sensible. Occupants add both sensible heat and moisture, and humid fresh air carries both sensible and latent heat; each therefore has a latent part.

5. A direct evaporative (desert) cooler is most effective in which climate, and what does it do to the room air?

   1. Hot-dry climate; it lowers the dry-bulb temperature but raises the humidity ratio
   2. Warm-humid climate; it lowers both the dry-bulb temperature and the humidity ratio
   3. Cold climate; it raises the dry-bulb temperature and lowers the relative humidity
   4. Monsoon weather; it lowers the dry-bulb temperature and keeps the humidity ratio constant
   Show answer

   Answer: A — Hot-dry climate; it lowers the dry-bulb temperature but raises the humidity ratio

   Evaporating water into the air is an adiabatic process: the dry-bulb temperature falls towards the wet-bulb temperature while the humidity ratio rises. It works where the wet-bulb depression is large, which means dry air. Lowering temperature and moisture together is what a coil below the dew point does, not an evaporative cooler.

6. Chilled water flows at 12 L/s through the cooling coils of a building and warms by 5 K. Take the specific heat of water as 4.2 kJ/kg·K, 1 L of water as 1 kg and 1 TR as 3.5 kW. The cooling duty, in TR, is ____.

   Numerical answer — type the value.

   Show answer

   Answer: 72

   Mass flow = 12 kg/s. Cooling = 12 × 4.2 × 5 = 252 kW. In TR: 252 ÷ 3.5 = 72. Check: 12 × 5 = 60; 60 × 4.2 = 252; 3.5 × 72 = 252.

7. A 12-storey hotel must keep a clean façade with no wall-mounted boxes. Its guest rooms are occupied at different times, riser space is small, and the owner does not want a chilled-water plant or a cooling tower. Which system fits best?

   1. Window units fitted in the wall of every guest room, each with its own thermostat and remote
   2. One constant-air-volume packaged rooftop unit with large supply and return ducts to every floor
   3. A water-cooled central chiller with a rooftop cooling tower and fan-coil units in every guest room
   4. A VRF system with heat-recovery outdoor units serving ducted or cassette indoor units
   Show answer

   Answer: D — A VRF system with heat-recovery outdoor units serving ducted or cassette indoor units

   VRF gives each room its own control and part-load efficiency, needs only small refrigerant pipes in the shafts, and has no chilled-water plant or tower. The central chiller breaks the owner's condition; window units are noisy, spoil the façade and are hard to run at scale; a single packaged unit cannot control rooms individually and needs very large ducts.

8. Which of the following statements about central chilled-water air conditioning are correct?

   1. A water-cooled chiller with a cooling tower is generally more efficient than an air-cooled chiller of the same duty
   2. A cooling tower should be sited so that its discharge and drift do not reach fresh-air intakes
   3. A chiller with a higher COP uses more kilowatts per ton of refrigeration
   4. A VAV system holds the air volume constant and varies the supply-air temperature with the load
   Show answer

   Answer: A — A water-cooled chiller with a cooling tower is generally more efficient than an air-cooled chiller of the same duty; B — A cooling tower should be sited so that its discharge and drift do not reach fresh-air intakes

   Water-cooled condensing runs at a lower condensing temperature, so the COP is better, and siting the tower away from intakes keeps its drift, which can carry Legionella, out of the fresh air. Higher COP means fewer kW per ton, since kW/TR = 3.517 ÷ COP. VAV is the opposite of the claim that air volume stays constant: in VAV the supply temperature is constant and the air volume varies.

9. A housing block has 60 flats with 5 occupants each, and the demand is taken as 135 litres per capita per day. The overhead tank is to hold one-third of one day's demand. Its capacity, in m³ correct to one decimal place, is ____.

   Numerical answer — type the value.

   Show answer

   Answer: 13.5

   Persons = 60 × 5 = 300. Daily demand = 300 × 135 = 40,500 L. One-third = 13,500 L = 13.5 m³. Check: 135 ÷ 3 = 45 L per person, and 300 × 45 = 13,500 L.

10. A booster pump delivers 20 L/s against a total head of 50 m. The overall efficiency is 75% and ρg is 9.81 kN/m³. The shaft power required, in kW correct to two decimal places, is ____.

    Numerical answer — type the value.

    Show answer

    Answer: 13.08

    Q = 20 L/s = 0.02 m³/s. Water power = ρgQH = 9.81 × 0.02 × 50 = 9.81 kW. Shaft power = 9.81 ÷ 0.75 = 13.08 kW. Check: 0.75 × 13.08 = 9.81.

11. In the water supply of a tall building, break-pressure tanks or pressure-reducing valves are provided mainly to

    1. raise the pressure at the top-floor fixtures so that showers and taps there work better than before
    2. limit the static pressure at lower-floor fixtures and pipes by splitting the height into zones
    3. remove hardness and dissolved salts from the water that is carried up to the upper floors
    4. let one fixed-speed pump feed every floor directly without any storage tank on the roof
    Show answer

    Answer: B — limit the static pressure at lower-floor fixtures and pipes by splitting the height into zones

    Static pressure is ρgh, about 1 bar per 10.2 m, so the lowest floors of a very tall building would see pressures that damage fittings. Zoning with break-pressure tanks or reducing valves keeps each zone within the limits of its fixtures and pipes. They do not raise top-floor pressure, which a booster or a higher tank does, and they do not treat water.

12. Which of the following statements about rainwater harvesting are correct?

    1. A first-flush diverter discards the initial roof runoff that carries dust and droppings
    2. A recharge borewell needs a silt chamber and filter in front of it, or it clogs
    3. The downpipes of a harvesting system should be connected to the sewer so that excess flow is removed quickly
    4. Annual yield is rainfall × catchment area × runoff coefficient, and the coefficient for a roof is less than 1
    Show answer

    Answer: A — A first-flush diverter discards the initial roof runoff that carries dust and droppings; B — A recharge borewell needs a silt chamber and filter in front of it, or it clogs; D — Annual yield is rainfall × catchment area × runoff coefficient, and the coefficient for a roof is less than 1

    The first flush washes the roof and is discarded; a bore takes only filtered water; and yield is the gross rain volume multiplied by a coefficient below 1 for losses. Sending roof water to the sewer is wrong: it wastes the harvest and, in a separate system, overloads the sewer and treatment plant.

13. A roof catchment of 800 m² receives 900 mm of rain in a year. With a runoff coefficient of 0.8, the volume that can be harvested in the year, in m³, is ____.

    Numerical answer — type the value.

    Show answer

    Answer: 576

    Rain depth = 0.9 m. Gross volume = 0.9 × 800 = 720 m³. Harvestable = 0.8 × 720 = 576 m³. Check: 800 × 0.8 = 640 m² effective; 640 × 0.9 = 576.

14. Which of the following is classed as greywater, suitable for reuse in flushing after simple treatment?

    1. Discharge from showers, wash basins and washing machines
    2. Discharge from WC pans and urinals, which carries faecal matter
    3. Overflow from the outlet chamber of a septic tank after settling
    4. Sludge drawn off from the bottom of a primary settling tank
    Show answer

    Answer: A — Discharge from showers, wash basins and washing machines

    Greywater is the relatively clean wastewater of bathing, hand-washing and laundry. WC and urinal discharge is blackwater, which carries faecal matter and needs full treatment; septic-tank overflow and settled sludge are strongly polluted.

15. Borewell water supplied to a building is hard, has a high TDS and shows bacterial contamination. Which treatment train best makes it fit for drinking?

    1. Sedimentation and rapid sand filtration followed by chlorination
    2. Aeration and an activated-carbon filter followed by a storage tank
    3. A softener and reverse osmosis followed by disinfection by UV or chlorine
    4. Coagulation with alum followed by a screen at the tank inlet
    Show answer

    Answer: C — A softener and reverse osmosis followed by disinfection by UV or chlorine

    Hardness and dissolved salts are removed by a softener and RO, and bacteria need a disinfection step. Sand filtration with chlorination handles turbidity and germs but leaves hardness and TDS; carbon and aeration remove iron, odour and taste, not dissolved salts; alum and a screen cannot do either job.

16. A 4 ha site has 1 ha of roofs (C = 0.9), 1 ha of paving (C = 0.8) and 2 ha of lawn (C = 0.35). For a design intensity of 90 mm/h, the peak storm discharge by the rational formula Q = CiA/360 (Q in m³/s, i in mm/h, A in ha) is

    1. 0.68 m³/s
    2. 0.60 m³/s
    3. 0.90 m³/s
    4. 1.00 m³/s
    Show answer

    Answer: B — 0.60 m³/s

    The coefficient must be weighted by area: ΣCA = 0.9 + 0.8 + 2 × 0.35 = 2.4 ha, so Q = 2.4 × 90 ÷ 360 = 0.60 m³/s. The tempting 0.68 comes from taking the plain average of the three coefficients, (0.9 + 0.8 + 0.35) ÷ 3 = 0.683, which ignores that lawn covers half the site.

17. The main purpose of an anti-siphonage pipe connected to the crown of a trap is to

    1. increase the flow velocity of the discharge in the horizontal branch pipe so that solids are carried
    2. carry rainwater from the roof gutters into the house drain during heavy storms
    3. lead foul air from the sewer directly into the room so that it escapes through the window
    4. admit air so that suction or pressure in the stack cannot break the water seal
    Show answer

    Answer: D — admit air so that suction or pressure in the stack cannot break the water seal

    The pipe lets air in at the crown of the trap so that a passing discharge cannot siphon the seal away, and relieves back pressure. A vent carries air, never liquid, and it ends above the roof so that foul air is released clear of the building.

18. Which of the following statements about building plumbing systems are correct?

    1. A two-pipe system has separate stacks for soil and for waste
    2. A single-stack system omits separate trap vents and relies on short branches, limited gradients and deep seals
    3. A one-pipe system takes soil and waste in one stack with individually vented traps
    4. In a single-stack system the basins and baths must be connected to a separate waste stack
    Show answer

    Answer: A — A two-pipe system has separate stacks for soil and for waste; B — A single-stack system omits separate trap vents and relies on short branches, limited gradients and deep seals; C — A one-pipe system takes soil and waste in one stack with individually vented traps

    The statements on separate stacks, omitted trap vents and individually vented traps define the two-pipe, single-stack and one-pipe systems. The claim that basins and baths need a separate waste stack contradicts the single-stack idea, which is that one stack takes all discharges; a separate waste stack belongs to the two-pipe system.

19. Which statement about a septic tank is correct?

    1. It settles solids and digests sludge, but its effluent still needs a soak pit or further treatment
    2. It produces effluent that meets drinking-water standards once the accumulated sludge has been pumped out
    3. It should also receive roof rainwater so that its contents are flushed through regularly
    4. It removes all pathogens, so the effluent may be discharged directly into a nearby stream or drain
    Show answer

    Answer: A — It settles solids and digests sludge, but its effluent still needs a soak pit or further treatment

    A septic tank is only a primary treatment unit: its effluent is still high in BOD and pathogens, so it goes to a soak pit or dispersion trench in suitable soil, away from wells. Rainwater must never enter it, because it would overload the tank and wash out the settled solids.

20. A town of 5,000 people is supplied at 135 litres per capita per day, and 80% of the supply is assumed to reach the sewers. Ignoring infiltration, the average sewage flow, in m³/day, is

    1. 675
    2. 432
    3. 108
    4. 540
    Show answer

    Answer: D — 540

    Supply = 5,000 × 135 = 675,000 L/day = 675 m³/day. Sewage = 0.8 × 675 = 540 m³/day. The figure 675 forgets the 80% factor; 432 applies it twice (0.64 × 675); 108 is the per-person sewage in litres, not the town total.