RAC
AHU stands for ……….
- Air Handling Unit
- Air Heating Unit
- Air Humidifying Unit
- None of these
In a low pressure duct system ……….
- Velocities < 600 mpm and static pressure $$\leq$$ 5 cm H2O gauge
- Velocities < 500 mpm and static pressure $$\leq$$ 5 cm H2O gauge
- Velocities < 400 mpm and static pressure $$\leq$$ 5 cm H2O gauge
- Velocities < 350 mpm and static pressure $$\leq$$ 5 cm H2O gauge
Pressure drop in ducts takes place because of adt ……….
- Duct friction
- Change of direction
- Change of velocity
- All of these
This type of duct requires least material for carrying air ……….
- Rectangular
- Square
- Circular
- Trapezoidal
What is the key distinguishing feature of a ducted split AC system compared to a non-ducted split AC?
- It uses a different type of compressor
- It has two separate fan motors
- It uses a network of ducts to distribute conditioned air to multiple rooms
- It operates on a different refrigeration cycle
Explanation:
- Ducted and non-ducted splits can use the same types of compressors (e.g., rotary, scroll).
- Both system types have fans in both the indoor and outdoor units.
- The primary difference is the indoor unit. A ducted split AC has a concealed indoor unit (like a blower coil) that connects to ductwork, distributing air through grilles and diffusers in various rooms. A non-ducted (or mini-split) system has a visible indoor unit that blows air directly into a single space.
- Both systems operate on the same basic vapor compression refrigeration cycle.
The indoor unit of a ducted split AC is most commonly installed in which location?
- Mounted on an exterior wall
- Hung on an interior wall
- In a concealed space like a false ceiling or loft
- Next to the outdoor condenser unit
Explanation:
- Mounting on an exterior wall is typical for a window or through-the-wall AC unit.
- Hanging on an interior wall is typical for a wall-mounted mini-split indoor unit.
- The central blower unit of a ducted system is designed to be hidden from view, often in a dropped ceiling, attic, crawlspace, or dedicated mechanical closet.
- The indoor and outdoor units are always connected by refrigerant lines, but the indoor unit is never installed right next to the outdoor unit.
In a multi-split AC system, what is the maximum number of indoor units typically connected to a single outdoor unit?
- 1
- 2
- Up to 5 or more, depending on the model
- 10
Explanation:
- A single indoor unit defines a standard split system, not a multi-split.
- 2 is a common configuration, but it is not the maximum.
- Multi-split systems are designed to connect multiple indoor units (e.g., 2, 3, 4, 5, or sometimes even more) to a single, powerful outdoor condenser. This allows for zoned cooling/heating from one external unit.
- While some advanced systems may support many zones, 5 is a more typical practical maximum for standard residential multi-split systems.
What is the primary function of the thermistor in an AC electrical circuit?
- To compress the refrigerant
- To move the fan blades
- To measure temperature and send a resistance signal to the control board
- To step down the voltage for the control circuit
Explanation:
- Compression is the function of the compressor.
- The fan motor moves the fan blades.
- A thermistor is a temperature-sensitive resistor. Its resistance changes predictably with temperature. The main control board measures this resistance to monitor temperatures (e.g., room air, evaporator coil, discharge line) and makes operational decisions (start/stop compressor, adjust fan speed) based on these readings.
- Stepping down voltage is the function of a transformer.
If a ducted split AC system is running but delivering very little cooling air from the vents, the most likely cause is:
- A faulty outdoor fan motor
- A clogged filter drier
- A dirty air filter or a failing indoor blower motor
- A faulty thermostat battery
Explanation:
- A faulty outdoor fan would cause high head pressure and poor cooling, but airflow from the vents would likely be normal.
- A clogged filter drier would cause a refrigerant restriction, affecting cooling capacity, but not directly reducing airflow volume.
- Low airflow is almost always an indoor airflow issue. A severely dirty air filter is the most common cause, restricting air from entering the system. A failing blower motor that cannot achieve its proper speed is the second most common cause.
- Weak thermostat batteries might cause the system not to start, but if it’s running, the airflow is a separate mechanical issue.
What electrical component is responsible for providing the initial rotational “kick” to the compressor motor?
- Run capacitor
- Contactor
- Start capacitor and potential relay (in many systems)
- Thermostat
Explanation:
- The run capacitor helps improve the efficiency and power factor of a running motor but does not provide the initial start torque.
- The contactor is a switch that sends power to the compressor but does not help it start rotating.
- Many compressors, especially those with single-phase motors, require a start capacitor to create a phase shift that provides high starting torque. A potential relay is often used to take the start capacitor out of the circuit once the motor reaches near-operating speed.
- The thermostat is a low-voltage control that signals the system to start, but it does not assist the motor mechanically.
A multi-split system where each indoor zone can choose between cooling, heating, or off independently is called a:
- Single-zone system
- Heat pump system
- Heat Recovery (VRF) system
- Dual-fuel system
Explanation:
- A single-zone system controls one space.
- A standard heat pump can only be in one mode (all heating or all cooling) at a time for the entire system.
- A Heat Recovery system, a type of VRF (Variable Refrigerant Flow), is an advanced multi-split system. It uses a special branch circuit (refrigerant piping) and controls that allow some indoor units to cool while others heat simultaneously by moving heat from the cooling zones to the heating zones.
- A dual-fuel system switches between a heat pump and a gas furnace based on outside temperature.
Which tool is essential for measuring the resistance of an electrical component like a capacitor or motor winding?
- Manifold gauge set
- Clamp meter
- Multimeter
- Vacuum pump
Explanation:
- A manifold gauge set measures refrigerant pressure.
- A clamp meter measures current (amperage).
- A multimeter, set to the ohms (Ω) function, is used to measure electrical resistance. This is critical for checking if windings are open (infinite resistance) or shorted (very low resistance), and for checking capacitor health.
- A vacuum pump is used to remove moisture and air from the refrigerant circuit before charging.
What is the purpose of the expansion valve in a ducted split AC system?
- To compress refrigerant vapor
- To reject heat to the outside air
- To meter the flow of liquid refrigerant and cause it to expand into a cold mist
- To move air across the evaporator coil
Explanation:
- Compression is the compressor’s job.
- Rejecting heat is the condenser’s job.
- The expansion valve (either a thermal expansion valve-TXV or electronic expansion valve-EXV) is a metering device. It creates a pressure drop, causing the high-pressure liquid refrigerant to rapidly expand and cool into a low-pressure, low-temperature liquid/vapor mixture before it enters the evaporator coil.
- Moving air is the blower fan’s job.
If one indoor unit in a multi-split system is not working, but all others are functioning correctly, the problem is most likely:
- A complete loss of refrigerant charge
- A faulty compressor in the outdoor unit
- An issue with that specific indoor unit’s PCB, fan, or its dedicated zone control
- A tripped main circuit breaker for the outdoor unit
Explanation:
- A complete loss of charge would cause all indoor units to stop cooling/heating.
- A faulty compressor would stop the entire outdoor unit, affecting all connected indoor units.
- Since the other indoor units are working, the outdoor unit is functional. The problem must be isolated to the non-working indoor unit. This could be a failed indoor PCB, a stuck/blown fan motor, a faulty remote or controller for that zone, or a wiring issue specific to that unit.
- A tripped main breaker would power down the entire outdoor unit and all indoor units.
What safety device is designed to shut off the compressor in the event of excessively high pressure?
- Low-pressure switch
- Thermostat
- High-pressure switch
- Overload protector
Explanation:
- A low-pressure switch protects against loss of charge or evaporator freeze-up.
- The thermostat is a control, not a safety device for compressor pressure.
- The high-pressure switch is a critical safety device that monitors the discharge pressure from the compressor. If pressure becomes dangerously high (e.g., due to a dirty condenser, fan failure, or overcharge), it will open the electrical circuit to the compressor to prevent mechanical failure.
- An overload protector (internal or external) protects against excessive electrical current and motor temperature, not directly from system pressure.
The electrical component that uses a low-voltage signal from the thermostat to switch high-voltage power to the compressor and fan motor is called a:
- Transformer
- Capacitor
- Contactor
- Relay
Explanation:
- The transformer steps down line voltage (e.g., 230V) to low voltage (e.g., 24V) for the control circuit.
- The capacitor helps start and/or run the motor.
- The contactor is an electromagnetic switch. Its coil is energized by the 24V signal from the thermostat. When energized, it pulls in heavy-duty contacts that close to send high-voltage power to the compressor and outdoor fan motor.
- Relays are used for lower-current switching within the control circuit.
Ice formation on the evaporator coil of a ducted system could be caused by all of the following EXCEPT:
- Low refrigerant charge
- Dirty air filter
- Failing indoor blower motor
- Excessively high outdoor ambient temperature
Explanation:
- A low charge can cause a pressure drop and temperature drop in the evaporator, leading to freezing.
- A dirty filter restricts airflow, reducing the heat load on the coil and causing its temperature to drop below freezing.
- A failing blower motor also results in reduced airflow, causing the coil to freeze.
- High outdoor temperatures cause high head pressure and strain on the system, but they do not cause the evaporator coil to become colder. They typically make it harder for the system to cool effectively, raising evaporator temperatures, not lowering them to the freezing point.
What is the purpose of the inverter technology used in many modern split and multi-split systems?
- To convert DC power to AC power for the compressor
- To vary the compressor speed to match the cooling/heating demand
- To increase the refrigerant pressure
- To control the fan speed in the indoor unit
Explanation:
- This is part of how it works (the inverter drive creates AC of variable frequency), but it is not the purpose. The purpose is speed control.
- The primary purpose of an inverter drive is to control the speed of the compressor motor. Instead of just turning on at full power and off (which is less efficient), an inverter compressor can run continuously at a variable speed that precisely matches the building’s load, greatly improving comfort and energy efficiency.
- Compressor speed affects pressure, but pressure change is a result, not the goal.
- While fan speeds can be variable, “inverter technology” specifically refers to compressor control.
When troubleshooting an electrical fault, the first step should always be to:
- Start replacing components one by one
- Check the refrigerant pressure
- Disconnect power to the unit completely
- Clean the condenser coil
Explanation:
- This is an inefficient and potentially costly approach without proper diagnosis.
- Refrigerant pressure is a mechanical check, not the first step for an electrical fault.
- Safety is always the highest priority. Before inspecting, testing, or touching any electrical components, the unit must be completely disconnected from its power source at the main disconnect switch or circuit breaker to prevent electrical shock.
- Cleaning the coil is a maintenance task, not a troubleshooting step for an electrical fault.
What is the primary function of an air filter in an HVAC system?
- To cool the air
- To heat the air
- To remove contaminants and particulates from the air
- To increase the humidity of the air
Explanation:
- Cooling is the function of the evaporator coil.
- Heating is the function of the furnace or heating coil.
- The core purpose of an air filter is to improve indoor air quality by capturing dust, pollen, mold spores, and other airborne particles.
- Humidification is handled by a separate humidifier unit, not the filter.
Which of the following is a common disposable air filter type constructed from spun fiberglass or polyester fibers?
- HEPA filter
- Electronic air cleaner
- Panel filter
- Carbon filter
Explanation:
- HEPA filters are made of dense, pleated paper-like material.
- Electronic air cleaners use an electrical charge to trap particles.
- Panel filters are flat, rectangular, and made from fibrous material like fiberglass. They are the most common type for residential HVAC systems.
- Carbon filters use activated carbon to adsorb gases and odors, not just particulates.
What does the acronym HEPA stand for?
- High Efficiency Particle Arrestance
- High Energy Particulate Air
- High Efficiency Particulate Air
- High Efficiency Particle Absorption
Explanation:
- This is a common misstatement.
- This is incorrect.
- This is the correct expansion. A True HEPA filter must remove at least 99.97% of particles that are 0.3 microns in size.
- HEPA filters work by trapping particles, not absorbing them.
A filter’s ability to remove particles of a specific size is measured by its:
- Pressure drop
- Airflow rate
- Efficiency rating (e.g., MERV)
- Thickness
Explanation:
- Pressure drop indicates resistance to airflow, not filtering ability.
- Airflow rate is the volume of air moving through the system.
- The Minimum Efficiency Reporting Value (MERV) is a standard scale from 1 to 20 that rates a filter’s ability to capture particles between 0.3 and 10 microns.
- Thickness can affect capacity but is not a direct measure of efficiency.
What is the most immediate effect of a severely clogged air filter?
- Improved indoor air quality
- Reduced airflow across the heat exchanger and coils
- Increased cooling capacity
- Decreased system static pressure
Explanation:
- A clogged filter cannot improve air quality; it becomes less effective and can harbor contaminants.
- A clogged filter creates a high-resistance barrier. The fan must work harder to pull air through it, resulting in significantly reduced airflow across the system’s coils and heat exchanger.
- Reduced airflow leads to decreased system capacity and performance.
- Static pressure across the fan increases due to the higher resistance.
Which type of air filter uses an electrical charge to attract and trap particles?
- Impingement filter
- HEPA filter
- Electronic air cleaner (Electrostatic precipitator)
- Viscous impingement filter
Explanation:
- Impingement filters rely on particles physically striking and getting stuck on fibers.
- HEPA filters use a dense mat of fibers for mechanical filtration.
- Electronic air cleaners ionize particles, giving them a charge so they are attracted to and collected on oppositely charged plates within the unit.
- Viscous impingement filters are coated with an adhesive oil to trap particles on contact.
A filter with a MERV rating of 16 is most likely to be used in:
- A residential window AC unit
- A standard residential furnace
- A commercial office building
- A hospital surgery room or pharmaceutical cleanroom
Explanation:
- Window units typically use very low MERV filters.
- Standard furnaces often use filters between MERV 5 and 11.
- Commercial buildings may use filters up to MERV 13 or 14.
- MERV ratings of 13-16 are used for superior performance in hospitals, and 17-20 (HEPA) are used in strict cleanrooms and surgery suites.
What is a common maintenance task for a permanent, washable air filter?
- Replacing it every month
- Applying a new electrical charge
- Cleaning it with water and mild detergent
- Recharging it with activated carbon
Explanation:
- Permanent filters are not replaced, they are cleaned.
- This is not a maintenance task for washable filters.
- Permanent filters are typically made of a durable, woven material that can be rinsed with water, sometimes with a mild detergent, allowed to dry completely, and then reinstalled.
- Carbon filters are typically disposable, not washable and rechargeable in this way.
A clogged air filter can lead to “short cycling” of an HVAC system because it causes:
- The compressor to overheat
- The limit switch to trip due to overheating
- The refrigerant pressure to drop too low
- The thermostat to malfunction
Explanation:
- While a clogged filter can contribute to compressor issues, it’s not the direct cause of short cycling.
- Reduced airflow from a dirty filter causes the heat exchanger to overheat. A safety device called a high-limit switch will trip to shut off the burner (in heating) or the entire system to prevent damage. Once it cools, the system restarts, leading to rapid on/off cycles known as short cycling.
- In cooling, reduced airflow causes low evaporator temperature and low refrigerant pressure, potentially tripping a low-pressure switch, but the limit switch is a more direct link to short cycling in heating.
- The thermostat operates independently of the filter’s condition.
What is the primary purpose of the pleats in a high-efficiency pleated filter?
- To make the filter look more modern
- To reduce the cost of the filter
- To increase the surface area for capturing particles
- To decrease the air velocity through the filter
Explanation:
- Appearance is not the primary engineering goal.
- Pleated filters are often more expensive than flat panel filters.
- Pleating creates a larger total surface area within the same frame size. This allows the filter to hold more dirt before becoming clogged and can improve efficiency without a drastic increase in airflow resistance.
- While velocity may change, it is a consequence, not the purpose.
Which of the following is a potential consequence of operating an HVAC system without any air filter?
- Increased system efficiency
- Dust and debris accumulation on the blower motor, coils, and ducts
- Lower electricity consumption
- Quieter system operation
Explanation:
- Efficiency may initially seem higher due to no pressure drop, but coil fouling will quickly reduce it.
- The filter protects the internal components of the HVAC system. Without it, dust will coat the blower assembly, clog the evaporator and condenser coils, and accumulate in the ductwork, leading to reduced performance, efficiency, and potential mechanical failures.
- Electricity consumption may decrease initially but will increase as the system struggles against fouled components.
- Operation may not be quieter, and component failure could lead to noise.
What does a “V” or “Pleated” in a filter’s name typically indicate?
- It is a viscous impingement filter
- It is an electronic filter
- It has an increased surface area due to its folded design
- It is a very low efficiency filter
Explanation:
- Viscous filters are usually flat and coated with oil.
- Electronic filters have collection plates, not pleats.
- The “V” or “Pleated” designation refers to the physical construction where the filter media is folded into a “V” or other pattern. This is done solely to pack more filtering media into the same frame size, increasing surface area and dust-holding capacity.
- Pleated filters are generally medium to high efficiency.
The pressure drop across a clean filter is 0.2 inches of water column (in. WC). When the filter becomes dirty, the pressure drop will most likely:
- Decrease
- Remain the same
- Increase
- Become zero
Explanation:
- Pressure drop decreases only if the restriction is removed.
- As particles load the filter, they block air passages.
- As a filter loads with dirt, the openings in the filter media become clogged. This increases the resistance to airflow, which is measured as an increase in pressure drop across the filter.
- Pressure drop will only be zero if there is no filter or no airflow.
Which type of filter is most effective at removing odors and gaseous pollutants from the air?
- HEPA filter
- Washable plastic filter
- Fiberglass panel filter
- Activated carbon filter
Explanation:
- HEPA filters are excellent for particles but ineffective against gases and odors.
- Washable and fiberglass filters are for particulate matter only.
- Fiberglass panel filters are for particulate matter only.
- Activated carbon (charcoal) filters contain a bed of carbon with a vast network of pores that adsorb (attract and hold) gas molecules and odorous compounds through a process called adsorption.
What is a typical recommended frequency for checking and potentially replacing a standard 1-inch disposable air filter in a residential system?
- Every week
- Every 5 years
- Every 1 to 3 months
- Only when the system breaks down
Explanation:
- Weekly is excessive and unnecessary for most homes.
- 5 years is far too long; the system would be severely damaged.
- This is the standard recommendation. The exact interval depends on factors like filter MERV rating, indoor air quality, presence of pets, and system runtime.
- Waiting for a breakdown will result in costly repairs.
A “choked” or clogged air filter in an AC system can cause the evaporator coil to:
- Overheat
- Freeze
- Increase in pressure
- Stop absorbing humidity
Explanation:
- The heat exchanger in a furnace overheats, not the AC evaporator coil.
- Reduced airflow from a clogged filter means less heat is absorbed by the evaporator coil. This can cause the coil’s temperature to drop below the freezing point of water, leading to condensation on the coil freezing into ice, which further blocks airflow.
- The refrigerant pressure in the evaporator will drop, not increase.
- It will still absorb humidity, but the resulting condensation may freeze.
What is the key construction difference between a standard panel filter and a HEPA filter?
- HEPA filters are always electronic
- Panel filters are always pleated
- HEPA filters have a much denser and deeper media pack
- Panel filters are made of paper
Explanation:
- HEPA filters are mechanical, not electronic.
- Panel filters can be flat or pleated.
- True HEPA filters are constructed from a very dense mat of very fine fibers, often arranged in a deep pleated pattern. This dense construction is what allows them to trap extremely small particles but also creates a higher initial pressure drop.
- Both can be made from paper-like materials; the density is the key factor.
The efficiency of an air filter that relies on particles being trapped after straying too close to a fiber is known as:
- Gravitational settling
- Impingement
- Interception
- Diffusion
Explanation:
- Gravitational settling is for large particles that fall out of the air stream.
- Impingement is when particles directly strike fibers.
- Interception occurs when a particle following an air stream around a fiber comes within one particle radius of the fiber and is captured by attractive forces.
- Diffusion affects very small particles that bounce around randomly (Brownian motion) and hit fibers.
Which component’s energy consumption increases the most due to a clogged air filter?
- The compressor
- The thermostat
- The ignition system
- The blower motor
Explanation:
- While the compressor may run longer, it is not the primary component working harder against the restriction.
- The thermostat is a low-power control device.
- The ignition system operates intermittently.
- The blower motor must work much harder to force air through the increased resistance of the clogged filter. This can significantly increase its electrical energy consumption and cause it to overheat.
A filter’s dust spot efficiency rating measures its ability to remove:
- Large visible dust bunnies
- Fine particles that soil surfaces
- Gaseous pollutants
- Bacteria and viruses
Explanation:
- Large particles are easily captured by most filters.
- The dust spot efficiency test measures a filter’s ability to remove the fine, smog-like particles (typically in the 0.3 to 10 micron range) that cause streaking and soiling of walls and furnishings.
- Gaseous pollutants require adsorption, not particulate filtration.
- While some fine particles may be biological, this test is not specific to them.
What is the purpose of the cardboard or metal frame on a disposable panel filter?
- To increase the filter’s efficiency
- To add weight to the filter
- To provide rigidity and prevent air bypass
- To absorb odors
Explanation:
- The frame itself does not filter air.
- Weight is not a functional goal.
- The frame holds the filter media taut and ensures a tight seal within the filter rack. This prevents unfiltered air from bypassing the media around the edges, which would defeat the purpose of the filter.
- The frame material is not designed for odor absorption.
In a system with a dirty filter, the temperature difference (ΔT) between the supply and return air will likely:
- Decrease
- Remain constant
- Increase
- Become unpredictable
Explanation:
- A decreased ΔT is a symptom of low refrigerant charge.
- The ΔT will change due to reduced airflow.
- With reduced airflow (CFM) across the coil due to a clogged filter, the same amount of heating or cooling capacity is put into or removed from a smaller volume of air. This results in a larger temperature change for that air, increasing the ΔT.
- The change is predictable and a key diagnostic clue.
Which of these is NOT a mechanical filtration mechanism?
- Straining
- Impingement
- Interception
- Ionization
Explanation:
- Straining is a mechanical process where particles larger than the gaps are caught.
- Impingement is a mechanical process where particles hit and stick to fibers.
- Interception is a mechanical process where particles touch fibers and are held by van der Waals forces.
- Ionization is an electrical process used in electronic air cleaners, not a mechanical one. It charges particles so they can be attracted to collector plates.
What is the primary function of a duct in an HVAC system?
- To heat the air
- To cool the air
- To convey or transport air
- To filter the air
Explanation:
- Heating is the function of a boiler or furnace.
- Cooling is the function of an evaporator coil or chiller.
- Ducts are pathways designed for the distribution and return of air throughout a building.
- Filtering is the function of air filters, not the ducts themselves.
Which of the following is a common type of duct shape known for its efficient airflow and structural rigidity?
- Rectangular duct
- Round duct
- Oval duct
- Square duct
Explanation:
- Rectangular ducts are common but have higher pressure drop and require more bracing.
- Round ducts offer less frictional resistance to airflow, are more rigid, and require less bracing compared to rectangular ducts.
- Oval ducts are a compromise between round and rectangular, used where space is a constraint.
- Square duct is a type of rectangular duct and shares similar properties.
Galvanized steel is a preferred material for ductwork primarily because of its:
- Light weight
- Flexibility
- Durability and resistance to corrosion
- Low cost
Explanation:
- While sturdy, steel is not the lightest material (e.g., compared to aluminum).
- Flexible ducts are made from materials like plastic and wire coils, not rigid steel.
- The zinc coating on galvanized steel protects the base metal from rust and corrosion, making it durable for long-term use.
- It is moderately priced but not always the cheapest option; cost is not its primary advantage.
The purpose of insulating ductwork is primarily to:
- Increase air velocity
- Reduce noise from the fan
- Prevent heat gain or loss and control condensation
- Make the ducts more aesthetically pleasing
Explanation:
- Insulation does not increase air velocity; it can slightly reduce it by increasing duct size.
- While it can provide some acoustic damping, that is a secondary benefit.
- Preventing thermal energy loss (in conditioned spaces) or gain (in unconditioned spaces) is the primary reason. It also keeps the duct surface temperature above the dew point to prevent condensation.
- Aesthetics are not a primary concern for functional insulation.
What does AHU stand for in HVAC systems?
- Air Handling Unit
- Air Heating Unit
- Automatic Handling Unit
- Axial Heating Unit
Explanation:
- This is the correct expansion. An AHU conditions and circulates air.
- An AHU can handle both heating and cooling, not just heating.
- This is not the correct term.
- This is not the correct term; axial refers to a type of fan.
In duct design, what does the term ‘pressure drop’ refer to?
- The loss in temperature of the air
- The loss in air velocity
- The loss in static pressure as air moves through the duct
- The increase in dynamic pressure
Explanation:
- Pressure drop is related to energy loss, not temperature loss.
- Velocity may change, but pressure drop is specifically the loss of pressure.
- Friction and turbulence in the duct system cause a loss of static pressure, which the fan must overcome.
- Dynamic pressure is velocity pressure; pressure drop is a loss of total pressure.
Which air distribution method is characterized by supplying air from outlets near the floor and returning it near the ceiling?
- Mixing ventilation
- Laminar flow
- Displacement ventilation
- Unidirectional flow
Explanation:
- Mixing ventilation aims to blend supply air with room air uniformly.
- Laminar flow is a smooth, ordered flow pattern, not a specific distribution method.
- Displacement ventilation supplies cool air at low velocity from floor level. As the air is warmed by heat sources, it rises and is exhausted at the ceiling level.
- Unidirectional flow is typically used in cleanrooms.
A centrifugal fan is typically used in HVAC systems for its ability to:
- Move large volumes of air at low pressure
- Generate high pressure to overcome resistance in duct systems
- Operate with minimal noise
- Function efficiently in axial flow applications
Explanation:
- Axial fans are better for moving large volumes at low pressure (e.g., ceiling fans).
- Centrifugal fans use a wheel to accelerate air radially, changing the direction of the airflow and generating higher static pressure, making them ideal for ducted systems.
- Noise levels depend on the design and application, not exclusively the fan type.
- Axial flow is characteristic of axial fans, not centrifugal fans.
What is the key difference between a ‘fan’ and a ‘blower’ in common HVAC terminology?
- A fan moves air, while a blower moves gases
- A fan is always centrifugal, while a blower is always axial
- A fan operates at lower pressure, while a blower operates at higher pressure
- There is no difference; the terms are interchangeable
Explanation:
- Both can move air or other gases.
- Both fans and blowers can be of either centrifugal or axial design.
- By ASHRAE definition, a fan has a pressure ratio up to 1.11, a blower has a pressure ratio from 1.11 to 1.2, and a compressor has a higher ratio. In common parlance, blowers are associated with higher pressure applications.
- While often used loosely, there is a technical distinction based on pressure generation.
Flexible ducts are typically made from:
- Galvanized steel
- Aluminum sheet
- A wire helix covered with plastic or foil
- Fiberglass reinforced plastic
Explanation:
- Galvanized steel is used for rigid sheet metal ducts.
- Aluminum sheet is also used for rigid ducts.
- Flexible ducts consist of a spring steel wire helix covered with a durable plastic sheet and often include insulation, making them easy to install in tight spaces.
- FRP is used for rigid ducts, often in corrosive environments.
The velocity of air inside a duct is measured in:
- Pascals (Pa)
- Meters per second (m/s) or Feet per minute (fpm)
- Cubic meters per second (m³/s)
- Degrees Celsius (°C)
Explanation:
- Pascals are the unit for pressure.
- This is the correct unit for velocity (e.g., typical duct velocities are 5-15 m/s or 1000-3000 fpm).
- This is the unit for volumetric flow rate.
- This is the unit for temperature.
What is the purpose of a turning vane inside a duct elbow?
- To increase the air temperature
- To reduce the duct size
- To guide the airflow and reduce pressure drop
- To filter the air
Explanation:
- Turning vanes do not affect temperature.
- They do not reduce the physical size of the duct.
- Turning vanes are installed inside sharp bends (elbows) to smooth the airflow around the corner, minimizing turbulence and the associated energy loss (pressure drop).
- They are not related to air filtration.
Which component is NOT typically a part of a standard Air Handling Unit (AHU)?
- Filters
- Heating and/or cooling coils
- Fan
- Compressor
Explanation:
- Filters are a standard component to clean the incoming air.
- Coils are standard for conditioning the air (heating with hot water/steam or electric, cooling with chilled water or direct expansion).
- A fan is the core component that moves the air through the AHU.
- The compressor is part of the external refrigeration cycle (condensing unit), not the AHU itself. An AHU using direct expansion (DX) cooling will have a DX coil, but the compressor is located remotely.
For a given airflow rate, a larger duct size will result in:
- Higher air velocity
- Lower air velocity and lower pressure drop
- Higher pressure drop
- Increased system noise
Explanation:
- Larger cross-sectional area results in lower velocity for a constant flow rate (Q = A * V).
- Lower velocity translates to reduced friction and turbulence, which significantly reduces the pressure drop in the duct system.
- Pressure drop decreases with lower velocity.
- Lower velocity generally results in lower noise generation.
What is a ‘plenum’ in an HVAC system?
- A type of fan
- A type of insulation
- A large air distribution box or chamber
- A type of air filter
Explanation:
- It is not a fan, though it is connected to fans.
- It is not insulation, though it may be insulated.
- A plenum is a compartment or chamber to which one or more ducts are connected, used to distribute or collect air from multiple pathways (e.g., a supply plenum right after the AHU).
- It is not a filter, though it may be near filters.
Which law of fluid mechanics is primarily used for calculating pressure drop in ducts?
- Newton’s Law of Cooling
- Bernoulli’s Principle
- Darcy-Weisbach Equation
- Ohm’s Law
Explanation:
- This law relates to heat transfer, not fluid flow pressure loss.
- Bernoulli’s principle relates pressure and velocity in a streamline but does not account for frictional losses.
- The Darcy-Weisbach equation is a fundamental formula used to calculate the major pressure loss (friction loss) in ducts and pipes.
- Ohm’s Law is for electrical circuits, though its concept is sometimes analogized to fluid flow.
Which material is commonly used for ductwork in environments where corrosion resistance is critical, such as swimming pools?
- Galvanized steel
- Stainless steel or PVC
- Flexible duct
- Aluminum
Explanation:
- Galvanized steel can corrode in highly humid and chlorinated environments.
- Stainless steel offers excellent corrosion resistance. PVC (polyvinyl chloride) plastic ducts are also highly resistant to moisture and chemical corrosion.
- The materials in standard flexible ducts are not suitable for such corrosive environments.
- Aluminum has good corrosion resistance but may not be the best choice for highly aggressive atmospheres compared to stainless or PVC.
The term ‘boot’ in ductwork refers to:
- The main trunk duct
- The AHU housing
- A fitting that connects a duct run to a diffuser or grille
- The fan housing
Explanation:
- The trunk duct is the main distribution duct.
- The AHU housing is the cabinet containing the components.
- A boot is a transition fitting, often at the end of a branch duct, that adapts the round or rectangular duct to the specific connection point of a floor, wall, or ceiling register/diffuser.
- The fan housing is called a volute or scroll, particularly in centrifugal fans.
What is the purpose of a damper in a duct system?
- To insulate the duct
- To clean the air
- To regulate, control, or stop the airflow
- To measure air velocity
Explanation:
- Dampers are not used for insulation.
- Dampers do not filter the air.
- Dampers are adjustable valves or plates installed in ducts. They are used to balance airflow to different branches, shut off flow to specific zones, or control the volume of air.
- An anemometer is used to measure velocity, not a damper.
Which type of duct system is characterized by a main trunk duct with smaller branches serving individual outlets?
- Radial system
- Loop system
- Extended plenum system
- Constant volume system
Explanation:
- A radial system has individual ducts running from a central plenum to each outlet.
- A loop system forms a complete circuit around the building.
- An extended plenum system uses a large main duct (trunk) that extends from the AHU, with smaller branch ducts tapped off to serve the diffusers. This is a very common layout.
- Constant volume refers to the operation of the fan, not the physical layout of the ducts.
Noise in ductwork can be reduced by:
- Increasing air velocity
- Using sharper elbows
- Lining the ducts with acoustic insulation
- Reducing the duct diameter
Explanation:
- Higher velocity increases noise from turbulence.
- Sharper elbows increase turbulence and noise.
- Acoustic (sound-absorbing) liner inside ducts absorbs sound energy, reducing noise transmission through the duct system.
- Reducing diameter increases velocity and, consequently, noise.
The static pressure in a duct is defined as the pressure:
- Exerted in the direction of airflow
- That causes the air to move
- Exerted perpendicular to the direction of airflow
- Measured with a Pitot tube facing the airstream
Explanation:
- Pressure in the direction of flow is related to velocity pressure.
- The fan causes the air to move; static pressure is the potential energy in the system.
- Static pressure is the component of the total pressure that is exerted in all directions perpendicular to the duct walls, whether the air is moving or not. It is the pressure that inflates a balloon if inserted into the duct.
- A Pitot tube facing the stream measures total pressure. The static port measures static pressure.
What is the primary advantage of a ‘VAV’ (Variable Air Volume) system?
- Simpler duct design
- Lower initial installation cost
- Energy savings by varying the airflow based on demand
- Constant air velocity in ducts
Explanation:
- VAV system design can be more complex due to the need for controls and VAV boxes.
- The initial cost is typically higher due to added controls and devices.
- Instead of running the fan at full capacity constantly, a VAV system reduces the airflow (often by slowing the fan with a VFD) when full cooling/heating is not needed, leading to significant fan energy savings.
- Air velocity in the main duct varies with the volume flow rate in a VAV system.
A ‘fire damper’ is installed in a duct penetration through a fire-rated wall to:
- Improve airflow
- Reduce noise
- Prevent the spread of fire through the ductwork
- Control temperature
Explanation:
- It restricts airflow when closed, it does not improve it.
- Noise reduction is not its function.
- Its sole purpose is life safety. It automatically closes (via a fusible link that melts at high temperature) to seal the opening in the fire barrier, containing the fire and smoke to its compartment of origin.
- It does not control HVAC temperature.