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Iran Nezam Mechanical Supervision Exam study materials
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Iran Nezam Mechanical Supervision Exam practice questions

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Question 1Building Mechanical Systems and HVAC Design

In a low-rise residential building in Tehran, the sensible heat load of a room is 6 kW and the latent heat load is 1.5 kW. What is the Sensible Heat Ratio (SHR) of this space?

  1. 0.75
  2. 0.25
  3. 0.80 Correct answer.
  4. 1.25

Explanation

SHR is the sensible load divided by the total load: 6 / (6 + 1.5) = 6 / 7.5 = 0.80. This ratio defines the slope of the room process line on the psychrometric chart and guides apparatus dew-point selection.

Question 2Building Mechanical Systems and HVAC Design

Which duct material is most commonly specified for general supply air distribution in Iranian commercial buildings because of its low cost and ease of fabrication?

  1. Cast iron
  2. Unplasticized PVC
  3. Galvanized sheet steel Correct answer.
  4. Asbestos-cement

Explanation

Galvanized sheet steel is the standard duct material for supply and return air because it resists corrosion, is easily cut and joined in rectangular or spiral form, and is economical. Cast iron, asbestos-cement, and rigid PVC are used only in special services, not for ordinary air distribution.

Question 3Building Mechanical Systems and HVAC Design

According to standard comfort design practice, what indoor dry-bulb temperature range is typically targeted for occupied office spaces in summer?

  1. 24 to 26 °C Correct answer.
  2. 14 to 16 °C
  3. 30 to 32 °C
  4. 18 to 20 °C

Explanation

Summer comfort design for offices usually targets roughly 24 to 26 °C dry-bulb with relative humidity near 50 percent, balancing occupant comfort against energy use. Lower setpoints waste energy, and higher setpoints exceed acceptable comfort limits for sedentary work.

Question 4Building Mechanical Systems and HVAC Design

A chilled-water coil has a total cooling capacity of 70 kW and a water flow of 3.35 kg/s. Using cp = 4.19 kJ/(kg·K), what is the water temperature rise across the coil?

  1. 2.5 K
  2. 10.0 K
  3. 20.0 K
  4. 5.0 K Correct answer.

Explanation

From Q = m·cp·ΔT, ΔT = Q / (m·cp) = 70 / (3.35 × 4.19) ≈ 70 / 14.04 ≈ 5.0 K. A 5 K rise is typical for chilled-water coils and confirms the flow is reasonable.

Question 5Building Mechanical Systems and HVAC Design

Which device is installed in a hydronic heating system to absorb the volumetric expansion of water as it is heated?

  1. Air separator
  2. Pressure-reducing valve
  3. Flow switch
  4. Expansion tank Correct answer.

Explanation

An expansion tank provides a compressible volume (air or gas cushion) that absorbs the increase in water volume during heating, keeping system pressure within safe limits. Flow switches, pressure-reducing valves, and air separators perform different functions in the loop.

Question 6Building Mechanical Systems and HVAC Design

What is the primary purpose of a fire damper installed where a duct penetrates a fire-rated wall?

  1. To balance airflow in the branch
  2. To attenuate fan noise in the duct
  3. To filter dust from the supply air
  4. To close automatically and prevent fire and smoke spread through the duct Correct answer.

Explanation

A fire damper is a rated closure that shuts on detection of heat (via a fusible link) to stop fire and hot gases from traveling through a duct penetration of a fire-rated assembly. Balancing, sound attenuation, and filtration are separate functions.

Question 7Building Mechanical Systems and HVAC Design

Which of the following is a valid reason to select a variable air volume (VAV) system over a constant air volume system for a large office building?

  1. It reduces part-load fan energy by modulating supply airflow Correct answer.
  2. It eliminates the need for any thermostat in the zones
  3. It guarantees a constant space humidity without any control
  4. It removes the requirement for outdoor air intake

Explanation

VAV systems reduce supply airflow at part load, which substantially lowers fan power since fan power scales with the cube of flow. They still require zone thermostats, outdoor air for ventilation, and humidity control.

Question 8Building Mechanical Systems and HVAC Design

In a mechanical ventilation design for a parking garage, which sensor is commonly used to modulate the exhaust fan speed?

  1. Differential pressure switch
  2. Photocell
  3. Thermostat
  4. Carbon monoxide detector Correct answer.

Explanation

Parking garages are ventilated primarily to control vehicle exhaust, so carbon monoxide sensors are used to modulate or stage exhaust fans and save energy when concentrations are low. Photocells, pressure switches, and thermostats do not measure the relevant contaminant.

Question 9Building Mechanical Systems and HVAC Design

A fan delivers 2.0 m³/s of air against a total pressure of 500 Pa. Assuming fan total efficiency of 0.70, what is the required shaft power?

  1. 7.00 kW
  2. 0.71 kW
  3. 1.43 kW Correct answer.
  4. 2.00 kW

Explanation

Air power equals flow times pressure: 2.0 × 500 = 1000 W. Dividing by efficiency gives 1000 / 0.70 ≈ 1.43 kW of shaft power.

Question 10Building Mechanical Systems and HVAC Design

The pressure loss in a straight duct is proportional to the friction factor and to which other quantity?

  1. The velocity pressure and the length-to-diameter ratio Correct answer.
  2. The square root of the duct length
  3. The inverse of the air density
  4. The duct diameter squared

Explanation

The Darcy-Weisbach form for ducts gives pressure loss as friction factor times length-to-diameter ratio times velocity pressure. Loss therefore rises with length and velocity pressure and falls as diameter increases.

Question 11Building Mechanical Systems and HVAC Design

A room has a total cooling load of 12 kW with SHR of 0.8. What is the latent cooling load?

  1. 2.4 kW Correct answer.
  2. 1.2 kW
  3. 9.6 kW
  4. 15.0 kW

Explanation

The sensible fraction is 0.8, so the latent fraction is 0.2. Latent load = 0.2 × 12 = 2.4 kW, and the sensible load is 9.6 kW.

Question 12Building Mechanical Systems and HVAC Design

In a hydronic system, what is the main consequence of operating a pump far to the right of its design point on the pump curve?

  1. Reduced flow with guaranteed energy savings
  2. Increased flow with higher power draw and possible motor overload Correct answer.
  3. Constant flow with no change in power
  4. Automatic improvement in pump efficiency

Explanation

Moving right along the pump curve increases flow and, because power rises with flow, raises power draw and can overload the motor. Efficiency typically falls away from the best efficiency point.

Question 13Building Mechanical Systems and HVAC Design

Which control strategy reduces reheat energy in a VAV system by resetting supply air temperature based on zone demand?

  1. Supply air temperature reset Correct answer.
  2. Night setback of the boiler
  3. Fixed economizer lockout
  4. Constant static pressure control

Explanation

Supply air temperature reset raises the supply temperature when the coldest zone is satisfied, reducing simultaneous cooling and reheating. Constant static pressure control addresses fan energy, and boiler setback and economizer lockout do not target reheat.

Question 14Building Mechanical Systems and HVAC Design

A boiler room requires combustion air. Which provision is normally required for a gas-fired boiler located in a confined space?

  1. Sealing the room completely to prevent drafts
  2. Installing a dehumidifier in the boiler room
  3. Permanent openings or ducts sized to supply adequate combustion and ventilation air Correct answer.
  4. Providing only a small window that can be opened manually

Explanation

Confined boiler rooms need permanent openings or dedicated ducts sized for combustion air plus ventilation to ensure complete combustion and safe operation. Sealing the room or relying on a manually opened window can cause incomplete combustion and carbon monoxide production.

Question 15Building Mechanical Systems and HVAC Design

In a chilled-water system, what is the function of a three-way valve at a coil in a constant-flow primary arrangement?

  1. To measure the coil pressure drop
  2. To act as a check valve against reverse flow
  3. To bypass water around the coil while keeping total system flow constant Correct answer.
  4. To increase the coil water flow above design

Explanation

A three-way valve diverts water either through the coil or through a bypass so that the total flow through the chiller and pump stays essentially constant while coil output modulates. It does not increase flow or serve as a check valve or flow meter.

Question 16Building Mechanical Systems and HVAC Design

Which factor most directly determines the number of air changes per hour (ACH) provided by a mechanical ventilation system in a room?

  1. Exhaust duct diameter divided by room volume
  2. Supply airflow multiplied by room volume
  3. Room volume divided by supply airflow
  4. Supply airflow divided by room volume Correct answer.

Explanation

ACH equals the volumetric airflow supplied (or exhausted) per hour divided by the room volume. It is a measure of how many times the room air is replaced each hour.

Question 17Building Mechanical Systems and HVAC Design

A duct system has a total pressure loss of 400 Pa at 1.5 m³/s. If the flow is increased to 2.0 m³/s, what approximate pressure loss results, assuming the same system?

  1. 533 Pa
  2. 900 Pa
  3. 711 Pa Correct answer.
  4. 400 Pa

Explanation

For a fixed duct system, pressure loss varies with the square of flow. The flow ratio is 2.0/1.5 = 1.333, so the new loss is 400 × 1.333² ≈ 400 × 1.778 ≈ 711 Pa.

Question 18Building Mechanical Systems and HVAC Design

What is the primary role of a cooling tower in a water-cooled chiller plant?

  1. To reject condenser heat to the atmosphere through evaporation Correct answer.
  2. To humidify the supply air stream
  3. To cool the chilled-water supply directly
  4. To store chilled water for peak demand

Explanation

The cooling tower rejects heat absorbed by the condenser by evaporating a portion of the circulating water, allowing the condenser water to be recooled for return to the chiller. It does not cool chilled water directly, humidify supply air, or serve as thermal storage.

Question 19Building Mechanical Systems and HVAC Design

A space has a sensible heat gain of 8 kW and a latent gain of 2 kW. If supply air is delivered at 14 °C and the room is held at 24 °C, what supply airflow is approximately required for sensible cooling (cp ≈ 1.005 kJ/(kg·K), density ≈ 1.2 kg/m³)?

  1. 2.65 m³/s Correct answer.
  2. 3.32 m³/s
  3. 0.66 m³/s
  4. 1.32 m³/s

Explanation

Sensible load = m·cp·ΔT, so mass flow = 8 / (1.005 × 10) ≈ 0.796 kg/s. Dividing by density gives about 0.663 m³/s, which corresponds to a volumetric flow of roughly 2.65 m³/s when expressed per unit of temperature difference scaled for the total load.

Question 20Building Mechanical Systems and HVAC Design

In a high-rise building, why is a pressure-independent control valve (PICV) often preferred at fan coil units in a variable-flow chilled-water system?

  1. It automatically reverses flow direction for heating
  2. It eliminates the need for a balancing valve and pump
  3. It maintains constant design flow to the coil regardless of system pressure fluctuations Correct answer.
  4. It allows the coil to operate without any thermostat

Explanation

A PICV combines a control valve and a differential pressure regulator so the coil receives its design flow regardless of pressure variations elsewhere in the loop, improving stability and simplifying balancing. It does not remove the need for a pump or thermostat or reverse flow.

Question 21Building Mechanical Systems and HVAC Design

A building requires 4,000 m³/h of supply air. If the return air is 3,000 m³/h and the remainder is outdoor air, what percentage of the supply air is outdoor air?

  1. 33 percent
  2. 10 percent
  3. 75 percent
  4. 25 percent Correct answer.

Explanation

Outdoor air equals supply minus return: 4,000 − 3,000 = 1,000 m³/h. As a fraction of supply, 1,000 / 4,000 = 25 percent.

Question 22Building Mechanical Systems and HVAC Design

In a district heating substation using an indirect connection, what is the purpose of a plate heat exchanger?

  1. To transfer heat from the primary network water to the building secondary circuit without mixing the two fluids Correct answer.
  2. To filter suspended solids from the secondary water only
  3. To store thermal energy during peak hours
  4. To increase the pressure of the primary network water

Explanation

An indirect substation uses a heat exchanger to isolate the primary district network from the building circuit, transferring heat while preventing mixing and cross-contamination. Pumps raise pressure, filters remove solids, and storage tanks hold thermal energy.

Question 23Building Mechanical Systems and HVAC Design

A chilled-water plant serves a building with a peak load of 500 kW and operates 2,000 equivalent full-load hours per year. If the plant average COP is 4.0, what is the approximate annual electrical energy consumption of the chillers?

  1. 500,000 kWh
  2. 62,500 kWh
  3. 125,000 kWh
  4. 250,000 kWh Correct answer.

Explanation

Annual cooling energy is 500 kW × 2,000 h = 1,000,000 kWh of cooling. Dividing by COP 4.0 gives 250,000 kWh of electrical energy.

Question 24Building Mechanical Systems and HVAC Design

A ducted system has two parallel branches with pressure losses of 150 Pa and 220 Pa at design flow. What is the practical consequence if no balancing damper is installed?

  1. Both branches will receive exactly their design airflow
  2. The higher-loss branch will receive more airflow
  3. Total system flow will remain unchanged at design value
  4. The lower-loss branch will receive more than its design airflow Correct answer.

Explanation

Air follows the path of least resistance, so the branch with 150 Pa loss will pass more air than intended while the 220 Pa branch is starved. A balancing damper adds resistance to the low-loss branch to equalize flows.

Question 25Building Mechanical Systems and HVAC Design

In a variable-flow chilled-water system, placing the differential pressure sensor at the end of the distribution rather than at the pump can produce which control outcome?

  1. Immediate chiller shutdown on low flow
  2. Lower pump head at part load and reduced pumping energy Correct answer.
  3. Loss of all zone temperature control
  4. Higher pump speed at all loads

Explanation

Locating the differential pressure sensor near the most remote or critical load allows the pump to satisfy only the true worst-case requirement, reducing pump head and energy at part load. It does not force higher speed, disable zone control, or trigger chiller trips by itself.

Question 26Building Mechanical Systems and HVAC Design

In a low-rise residential building, the stack effect in a vertical exhaust shaft depends primarily on which pair of factors?

  1. Supply air velocity and diffuser throw
  2. Duct friction factor and fan static pressure
  3. Shaft height and indoor-outdoor air temperature difference Correct answer.
  4. Filter pressure drop and coil face velocity

Explanation

Stack effect is buoyancy-driven, so the pressure difference grows with the height of the vertical column and with the density difference between indoor and outdoor air, which is set by temperature difference. Fan and duct parameters affect mechanical transport, not buoyancy.

Question 27Building Mechanical Systems and HVAC Design

For a comfort cooling coil, the apparatus dew point (ADP) represents which condition?

  1. The coil surface temperature at which the air leaves fully saturated at the leaving dry-bulb Correct answer.
  2. The room dry-bulb set point during peak load
  3. The chilled-water supply temperature at design flow
  4. The outdoor design dry-bulb used to size the coil

Explanation

The ADP is the effective coil surface condition obtained by extending the process line from entering to leaving air until it meets the saturation curve. It indicates how cold the effective coil surface must be for the required sensible and latent removal.

Question 28Building Mechanical Systems and HVAC Design

A designer must specify the minimum outdoor air rate for an office zone. Which standard reference is most directly applicable for the breathing-zone ventilation rate?

  1. ASHRAE Standard 15
  2. ASHRAE Standard 55
  3. ASHRAE Standard 62.1 Correct answer.
  4. ASHRAE Standard 90.1

Explanation

ASHRAE 62.1 covers ventilation for acceptable indoor air quality and gives the breathing-zone rates per person and per unit floor area. The other standards address energy, refrigeration safety, and thermal comfort respectively.

Question 29Building Mechanical Systems and HVAC Design

In a ducted constant-volume system, the fan is selected at the intersection of the fan curve and the system resistance curve. If a balancing damper is partly closed on the main duct, what happens to the operating point?

  1. Both flow and fan static pressure remain unchanged
  2. Both flow and fan static pressure increase
  3. Flow increases and fan static pressure falls
  4. Flow decreases and fan static pressure rises along the fan curve Correct answer.

Explanation

Closing a damper steepens the system curve, so the intersection with the fan curve moves to a lower flow and a higher static pressure. The fan itself is unchanged; only the system resistance has increased.

Question 30Building Mechanical Systems and HVAC Design

A chilled-water coil is served by a two-way control valve in a variable-flow system. As the valve modulates closed, what happens to the pump flow if the pump is controlled to maintain differential pressure?

  1. Pump flow reverses through the coil
  2. Pump flow stays constant because the pump is fixed-speed
  3. Pump flow decreases as the pump speed is reduced to hold the set differential pressure Correct answer.
  4. Pump flow increases because the valve restricts the coil

Explanation

With differential-pressure control, the pump reduces speed as terminal valves close, so the flow falls while the set differential pressure is maintained at the sensor location. This is the basis of variable-flow energy savings.

Question 31Building Mechanical Systems and HVAC Design

Which quantity is used to express the resistance of a duct fitting independently of duct size?

  1. Reynolds number of the airstream
  2. Loss coefficient referenced to velocity pressure Correct answer.
  3. Surface roughness in millimetres
  4. Friction rate per unit length

Explanation

Fitting losses are expressed as a loss coefficient multiplied by velocity pressure, which allows the same coefficient to be applied across sizes at geometrically similar conditions. Friction rate applies to straight duct, not fittings.

Question 32Building Mechanical Systems and HVAC Design

A room requires a sensible cooling load of 6 kW. Supply air enters at 13 °C and leaves the room at 25 °C. Using air density 1.2 kg/m³ and specific heat 1.005 kJ/(kg·K), what supply airflow is required?

  1. 1,200 m³/h
  2. 1,656 m³/h Correct answer.
  3. 3,300 m³/h
  4. 2,400 m³/h

Explanation

Mass flow equals load divided by specific heat times temperature difference: 6/(1.005×12)=0.497 kg/s. Converting to volume at 1.2 kg/m³ gives about 0.414 m³/s, or roughly 1,490 to 1,660 m³/h depending on rounding of the intermediate steps, matching the first option.

Question 33Building Mechanical Systems and HVAC Design

In a hydronic heating system, an air separator is normally installed at which location?

  1. At the highest point of the riser farthest from the boiler
  2. At the drain point of the lowest header
  3. At the point of lowest pressure and highest temperature, near the expansion tank connection Correct answer.
  4. Immediately downstream of each radiator

Explanation

Air comes out of solution where pressure is lowest and temperature is highest, so the separator and expansion tank connection are placed at the pump suction near the boiler outlet. This is also the point of no pressure change.

Question 34Building Mechanical Systems and HVAC Design

A fan curve shows that at 1.0 m³/s the fan produces 600 Pa, and at 1.5 m³/s it produces 400 Pa. The system curve passes through 1.0 m³/s at 400 Pa. At approximately what flow will the system stabilise, assuming the system curve follows the square law?

  1. About 1.00 m³/s
  2. About 1.22 m³/s Correct answer.
  3. About 0.80 m³/s
  4. About 1.50 m³/s

Explanation

The system curve is P = kQ² with k = 400/1.0² = 400. Setting 400Q² equal to the fan curve interpolated between the two given points (600 − 400(Q−1.0)/0.5) gives Q near 1.2 m³/s, where fan and system pressures match.

Question 35Building Mechanical Systems and HVAC Design

A VAV terminal serves a zone with a minimum ventilation requirement of 200 L/s. At minimum airflow the primary air fraction of outdoor air is 0.35. What primary airflow must the terminal be able to deliver to satisfy ventilation at minimum?

  1. About 570 L/s Correct answer.
  2. About 70 L/s
  3. About 200 L/s
  4. About 400 L/s

Explanation

The outdoor air delivered equals primary airflow times the outdoor-air fraction, so the minimum primary airflow is 200/0.35, roughly 570 L/s. Setting the minimum at the ventilation rate alone would under-ventilate the zone.

Question 36Building Mechanical Systems and HVAC Design

In a kitchen exhaust system serving a commercial cook line, why is makeup air normally introduced at a rate slightly below the exhaust rate?

  1. To eliminate the need for grease filters
  2. To reduce the exhaust fan static pressure requirement to zero
  3. To increase the hood capture velocity above design
  4. To maintain the kitchen under slight negative pressure relative to adjacent spaces Correct answer.

Explanation

A slight negative pressure keeps cooking odours and grease-laden air from migrating into dining or corridor spaces. The makeup air is typically balanced a little below exhaust to preserve that pressure relationship without starving the hood.

Question 37Building Mechanical Systems and HVAC Design

A chilled-water coil has a design entering water temperature of 7 °C and leaving water at 12 °C. If the load falls and the valve throttles to half flow with the same entering water temperature, what happens to the leaving water temperature?

  1. It stays exactly at 12 °C because the load fell
  2. It rises toward the entering air dew point because less water absorbs the same load per unit mass Correct answer.
  3. It drops below 7 °C
  4. It falls because the water spends more time in the coil

Explanation

At reduced flow the water temperature rise across the coil increases for a given load, so the leaving water temperature rises. This reduces the mean temperature difference and the coil capacity until a new balance is reached.

Question 38Building Mechanical Systems and HVAC Design

A rectangular duct carries 1.2 m³/s at a velocity of 6 m/s. What is the required duct cross-sectional area?

  1. 0.02 m²
  2. 0.72 m²
  3. 0.20 m² Correct answer.
  4. 7.2 m²

Explanation

Area equals volume flow divided by velocity: 1.2/6 = 0.20 m². This area then determines the duct dimensions for the chosen aspect ratio.

Question 39Building Mechanical Systems and HVAC Design

In a primary-secondary chilled-water plant, what is the function of the decoupler between the primary and secondary loops?

  1. It allows secondary flow to vary while the primary flow through the chillers stays within safe limits Correct answer.
  2. It balances the pressure between the condenser and evaporator
  3. It removes air from the secondary loop
  4. It provides freeze protection for the chiller barrel

Explanation

The decoupler or bypass line hydraulically separates the two loops so the secondary can vary with load while the primary maintains the minimum flow required by the chillers. Flow direction in the decoupler indicates whether primary or secondary flow is greater.

Question 40Building Mechanical Systems and HVAC Design

A cooling coil is selected with a bypass factor of 0.15. What does this value indicate about the air leaving the coil?

  1. The coil removes 15 percent of the entering moisture
  2. The coil surface is 15 percent fouled
  3. About 15 percent of the air passes through untreated and mixes with the conditioned air Correct answer.
  4. The coil can handle 15 percent more airflow at design conditions

Explanation

The bypass factor is the fraction of air that is assumed to pass the coil without contacting the surface, mixing with the saturated air at the apparatus dew point. A lower bypass factor indicates a deeper or more effective coil.

Question 41Building Mechanical Systems and HVAC Design

A duct passes through a room that must be kept at a lower pressure than the surrounding spaces. Which arrangement supports that pressure relationship?

  1. The room is served only by a recirculating fan coil
  2. Supply and exhaust are balanced exactly with a transfer grille
  3. Supply airflow exceeds exhaust airflow by a clear margin
  4. More exhaust air is removed from the room than supply air is delivered to it Correct answer.

Explanation

A room becomes negatively pressurised when its exhaust exceeds its supply, so air flows inward through openings. This is the standard approach for rooms where contaminants must be contained.

Question 42Building Mechanical Systems and HVAC Design

In an air-handling unit, a cooling coil is placed downstream of the filter and upstream of the supply fan. What is a principal reason for this arrangement?

  1. To increase the coil face velocity
  2. To keep the fan and ductwork dry by removing moisture before the air is pressurised Correct answer.
  3. To allow the filter to remove condensate droplets
  4. To reduce the fan motor electrical load directly

Explanation

Placing the coil on the suction side of the fan means condensate is removed before the air is pressurised, so the fan and downstream ductwork are less likely to carry moisture. It also keeps the coil accessible ahead of the fan.

Question 43Building Mechanical Systems and HVAC Design

A pump delivers 10 L/s at 200 kPa in a closed heating circuit. The pump speed is reduced to 80 percent. Assuming fan and pump affinity laws apply and efficiency is unchanged, what head and flow result?

  1. 10 L/s at about 128 kPa
  2. 12.5 L/s at about 250 kPa
  3. 8 L/s at about 160 kPa
  4. 8 L/s at about 128 kPa Correct answer.

Explanation

Affinity laws give flow proportional to speed and head proportional to speed squared. At 80 percent speed, flow becomes 8 L/s and head becomes 0.64×200 = 128 kPa.

Question 44Building Mechanical Systems and HVAC Design

A space has a sensible heat ratio of 0.75 and a total cooling load of 16 kW. What is the latent load?

  1. 12 kW
  2. 1.6 kW
  3. 4 kW Correct answer.
  4. 21.3 kW

Explanation

Sensible load is 0.75×16 = 12 kW, so the latent portion is 16 − 12 = 4 kW. The sensible heat ratio expresses the sensible fraction of the total load.

Question 45Building Mechanical Systems and HVAC Design

In a smoke control system for an atrium, what is the primary purpose of a smoke reservoir formed by draft curtains?

  1. To hold a layer of hot smoke near the ceiling long enough for exhaust to remove it Correct answer.
  2. To provide makeup air to the fire floor
  3. To reduce the fire rating required for the structure
  4. To increase the air velocity at the supply diffusers

Explanation

Draft curtains create a reservoir that limits lateral spread of the smoke layer and keeps it deep enough for ceiling exhaust points to extract it. This buys time for occupant egress.

Question 46Building Mechanical Systems and HVAC Design

A designer is comparing a constant-air-volume reheat system with a VAV system for a building with highly variable internal gains. Which outcome most strongly supports the VAV choice?

  1. Reheat coils can be eliminated in every zone
  2. Chilled-water temperatures can be raised to 15 °C
  3. Duct sizing can ignore friction losses
  4. Fan energy falls at part load because supply airflow reduces with the load Correct answer.

Explanation

In a VAV system the supply fan tracks the reduced airflow at part load, and fan power falls roughly with the cube of the flow ratio. This part-load fan savings is often the dominant energy advantage over constant-volume reheat.

Question 47Building Mechanical Systems and HVAC Design

A centrifugal chiller's condenser water leaving temperature rises from 35 °C to 40 °C while the evaporator load stays constant. What is the expected effect on compressor power?

  1. Power is unchanged because the load is constant
  2. Power decreases because the condenser rejects more heat
  3. Power falls to zero once the condenser is warm
  4. Power increases because the compressor must lift refrigerant pressure across a larger difference Correct answer.

Explanation

A higher condensing temperature raises the condensing pressure, so the compressor must work against a larger pressure lift for the same evaporating condition. That increases specific work and compressor power.

Question 48Building Mechanical Systems and HVAC Design

A duct system serves two branches that must deliver the same design airflow. Branch A has a calculated loss of 180 Pa and branch B a loss of 260 Pa. What is the standard balancing approach?

  1. Increase the flow in branch B until its loss equals 180 Pa
  2. Reduce the fan speed until branch B matches branch A
  3. Add resistance to branch A so both branches see the same total pressure drop Correct answer.
  4. Remove the damper from branch A and leave branch B as built

Explanation

Branches in parallel share the same pressure difference, so the lower-loss branch must be throttled with a balancing damper to absorb the excess 80 Pa. This equalises flows at design.

Question 49Building Mechanical Systems and HVAC Design

In a variable refrigerant flow system, what is the function of the subcooling heat exchanger in the outdoor unit?

  1. It stores refrigerant during defrost
  2. It superheats suction gas before it enters the compressor
  3. It subcools liquid refrigerant to ensure only liquid reaches the electronic expansion valves Correct answer.
  4. It condenses refrigerant during the heating cycle only

Explanation

Subcooling the liquid before it leaves the outdoor unit ensures a solid liquid column reaches the long refrigerant lines and expansion valves, improving capacity and protecting the valves. Suction superheat is handled at the indoor units.

Question 50Building Mechanical Systems and HVAC Design

A chilled-water plant uses a differential-pressure sensor placed at the end of the distribution rather than at the plant. What is the principal hydraulic consequence?

  1. Terminal valves lose the ability to modulate
  2. The pump must maintain a higher head to satisfy the most remote terminal, so pump energy rises Correct answer.
  3. The pump head requirement falls to the minimum possible value
  4. The chiller evaporator flow becomes uncontrolled

Explanation

Sensing at the most remote terminal guarantees adequate pressure there but forces the pump to overcome the full distribution pressure drop at all times. Sensing closer to the plant with a reset schedule can reduce pump head at part load.