Top 100 HVAC Design Engineer Interview Questions & Answers
Why are water-cooled chillers generally more efficient than air-cooled chillers?
Water has a higher specific heat capacity and thermal conductivity compared to air, allowing it to absorb and reject heat more efficiently. Additionally, water-cooled chillers operate with more stable and lower condenser temperatures due to the use of cooling towers, which improves their efficiency.
What is the purpose of a cooling tower?
To reject heat from the building's HVAC system to the atmosphere by cooling water through evaporation.
What is the difference between open and closed cooling towers?
Open towers expose water directly to air; closed towers keep water inside tubes, using air to cool them indirectly.
What is approach temperature in a cooling tower?
Difference between cold water temperature leaving the tower and the ambient wet bulb temperature.
What is range in a cooling tower?
Temperature difference between hot water entering and cold water leaving the tower.
How do you calculate cooling tower efficiency?
Efficiency (Range) / (Range + Approach) x 100%
What factors affect cooling tower performance?
Ambient wet bulb temperature, water flow rate, airflow rate, fill type, drift loss, and fouling.
Why is water treatment important in cooling towers?
To prevent scale, corrosion, and biological growth, ensuring efficient heat transfer and equipment longevity.
What is drift loss?
Small water droplets carried away with the air leaving the tower, causing water loss.
How do you size a cooling tower?
Based on cooling load, approach, range, water flow rate, and ambient wet bulb temperature.
What is a drift eliminator?
A device inside the cooling tower that captures water droplets to minimize drift loss.
What is the typical COP for water-cooled chillers?
4.5 to 7.0 depending on conditions and efficiency.
What is the typical COP for air-cooled chillers?
3.0 to 4.5, generally lower than water-cooled due to higher condensing temperatures.
What does COP stand for?
Coefficient of Performance - the ratio of cooling capacity to power input.
How is AHU different from FAHU?
FAHU includes fresh air intake with filters and pre-treatment: AHU may recirculate internal air.
Why are water-cooled chillers generally more efficient than air-cooled chillers?
Because water has a higher heat transfer rate and stable condenser temperatures.
What causes chiller short cycling and how to avoid it?
Frequent on/off operation; avoid by proper sizing and using variable speed drives.
Why is pre-cooling used in FAHU?
To remove latent heat from humid outside air before it mixes with return air.
Why is reheating done after cooling in FAHU?
To avoid overcooling and maintain set supply air temperature and humidity.
What is the function of a VFD in HVAC?
Variable Frequency Drives control motor speed, improving energy efficiency in pumps and fans.
What is the delta-T in chilled water systems?
Temperature difference between chilled water supply and return; typically 10°C.
What is a DOAS?
Dedicated Outdoor Air System - handles 100% outdoor air ventilation, separate from space conditioning.
What is an RTU?
Rooftop Unit a packaged HVAC system mounted on the roof that provides heating and cooling.
DOAS vs RTU - which is better?
DOAS is better for precise ventilation and humidity control; RTU is simpler and cost-effective.
How do you select an FCU?
Based on cooling/heating load, air flow, available space, and static pressure requirements.
How to size an AHU?
Based on total air flow, external static pressure, cooling/heating coil capacity, and filtration needs.
What is sensible and latent heat?
Sensible changes air temperature: latent changes moisture content (humidity).
How do you calculate total heat?
Total heat Sensible heat + Latent heat
What is SHR?
Sensible Heat Ratio = Sensible Heat / Total Heat: used for coil and equipment selection.
Why do we need insulation on chilled water pipes?
To prevent condensation and reduce energy loss.
What are balancing valves used for?
To ensure proper flow rate to each terminal unit in hydronic systems.
What is an expansion tank used for?
To accommodate volume changes in water due to temperature variations.
What is air balancing?
Adjusting airflow in ducts to achieve design flow rates at diffusers and grilles.
What are the types of diffusers?
Slot diffuser, square diffuser, round diffuser, jet nozzle, etc.
What is a return air plenum?
A space used to collect return air and direct it to the AHU/RTU.
What is a chilled beam?
An HVAC device using convection and chilled water to cool indoor spaces.
What are common control strategies for chillers?
Lead-lag control, demand limiting, temperature reset, VFD control.
What is BMS?
Building Management System-controls HVAC, lighting, and other systems for efficiency.
How is chilled water flow rate calculated?
Q(TRx3024)/(AT x 60), where TR = Tons of Refrigeration, AT = temperature difference.
What is a primary-secondary pumping system?
Uses separate pumps for chiller loop (primary) and building loop (secondary) to maintain flow.
What is free cooling?
Using outdoor air or water directly for cooling when ambient conditions permit.
What is thermal storage in HVAC?
Storing chilled water or ice during off-peak hours to reduce peak demand.
What is a VAV system?
Variable Air Volume-adjusts airflow to maintain temperature, saving energy.
What is a constant volume system?
Delivers fixed airflow regardless of load, less efficient than VAV.
What is heat recovery?
Capturing waste heat from exhaust air to pre-heat incoming air.
Why do we use economizers?
To reduce mechanical cooling by using outdoor air when conditions are favorable.
What is an enthalpy wheel?
A rotating device that transfers heat and moisture between exhaust and supply air streams.
What is the function of a humidifier in AHU?
Adds moisture to air to maintain required humidity levels.
Why do we use filters in AHU/FAHU?
To remove dust, particulates, and pollutants, ensuring indoor air quality.
What are types of HVAC filters?
Pre-filters, Fine filters, HEPA filters, Carbon filters.
What is thermal comfort?
Condition of mind expressing satisfaction with the thermal environment: ASHRAE defines it via PMV and PPD.
What is the ideal indoor comfort temperature?
Typically 22-24°C with 50-60% relative humidity as per ASHRAE.
What is a packaged unit?
A complete HVAC system in a single casing, used for medium to large buildings.
What is COP for VRF systems?
3.5 to 5.0, depending on manufacturer and operating conditions.
What is integrated part load value (IPLV)?
Weighted average efficiency of HVAC equipment at various load conditions.
What is a psychrometric chart?
A graphical representation of air properties like temperature, humidity, enthalpy, etc.
What is the function of a chemical dosing system in chilled water?
To prevent scaling, corrosion, and biological growth.
What is a differential pressure switch?
Used to detect clogged filters or monitor pressure drop across coils/valves.
Why is static pressure important in duct design?
To size fans correctly and ensure adequate airflow to all zones.
What are the types of fan in HVAC?
Centrifugal, axial, mixed flow - selected based on airflow and pressure.
51. What is a VRF System?
VRF (Variable Refrigerant Flow) is an advanced HVAC technology that controls refrigerant flow to supply different temperatures to different indoor units.
52. What is the difference between VRF and VRV?
Technically both are the same; VRV (Variable Refrigerant Volume) is Daikin’s trademark, while VRF is the generic name.
53. Why is a pre-filter placed before the AHU coil?
To trap dust and large particles so that the coil remains clean and heat transfer efficiency is maintained.
54. Why does static pressure drop occur?
Due to air duct friction, fittings, filters, coils, and diffusers causing resistance to airflow.
55. What are the methods used for duct sizing?
Equal friction method, velocity reduction method, and static regain method.
56. What is the purpose of a psychrometric chart?
To analyze air properties such as temperature, humidity, enthalpy, dew point, and moisture content.
57. How is the arrangement of cooling coil and heating coil in an AHU decided?
Based on application requirements, climate conditions, and coil maintenance accessibility.
58. What are the types of dampers in HVAC systems?
Volume control damper, fire damper, smoke damper, and motorized damper.
59. What is the function of a fire damper?
Installed in ducts to prevent the spread of fire; closes automatically in fire conditions.
60. What is the purpose of a smoke damper?
Installed in ducts to control and isolate smoke, especially during a fire emergency.
61. What is Air Changes per Hour (ACH) in HVAC?
A measure of how many times the air within a room is replaced in one hour.
62. What is the typical ACH requirement in hospital HVAC design?
Operating rooms: 15–20 ACH; Isolation rooms: 12 ACH (as per ASHRAE guidelines).
63. Why is redundancy important in HVAC?
To ensure backup equipment is available in critical applications, preventing downtime in case of system failure.
64. What is the typical condenser water temperature range?
From cooling tower to chiller condenser inlet: 27–32°C; outlet: 35–37°C.
65. How can chiller efficiency be improved?
Through regular maintenance, tube cleaning, VFD usage, correct load management, and optimizing condenser water temperature.
66. Why is a duct leakage test conducted?
To check for air leakage, reducing energy loss and pressure drop.
67. Why is UV light installed in AHUs?
To prevent microbial growth on coils and drain pans.
68. What is the difference between GA drawing and SLD in HVAC?
GA (General Arrangement) shows layout and positioning; SLD (Single Line Diagram) shows piping or ducting schematics.
69. What is a heat pump?
A device that provides both heating and cooling by reversing the refrigerant cycle.
70. What is the advantage of an inverter compressor in HVAC?
Operates at variable speed, increasing energy efficiency and comfort control.
71. Why is a condensate drain trap necessary?
To prevent backflow of condensate water in negative pressure AHUs.
72. Why is a bypass line provided in a chiller plant?
To maintain minimum flow requirements and handle load variations.
73. What is an air washer unit?
An HVAC unit used for evaporative cooling and humidification.
74. What is the purpose of a face & bypass damper in an AHU?
To control supply air temperature by bypassing the coil.
75. What factors are included in heat load calculation?
External load (solar, wall conduction, window gain), internal load (people, lights, equipment), ventilation load, and infiltration load.
76. What is the function of a Thermal Expansion Valve (TXV)?
To control refrigerant flow into the evaporator and maintain proper superheat.
77. What is the difference between a Packaged Air Conditioner (PAC) and a Split AC?
PAC is centralized and serves multiple spaces via ducts; Split AC serves a single space.
78. What is commissioning in HVAC?
Testing, adjusting, and verifying the system according to design specifications.
79. Why is filter pressure drop limit important in an AHU?
Excessive drop reduces airflow and increases fan energy consumption.
80. What is the tube fouling factor in chillers?
Thermal resistance due to dirt or scaling on heat transfer surfaces, which must be minimized for efficiency.
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81. Why is there a refrigerant piping length limit in VRF systems?
Due to manufacturer design limitations to ensure proper refrigerant pressure drop and oil return.
82. What is the difference between passive and active chilled beams in HVAC?
Passive beams cool by natural convection; active beams also use supply air, increasing cooling capacity.
83. Why is duct insulation used in HVAC?
To prevent heat gain/loss and avoid condensation.
84. What is the purpose of fresh air and return air dampers in an AHU?
To control the proportion of fresh air intake and return air.
85. What are the main criteria for exhaust fan selection?
Airflow (CFM), static pressure, temperature resistance, and noise level.
86. What is the purpose of a negative pressure room?
To prevent contaminated air from escaping, such as in isolation rooms.
87. Why is a positive pressure room used?
To maintain clean air inside, such as in operating theatres.
88. What is chiller surge?
An unstable flow condition in a centrifugal chiller causing vibration and noise.
89. How can surge be prevented?
By proper load control, using VFDs, and installing anti-surge control systems.
90. Why is there a cooling coil face velocity limit in an AHU?
High velocity can cause water carryover and increase pressure drop.
91. What is thermal zoning in HVAC?
Dividing a building into zones so each can have independent temperature control.
92. What is the main difference between inverter and non-inverter Split ACs?
Inverter compressors run at variable speed, while non-inverters run at fixed speed.
93. What is the difference between 2-pipe and 4-pipe fan coil systems?
2-pipe systems provide either cooling or heating at a time; 4-pipe systems can provide both simultaneously.
94. What is approach temperature in a chiller?
The difference between chilled water leaving temperature and evaporator refrigerant saturation temperature.
95. Why is glycol used in HVAC?
To prevent water from freezing in low-temperature applications.
96. What is the purpose of blowdown in a cooling tower?
To control dissolved solids concentration in the water.
97. What is a Variable Primary Flow system?
A system that uses variable speed pumps in the primary chilled water loop without a secondary loop.
98. What is the benefit of microchannel coils in air-cooled condensers?
Higher heat transfer efficiency, reduced refrigerant charge, and better corrosion resistance.
99. What is the difference between an Energy Recovery Ventilator (ERV) and a Heat Recovery Ventilator (HRV)?
ERV recovers both heat and moisture; HRV recovers only heat.
100. What is coil bypass factor in an AHU?
The portion of air that bypasses the coil without passing through it, reducing effective cooling capacity.
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