70 HVAC Engineer Interview Questions & Answers (Mechanical)
Q1. What is the Unit of Heat?
BTU (British Thermal Unit) – The amount of heat required to raise the temperature of 1 pound of water by 1°F.
- 1 lb = 0.45 kg
- 1 kg = 2.2 lb
Types of Heat:
- Sensible Heat: Measured by thermometer (e.g. body temperature).
- Latent Heat: Hidden heat absorbed or released during phase change.
Q2. What is TON of Refrigeration?
TON is the amount of heat required to melt 1 ton of ice in 24 hours.
Q3. Volume of Air is Measured In?
- CFM – Cubic Feet per Minute
- CMH – Cubic Meter per Hour
- m³/sec – Cubic Meter per Second
- Liters per second
Conversions:
- CMH = 1.7 × CFM
- CFM = 2.119 × L/s
Q4. What is ASHRAE?
American Society of Heating, Refrigeration and Air-Conditioning Engineers.
Q5. Standard Room Temperature?
24°C or 76°F
Q6. CFM Formula
CFM = Velocity × Area
Q7. 1 Ton Equals
1 TR = 12,000 BTU/hr = 3.516 kW
Q8. Heat Transfer Coefficient (U)
1 Btu/hr·ft²·°F = 5.6782 W/m²·K
Q9. What is CFM?
Cubic Feet per Minute – unit of airflow.
Q10. Aspect Ratio
Ratio of duct height to width (generally 1:2, max 1:4).
Q11. Collar Length
Strip left at the joint of duct and machine.
Q12. AHU & Chiller Selection Factors
- CFM
- Static Pressure
- Capacity (TR)
- Air & Water Temperature
Q13. Pump Supply Fittings
- Thermometer
- Pressure Gauge
- Strainer
- Valve & Flexible Connector
Q14. Pump Return Fittings
- Eccentric Reducer
- Check Valve
- Gate Valve
Q15. CFM Distribution
Use Ductulator.
Q16. Load Calculation Software
HAP 4.32 or equivalent.
Q17. Types of Dampers
- Fire Damper
- Smoke Damper
- Motorized Damper
- Gravity Damper
Q18. Types of Motorized Dampers
- Volume Control Damper
- Fire & Smoke Damper
Q19. Types of Diffusers
1-way, 2-way, 3-way, 4-way, Jet Diffusers.
Q20. What is VFD?
Variable Frequency Drive – used to control motor speed.
Q21. Chilled Water Balancing
Using Double Regulating Valve (DRV).
Q22. Noise Control in Duct
Duct lining (NC-35) and reinforcement.
Q23. Chiller & AHU Water Temperature
- Chiller Outlet: 6–7°C
- AHU Inlet: 7–8°C
- AHU Outlet: 10–11°C
- Chiller Inlet: 13–15°C
Q24. Primary & Secondary Pumps
Primary: Chiller to building
Secondary: Within building
Q25. FCU & AHU Valve Connection
Refer to AHU installation drawings.
Q26. CFM Value
1 CFM = 0.417 L/s
Q27. Refrigerants Used
R22, R12, R134a (depends on system).
Q28. Duct Heaters
Used for heating in specific areas.
Q29. What is VAV?
Variable Air Volume system used to control airflow and temperature.
Q30. 2-Way & 3-Way Valve
- 2-Way: Regulates flow
- 3-Way: Bypass during minimum load
Q31. Duct Gauge
Supply: 22G | Exhaust: 18G
Q32. Types of Duct Joints
Slip joint, Flange joint.
Q33. HVAC Standard Books
ASHRAE & SMACNA
Q34. Smoke & Light Test
Used for duct leakage testing.
Q35. Duct Sizing Methods
- Equal Friction
- Velocity Method
- Static Regain
Q36. Noise Measurement
Measured using dB meter.
Q37. Pressure Drop in Chilled Water Pipes
210 Pascal/meter
Q38. Fire, Splitter & NR Dampers
Used for fire safety, air balancing, and preventing backflow.
Q39. Exhaust Fans
Axial, Centrifugal, Twin fans.
Q40. Duct Shapes
Rectangular, Round, Spiral, Oval.
Q41. Vibration Control
Spring isolators, rubber pads, flexible bellows.
Q42. VAV vs CAV
Variable Air Volume vs Constant Air Volume.
Q43. Chiller Valve Arrangement
Standard HVAC valve layout.
Q44. Plant Room Layout Considerations
Space, orientation, unit type.
Q45. Types of Louvers
Fresh air, sand trap, acoustic, back draft.
Q46. Types of Chillers
Air-cooled & Water-cooled (Reciprocating, Screw, Centrifugal).
Q47. Chilled Water Pipe Test Pressure
1.5 times working pressure.
Q48. Chiller Safety Devices
Flow switch, HP cut-out, oil pressure switch.
Q49. Human Comfort Conditions
22 ±2°C & 50% RH
Q50. Temperature Where °C = °F
-40°C
Q51. Gas Used for Brazing
Nitrogen
Q52. 1 TR Equals
365–400 CFM, 3.516 kW
Q53. Main Duct Velocity
900–1100 FPM
Q54. Duct Insulation
Inside: 25 mm | Outside: 50 mm
Q55. Nominal vs Actual Capacity
Depends on operating conditions.
Q56. Nominal Capacity Same Worldwide?
Yes.
Q57. Weight of Duct
Wt = 8.0318 × Thickness × Perimeter
Q58. Underground Chilled Water Pipe Material
Pre-insulated black steel pipe (Sch-40).
Q59. Velocity Calculation
Using GPM, friction & ASHRAE charts.
Q60. What is FCU?
Fan Coil Unit used for cooling/heating.
Q61. Function of AHU
Condition and circulate air.
Q62. Types of AC Systems
Window, Split, Packaged, Central AC.
Q63. What Does an HVAC Engineer Do?
Design, installation, maintenance of HVAC systems.
Q64. What is HVAC?
Heating, Ventilation & Air Conditioning.
Q65. Meaning of BTU
Heat required to raise 1 lb water by 1°F.
Q66. Variable Air Volume System
Controls airflow at constant temperature.
Q67. Constant Air Volume System
Constant airflow, variable temperature.
Q68. Centralized Air System
Central cooling using condenser and evaporator.
Q69. What is FAHU?
Fresh Air Handling Unit.
Q70. CFM & Infiltration
Outside air entering a building measured in CFM.
https://brotechnicalbro.blogspot.com