HVAC INTERVIEW QUESTIONS FOR DUCTING

1.What are the types of ducting?

• Factory Fabricated
• Site Fabricated

2. What are the types of duct coating?

• Zinc Coating
• Powder Coating

3. What are the types of ducting material?

• GSS – Galvanised Stainless Steel
• Aluminium
• SS – Stainless Steel
• FRP – Fibre Re-enforced Plastic
• Pre-insulated
• Porous Fabric
• Non-Porous Fabric

4. What are the types of ducting shapes?

• Rectangular
• Round
• Oval
• Elliptical

5. What are the types of ducting gauges? Accordingly, thickness?

The standard gauge of ducting are:

• 24G (0.63 mm)
• 22G (0.81 mm)
• 20G (1.00 mm)
• 18G (1.20 mm)

6. What are the types of duct flange?

• TDF Flange
• Angle Flange

7. What is a Duct sealing?

Duct sealing is a process of applying approved materials or sealants on the duct joints prone to air leakages. Locations, where sealing is typically performed, include the supply and return plenums, starting collars where ducts are connected to the plenums, fittings in the duct run, splices within the duct run, and terminations where the ducts connect to the supply registers.

8. What are the types of gasket? Why it is required?

The types of gaskets are:

• Rubber Gasket
• Neoprene Rubber Gasket,
• Food Grade Gasket

Its reaquire when two ducts are connecting to each other

9. What are the different types of Duct TEST?

• Smoke Test,
• Light Test
• Robot Test

10. What are the different duct Design Methods?

• Equal friction method/ Constant pressure method
• Velocity reduction method
• Constant velocity method
• Static regain method

11. What is the Aspect Ratio?

It is a ratio of the longer side to the short side of the duct as the aspect ratio increases; more metal surface is required for the duct of the same cross-section area

Aspect Ratio: Long Side / Short Side

Note: Aspect ratio is applicable to only rectangular and square duct only

• As per SMACNA Aspect Ratio is less than 4 (A.R < 4)

12.What is Full Form of SMACNA?

Sheet Metal and Air Conditioning Contractors’ National

13. Explain the Term CFM, FPM, SQFT

• CFM: volume of air flow; cubic feet/minute
• FPM: velocity or speed of air flow; feet/minute
• Sq.ft: duct size or cross-sectional area; square feet

14.What are Different Type of Pressures in Duct?

• Static Pressure
• Velocity Pressure
• Total Pressure

15. What is Static Pressure ?

Static Pressure is the pressure that causes air in the duct to flow. Static pressure is the outward push of air against duct surfaces and is a measure of resistance when air moves through an object like ductwork. Measured in inches of water column (in-wc), it acts equally in all directions and is independent of velocity.

16. What is Velocity Pressure ?

Velocity pressure is the pressure caused by air in motion.

17. What is Total Pressure?

Total pressure is the algebraic sum of velocity pressure and static pressure.

                    TP = VP + SP

18. What is the Classification of Duct?

Velocity Classification

1. Low Velocity Systems: They are characterized by air velocities up to 2000 fpm.
2. Medium Velocity Systems: They are characterized by air velocities in the range of 2,000 to 2,500 fpm.
3. High Velocity Systems: They are characterized by air velocities greater than 2,500 fpm.

Pressure Classification

• Low Pressure: The term low-pressure applies to systems with fan static pressures less than 3 inches WC. Generally, duct velocities are less than 1,500 fpm.
• Medium Pressure: The term medium pressure applies to systems with fan static pressures between 3 to 6 inches WC. Generally, duct velocities are less than or equal to 2,500 fpm.
• High Pressure: The term high pressure applies to systems with fan static pressures between 6 to 10 inches WC. Usually, the static pressure is limited to a maximum of 7 inches WC, and duct velocities are limited to 4,000 fpm.

19.What are the Different Duct Components?

• Plenum or Main Trunk
• Take Off:
• Air Terminals Devices
• Runouts
• Dampers

20. What are the type of duct Depending on Function?

1. Supply air ductwork supplies conditioned air from the air handling unit to the conditioned area.
2. Return air ductwork removes air from the conditioned building spaces and returns the air to the air handling unit, which reconditions the air. In some cases, part of the return air in this ductwork is exhausted to the building exterior.
3. Fresh air ductwork supplies outdoor air to the air handling unit. Outdoor air is used for ventilating the occupied building space.
4. Exhaust (relief) air ductwork carries and discharges air to the outdoors. Exhaust air is taken from toilets, kitchen, laboratories and other areas requiring ventilation.
5. Mixed air ductwork mixes air from the outdoor air and the return air then supplies this mixed air to the air handling unit.

21. What is Effect of Aspect ratio?

As the aspect ratio decreases, less metal surface in contact with air results in decreases in friction and running cost of the project.

22. What are the various Supply Duct Systems?

• Extended Plenum Systems
• Reducing Plenum System
• Reducing Trunk System
• Spider System
• Radial System
• Perimeter Loop System

23. What are the various Return Duct Systems?

• Distributed Return
• Central Return

24. What are the Duct Cleaning Methods?

• Contact Vacuum Method
• Air Sweep Method
• Power (Mechanical) Brushing Method

25. General Guidelines for Duct Design?

• Air should be conveyed as directly as possible to save space, power, and material.

• Sudden changes in directions should be avoided. When not possible to avoid sudden changes, turning vanes should be used to reduce pressure loss.
• Diverging sections should be gradual. The angle of divergence should be ≤ 20°.

• Aspect ratio should be as close to 1.0 as possible. Normally, it should not exceed 4.

• Air velocities should be within permissible limits to reduce noise and vibration.

• Duct material should be as smooth as possible to reduce frictional losses.

• Round ducts require 27% less metal per unit of air-handling capacity than rectangular ducts and have lower installation costs, which may result in substantial capital cost savings.

• Rectangular metal ducts are the most flexible when it comes to fitting them within limited spaces and are used in many duct systems with pressures under 2.0 in-WC, particularly when the installing contractor has a sheet metal shop.

26. What are the different Duct Vendors

• Asawa
• Ducto Fab
• Rolastar
• Radiant Duct
• Zeco
• Prime Duct

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