HVAC INTERVIEW QUESTIONS AND ANSWERS Q1) What is the Unit of Heat?(1M) ANS) Units: BTU (British Thermal Units) 1BTU: It is the amount of energy required to raise 1 Degree Fahrenheit temperature of 1 Pound of water (OR) It is the amount of heat when added to 1 Pound of water increases the temperature of water to 1 Degree Fahrenheit 1 Lb = 0.45 Kgs (or) 1kg = 2.2 Lbs. Types of Heat:Sensible Heat: It can be sensed or measured by an ordinary dry bulb thermometer. Eg: Human body temperature can be measured using thermometer Latent Heat: It can’t be sensed or measured by an ordinary dry bulb thermometer. It is a hidden heat. Latent heat is absorbed or released when ever there is a change in phase. Eg: When the water becomes vapor and vice versa Q2) What is TON & 1 TON = ?(2M) ANS) TON or Ton of refrigeration: It is the amount of energy required to Freeze 1 pound of water in 24 hours (or) It is the amount of energy required to Melt 1 Ton of Ice in 24 Hours Q3) Volume Of Air is measured in?(2M)
ANS) Volume of air is measured in terms of: CFM = Cubic Feet per Minute CMH = Cubic Meter per Hour M3/Sec = Cubic Meter per Second L/Sec = Liters per second CMH = 1.7 x CFM CFM = 2118 x m3/sec CFM = 2.119 x l/sec Q4) Whai is ASHRAE?(1M) ANS) ASHRAE: American Society of Heating Refrigeration and Air Conditioning Engineers Q5) What is the Room Temperature that you maintain in any room?(1M) ANS) 24 Degree Celcius or 76 Degree Fahreinheit
Q6) How much Fresh Air do you provide in a Bedroom?(2M) ANS) Fresh Air: According to ASHRAE 2004 62.1 Ventilation Standards 5 CFM/PERSON + AREA x 0.06 CFM/SQ FT 1. CFM/PERSON x PEOPLE 2. CFM/SQ FT x AREA No. Of People = 2 CFM per Person = 5 Area = 200 SQ FT (Example) CFM per Sq. Ft = 0.06 Therefore 5 x 2 = 10CFM 0.06 x 200 = 12 CFM Therefore Total Fresh Air CFM = 10 + 12 = 22CFM Q7) What are the Air Terminal Devices?(1M) ANS) DIFFUSERS, GRILLS, LINEAR SLOT DIFFUSERS, LOUVERS Etc..
Q8) How do you design the sizes of Air Terminal Devices?(1M) ANS) Two parameters are Required: 1. Air Quantity CFM and 2. Air Velocity FPM. The Following Formula can be used Q = Q9) How much Noise Criteria do you maintain in BETA for Linear Bar Grill & units?(2M) ANS) 35 Decibels Q10) What is Single Skin & Double Skin Hoods?(2M) ANS) Single Skin: Has only the Exhaust System. Double Skin: Has both Exhaust and Fresh Air System
Q11) Calculate Hood Size,Exhaust Air CFM,Exhaust Air Duct Size, Fresh Air CFM & Fresh Air Duct Size(25M)? ANS) EXAMPLE PROBLEM ON KITCHEN VENTILATION SYSTEM:Q) Length of the stove = 750mm Width of the stove = 500mm Height between stove and Hood = 800mm Find (a):Hood Size (b): Exhaust Air CFM (c): Exhaust Air Duct Size (d): Fresh Air CFM (e): Fresh Air Duct Size For a wall Type hood and medium cooking? Solution:- (a): Hood Size: L = l + 0.8H = 0.75 + 0.8(0.8) = 1.39M M x 3.28 = FT 1.39 x 3.28 = 4.55FT W = w + 0.8H = 0.5 + 0.8(0.8) = 1.14M M x 3.28 = FT 1.14 x 3.28 = 3.73FT Therefore Hood Size: L = 4.55Ft and W = 3.73Ft (b):- Exhaust Air CFM: CFM = P x 150 Where P = 2L + W for wall type of Hood Therefore P = 2(4.55) + 3.73 = 12.83 Exhaust Air CFM = P x 150 = 12.83 x 150 = 1925CFM (c): Exhaust Air Duct Size: Q = A x V 1925 = A x 1800 A = 1925/1800 = 1.06 Sq.Ft (Area of the Duct) Sqft x 0.093 = Sq.M 1.06 x 0.093 = 0.098Sq.M (1):- Round Duct: Area = Pi x D2/4 0.098 = 3.14 x D2/4 Therefore Diameter D = Under Root of (0.098 x 4)/3.14 = 0.35M = 350mm (2):- Square Duct: Area = 0.098 Therefore Under Root of Area = Under Root of 0.098 = 0.30M = 300mm Hence the Width and Height of Square duct is 300 x 300mm
(3):- Rectangular Duct: Area = W x H 0.098 = W x 0.3 W = 0.098/0.3 = 0.32M ~ 350mm Hence the Width and Height of Rectangular duct is 350 x 300mm (d): Fresh Air CFM = Exhaust Air CFM x 0.8 = 1925 x 0.8 = 1540CFM (e): Fresh Air Duct Size: Q = A x V 1540 = A x 1800 A = 1540/1800 = 0.855 Sq.Ft (Area of the Duct) Sqft x 0.093 = Sq.M 0.855 x 0.093 = 0.08Sq.M (1):- Round Duct: Area = Pi x D2/4 0.08 = 3.14 x D2/4 Therefore Diameter D = Under Root of (0.08 x 4)/3.14 = 0.319M = 350mm (2):- Square Duct: Area = 0.08 Therefore Under Root of Area = Under Root of 0.08 = 0.28M = 300mm Hence the Width and Height of Square duct is 300 x 300mm (3):- Rectangular Duct: Area = W x H 0.08 = W x 0.25 W = 0.08/0.25 = 0.32M ~ 350mm Hence the Width and Height of Rectangular duct is 350 x 300mm Q12) How do you calculate CFM for Toilets?(2M) ANS) CFM = M3 x A.C.H/ 1.7 Q13) To select a fan in Commercial Toilets, what are the things that are required?(2M) ANS) 1) Volume of Air and 2) Static Pressure Q14) Difference between Axial and Centrifugal Fans ANS) Axial Fan: Has the Parallel Movement of Air. Centrifugal Fan: Has the Perpendicular Movement of Air Q15) Explain Vapour Compression Cycle?
ANS) Cooling take place when liquid evaporates to become gas. COMPRESSOR: Most refrigerant gases can be made into the liquid form by raising its pressure (and cooling it, which I handled by the condenser). CONDENSER: During compression the refrigerant becomes hot because of two reason: (a) work done on it. (b) Refrigerant is converted from gas to liquid releasing its latent heat. This heat has to be removed to enable the gas to condense into a liquid easily. EVAPORATOR: The liquid refrigerant from the condenser is injected through capillary (or) expansion valve into the cooling coil which has a bundle of tubes. Inside the cooling coil pressure is low because of throttling device on one side & the compressor suction on the other side. In the low pressure, the liquid refrigerant tarts evaporating rapidly by soaking surrounding sensible heat & this is what causes the cooling. Q16) What is COP and EER? ANS) COP: Coefficient of Performance. EER: Energy Efficiency Ratio Q17) What is Partition, Ventilation & Infiltration? ANS) Partition: It is common wall between Conditioned and Unconditioned Space. Quality through Window Cracks and Door Openings Q18) How to you Design a Duct? ANS) There are Three Standard Methods to Design a Duct 2. Velocity Reduction Method Q19) Design a Duct using Equal Friction Method ANS) Find out the CFM and Maintain the Constant Friction Throughout the Ducting to follow Equal Friction Method Q20) What should be the HWS & HWR Temperatures in a Cooling Tower? ANS) HWS = 106 & HWR = 98 Degree fahreinHeit based on the Ambient Conditions Q21) What should be the CWS & CWR in any Chiller? ANS) CWS = 45 & CWR = 55 Degree FahreinHeit as per the Standards Q22) What is VAV & CAV ANS) VAV: Variable Air Volume & CAV: Constant Air Volume Q23) What are the things we have in Indoor unit & Outdoor Unit ANS) Indoor Unit: Evaporator Coils and Blower Fan Q24) Which units consume power ANS) COMPRESSOR, AXIAL FAN & BLOWER FAN
Q25) What is the advantage if I use VAC instead of VCC? ANS) Power Consumption on the daily basis will be lowered down Q26) What are the ASHRAE Recommendations for Pipe Designing? ANS) Maximum Velocity is 4 FPS & Maximum Head Loss is 4 FT/100FT Q27) Formula for GPM ANS) GPM = Ton x 12000/ 500 x Delta T
Q28) Decoupler Sizing: How do you size a Decoupler? ANS) Decoupler Sizing: Minimum Length of the Decoupler should be 3 times the diameter of the pipe & the Friction Loss Q29) Where do we use "U" trap? Why do we use "U" trap? ANS) U Traps are used in almost all Air Conditioning Systems like Split Ac's, Casette AC's, Fcu's, AHU's Etc… units Q30) Types of false ceiling with section view. ANS) 1) Plain or Flat False Ceiling 2) Step False Ceiling 3) Tiles False Ceiling Q31) If a Bed room of Area 10 x 15 then what will be the capacity of AC in that room? ANS) 10Ft = 3M and 15Ft = 4.5M. Therefore L x W = 3M x 4.5M = 13.5 M2. Ton = 400CFM Q32) What is Qs & QL. Explain? ANS) Qs is the Sensible Heat and QL is the Latent Heat Q33) Name the types of insulation used for ducting & why do we use insulation. ANS) Fibre Glass Insulation, Rubber Insulation and Canvas Type Insulation. Outside Hot Air should not effect the Air moving inside the Duct which is chilled. Q34) Name the different types of material used for duct fabrication. ANS) GI Duct, Aluminium Sheet Metal, Fibre Glass Duct, Thick Black Mild Steel, Galvanized Steel Sheet & PUF Q35) What are the different gauges of duct. ANS) 16,18,20,22,24 & 26 with the Thickness 1.8, 1.2, 0.9, 0.7, 0.6 & 0.5mm
