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How to prepare RAC for GATE Mechanical Engineering

Hello all,

RAC (Refrigeration & Air Conditioning) is a subject which many aspirants leave for the GATE exam which is not good. Every year in GATE 3-5 marks questions come from this subject. This blog will be very useful for those aspirants who are doing self-study. Follow this blog to prepare effectively for this subject. This is the subject where you can gain more marks as it is easy. Air Conditioning is quite different and interesting topic because it deals with the human comfort .This is the basic difference between Refrigeration & Air conditioning as Refrigeration deals with the cooling only but Air conditioning deals with the human comfort.

Note: In this blog, the time duration and day wise analysis are given. So, the time taken by any aspirant may vary that depends on the aspirant’s intellectual.

Marks analysis of previous year GATE questions has been shown below:

Refrigeration Systems and refrigerants 92 marks

Marks Category No. of questions
1 Mark 6
2 Marks 16
5 Marks 12

Psychrometry and Air conditioning 78 marks

Marks Category No. of questions
1 Mark 13
2 Marks 20
5 Marks 5

Refrigeration Duration-2 days


Refrigeration machine, Refrigeration effect, Refrigeration Capacity, Unit of Refrigeration, Ideal or Best Refrigeration Cycle/The Carnot principle, Solve some numericals.

The refrigerating Machine-the second law Interpretation, Heat Engine Heat Pump & Refrigerating machine, Vapour as a refrigerant in reversed Carnot cycle, gas as a Refrigerant in reversed Carnot cycle, actual refrigeration systems, the Volumetric efficiency of Reciprocating Compressor.

Vapour Compression Refrigeration system (VCRS)-

Its components-Compressor, Condenser, Throttling valve or Expansion device, Evaporator Its COP calculation, Effect of various properties on the performance of Vapour Compression cycle- Effect of decrease in evaporator pressure, Effect of increase in condenser pressure, Effect of superheating, Effect of sub-cooling, Solve numericals.

Tips & instruction:

This Subject is very easy & scoring. The topics which I have mentioned above are important and try to learn everything. Generally, aspirants don’t complete this subject and leave it, but if you complete it then that will be an extra edge for you.

Observe and analyze the T-s and P-v diagrams and learn each process which are occurring that will help you in solving numericals. Most of the questions are from Vapour Compression Refrigeration system so practice numerical problems a lot. Also, don’t forget to solve previous year question of GATE which is very important to solve, many good questions have come in the previous year. Learn about the components of VCRS and that concept will help you in solving numerical. Practice numerical as much as possible from the standard book

Practice heat pump numericals from the standard book and from the previous year also. The heat pump is not so important.

Other cycles-

Vapour Absorption Refrigeration Cycle-

Learn the Flow diagram, learn the overall COP formula, Solve only basic numericals.

Reverse Brayton cycle/Bell-Coleman Cycle/Gas Refrigeration Cycle-

Learn T-s & P-v diagram, COP formula is very important and derivation of COP formula is advisable because it will help you to understand each process.

Cascade Refrigeration-Learn the basics of it and learn the COP formula.

Standard book- Khurmi & Gupta and PK Nag



Type of Refrigerants, Designation of Refrigerants- Saturated hydrocarbon, unsaturated hydrocarbon, Inorganic compound, Desirable properties of Refrigerant.

Tips & instruction:

Very easy and scoring topic. You should know how to convert from chemical formula to designations & vice versa. One mark questions can be asked about the properties.

Psychrometry and Air conditioning Duration-3 days


Specific Humidity or Humidity ratio or Absolute Humidity, Relative humidity, DBT, WBT , DPT , Wet Bulb Depression, Degree of Saturation, Enthalpy of Moist air, Psychrometric chart, Psychrometry of Air-conditioning Process,

Basic Air conditioning process like sensible Heating, Sensible Cooling, Humidification, Dehumidification, Heating and Humidification, Cooling and Dehumidification, Cooling and Humidification, Heating and Dehumidification.

Sensible Heat Factor, bypass factor, the efficiency of the coil.

Tips & instruction:

I would suggest you to give some days of your preparation to this subject also. This subject is left by most of the aspirants but it is very easy and direct questions have been asked from the past 3-4 years. Learn all the formulae of the Specific Humidity, Relative humidity, Wet Bulb Depression, Degree of Saturation, Enthalpy of Moist air, Sensible Heat Factor, bypass factor, the efficiency of coil and also don’t forget to study the formula validation means where can you apply that particular formulae. The best way to understand the application concept is to solve numericals as much as possible.

Learn basic Air conditioning process and its example and also learn how to draw on Psychrometry chart.

Observe and learn the psychrometry chart perfectly so that you can solve numerical problems. Representation of Psychrometric properties on the psychrometry chart should be learned.

Standard book- Khurmi & Gupta and PK Nag

NOTE- After every completion of Topic & subject topic wise & subject wise respectively TEST SERIES should be given.

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