MRAC Notes

• Notes: Introduction to basic refrigeration, job opportunities, Safety precautions and first aids, Applications and History of Refrigeration and Air conditioning. 
• FITTING Notes: Study the different types of Fitting hand tools, power tools, precision measuring instruments & their use. Equipments used in fittings like drilling machines, grinding machines, types, specifications and care and maintenance. Vernier caliper and micrometer. 
• SHEET METAL  Notes: Sheet metal tools and equipment type specification, care and safety. Types of sheet metal joints and their use. Rivet & riveting- their types and use. Calculation of Blank sizes from component drawing. 
• ELECTRICAL Notes: Electrical hand tools & measuring instruments, types, specifications, use, care and safety. Common terms used in the trade. Conductors and insulators. Selected letters symbols and sign as per 1. S. I. Rules for medium voltage. 
• Introduction to Electricity Notes: Safety precaution and first aid. Electric current, voltage, Resistance and their units. Simple electrical circuit, essential requirement of electrical circuit, series and parallel circuit. Different Different types of resistances. Earthing and fuses. Types,  grades and sizes of insulated wire and cables their selection and use. List of material for wiring. Switches, A.C. Motor, starters and transformer. Their working principles, specification & use. Care & safety. Run/start capacitors and PTCs. Motor Protection devices. Temperature rise of windings
• ELECTRONICS  Notes: Introduction to Electronics. Basic Principles of semiconductors, Principles and application of Diodes Rectification, Zener diode as voltage regulator transistors parameters- CB, CE, CC, configuration, amplification. SCR Photo diodes, photo transistors, multi – vibrator, CR & LR circuit. SCRS, UJTS, ICs. 
• WELDING Notes: Introduction to basic principles of commonly used Welding processes, Arc welding, oxy fuel gas welding, brazing. Welding tools and equipment type specification and use. Safety method in welding. Method of gas welding, gas used and flames adjustment. Difference between soldering and Brazing in terms of temperatures, filler materials, joint strengths and applications. Use of Oxy Acetylene, Oxy LPG and Air LPG for brazing/soldering 
• BASIC REFRIGERATION Notes: Fundamentals of Refrigeration, units and measurements, Pressure & its Measurements. Heat and Temperature, Different temperature scales, Ton of Refrigeration working of vapour compression cycle, low side & high side components of vapour compression system  and functions and applications of components. 
• REFRIGERANT Notes: Classification of refrigerants, Properties, Chemical name and formulas, HFC, CFC. Ozone rule, substitute of CFC, Montreal protocol & India’s CFC/HCFC phase out schedules. Ozone rules 2000. Substitute refrigerants in lieu of CFC’S their properties & comparison with CFCs, HFCs and HCs. 
• REFRIGERATOR (SINGLE DOOR) Notes : Study the types of construction & working of compressor, Condenser, Capillaries & evaporators,  suction Heat exchanger, door, gaskets, Heat Insulation materials, Electrical components. Importance of flushing in evaporator and condenser, necessity of replacing capillary and drier. Evacuation, leak testing, gas charging method in refrigerator, Refrigerants used in Refrigerators and its properties. 
• FROST FREE REFRIGERATOR Notes: Study the construction of Frost Free (2 or 3 door) Refrigerator parts particularly, the forced draft cooling, Air Duct circuit, temperature control in Freezer & cabinet of Refrigerator, the automatic defrost system. Study of Electrical accessories & their functions (Timer, Heater, Bi-Metal, Relay, OLP, T/S etc,.) Refrigerator cabinet volume calculation. Study the faults, Causes and their remedies of Refrigerator.
• WINDOW AIR CONDITIONER Notes: Study the construction and working of window A.C, Care and Routine maintenance, installation procedure. Study the faults, Causes and their remedies of  Window AC. 
• SPLIT A.C (Duct) & MULTI SPLIT A.C Notes: Study the construction and working, various components, electrical circuits, testing components, fault detection, leak testing, evacuation, gas charging, Installation, trouble shooting. VRV & VRF system. 
• COMPRESSOR Notes: Types & working principle of compressor like, Reciprocating, rotary, scroll, wobble, swash plate, lubrication method, Compressor efficiency factors, wet compression, oil, properties, lubrication methods. 
• CONDENSER Notes: Function of condenser, types, Construction of air cooled condenser, calculating Capacity of air cooled Condenser. Effect of chocked condenser. Advantages, decaling air cooled condenser
• EVAPORATOR Notes : Working principle evaporators, Function of evaporators, types of evaporators used in refrigerator, water coolers, bottle coolers, window and split A.C, Super heating in evaporators, Function of accumulator and types. Methods of defrosting, heat exchanger.
• EXPANSION VALVE AND DRIER Notes: Function of drier & Expansion valve used in domestic refrigeration and air conditioning systems. Capillaries, Automatic and Thermostatic Ex. Valves.
• THERMAL INSULATION Notes:  Function, types, thermodynamic properties of heat insulation materials used in refrigeration and Air Conditioning systems. 

• CABINET Notes:  Details about components, their functioning, working principle, Circuit diagram, capacity & types. Care and maintenance. 
• COLD STORAGE Notes: Study of cold storage plant, parts, Construction, applications, controls & electrical diagram used in cold storage plant. Food preservation spoiling agents- controlling of spoiling agents, preservation by refrigeration system, maintaining temperature in different places. Types of cold storage and its details. 
• CAR AIR CONDITIONING Notes: Study various components, electrical circuits, testing components, fault detection, leak testing, evacuation, gas charging, Installation, trouble shooting, Magnetic clutch operation, free wheeling 
• COMMERCIAL COMPRESSOR Notes: Types, Construction & applications of Open type compressor and working, Performance of reciprocating compressor volumetric efficiency, Capacity control, factor influencing volumetric efficiency. Selection of lubricant, Function and characteristic of lubricant, types of lubrication methods such as splash, forced feed. Construction and working principle of Centrifugal and Screw compressor. 
• WATER COOLED CONDENSER & Evaporator  Notes: Condenser its type and capacity, water cooled condenser, construction and working, de scaling, application. Evaporative condenser-their function, construction and application. 
• COOLING TOWER & water treatment Notes: Cooling tower, types, Construction, capacity, advantage & disadvantages of different types of cooling tower. Efficiency, approach and Cooling tower range. Water treatment necessary, Causes of water contamination control of scale deposit, corrosion, Slime and algae, Water softening and De-scaling method, pump and fan used, 
• ICE PLANT/CANDY Notes: Function, construction, working principle, Circuit diagram, capacity & types of compressor used. Brine Brine composition to maintain required temperature. Operation, maintenance. 

• Refrigeration is the process of removing heat from a space to lower its temperature.
• Principle of Refrigeration
  • Based on the Second Law of Thermodynamics: “Heat flows from a higher temperature body to a lower temperature body.”
  • In refrigeration, heat is artificially extracted using mechanical work.
  • Air conditioning is the process of controlling temperature, humidity, and air quality.
• Types of Refrigeration Systems

1. 1. Vapor Compression System (VCRS) – Used in refrigerators, ACs, and industrial cooling.
   2. Vapor Absorption System (VARS) – Used in large-scale cooling (uses ammonia-water or lithium bromide-water as working fluid).
   3. Gas Cycle Refrigeration – Used in aircraft cooling.
   4. Thermoelectric Refrigeration – Uses Peltier effect, found in portable coolers.

• Vapor Compression Cycle consists of four main components:
  • Compressor – Increases refrigerant pressure and temperature.
  • Condenser – Converts high-pressure vapor into a liquid by rejecting heat.
  • Expansion Valve – Reduces pressure and temperature of refrigerant.
  • Evaporator – Absorbs heat, cooling the surroundings.
• Refrigerants Used
  • R-134a (Tetrafluoroethane) – Used in domestic refrigerators.
  • R-22 (Chlorodifluoromethane) – Phased out due to ozone depletion.
  • R-600a (Isobutane) – Used in eco-friendly refrigeration.
  • R-410A – Used in modern air conditioners.
• Units of Refrigeration
• Common Units of Refrigeration

• Ton of Refrigeration (TR) –  A Ton of Refrigeration (TR) is the amount of heat required to freeze 1 ton (2000 lbs) of water at 0°C into ice in 24 hours.
  • 1 TR = 211 kJ/min = 3.517 kW = 12,000 BTU/hr
• British Thermal Unit (BTU)- 1 BTU is the amount of heat required to raise the temperature of 1 lb of water by 1°F.
  • 1 TR = 12,000 BTU/hr
• 1 kcal = 4.186 kJ
• 1 TR = 50 kcal/min = 0.86 kcal/s
• 1 HP = 0.746 kW
• Used in compressors and motors in refrigeration systems.
• Household ACs: Rated in Ton of Refrigeration (TR).
• Industrial Chillers: Measured in kW or TR.
• HVAC Systems: Use BTU/hr or kW for cooling capacity.
• Sensible heat is the heat energy added to or removed from a substance that changes its temperature but does not change its phase (solid, liquid, gas).
  • This means it can be measured using a thermometer.
  • amount of sensible heat Q=mcΔT
    • Where:
    • Q = Sensible heat (Joules or kcal)
    • $\mathrm{m\; =\; Mass\; of\; the\; substance\; (kg)}$
    • c = Specific heat capacity (J/kg·K)
      • Water (liquid) 4.1 KJ/kg·K
      •  Air = 1 KJ/kg·K
    • ΔT = Change in temperature (°C or K)
  • Example of Sensible Heat
    • Heating water from 20°C to 60°C without boiling.
    • Cooling air in an air conditioning system without condensation.

Difference Between Sensible Heat and Latent Heat

• Types of Air Conditioning Systems
  • Window AC – Single unit for small rooms.
  • Split AC – Separate indoor and outdoor units, common in homes.
  • Central AC – Used in large buildings.
  • Packaged AC – For medium-sized spaces like hotels and offices.
• Working of Air Conditioner
  • Works on the Vapor Compression Cycle similar to refrigeration.
  • Cools air using an evaporator coil, then circulates it using a blower.
• Air Conditioning Terms
  • Tonnage of AC – Cooling capacity (1 Ton = 3.5 kW cooling).
  • COP (Coefficient of Performance) – Efficiency ratio (Higher COP = More efficient).
  • Relative Humidity – Percentage of moisture in the air.

• Railway Air Conditioning System
  • Self-Generating System: Used in passenger coaches.
  • End-on-Generation (EOG) System: Power supplied from a generator in the train.
  • Roof-Mounted Package Unit (RMPU): Used in modern trains for efficient cooling.

• Common Issues in Refrigeration & AC Systems
  • Compressor Not Working – Due to electrical failure or overheating.
  • Low Cooling Efficiency – Caused by low refrigerant, clogged filters, or dirty coils.
  • Water Leakage – Due to blocked drain pipe or frozen coils.
  • Noisy Operation – Loose components, faulty fan motors.

• Preventive Maintenance
  • Clean condenser and evaporator coils.
  • Check refrigerant levels.
  • Inspect electrical connections.
  • Clean air filters regularly.

• Inverter AC- An Inverter Air Conditioner is a type of AC that uses variable-speed technology to regulate the compressor’s speed, providing efficient cooling and energy savings. Unlike conventional ACs, which turn ON and OFF frequently, an inverter AC adjusts its speed based on cooling demand.

• Working Principle of Inverter AC
  • The inverter technology controls the compressor motor speed to adjust the cooling output.
  • It works on Variable Frequency Drive (VFD) technology, where the AC (Alternating Current) is converted into DC (Direct Current) and then back into AC at variable frequency to regulate compressor speed.
  • More load = Higher compressor speed, Less load = Lower compressor speed.

Comparison: Inverter AC vs Non-Inverter AC

• Advantages of Inverter AC
  • Energy Efficient – Saves up to 50% electricity.
  • Faster Cooling – Reaches desired temperature quickly.
  • Stable Temperature – No frequent ON/OFF cycles.
  • Lower Noise – Compressor runs at optimal speed.
  • Longer Lifespan – Less wear and tear on components.
  • Eco-Friendly – Uses refrigerants like R-32, which have low Global Warming Potential (GWP).

• Disadvantages of Inverter AC
  • Higher Initial Cost – More expensive than regular ACs.
  • Expensive Repairs – Inverter circuit repairs cost more.
  • Sensitive to Voltage Fluctuations – Requires a stabilizer in areas with frequent voltage drops.

Common Problems & Solutions

• Best Practices for Using Inverter AC
  • Set Temperature to 24-26°C for best efficiency.
  • Keep doors & windows closed while running AC.
  • Clean filters every 2 weeks to maintain airflow.
  • Service AC annually to prevent major issues.

• Popular Inverter AC Brands in India
  • Daikin
  • LG
  • Samsung
  • Voltas
  • Blue Star
  • Hitachi

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