20 Central AC

Central Air Conditioning (AC) System: 

Central AC systems are designed to provide cooling, dehumidification, and air filtration for large buildings or multiple rooms from a single, centralized location.

1. Core Working Principle

• Cycle: Operates on the Vapor Compression Refrigeration Cycle (VCRC).

• Process: Absorbs heat from indoor air via an evaporator, transports it through refrigerant lines, and rejects it to the outside environment via a condenser.

2. Major Components

• • Condensing Unit (Outdoor): Contains the Compressor (the heart of the system) and the Condenser Coil with a cooling fan.

  • Evaporator Coil (Indoor): Located inside the plenum of the furnace or air handler; responsible for heat absorption.

  • Expansion Valve (TXV/Capillary Tube): Meters the liquid refrigerant flow into the evaporator, causing a pressure drop.

  • Air Handler/Blower: Circulates air through the ducts.

  • Ductwork: The “circulatory system” that distributes conditioned air and returns warm air.

3. Types of Central AC Systems

4. Technical Parameters for Exams (ISRO/DRDO)

• Refrigerants: Modern systems primarily use R-410A or R-32 (low GWP). Older systems used R-22 (now phased out).

• Energy Ratings:

  • SEER (Seasonal Energy Efficiency Ratio): Total cooling output / Total electrical energy input over a season.

  • EER (Energy Efficiency Ratio): Instantaneous efficiency at specific conditions (usually 35°C outdoor temp).

• Capacity: Measured in Tons of Refrigeration (TR).

  • $1 TR = 3.517 kW = 12,000 BTU/hr = 3024 kcal/hr$.

• Sensible vs. Latent Heat: Central AC must handle both—Sensible Heat (lowering temperature) and Latent Heat (removing moisture/humidity).

5. Maintenance & Troubleshooting (RAC Mechanic Focus)

• Short Cycling: Rapid turning on/off; often caused by an oversized unit, frozen coils, or a faulty thermostat.

• Superheat & Subcooling:

  • Superheat: Measured at the evaporator outlet to ensure no liquid enters the compressor.

  • Subcooling: Measured at the condenser outlet to ensure a full column of liquid reaches the expansion valve.

• Charging: Always charge by weight (using a scale) or by calculating subcooling/superheat for precision.

• Airflow: Restricted airflow (dirty filters/blocked ducts) is the #1 cause of Evaporator Icing.

6. Important Terms to Remember

• CFM (Cubic Feet per Minute): Volume of air moved by the blower. Standard is roughly 400 CFM per Ton.

• Relative Humidity (RH): Ideal indoor RH for comfort is 40% – 60%.

• Plenum: The main chamber of the duct system where air is collected for distribution.

Are you focusing more on the mathematical problems of the refrigeration cycle or the practical mechanical troubleshooting?