23 Commercial AC Plant and Chiller

Updated on 01/04/26 by Mananjay MahatoShare on WhatsApp

1. Core Concept: The Chiller System

A chiller is a machine that removes heat from a liquid (usually water or a water-glycol mixture) via a vapor-compression or absorption refrigeration cycle. This “chilled water” is then circulated through a building to cool the air.

  • Refrigeration Cycle: Operates on the Vapour Compression Refrigeration System (VCRS).

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

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

  • COP (Coefficient of Performance): $\text{COP} = \frac{\text{Net Refrigeration Effect}}{\text{Work Done}}$. Always $> 1$ for chillers.

2. Main Components of a Chiller Plant

A. Compressor (The Heart)

In commercial plants, four main types are used:

  1. Reciprocating: For small capacities ($<100$ TR). High pressure, low volume.

  2. Scroll: Quiet, efficient, used in modular chillers ($10-50$ TR).

  3. Screw: High efficiency, medium-to-large capacity ($50-500$ TR). Uses helical rotors.

  4. Centrifugal: Used for very large capacities ($>500$ TR). Uses high-speed impellers.

B. Evaporator (Chiller Barrel)

  • Function: Cools the water. Refrigerant evaporates here by absorbing heat from the “return water.”

  • Type: Mostly Shell and Tube. Refrigerant is usually in the shell, and water flows through the tubes (or vice-versa).

C. Condenser

  • Air-Cooled: Uses fans to blow air over coils. No water needed, but less efficient in hot climates.

  • Water-Cooled: Uses water to carry heat away. Requires a Cooling Tower. Much more efficient for large plants.

3. Auxiliary Equipment (The “Plant” Side)

  • AHU (Air Handling Unit): The “indoor” part. Chilled water flows through coils inside the AHU, and a large blower blows room air over these coils to cool the room.

  • Cooling Tower: Used only with water-cooled condensers. It rejects heat from the condenser water to the atmosphere via evaporation.

  • Pumps:

    • Primary Pump: Circulates water through the chiller.

    • Secondary Pump: Distributes water to the AHUs across the building.

    • Condenser Pump: Circulates water between the condenser and the cooling tower.

4. Important Technical Facts for Exams

Parameter Typical Value / Fact
Chilled Water Supply Temp Usually $7^\circ\text{C}$ ($45^\circ\text{F}$)
Chilled Water Return Temp Usually $12^\circ\text{C}$ ($54^\circ\text{F}$)
Approach Difference between refrigerant temp and leaving water temp. (Lower is better).
Secondary Refrigerant Water or Brine (used when cooling below $0^\circ\text{C}$).
Expansion Device Usually Electronic Expansion Valve (EEV) or Thermostatic (TXV) for precise control.
Purge Unit Found in Low-pressure Centrifugal Chillers (like R-123) to remove non-condensable gases (air).

5. Maintenance & Troubleshooting (RAC Mechanic Focus)

  • Scaling: Hard water minerals build up inside condenser tubes. Requires Descaling (chemical or mechanical brushing).

  • Cavitation: Occurs in pumps when suction pressure is too low, causing bubbles to implode and damage the impeller.

  • Low Pressure (LP) Cutout: Trips the compressor if refrigerant pressure drops (due to leak or low load).

  • High Pressure (HP) Cutout: Trips the compressor if the condenser is dirty, the fan fails, or the cooling tower water is too hot.

  • Oil Separator: Essential in large plants to ensure oil returns to the compressor crankcase and doesn’t clog the evaporator.

6. Quick Comparison: Air-Cooled vs. Water-Cooled

  • Air-Cooled: Lower initial cost, higher maintenance (cleaning fins), higher electricity bill, no water consumption.

  • Water-Cooled: Higher initial cost (needs pumps/towers), higher efficiency (lower $kW/TR$), requires water treatment for scale/algae.

23 Commercial AC Plant and Chiller