All Major Cycles in a Chilled Water System

All Major Cycles in a Chilled Water System  

In a chilled water HVAC system, the major thermodynamic and fluid-flow cycles can be understood as follows:

1. Chilled Water (CHW) Cycle

This is the primary HVAC water circulation loop:

Chiller → CHW Pump → AHU/FCU Cooling Coil → Return CHW → Chiller

• Chiller produces chilled water, typically around 6–7°C.
• CHW pump circulates water to AHUs/FCUs.
• Cooling coils absorb heat from the air.
• Water returns warmer, typically 11–12°C.
• The chiller removes the absorbed heat and repeats the cycle.

Typical ΔT: 5–6°C.

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2. Refrigerant Cycle

Inside the chiller, the refrigerant follows the basic vapor-compression cycle:

Compressor → Condenser → Expansion Valve → Evaporator → Compressor

Component	Main Function
Compressor	Raises refrigerant pressure and temperature
Condenser	Rejects heat from refrigerant
Expansion Valve	Reduces refrigerant pressure/temperature
Evaporator	Absorbs heat from chilled water

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3. Condenser Water Cycle

For a water-cooled chiller:

Condenser → Condenser Water Pump → Cooling Tower → Condenser

• Warm condenser water leaves the chiller.
• Pump sends it to the cooling tower.
• Cooling tower rejects heat to atmosphere.
• Cooled condenser water returns to the chiller.

Typical example:

32°C → Cooling Tower → 27°C → Chiller

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4. Cooling Tower Heat-Rejection Cycle

The cooling tower removes heat from the condenser water:

Hot Condenser Water → Spray/Distribution → Air Contact → Evaporation → Cooled Water

Heat is rejected through:

• Sensible heat transfer
• Evaporation
• Air movement

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5. Air-Side Cooling Cycle

At an AHU:

Return Air + Fresh Air → Filter → Cooling Coil → Supply Fan → Conditioned Space → Return Air

The cooling coil transfers heat from the air to the chilled water.

Example:

Return Air: 24°C → Cooling Coil → Supply Air: 13–14°C

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6. Building Heat-Transfer Cycle

The overall process is:

Room Heat → Air → AHU Cooling Coil → Chilled Water → Chiller Evaporator → Refrigerant → Condenser Water → Cooling Tower → Atmosphere

This is the complete heat-transfer path.

Overall System

        BUILDING
           ↓
      Return Air
           ↓
      ┌─────────┐
      │   AHU   │
      │ Cooling │
      │  Coil   │
      └────┬────┘
           │
     Warm CHW Return
        11–12°C
           ↓
      ┌─────────┐
      │ CHILLER │
      │Evaporator│
      └────┬────┘
           │
     Cold CHW Supply
         6–7°C
           ↓
      CHW PUMP
           ↓
       AHU / FCU
           
────────────────────────

        CHILLER
           ↓
   Hot Condenser Water
           ↓
   Condenser Water Pump
           ↓
     COOLING TOWER
           ↓
   Cooled Condenser Water
           ↓
        CHILLER

The 4 Major Cycles to Remember

1. CHW Cycle: Chiller ↔ AHU/FCU
2. Refrigerant Cycle: Compressor → Condenser → Expansion Valve → Evaporator
3. Condenser Water Cycle: Chiller ↔ Cooling Tower
4. Air Cycle: Space ↔ AHU/FCU

For a hospital or large facility, there are also important secondary systems such as primary/secondary CHW pumping, variable-primary flow, differential-pressure control, BMS control, chemical treatment, expansion/pressurization, air separation, and water filtration.