HVAC Systems Encyclopedia

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Chiller Performance Analysis for HVAC Engineers

Chiller Performance Analysis for HVAC Engineers

Chiller efficiency determines 40-60% of total HVAC energy consumption. Understanding performance metrics and optimization strategies enables equipment selection and operation for minimum life cycle cost.

Efficiency Metrics

kW/ton:

$$kW/ton = \frac{Compressor\ kW}{Tons\ Cooling}$$

Lower is better. Typical values:

  • Air-cooled chiller: 0.9-1.2 kW/ton
  • Water-cooled centrifugal: 0.50-0.65 kW/ton
  • Magnetic bearing centrifugal: 0.45-0.55 kW/ton

COP (Coefficient of Performance):

$$COP = \frac{12,000 \times Tons}{3,413 \times kW} = \frac{12}{3.413 \times kW/ton} = \frac{3.516}{kW/ton}$$

Higher is better. Typical: COP 4.0-6.5

EER (Energy Efficiency Ratio):

$$EER = \frac{Cooling\ Capacity\ (Btu/h)}{Power\ Input\ (Watts)} = 12 \times COP$$

Part-Load Performance

Integrated Part Load Value (IPLV):

Weighted efficiency at four load points:

  • 100% load: 1% weight
  • 75% load: 42% weight
  • 50% load: 45% weight
  • 25% load: 12% weight

$$IPLV = 0.01A + 0.42B + 0.45C + 0.12D$$

Where A, B, C, D are kW/ton at respective load points.

Significance: Chillers rarely operate at full load; IPLV better predicts annual energy.

Temperature Lift Effect

Chiller efficiency degrades with increased temperature lift:

$$Lift = T_{leaving,cond} - T_{leaving,evap}$$

Every 1°F lift increase: ~0.015-0.025 kW/ton efficiency penalty

Optimization strategies:

  1. Lower chilled water temperature only when needed (reset based on load)
  2. Maximize condenser water temperature in winter (free cooling)
  3. Keep condenser clean (fouling increases lift)

Condenser Water Optimization

Cooling tower approach:

$$Approach = T_{leaving,tower} - T_{wetbulb,outdoor}$$

Tower range:

$$Range = T_{entering,tower} - T_{leaving,tower}$$

Typical: 10°F range, 7-10°F approach

Energy tradeoff:

  • Lower condenser water temp: Better chiller efficiency, more tower fan power
  • Optimal condenser water temp: 65-75°F (varies by climate, equipment)

Free Cooling

Waterside economizer: Uses cooling tower to cool chilled water directly (bypasses chiller)

Conditions: Outdoor wet bulb < 45-50°F

Energy savings: 50-90% when active (winter months in most climates)

Practical Applications

  1. Equipment selection: Specify IPLV > 15.0 EER for ASHRAE 90.1 compliance
  2. Sequencing: Load most efficient chiller first at part load
  3. Condenser water reset: Raise leaving temperature in cool weather
  4. Chilled water reset: Raise leaving temperature when loads decrease

Related Technical Guides:

References:

  • ASHRAE Handbook of HVAC Systems and Equipment, Chapter 38: Compressors
  • ASHRAE Handbook of HVAC Systems and Equipment, Chapter 43: Liquid Chillers
  • AHRI Standard 550/590: Performance Rating of Water-Chilling and Heat Pump Water-Heating Packages