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Understanding "Approach" in Chiller Systems: Causes, Impacts, and ASHRAE-Based Best Practices

Understanding "Approach" in Chiller Systems: Causes, Impacts, and ASHRAE-Based Best Practices


In chiller system performance, one critical parameter that is often overlooked but highly indicative of system health is the Approach Temperature.


What is Approach?

Approach refers to the temperature difference between:


Evaporator Approach: Leaving chilled water temperature vs. refrigerant evaporating temperature Condenser Approach: Leaving condenser water temperature vs. refrigerant condensing temperature


A low approach indicates efficient heat transfer, while a high approach signals performance degradation.


ASHRAE Guidance

What is a "Good" Approach?

Based on industry best practices and references from


ASHRAE:


Evaporator Approach (Typical Range):

  • 1-3°C (1.8-5.4°F) →Excellent / Clean condition
  • 3-5°C (5.4-9°F)  → Acceptable, monitor trend
  • > 5°C (9°F)  →Indicates fouling or performance issue

Condenser Approach (Typical Range):
  • 2-4°C (3.67.2°F) →Good performance
  • 4-6°C (7.2-10.8°F)  →Acceptable, needs observation
  • > 6°C (10.8°F) → Likely scaling, fouling, or airflow/water issue

Note: Values may vary depending on chiller type (air-cooled vs water-cooled), design conditions, and manufacturer specifications.

Always compare with commissioning baseline data.

Why Does High Approach Occur?
Several operational and maintenance related issues can increase approach values:
  • Fouling / Scaling on heat exchanger tubes
  • Poor water treatment leading to biofilm or deposits
  • Low water flow rate (pump issues, valve restrictions, airlocks)
  • Non-condensable gases in the refrigerant circuit (for condenser side)
  • Refrigerant issues (low charge, oil contamination)
  • Sensor inaccuracies or calibration drift Impact of High Approach
A rising approach is not just a number, it directly affects system efficiency and cost:
  • Reduced heat transfer efficiency
  • Increased compressor workload and energy consumption
  • Higher operating costs (kW/RT increases)
  • Potential for system trips under high load conditions
  • Shortened equipment lifespan
How to Handle High Approach

Proactive monitoring and corrective actions are essential:
⏩Regular tube cleaning (mechanical or chemical)
⏩Maintain proper water treatment program
⏩Verify design flow rates (check pumps, valves, strainers)
⏩Purge non-condensables from the system
⏩Check refrigerant charge and oil condition
⏩Calibrate temperature and pressure sensors periodically
⏩Trend approach values-early detection prevents major failures

Approach temperature is a simple yet powerful KPI in chiller systems.

Aligning your operation with ASHRAE recommended ranges ensures optimal efficiency, lower energy consumption, and longer equipment life.


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