A rising approach temperature is one of the earliest warning signs that your chiller is losing efficiency. Every 1 °C increase in condenser approach can push compressor energy up by roughly 2–3%, often long before anyone notices a loss of cooling capacity.
The good news: most approach problems are preventable. This guide explains what approach temperature is, how to calculate it, what makes it rise, and the practical steps that keep it low and stable.
What Is Chiller Approach Temperature?
Approach temperature is the difference between the leaving-water temperature and the refrigerant saturation temperature inside a chiller heat exchanger. It is the fastest single indicator of how well the evaporator and condenser are transferring heat.
- Low and stable approach: heat transfer is efficient.
- Rising approach: something is blocking heat transfer — typically fouling, scale, low refrigerant charge, poor water flow, non-condensables or a faulty sensor.
- Why it matters: trending approach reveals efficiency losses before they become capacity or energy problems.
Evaporator vs. Condenser Approach
| Evaporator approach | Condenser approach | |
|---|---|---|
| Formula | Leaving chilled-water temp − evaporating refrigerant saturation temp | Condensing refrigerant saturation temp − leaving condenser-water temp |
| What a low, stable value means | Heat is moving efficiently from water to refrigerant | Heat is rejected efficiently from refrigerant to condenser water |
| Common causes of a rise | Tube fouling, scale or oil film, low refrigerant charge, restricted refrigerant flow, poor chilled-water flow, sensor error | Scale and biofilm in tubes, low condenser-water flow, cooling-tower problems, air or other non-condensables, low refrigerant charge, sensor error |
Important: never judge a chiller against a generic threshold alone. Compare readings at similar load, water temperature and flow, and use the manufacturer’s design or commissioning data as your baseline.
How to Calculate Chiller Approach
Evaporator example: chilled water leaves at 6.7 °C (44 °F) and the evaporating refrigerant saturation temperature is 5.6 °C (42 °F). The evaporator approach is 1.1 °C (2 °F).
Condenser example: the condensing refrigerant saturation temperature is 36.1 °C (97 °F) and condenser water leaves at 35.0 °C (95 °F). The condenser approach is 1.1 °C (2 °F).
Record the value at every log round. A single reading tells you little; the trend tells you almost everything.
Troubleshooting a Changing Approach
| Observed trend | Possible cause | Recommended check |
|---|---|---|
| Evaporator approach rising | Fouling, low charge, restricted flow, sensor drift | Verify sensor accuracy, water flow, refrigerant condition and tube cleanliness |
| Condenser approach rising | Scale, biofilm, poor tower operation, low water flow, non-condensables | Inspect tubes, confirm pump and tower operation, verify flow and purge performance |
| Approach changes sharply | Change in operating conditions or measurement | Compare load, entering-water temperatures, flow rates and sensor readings |
| Approach unusually low | Sensor offset or invalid calculation | Calibrate instruments; confirm correct leaving-water and saturation temperatures |
How to Prevent Approach Temperature from Rising
In most plants, a rising condenser approach comes down to one thing: deposits on the tube surface. Just 0.8 mm of calcium carbonate scale can raise energy costs by about 10%, and a 1/4-inch (6.35 mm) layer by 40–80%, and biofilm insulates even more effectively than mineral scale. Prevention means keeping the water side clean, the flow right and the refrigerant side healthy.
The cost grows with chiller size. Even a thin 0.036-inch (0.9 mm) fouling layer can add around US$95,000 a year to the energy bill of a 2,000-ton chiller.
- Stop scale and biofilm at the source. Control the cooling-tower water so hardness, silica and microbes cannot build up on condenser tubes. Physical treatment such as Electrolysis Scale Treatment (EST) prevents scale and biofilm formation without chemical dosing, keeping tubes clean between shutdowns.
- Maintain correct water flow. Check pump performance, strainers and valve positions. Low flow through either barrel raises approach and reduces capacity.
- Keep the cooling tower in good condition. Clean fill and basins, check nozzles and fans, and control blowdown so cycles of concentration stay within design.
- Keep the refrigerant side healthy. Maintain correct charge, manage oil carry-over and make sure the purge unit removes air and non-condensables.
- Calibrate sensors regularly. A drifting sensor can trigger unnecessary tube cleaning or hide a real problem.
- Clean tubes based on data, not the calendar. Let the approach trend tell you when cleaning is genuinely needed.
Best Practices for Monitoring Approach Temperature
- Record evaporator and condenser approach at consistent operating conditions.
- Trend the values over weeks and months instead of relying on a single reading.
- Compare current data with commissioning, startup or manufacturer performance data.
- Verify temperature sensors before authorizing cleaning or refrigerant work.
- Act promptly on a sustained upward trend — lost heat transfer means higher energy bills and less cooling capacity.
Keep Your Chillers Running at Design Efficiency
Water Treatment Asia helps plants across Southeast Asia keep condenser approach low with Elgressy Electrolysis Scale Treatment — no chemicals, less cleaning downtime and lower energy consumption. Contact our team to discuss your cooling-tower and chiller performance.
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