Engineering practice · Air conditioning
Article 08 / 10
Water returning too cold: what low ΔT means in a chiller system
The design water temperatures are +7/+12 °C. The BMS shows +7/+10 °C, the pumps are working hard, yet some rooms are still warm.
The first instinct is to lower the supply temperature or add another chiller. I would check something else first: what load is the water carrying, and by which route does it return?
Start with the heat balance
For water:
Assume a load of 300 kW. At a temperature difference of 5 K, the required flow rate is:
If the temperature difference falls to 3 K, carrying the same 300 kW requires:
That is 67% more water for the same thermal result. It does not automatically mean 67% more electricity consumption: pump power also depends on head and efficiency.
Low ΔT is not always a fault
Suppose the flow rate remains at 51,6 m³/h while the load falls to 150 kW. The same formula then gives a temperature difference of approximately 2,5 K.
For the assumed constant-flow operating mode, this is the expected heat balance. The figure of 2,5 K alone does not prove a defect.
A different situation is high cooling demand, insufficient zone cooling and excessive circulation. In that case, we need to establish why the water is not carrying the required amount of heat at the expected temperature difference.
Where to look for the cause
Water may pass through an open bypass or leaking valves, mixing with the return flow without usefully cooling the rooms. Excessive flow rates and imbalance can also reduce the temperature difference. Belimo discusses these scenarios in its materials on system branch control. 1
We would check not only the central temperatures but also individual terminal units: valve position, flow rate, air temperature and heat exchanger condition.
It is best not to explain this by saying ‘the water flows too fast and does not have time to pick up heat’. A smaller temperature change in one kilogram of water does not, by itself, mean less total heat transfer. The full mass flow rate must be taken into account.
Sensors can create a problem on the trend chart
If each of two temperature sensors has a worst-case error of ±0,2 °C, the error in their difference can reach ±0,4 K. For a temperature difference of 2 K, that is already 20% — before accounting for flow meter error.
This is a calculated illustration of error limits, not a statistical assessment of the accuracy of specific instruments. The measurement chain must be checked before drawing financial conclusions.
Why you cannot simply throttle the water flow
The loads at the terminal units must be met while maintaining an acceptable flow rate through the evaporators. The rate of change of flow and sufficient circuit water volume also matter for the chiller. Trane shows how errors involving minimum flow, bypass arrangements and water volume disrupt the operation of the whole system. 2
Increasing ΔT therefore cannot be an end in itself. The aim is to deliver the required cooling at a sensible flow rate without breaching the equipment’s operating requirements.
Two thermometers show a symptom. Diagnosis comes from the relationship between temperatures, flow rate, load and the actual operation of the terminal units.