Chilled Water Flow Rate Calculator (SI Units)
Calculate the required volumetric flow rate based on Cooling Capacity and ΔT.
About Chilled Water Flow Rate Calculations
In HVAC engineering and plant design, determining the correct chilled water flow rate is fundamental to sizing pumps, pipes, and control valves. This calculator uses SI units (International System of Units) to derive the flow rate based on the thermodynamic relationship between heat load, specific heat capacity, and temperature differential.
The SI Formula
The fundamental equation for heat transfer in a fluid is:
Where:
- Q = Cooling Capacity or Heat Load (kW)
- m = Mass Flow Rate (kg/s)
- Cp = Specific Heat Capacity of the fluid (kJ/kg·K)
- ΔT = Temperature difference between Return and Supply (°C or K)
To solve for the Flow Rate, we rearrange the formula. Since water density is approximately 1 kg/L (at standard chilled water temperatures), the calculation simplifies to:
Key Variables Explained
Cooling Capacity (kW): This represents the total amount of heat that the chiller needs to remove from the system. It is often calculated based on the building's cooling load.
Specific Heat Capacity (Cp): For pure water, this value is approximately 4.186 kJ/kg·K. If you are using a glycol mixture (for freeze protection), this value will decrease, requiring a higher flow rate to achieve the same cooling effect.
ΔT (Delta T): The design temperature difference between the supply water (leaving the chiller) and the return water (entering the chiller). Standard HVAC designs often use a ΔT of 5°C to 6°C.
Example Calculation
Consider a chiller with a capacity of 500 kW. The system is designed for a return temperature of 12°C and a supply temperature of 7°C.
- ΔT: 12 – 7 = 5°C
- Cp: 4.186 kJ/kg·K (Standard Water)
- Flow (l/s): 500 / (4.186 × 5) = 23.89 l/s
- Flow (m³/h): 23.89 × 3.6 = 86.0 m³/h