02/09/2026
Day 54: Calculating Flow Rate💧
To calculate the chilled water flow rate of a cooling coil (FCU/AHU), you need two parameters: q & ∆T.
Now, what's q?
q is the total heat load of a given room or space. It represents the sum total of internal & external heat loads. 🔥
In other words, the cooling coil needs to remove heat from the space at a rate of "q" in order to bring it to the desired temperature.
And who's carrying that q? It's the flowing chilled water, of course.
The other parameter, ∆T, is the temperature difference between the supply and return chilled water pipes. ∆T, as discussed earlier, is best used at 7-10 °C.
For example, if chilled water supply was at 5°C and the return is at 12°C, then your ∆T is 7°C.
The higher the delta T, the less the required water flow.🧐
So, the question becomes:
How much water flow is needed to carry the total heat load, at a certain temperature rise?🤔
How to calculate that flow:
q = Q*c*∆T
Q: Flow rate of chilled water (L/s)
c: Heat capacity of water (kJ/L.°C, assumed constant at 4.184)
∆T: Temperature rise between supply & return (usually 7-10°C, depends on system)
So,
Q = q / (c*∆T)
Let's do an exercise:
A fan coil unit is supposed to cool a computer room having a total heat load of 100kW.
Determine the flow rate of chilled water required at ∆T of 8°C and at 10°C.
Q = q / (c*∆T)
∆T = 8°C,
Q = 100/(4.184*8) = 2.987 ~ 2.99 L/s
∆T = 10°C,
Q = 100/(4.184*10) = 2.39008 ~ 2.39 L/s
You can see clearly how changing the ∆T can change flow rates, and consequently, pipe sizing, pump sizing, etc.
Aim to keep your equipment at a ∆T of 10°C to run a highly efficient system.
