Calculating Flow Rate

To calculate the chilled water flow rate of a cooling coil (FCU/AHU), you need two parameters: q & ΔT.

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, ΔT, is the temperature difference between the supply and return chilled water pipes. ΔT, 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 ΔT 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*ΔT

Q: Flow rate of chilled water (L/s)

c: Heat capacity of water (kJ/L.°C, assumed constant at 4.184)

ΔT: Temperature rise between supply & return (usually 7-10°C, depends on system)

So,

Q = q / (c*ΔT)

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 AT of 8°C and at 10°C.

2 = q / (c*ΔT)

ΔT = 8°C,

Q = 100/(4.184*8) = 2.987~2.99 L/s

ΔΤ = 10°C, Q 100/(4.184*10) = 2.39008 ~ 2.39 L/s

You can see clearly how changing the AT can change flow rates, and consequently, pipe sizing, pump sizing, etc.

Aim to keep your equipment at a ΔT of 10°C to run a highly efficient system.

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