MEP Engineers Club

Centrifugal Pumps
Everything an Engineer Must Know.

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2.4 GPM per TR Works Until ΔT Changes!!

I often see engineers using a quick rule of thumb for chilled water flow in FCU and AHU design: “2.4 GPM per TR.”

It’s simple, fast, and works in early calculations. But it only represents a special case, not the full picture. At the core of all these shortcuts is the real heat transfer relationship:
GPM = Q/ (500 x ΔT)

This is the fundamental equation used in chilled water systems. It links the actual cooling load (Q) directly to water flow and temperature difference.
If you rearrange it using tons of refrigeration, you get the familiar shortcut:
GPM = (TR x 24) / ΔT

And when ΔT is assumed as 10°F, it becomes:
GPM = TR x 2.4
So the “2.4 per TR” rule is not wrong. It is just the result of fixing one variable in the equation.

The important part is that the original equation depends on ΔT. And ΔT is not a constant in real systems. It shifts depending on coil selection, control stability, balancing, and actual system operation.

That’s where the difference starts to matter. Two systems with the same TR can have completely different flow rates if their ΔT is different.

So while shortcuts like 2.4 per TR are useful for quick sizing, the real design logic always comes back to:
Required water flow depends on two things: cooling capacity and ΔT. As ΔT changes, the required GPM changes with it
GPM = (TR x 24) / ΔT
In other words, the system always follows the physics, not the shortcut.

Let’s take a simple example.
A 2 TR FCU:
Using the quick method at 2.4 GPM per TR,
GPM = 2 x 2.4= 4.8 GPM

Now using the actual equation, but assuming a more realistic ΔT of 12°F:
GPM = (2x24)/12 = 4 GPM

Then you can clearly see the difference. Just by changing ΔT, the required flow drops from 4.8 GPM to 4 GPM.

That difference might look small on a single FCU, but across a full system it becomes significant in pump sizing, valve selection, and energy consumption.

So the takeaway is simple. The 2.4 rule is useful for quick sizing, but the real driver is always ΔT. When ΔT changes, everything else changes with it.

Water Supply System Layout.