The simple formula which is used to calculate chilled water Btu is

Btu/hr = 500 * gpm * $\Delta$T

Where,

gpm - Water flow rate measured in gpm (gallons per minute)

$\Delta$T - Chilled water Temperature difference between inlet and outlet in °F

The above equation is derived from

Q = m$C_{p}$$\Delta$T

Where,

Q - Total enthalpy

m - Mass flow rate

$C_{p}$ - Specific heat of water

$\Delta$T - Chilled water Temperature difference between inlet and outlet

We want to get the total enthalpy in Btu/hr by entering the water flow rate in gpm. To achieve that we need the vairables in below specified unit of measurements

mass flow rate in lb/gallons-hr,

Specific heat in Btu / lb-°F,

Temperature difference in °F

- Mass flow rate of water = mass/volume
- 1 gallon of water = 8.33 lb
- 1 gpm = 60 gallons/hour
- Specific heat of water = 1.0 $Btu/lb-^{o}F$

Substitute the above values and we get

Q = (8.33 lb * gpm * 60) * 1.0 * $\Delta$T

= 499.8 * gpm * $\Delta$T

$\thickapprox$ 500 * gpm * $\Delta$T

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