What is The Formula to Measure The Energy Content of Liquid Flow

The Formula to Measure the Energy Content of Liquid Flow is called the Btu formula. The Btu formula assumes that Liquid is flowing in a loop to and from a point that requires either heating or cooling. The Btu formula is designed to measure the difference in energy from the feed liquid flow versus the return liquid flow. This formula is expressed as follows: 

P= V x ρ x (hTV –hTR)

Where  P = Power (BTU/Hr)

V = Volumetric Liquid Flow rate (Gal/Hr)

ρ = Density (Lb/Gal)

hTV = Specific Enthalpy (BTU/Lb) – Feed Temperature

hTR = Specific Enthalpy (BTU/Lb) – Return Temperature

The Btu formula requires three dynamic measurements in order to calculate the energy content. These measurements are made up of a volumetric liquid flow installed in either the feed or return line, a feed line temperature, and a return line temperature. With these parameters and a flow computer using the Btu formula, a dynamic calculation is made to calculate the instantaneous energy usage in a heating hot water or chilled water loop.

For a chilled water system the energy consumption can be calculated in Btu’s or TON’s, where the conversion calculated inside the Btu meter is as follows:

1 Ton x Hour (Refrigeration) = 12,000 BTU’s

Considering the Volumetric liquid flow measurement occurs in either the Feed or Return line, the installation location must be identified so that the proper Density (ρ) value is applied in the Btu calculation for the highest system accuracy. Also, since some Btu calculations are made at subfreezing water temperatures, the liquid media may be a Water/Glycol mixture. In this situation the density values used must incorporate the glycol influence on the media density. The effect of Glycol will keep the solution from freezing, but lowers the density of the solution in comparison to just water. This requires that the (%) by weight of the glycol present be known and adjustments be made in the Btu formula calculation in the form of a look-up table or curve fit calculation at varying temperatures. See table on previous page. 

This calculation may also be simplified as an approximation by taking the volumetric flow in (Gal/Min) and multiplying it by the difference between feed and return temps (∆T) and a rounded constant value of 500. While this is not an exact calculation it will give a good approximation for a quick check of energy usage.

Btu/hr = Flow (GPM) * (∆T)* (500) – Chilled Water

Btu/hr = Flow (GPM) * (∆T)* (490) – Heating Hot Water

The rounded multiplier of 500 comes for the conversion from GPM (Gal/Min) to GHP (Gal/Hr) or a factor of 60 multiplied by a generalized Density of Water (8.333 Lbs/Gal @ 65°F), which would provide a very good approximation for a chilled water energy measurement. For a hot water system a Density value (8.255 Lbs/Gal @ 125°F) could be used for a constant of 490 as the multiplier.

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