Chilled water ton-hours measurement for billing
We are in a commercial building and are being billed per-ton hour for the chilled water usage on our 20-ton, chilled water fed Liebert upflow A/C unit. We are trying to figure out how much we are paying for how much tonnage we are actually using in a month. Our last bill said we used 2,904 ton hours, and the amt was just shy of $500. This 20-ton unit runs 24/7 and maintains a temp of around 70 degrees, and the chilled water valve is open 100% of the time to maintain that temp. So, we are trying to figure out how the ton hours are being calculated on the meter -- with 720 hours in a month, we expected something like:
720 * 20 = 14,400 ton hours
At 2,904, that comes out to around 4 tons, which we know we are using more than that.
Where are we going wrong here?
You would be better off measuring the entering & leaving water temps to the coil, and entering and leaving air temps over the cooling coil. With this info you could calculate BTUs better.
Assuming the coil is really putting out 20 tons because it is rated at 20 tons is a WAG as best. The coil could be dirty, low airflow, low water flow, etc that would affect what the coil is really doing. How sure are you that the valve is allways 100% open?
Most are going to use an energy meter of some sort that will measure flow, inlet and outlet temperatures. Flow in GPM is converted to mass and the difference between entering and leaving temperature is added to the equation to calculate thermal energy usage. See link for an example.