As energy conservation becomes more prevalent throughout society, the requirement arises for most users/industries to first measure energy usage in order to conserve.
For heating and chilled water applications, this requires the installation of flow and temperature measuring devices tied to a BTU computer. From this information, the end user can assess where and how energy is being consumed and then take steps to improve efficiency.
The issue of concern is what is the weakest link in this Energy system, and how does it impact overall measurement accuracy and the following decisions taken from the information gathered?
Almost without exception, this is the flow technology chosen as part of the system. The two most typical technologies used are mechanical flow devices (turbine, paddle, and nutating disc) or differential pressure producers (orifice plates, pitot tube, flow bars, v-cone, target meters).
Since Mechanical meters by their nature have moving parts, they will experience wear and drift over time. So if a Mechanical device has a stated accuracy at installation, it pretty much ends at that point. In addition mechanical meters are very limited in their turndown or ability to measure low flows. In low flow conditions Mechanical meters either cease to function, or the accuracy decreases logarithmically until the reading is essentially useless. Accuracy is (+/-) 1.0% at best and once installed and running for any period of time (+/-) 5-20%.
Differential producers create another issue. Since the calculation for flow requires a square root math calculation in order to convert to a flow reading, any error is compounded by the calculation. Turndown is again a major concern as that the differential pressure measurement becomes more and more difficult as the velocity of the flow drops making induced error even more significant. Accuracy (+/-) 1.0 % at best and at low flows or over time (+/-) 10-15% at best
The bottom line is that with flow modulation, which is the typical condition found with hot and chilled water distribution as created with VFD’s or modulation control valves, both of these flow technologies (Mechanical & Diff Pressure) should be avoided if reliable and accurate Energy measurements are expected!