As far as hot/chilled water BTU measurement systems goes, panel mounted energy computers, volumetric flow meters, and high precision temperature elements have become the technology solution of choice for most facilities and plants.
With that being said, in most cases, these facilities will experience significant savings on energy, maintenance, and processing costs. While the accuracy of most flow computers are quite high, the sensor technology—flow and temperature—used in conjunction can be an area of concern, especially with LEED certified installations for energy conservation, and where sub billing is occurring.
Below are three things to keep in mind to achieve optimal system performance:
1. Flow Technology Selection
When selecting a flow meter in a hot/chilled water BTU measurement system, it can vary from a number of volumetric technologies, including magnetic, turbine, vortex, differential pressure, and ultrasonic flow meters.
It is important to be aware that all technologies vary greatly in performance, maintenance, accuracy, and capability. As the BTU market has matured, flow measurement has become recognized as contributing the single greatest inaccuracy to overall system performance.
When selecting a BTU measurement technology for a hot/chilled water energy system, it is important to understand the operating conditions of the application. In particular, consideration must be given to the low-flow requirements created to help conserve energy when moving water throughout the system.
Low-flow conditions in hot and chilled water systems now approach velocities well below 1.0 ft/s. At this point, many volumetric flow technologies cease to operate or become so inaccurate that they are no longer a valid option.
In this environment, an inline magnetic flow meter can provide reliable and accurate measurement and is quickly becoming the technology of choice. In addition, magnetic meters require minimal straight runs of pipe to function properly, allowing them to overcome the traditional flow-profiling requirements of other volumetric flow technologies.
Additional tip: Typically, you will need a straight run of at least 10 pipe diameters upstream and 5 pipe diameters downstream for most magnetic flow meters. However, some manufacturers have improved the operating design of the flow tube and electronics, significantly reducing this requirement to 1 pipe diameter or less upstream and downstream.
2. Temperature Sensors
Typical temperature sensors include thermocouples, thermostats, resistance temperature detectors (RTDs), and temperature-sensing integrated circuits.
Without going into the function of each device, RTDs are considered the most accurate and stable option for these applications. The grade of an RTD will vary, and the higher the grade, the greater the precision.
In addition, while linearity is good, it is important when selecting a pair of RTDs for this application that the sensors are matched or selected with similar resistance curves. This is called precision matching and is identified when the sensors are calibrated.
Precision-matched temperature sensors should be selected with a matching accuracy of ±0.1% or better.
Why Is Precision Matching Important?
Temperature measurement is critical when calculating enthalpy and density for energy measurement in hot or chilled water. Reducing temperature-measurement inaccuracy improves the accuracy of the overall system, including the flow computer, flow meter, and temperature sensors.
The combined system accuracy should be maintained at or below ±1.0%.
3. Installation Considerations
Care should be taken when installing the components into the process system. Flow meter installation is critical for proper flow profiling.
Most volumetric flow technologies require a minimum of 10 pipe diameters of straight piping upstream and 5 pipe diameters downstream to allow for a proper flow profile.
As mentioned earlier, when using some improved or advanced magnetic flow meters, this requirement may be reduced significantly—as much as 1 pipe diameter on either side of the meter—allowing for much greater installation flexibility.
Temperature sensor installation provides more flexibility. However, the sensor mounted on the same side of the system—feed or return—as the flow meter should be installed as close as possible to the flow meter without affecting the meter’s required flow profile.
This positioning will provide the highest calculation accuracy for fluid density within the energy flow computer.