District Energy - 2006 First Quarter

Inside
Insights

of the energy meter installation. So how do these inaccuracies all combine? First let’s look at each of the major components.

How does your
energy meter
measure up?
Steve Tredinnick, P.E., Mechanical Systems, Affiliated Engineers Inc.

Editor’s Note: “Inside Insights” is a column designed to address ongoing issues of interest to building owners, managers and operating engineers who use district energy services.

Instrumentation devices in district energy systems are usually of high quality, very accurate and very reliable due to the long life expectancies and sensitivities of accounting for the energy used. One of the key components to the successful relationship between the district energy provider and customer is the energy meter, which is generally supplied by the provider. That’s why I thought it would be helpful to give district energy customers an inside look at how meters work.

The accuracy of the energy measured and calculated is directly related to the precision of the components selected and how they are installed and maintained. There are many technologies available to choose from and all cannot be covered here, so I have focused on current state-of-the-art selections for water-based systems.

The major components of an energy meter are one flow element (meter), one flow transmitter,

two temperature sensors (one on the supply and one on the return),

two temperature transmitters (one on the supply and one on the return), and

one energy (Btu) calculator.

Each of the above devices has its own level of accuracy, which is either inherent to the device or can be affected by not properly following the manufacturer’s installation and maintenance recommendations for clearances and calibration requirements. However, there also is an additive effect of each component’s inaccuracy that ultimately defines the accuracy

Flow Meters and
Transmitters

Flow is usually measured indirectly by first measuring differential pressure or fluid velocity in a pipe; then the flow rate is calculated by using the known pipe area.

Flowmeters can be grouped into four generic types:

positive displacement (gear or nutating disc) meters

head meters (orifice plates, venturis, target),

velocity meters (turbine, electromagnetic, vortex shedding, ultrasonic)

mass meters (thermal, coriolis)

See table 1 for a sample of flowmeter characteristics.

Usually velocity meters are used for chilled water, with a focus on either electromagnetic meters or ultrasonic. Standard magnetic flow meter inaccuracies are ±0.5 percent of span, but with a matched transmitter and additional bench calibration at the manufacturer, the value drops to ±0.25 percent of span.