April 15, 2007

Focus on O&M  (April 2007)

Metering H₂O, not kWh

Few cities of Toronto's size and density are blessed to have such a large energy-equivalent resource on its doorstep. At least in local terms, Steve Perkins, Enwave's vice president of engineering, is correct in saying, "We're changing the paradigm of how buildings provide air conditioning." As Perkins explains, "Instead of buying electricity to power their own cooling system, our customers now outsource the process to our district cooling operation. They receive cooling energy in the form of cold water, and they need only distribute that cooling energy within their building."

Enwave bills end users based on both their water consumption and the temperature difference between their supply and return lines. "Our customers are used to buying precisely metered electricity, so it is critical that we duplicate that accuracy," says Perkins. "Some spend millions of dollars on cooling each year."

For its water-metering system, Enwave turned to SRB Controls Inc. of Markham, Ontario, which recommended IFS 4000 KC Enviromag electromagnetic flowmeters (EMFs, or magmeters). The meters, which range in diameter from 3 inches to 20 inches, are made both by Krohne Inc., based in Peabody, Mass., and Krohne Toronto (www.krohne-mar.com). Krohne has been a leader in the development of magmeters since it introduced the first EMF for industrial applications in 1952.

Mag wheels turn

"We use only magmeters, because they're economical and very accurate and reliable over a wide range of diameters and flow rates," says Perkins. "Accuracy is especially important, considering that the flow to a customer might fall from 5,000 gallons per minute on a hot summer afternoon to only 200 gpm that same night."

EMF technology relies on a fairly simple principle of electromagnetism. Inside the meter, an electrically conductive fluid is pushed through an insulated measuring tube in the presence of a constant magnetic field. The voltage induced by the movement is proportional to the mean velocity of the fluid flow and detected by electrodes. Microprocessor-based electronics converts the voltages into scaled pulses at the rate of 1 pulse for every gallon that flows through the measuring tube. Enwave uses Krohne IFC 020 electromagnetic flow converters to store and aggregate data and to perform self-diagnostics.

In addition to providing billing information, the magmeters are networked over telephone lines to provide real-time information for monitoring the flow to each customer. For example, if a meter indicates that a customer is consuming a lot more water than usual, that may be a sign of a leak in the building's distribution system. Or, it may simply mean that Enwave needs to lower the temperature of its supply.

Enwave is connecting DLWC customers to its distribution loop at the rate of about 18 new buildings per year. Because of the wide variety of meter diameters required by different customers, it is not economical to carry any meter inventory. "But most important of all," says Perkins, "is the accuracy of the magmeters. We hate it when customers dispute bills, and that's why we're so fussy about the quality of the meters we use."

—Visit Krohne on the web at www.krohne.com.