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220 Geothermal Energy: Renewable Energy and the Environment
District heating
distribution
line
Discharge line
500 meters
Discharge point
FIGUre 11.11 Air photo of the town of Canby, showing the district heating layout (white lines), production
well location and discharge line (black line). Individual buildings served by the district heating system are
outlined in white. The location of the production well, which is co-located with the control facility and GAC
filter system, is indicated. (Modified from diagram provided by Dale Merrick. With permission.)
Heat
exchanger Backup
Production boiler Hot water
well
supply
Hot water
return
Hot water
return
GAC
Heat filter
exchanger Discharge
FIGUre 11.12 Schematic of the production well, distribution system, and disposal system for the Canby
district heating system. (Modified from an original by Dale Merrick. With permission.)
2.3 l/s at a temperature of 88°C. When the heat exchanger operates optimally, it is capable of heat-
ing 38°C return water from the district heating system to 66°C. The geothermal fluid that leaves
the heat exchanger at 43°C flows to a secondary heat exchanger which preheats return water, thus
maximizing heat transfer and use.
It was found that optimal performance for the heat exchanger was not achieved because the
water has an iron content of about 3 mg/l. The iron deposits on the surfaces of the heat exchanger
plates, reducing the thermal conductivity of the plates. Within a period of about three months the
heat transfer efficiency of the primary heat exchanger drops by about 15%, requiring that the heat
exchanger be disassembled and cleaned. This periodic maintenance has been incorporated into the
operational schedule for the system.
Experience has shown that, on average, the district heating system load is about half that of the
design load. Since the computer monitoring and control system functions on a load-following basis,
this has allowed the resource to be minimally affected. Experience has shown that it is only for brief
winter periods that the design load must be met. On a few occasions, flow rates as high as 2.5 l/s