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Direct Use of Geothermal Resources                                          219































            FIGUre 11.10  (See color insert following page 17.0..) Drill rig used for the Canby geothermal project. The
            drilling rig was a rotary platform  system with a tower height of about 18.5 m. (Photograph courtesy of Dale
            Merrick and used with permission.)

              The design heating load for the district heating system was calculated based on the number
            of buildings (34) to be included in the system, building size (nearly 5000 m  of total floor space),
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            and construction type (insulated and uninsulated buildings). The peak heating load was based on
            American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) heating
            design standards for 99.6% of the climate conditions in the area, which was 18°C inside temperature
            and −15°C outside temperature. This combination of climate conditions and building population
            resulted in a computed heating load of 389,784 J/s.
              Analysis of the hydrological state of the aquifer suggested that long-term production capacity was
            about 2.3 l/s with a potential maximum draw down of the water table of about 75 m. In order to assure
            that the system would be operated in a sustainable way, a computer control system was installed to
            monitor and control flow rates so that the actual load required by the thermostatic controls in the
            buildings was matched by the flow rate in the system. This necessitated emplacing a pump in the
            well, which was seated at a depth of about 73 m, which allows precise control of flow rates.
              Such a control capability is an important element in a system in which a limited resource must
            be carefully managed. By monitoring demand, the system can follow the load, thus eliminating or
            reducing production when there is no or little demand. Such an approach minimizes draw down and
            allows recovery of the system when demand is low.
              The district heating system (Figure 11.11) required approximately 2050 m of preinsulated copper
            pipe for the main distribution lines. Approximately 550 m of 2.5 cm diameter preinsulated cross-linked
            polyethylene pipe was used for connecting supply and return pipes to each building. Propane-fired fur-
            naces that were in most of the buildings were retrofitted with hot water—air heat exchange systems
            and blowers. In-floor radiant heating was installed in one building. It was realized that the hot water in
            the district heating system could also be used for domestic hot water heating and that system was also
            emplaced. Approximately 1550 m of PVC pipe was used for the discharge line (Figure 11.11).
              Figure 11.12 shows the component layout of the control and production well facility. Production
            from the well flows to a heat exchanger capable of transferring 433,745 J/s to the heating fluid in
            the closed-loop district heating system, which is more than sufficient to meet the design load of
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