A pressure gage located on the last probe in each row monitors the C0 2 pressure. 

 The throttle and relief valves are adjusted to maintain the desired pressure in the 

 system and minimize the loss of C0 2 through the relief valve. A second relief valve 

 is positioned on the first probe to relieve excessive pressure (over 100 psi) if 

 blockage should occur in intermediate probes. Such a blockage could allow full tank 

 pressure to be applied to the probes ahead of the blockage. 



Maximum efficiency in this system requires the liquid and gas CO2 to be held at 

 a uniform pressure and therefore, temperature, while it continually circulates in the 

 tubes and probes providing the turbulence necessary for efficient heat transfer. Also, 

 the probes are insulated above the stream channel to reduce cooling losses to water 

 and air. A uniform probe wall temperature is maintained because the C0 2 is at a 

 constant temperature for a given operating pressure. 



Probe and Water Flow Shield 



The probes are driven into the riverbed to the desired freezing depth exposing 

 the C0 2 cooled probe wall directly to the substrate. The probes are arranged in 

 four rows of eight probes each and spaced 6 inches (15.2 cm) apart so that an area 

 2 by 4 ft (0.61 by 1.22 m) will be frozen. This provides good control of C0 2 con- 

 sumption but requires 1 to 1-1/2 hours freezing time. The water circulation must be 

 reduced to low flows for complete freezing to take place. To reduce water circula- 

 tion, a metal shield and canvas was worked into the streambed around the probes. If 

 high water temperatures become a problem they can be overcome by using more C0 2 and 

 extending the time of C0 2 circulation. 



DESCRIPTION OF EQUIPMENT 

 Probes and Caps 



The probe and cap construction is shown in figures 3 and 4. The brass flanges 

 were wetted over the entire joint area while being soldered to the top of the 

 probe. This required pretinning the inside of the flange and the outside of the 

 probe. The probe parts not used directly for freezing were insulated with Cell-0- 

 Flex pipe and insulation. DWV pipe was installed loosely over the probe and held 

 in place with spacers. With the bottom cap removed, insulation was poured between 

 the pipe and probe. 



Three different probe cap configurations were used depending on the location 

 of the probes (fig. 5). The basic cap consists of a piece of brass hex machined 

 to the dimensions shown in figure 4. The copper tube is on the inlet side of the 

 cap. The caps for the upstream probe in each row have a 1/4-inch (6.35 mm) hydraulic 

 swivel fitting, a throttle valve, and a relief valve connected to the inlet side of 

 the cap. The downstream cap in each row has 100 psia (0.689 MP a) gage and a relief 

 valve on the outlet side of the cap. All other cap connections have tube fittings as 

 indicated. The probes are connected with flexible nylon tubing rated for 700 psia 

 (4.83 MPa) . The downstream probe in each row has a 1/8-inch (3.3 mm) stainless steel 

 clad thermocouple mounted on it to record probe wall temperatures. The thermocouple 

 is equipped with a standard plug near the top of the probe. 



6 



