The cooling available from liquid C0 2 is approximately 80 Btu/lb (0.186 MJ/kg) 

 from heat of vaporization and 22 Btu/lb (0.051 MJ/kg) from heat absorbed between -11°F 

 and -0°F (-79°C and -18°C) for a total of 102 Btu/lb (0.237 MJ/kg) (Ashrae 1972). 

 The amount of liquid C0 2 was calculated, assuming 70 percent efficiency, as follows: 



kg of C0 2 = (36,479) (102) (0.7) = 510 lb or 232 kg 



If the conditions described in the paragraph above could be maintained, then the 

 C0 2 consumption per kg of dry sample would be approximately 0.368 lb C0 2 /lb sample 

 (0.368 kg C0 2 /kg) or about 25 percent of the C0 2 necessary in Walkotten's (1976) system. 



Three variables can cause large discrepancies in the cooling values calculated 

 above. The first is water temperature. If the water temperature was 59°F (15°C) 

 instead of 40°F (4.5°C), the C0 2 required for cooling would increase 22 percent. The 

 second variable affecting the C0 2 requirement is the amount of water circulated 

 through the sample area. As an example, a volume flow rate of 1.05 ft 3 /min 

 (494 cm 3 /s), which would be equivalent to a velocity of 0.02 ft/s (0.61 cm/s) 

 through the void area in the sample, could add 28,156 Btu (29.7 MJ) of heat to the 

 sample with a 7.2°F (4°K) temperature change in 1 hour. The above velocity would be 

 about 1/100 of normal stream velocity. If very rapid cooling cannot be achieved 

 within 10 minutes, the surface water circulation needs to be reduced to near zero-- 

 if possible. The third variable is the cooling efficiency of the C0 2 . At least 70 

 percent of the C0 2 cooling capacity should be released into the sample. 



DESCRIPTION OF NEW METHOD 

 C0 2 Liquid-Gas Phase 



The freeze methods used today (Walkotten 1976; Lotspeich and Reid, in press) 

 expand C0 2 liquid to atmospheric pressure producing a gas of -110°F (-78°C) . This 

 causes large amounts of solid C0 2 (dry ice) to form. Dry ice is a poor heat transfer 

 medium and as it builds up in the probe it decreases the cooling capacity of the expand- 

 ing C0 2 . Our new method expands C0 2 to a pressure of 78 psia (540 kPa) producing a temp- 

 erature of -68°F (-55°C). At this temperature C0 2 is a boiling liquid-gas medium which 

 provides a much higher rate of heat transfer than dry ice. Initially, the heat trans- 

 fer rate is less because of the higher temperature (-68°F or -55°C) of the liquid-gas 

 rather than gas at atmospheric pressure (-110°F or -78°C). Liquid C0 2 allows for 

 better gas control, higher cooling efficiency, and fewer clogged probes and lines by 

 dry ice. 



C0 2 in the System 



A schematic of the system and probe layout is provided in figures 1 and 2. The 

 tanks containing the C0 2 are stationed on the bank. The C0 2 in these tanks is 

 maintained under a pressure of 500 to 850 psia (3.45 to 5.85 MPa) depending on ambient 

 temperature. The tanks release C0 2 through high pressure hoses to a distribution 

 manifold where it is filtered. The C0 2 then flows through opened throttle valves 

 attached to each of the four rows of probes and enters the first probe in each row. 

 These probes rapidly fill with C0 2 liquid and the overflow travels into the next 

 probe in each row. This procedure is repeated until all probes are filled. The 

 excess C0 2 leaves the last probe through an adjustable relief valve. The liquid is 

 then throttled to a pressure of 79 to 84 psia (540 to 580 kPa) providing a temperature 

 of -68°F (-55°C). By maintaining C0 2 in liquid form all the available heat of 

 vaporization for cooling the sample is used. This represents ■ nearly 80 percent of 

 the cooling capacity of the C0 2 . 



4 



