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Cryocooler Contamination (Cryogenics Refrigerators, Coolers, Engines)
Outgassing properties of various gaseous and liquid contaminants in cryocoolers have been investigated in Reference 1. Due to the lack of outgassing data as a function of temperature, a linear function was adopted, assuming desorption of gases from a monolayer. Further testings indicated that the temperature dependence of outgassing is nonlinear. The test setup and procedure are described in Reference 1, except that the test chamber temperature was varied between 23 C and 100 C. An exponential dependence of concentration on temperature was found, with much higher outgassing rates at elevated ambient temperatures (Reference 2).
The high outgassing rate at elevated temperature was first attributed to the possibility of phase transition. >As the boiling points of the liquids are exceeded, vaporization of the liquids may have resulted in the high contents of concentrations being measured. Further testing on the outgassing property of air shows that it exhibits similar exponential behavior. This confirms that the exponential outgassing property as a function of temperature is intrinsic to all gases and liquid tested.
Using a least squares curve approach, the above data can be fitted with the following equation.
m = K1 exp (BT) + K2......................................Eq. (1)
Where m is the concentration, T is the ambient temperature, B, K1, and K2 are constants.
Knowing the temperature dependence of the outgassing property, one can then combine with the time dependence function reported in Reference 1 to arrive at the following equation.
m = mo exp (BT) [1-exp (-t/A)]..............................Eq. (2)
Where mo is a function of the initial contaminant level in the system (i.e., the larger the value of mo the higher the contamination level), B is the temperature dependence constant found in Equation (1), and A is the time dependent constant found in Reference 2. The values of these constants are listed in Table 1.
Table 1- Outgass parameters of acetone, alcohol and water.
Care must be taken in applying values in Table 1 for Equation 2, for the contamination level inside a cooler is a function of the amount of gas in the cooler.
References:"Cryocooler Contamination Study", in the proc. of Adv. in Cryogenic Engineering, vol. 45A, P275, 1999. (with D.T.Kuo and A.S. Loc) Abstract Download
"Cryocooler Contamination Study: Temperature Dependence of Outgassing", to be published in the proc. of the 11th International Cryocooler Conference, 2000. (with D.T.Kuo) Abstract Download
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About the author- Dr. Sidney Yuan is a consultant in the field of Low Temperature Physics and Cryogenics, and has written a Book on Cryogenics and published extensively in the field. E-Mail.
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