Heat transfer fluids and methods of making and using same...

Details Heat transfer fluids and methods of making and using same... Heat transfer fluids and methods of making and using same...

2001-10-31

2003-11-25

Lazarus, Ira S. (Department: 3749)

Drying and gas or vapor contact with solids

Process

Cooling by gas or vapor contact

C034S072000, C034S075000, C034S435000, C252S071000

Reexamination Certificate

active

06651358

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to novel compositions comprising substantially pure hydrogen and substantially pure helium and their use as heat transfer fluids in a variety of applications.
2. Related Art
Pure or substantially pure helium has excellent heat transfer properties. For example, helium is typically employed to enhance fiber cooling during the optical fiber drawing process because it is chemically inert and because of its heat transfer properties. Of the common pure gases, only pure hydrogen has a higher thermal conductivity than pure helium. However, hydrogen is not as inert as helium and it is more hazardous to employ in certain gas-related heat transfer applications than any inert gas. Therefore, hydrogen is typically avoided as a gaseous heat transfer medium in some (but not all) cooling or heating process applications.
It is generally accepted that binary mixtures of helium (or hydrogen) with other gases will have better heat transfer coefficients than the pure gases themselves. See, for example, M. R. Vanco, “Analytical Comparison of Relative Heat-Transfer Coefficients and Pressure Drops of Inert Gases and Their Binary Mixtures, NASA TN D2677 (1965); F. W. Giacobbe, “Heat Transfer Capability of Selected Binary Gaseous Mixtures Relative to Helium and Hydrogen”,
Applied Thermal Engineering
Vol. 18, Nos. 3-4, pp.199-206 (1998); R. Holoboffet al., “Gas Quenching With Helium”,
Advanced Materials
&
Processes
, Vol.143, No. 2, pp.23-26 (1993). In particular, Holoboff et al. noted that in the context of a heat treating furnace, by changing to an optimum helium/argon mixture, a customer was able to heat treat parts that could not be processed as rapidly as when using argon alone, while maintaining lower operating costs than normally required when using 100% helium. In a separate example, the same authors also recognized the benefits of increasing the fan speed (gas circulation velocity) on specimen cooling rates when using pure helium and pure nitrogen in cooing applications. However, there is no teaching or suggestion of the influence of heat transfer fluid mixture velocity on cooling rate for optimized mixtures of heat transfer fluids.
For illustrative purposes, according to earlier theories the relative heat transfer capability of helium plus one other noble gas compared to pure helium may be seen in FIG.
1
. In
FIG. 1
, pure helium has been arbitrarily assigned a relative heat transfer capability of 1.0 in order to deliberately avoid the use of a more complicated system of SI heat transfer units. So, if a binary gas mixture containing helium has a heat transfer capability of 2.0 (relative to pure helium), it is assumed from this data that gas mixture will be 2.0 times more effective in any heat transfer process employing that gaseous mixture instead of pure helium alone. And, as a simplified illustration of the potential helium savings using this data, if the best binary gas mixture contained only 50 percent (by volume or mole fraction) helium plus 50 percent of some other gas, only ½ of that gas mixture would be needed to perform the same cooling function as the pure helium alone. Therefore, only 25 percent of the helium that would have been required for a particular heat exchange process using pure helium would be needed during the same cooling process employing the gas mixture.
In
FIG. 2
, and also according to earlier theories, the optimum composition and approximate relative heat transfer capability of hydrogen plus one noble gas with respect to pure helium is illustrated. In
FIG. 2
, pure helium has also been arbitrarily assigned a relative heat transfer capability of 1.0. So, if a binary gas mixture containing only hydrogen and argon (but no helium) has a heat transfer capability of 1.4 (relative to pure helium), that gas mixture presumably will be 1.4 times more effective in any heat transfer process employing that gaseous mixture instead of pure helium alone. And, since no helium is required to produce this effect, the helium usage is cut to zero. Furthermore, since hydrogen and argon are typically much less expensive than helium, the overall cost of the hydrogen/argon coolant gas stream will tend to be negligible compared to a pure (or relatively pure) helium coolant gas steam.
It should be emphasized that the data presented in
FIGS. 1 and 2
are theoretical and based on turbulent flow for all gases and gas mixtures considered. However, in the seminal work of R. B. Bird, W. E. Stewart, and E. N. Lightfoot,
Transport Phenomena
, pp. 392-393 (1960) it was pointed out that “the heat-transfer coefficient depends in a complicated way on many variables, including the fluid properties (k, &mgr;, &rgr;, C
p
), the system geometry, the flow velocity, the value of the characteristic temperature difference, and the surface temperature distribution.” In engineering design, therefore, use of constant property idealization frequently leads to either a greater built in safety factor, or a dangerous situation if the other extreme is taken. See D. M. McEligot, et al., “Internal Forced Convection to Mixtures of Inert Gases”,
Int. J. Heat Mass Transfer
, Vol. 20, pp. 475-486 (1977).
In light of the unexpected nature of heat transfer coefficients of fluids, it would be advantageous in many heat transfer situations common in engineering to employ a heat transfer fluid mixture consisting essentially of pure hydrogen and helium that can easily be changed in composition to take advantage of the heat transfer properties of hydrogen, without the dangerous explosive characteristics of pure hydrogen, or to reduce the cost of using pure helium.
SUMMARY OF THE INVENTION
In accordance with the present invention, compositions consisting essentially of substantially pure hydrogen and substantially pure helium are presented (that can be advantageously employed in heat transfer applications, such as glass fiber cooling applications) which significantly reduce the danger of using pure hydrogen while providing nearly the same heat transfer properties as pure hydrogen. As used herein the term “hydrogen” means molecular hydrogen, or H
2
. It has been discovered, quite unexpectedly, that heat transfer fluid mixtures consisting essentially of hydrogen and helium, plus such optional fluids such as argon, when flowing past a heat transfer surface at very low bulk velocity or very high bulk velocity, exhibit heat transfer coefficients that are less than but close to that of the pure hydrogen flowing at the same bulk velocity. Therefore, while compositions of the invention might require slightly more heat transfer area than pure hydrogen to achieve the same characteristic temperature difference in a fluid being heated or cooled, since the inventive compositions are much less explosive than pure hydrogen, there is an opportunity for better overall safety and longevity of equipment. Alternatively, if the designer allows for a slightly higher characteristic temperature difference, no change in heat transfer area is required. Furthermore, due to significant improvements in the heat transfer coefficients of these gas mixtures over substantially pure hydrogen when flowing at bulk velocities between very low and very high bulk velocity, the heat transfer designer may decide to use the inventive compositions and vary a parameter, such as concentration, bulk velocity, system pressure, characteristic temperature difference, and the like, to suit high demand time periods. For example, during times of high cool air demand in the summer months, a refrigeration unit employing one of the compositions may vary the concentration ratio of gases and the bulk velocity to achieve a higher characteristic temperature difference (better cooling).
As used herein the term “cooling” includes freezing. The term “heating” includes boiling, vaporizing, and the like.
The term “substantially pure hydrogen” means a composition that includes only impurities or additives in such amounts that do not substantially lesson the heat transfer characteristics of pure hydrogen

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