A Compact 1900" F. Resistance-Wound Combustion Furnace for Remote Operation Donald R. Farmeloll Pratt & Whitney Aircraft-CANEL,
Division of United Aircraft Corp., Middletown, Conn.
of reactor fuel elements Iimportant and components, frequently it is to know the post-irradiation N THE DESIGN
carbon content of these materials. Conventional carbon analysis techniques can be employed if sufficient shielding is used to reduce the radiation levels t o practical limits. However, under the conditions imposed by the fuels-irradiation program a t CANEL, it was deemed more feasible to perform the analyses remotely in a chemistry hot cell. Conventional resistance furnaces were considered impractical because of the difficulty of loading samples remotely and keeping them contamination-free. Induction methods either required installation of a large furnace in the cell or expensive design modifications to extend the heating coil through the cell wall. Also, both methods required a new combustion crucible for each sample, a requirement which is tedious and time consuming for remote operation. Induction methods also have the disadvantage of depending on a metal susceptor for operation which can result in incomplete combustion of the sample. The furnace described herein is simple to operate, allows combustion of 8 to 10 samples without changing crucibles or interrupting the gas flow, and has a continuous heat source for better combustion. Furnace Design. The design consists of an upright ceramic pedestal surrounded by a gas-tight vycor tube (Figure 1). Samples in tin capsules are introduced through a modified Leco (Laboratory Equipment Corp.) sample loading stopcock, a n integral part of the outer glass vessel. The inner pedestal is mounted on the base while the outer vessel is attached to the lifting mechanism. A gas-tight seal is made between the outer and inner parts with an O-ring.
LECO SWPLE LOADING STOPCOCK
PYREX
GRADED SEAL
51
MJ WCOR TUBING
13 Rh MEWCOUPLE
Figure 1 .
Pure platinum wire wound on spiralgrooved, high purity alumina is the furnace core. The resistor has a room temperature resistance of 0.28 ohm and requires about 20 amperes a t 20 volts to maintain 1950' F. The design is such that the core can be replaced, if necessary, without dismantling the rest of the furnace. Also, the ceramic pedestal or the screw-type base containing a Pt-Pt, 13 Rh thermocouple and power leads can be changed separately.
Carbon Analysis of Standards No. of Av samples p.p.m. C Cb-1Zr secondary standard 18 756 NBS steel standard 121c 8 369 (18Cr-lONi-0.4Ti-0.038C) Table 1.
Diagram of carbon furnace
.
Rel. std. dev., %
RESULTS AND CONCLUSIONS
A Leco conductometric analyzer was used to detect the carbon dioxide produced upon combustion of the sample, Standardization of the analyzer is accomplished by introducing known amounts of COz gas from a calibrated manifold. The results of several remote analyses on a columbium alloy are shown in Table I. A standard deviation of f5.9Y0 was obtained on these samples, which contained about 200 pg. of carbon. Also shown in Table I are the results of analyses on the National Bureau of Standards steel standard No. 1 2 1 ~ .The average value was within 3Y0 of the reported value, indicating the accuracy of the determination. These samples contained
f 5.9
f11.4
Present address, Materials Development Laboratory, Pratt & Whitney Aircraft, East Hartford, Conn. VOL. 38, NO. 2, FEBRUARY 1966
365
about 100 pg. of carbon and were measured with a precision of =t11.4%. All samples weighed between 250 and 400 mg. Included in the above errors are those inherent in detection of the carbon dioxide produced. One major source of error is caused by uncertainty in the COz background rate over the required 15-minute collection time. Normally in
bench-top operation a sample can be analyzed in 5 to 6 minutes, but in this case the furnace is 25 feet from the analyzer. These errors can be reduced, particularly for bench-top application, by burning larger samples and minimizing the length of the gas line between the furnace and analyzer. However, it is considered sufficient proof that this furnace is directly applicable for the
combustion of steels and refractory metals for carbon or sulfur analysis. ACKNOWLEDGMENT
The author thanks Frank F. Felber, Jr., for his guidance in the project, and W. E. Hatch and R. A. Norton for their help in the mechanical development. Work performed for the U. S. Atomic Energy Commission under contract No. AT(30-1)-2789.
A Gas Sampling and Injection Valve for Vacuum Service Martin Alperstein and Ronald STUDY
1. Bradow, Texaco Research Center, Beacon, N. Y.
of slow combustion reactions
@ NUT
A in the end gas of an Otto-cycle engine in progress in this laboratory uses a unique gas-chromatograph column introduction valve. In one phase of the experimental program hot, reacting gases are withdrawn from the engine through a special valve (1) and are rapidly quenched by expansion into a low-pressure receiver. The quenched gases a t pressures less than 40 torr are qualitatively and quantitatively analyzed by flame-ionization gas chromatography. The analysis system requires a gas handling apparatus that would maintain sample integrity while introducing the sample into a flowing carrier gas stream a t pressures up to 100 p s i Specific requirements for such a gas sampling valve include: capability of leak free operation with collection torr; low volume pressures of internal pressure drops; leak-free carrier gas circuit; no exposure of sample t o the atmosphere during switching; no materials which absorb sample or outgas to contaminate sample. Spool or linear valves are unsatisfactory because of their high pressure drops. When ports are increased in diameter, damage of the highly loaded edge seals occurs ( 7 ) . Rotary indexed face-sealed valves (4,6,8), which possess the advantage of large ports and variable loading of the seals, are commonly constructed with hard seals which either do not effectively seal the pressure extremes or quickly deteriorate on repeated switching. Systems of glass (2, 6) or metal stopcocks (3) either expose the sample to stopcock lubricant or do not have sufficiently rapid injection characteristics to be used with low-flow, high-resolution columns. Ideally, a valve meeting the system requirements should have the injection and variable seat loading characteristics of a rotary indexing valve and the low pressure sealing capability of a spool valve. A valve was designed and constructed using internal flow paths similar to the rotary valves but using easily replaced, 366
ANALYTICAL CHEMISTRY
A W E PORT PLATE
SEAL (6) TWO CONCENTRIC O-RINGS
INDEXING GUIDE
SURROUNDING O-RING GROOV
Figure 1.
VACUUM PORT
Low-pressure service six-port sampling valve
inexpensive O-rings as the face seals. The figure illustrates a typical design with this unique sealing configuration. The isometric assembly sketch shows a six-port valve; however, other porting arrangements should be equally feasible. As constructed, the smallest port passage used has a 1/16-inchdiameter and the valves were machined from 304 stainless steel. Each port seal consists of two concentric O-rings ( W o n A series 2-10 outer and series 2-5 inner) held in 0.047-inch deep recesses machined in the valve body. In assembly, the valve port plate slides easily over the valve stem, and is indexed t o the valve body so that all ports align a t valve motion extremes (in this case 60" movement). The valve port plate is loaded toward the valve body by variable spring tension so that the O-rings are squeezed approximately 0.009 inch. This deformation is sufficient to seal over the required pressure ranges, yet permits handoperated orientation of port positions.
In construction, it is not necessary to lap the valve port plate face; tolerances of 3tO.001 in. and =t are maintained. The port edges of the valve port plate are rounded approximately 0.005 inch to prevent O-ring cutting. The valve port plate is self leveling relative to the valve body and equalizes loadings on all O-rings. The valve illustrated is suitable for analyses of air components because the port seals could be surrounded by a vacuum environment formed by series 2-33 outer and series 2-15 inner O-rings. Such arrangements prevent dilution of sample by entrapped air in the valve port plate passages upon flow path orientation. This precaution is unnecessary for other analyses, and the surrounding O-rings and vacuum port are not provided in valves used for hydrocarbon analyses. The valve stem illustrated can be extended and keyed to the valve port plate for remote operation or immersion of valve body in a controlled-temperature air bath.
