A Simple Portable Aradiant Convection Pyrometer . FRANK T. BARR’
ALTHOUGH
AND
RICHARD F. BERGER,* Armour Institute of Technology, Chicago, Ill.
radiation errors in the measurement of temsuction mechanism may be necessary. To reduce radiation, an insulating shield should be placed around the element. perature of gases are common and frequently of comparatively great magnitude] correction for these errors is a t Any type of element may be used, providing it registers by conduction and its impulses can be transmitted by leads; best a rough approximation and involves considerable inconthis limits the selection, for practical purposes, to the thennovenience. This is due to the lack of sufficient data on emission coefficients and the difficulties met in accurately meascouple, although the resistance thermometer element might uring the temperature of all surfaces which may be radiating be used. Finally, all parts should be small, to allow use in heat to or receiving heat from a static thermocouple. It is cramped quarters, and as light as consistent with structural evident that the development of a convenient form of pyromeand thermal strength. ter for measuring the true temperatures of gases without the Design of Instrument necessity for attempting corrections is much needed. Various suggestions for eliminating the effect of radiation on The instrument developed has been made in two forms, diapyrometer readings have been made. Methods of shielding grams of which are shown in Figure l. Each has certain conthe measuring element from radiation from the walls by means structional advantages, but neither construction involves the of polished metal surfaces and the use of a shield with regusacrifice of any desired characteristics of performance. lated temperature have been described (4). Heat-insulating The fist was built up from standard parts obtainable from any shields have also been suggested] and the use of small measurlaboratory supply house. A 2&inch length of 0.375-inch cop er ing elements has been investigated (6). This latter method tubing was flared and fitted with a T-fitting at one end ani! a partially accomplishes the desired radiation correction because standard coupler at the head. The thermocouple leads were carried from the head in standard 0.1875-inch two-hole alundum radiant heat transfer takes place a t the surface of the element insulators placed inside the cop er tube. The thermocouple while transfer of heat by convection takes place a t the surface junction itself was fused to a smafbead and placed in the center of a stagnant film around the measuring element and of of a 0.0938-inch hole in a slate disk cut for the purpose and held greater area than the element. For very small elements it is in position by the cap and asbestos packing placed over it. An inlet hole 0.3125 inch in diameter was drilled in the c m . To h e b obvious that this difference in heat transfer area mag become proportionately large. These methods, however, are accurate only under a comparatively narrow range of c o n d i t i o n s and are not so generally applicable as to lead to the development of a convenient portable temperaturemeasuring instrument. It was early experimentally discovered that the velocity of the gas p a s t t h e p y r o m e t e r ASBLLrTos element affected the reading and that a higher speed decreased the error due to radiation (12). In nearly all measurements of gas temperature, heat from the substance whose temperature is to be measured is transmitted to the measuring element by convection only. It is, therefore] advantageous to use methods which increase the heat transfer by convection as well as decrease that by radiation. The use of suction to increase the telocity of the gas past the therFIGURE1. Two TYPESOF ARADIANT CONVECTION PYROMETER mometer element accomplishes this, since the coefficient of heat transfer increases with the prevent the effects of conduction of heat along the lead wires, velocity of the gas past the heat-transfer surface (IO). Several they were left unsupported for the last 0.75 inch. The alundum attempts to apply this principle have been made (1-3,6-9,11), tubing was fixed at the center of the copper tube by crimping the but none of these so-called ‘(high velocity” thermocouples or end at 3 points. The lead wires were carried out throu h the T“suction” pyrometers has been developed into an instrument fitting. This oint had the alundum insulator packet in with asbestos and tge holes filled in with high-temperature cement to which is of general utility and is still dependable and easily prevent air leakage. To the remaining end of the T-fitting was handled. Most are unwieldy and lack sufficient simplicity in attached a 7-inch doubie coil of the same s i ~ tubing, e which served construction to be widely used. to cool the gases before they reached the pump. The design of the proposed instrument should embody the The second design was the result of a suggestion by Harry McCormack, Department of Chemical Engineering, Armour Infollowing features: The gas velocity past the measuring elestitute of Technology, Chicago. In this design all screwed coument should be sufficiently large to make heat transfer by plings were eliminated and taper joints substituted. The Tradiation an insignificant part of the total amount; likewise, fitting in the first design was replaced by a special brass casting leads from the element should be so small that conduction with the angle changed t o 30’. The thermocouple was carried as before in 0.375-inch copper tubing, but the coupling at the along them is negligible. The high gas velocity must be obhead was replaced by a 1.75-inch length of 0.6-inch copper tube. tained by the diversion of the smallest feasible amount of the The disk in this case was cut from standard 0.25-inch alundum gas from its normal path. Cooling the gas before reaching the single-hole insulator and was held in a 0.1875-inch length of 1 Present address, Standard Oil Company of Louisiana, Baton Rouge, La. 0.375-inch tubing whose ends were rolled in. This assembly was * Present address, Universal Oil Product. Company, Riverside, Ill. a force fit in the 0.5-inch tube. The three main joints were made 393
INDUSTRIAL AND ENGINEERING CHEMISTRY
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FIGURE2. TYPICAL CALIBRATION CURVES by machining the 0.375-inch copper tube for a taper fit, 0.1875 inch taper per foot being used. A 7-inch double cooling coil was fitted as before. The lead wires were sealed at their point of exit by counterboringfor 0.125 inch and filling with vinylite which was baked in. This design has the advantage of very small size at the head, the greatest outside diameter at that point being 0.5 inch. The heads of both instruments are small, however, so they may be used in the determination of temperatures in large spaces or as a probe between the sections of a pipe coil. For use with temperatures higher than copper will stand, a large variety of metal tubings is available. Since the gases are cooled practically to room temperature, it is possible to use any convenient pumping device, from a laboratory vacuum pump to a steam ejector, with rubber tubing as the connector. I n an experimental gas-fired furnace, tests on the reliability of the apparatus described were made. A small laboratory rotary vacuum pump was used as the source of vacuum, and a Leeds & Northrup student potentiometer, with chromelalumel thermocouple, giving readings accurate to 1" F., was used to measure the temperature. In all cases equilibrium temperature was reached when the rate of gas flow was 12 cubic feet per hour, and in some cases the necessary amount of gas to be removed from the furnace was only 8 cubic feet per hour.
Discussion Graphs of the effect of gas velocity on the approach to true temperature are shown in Figures 2 and 3. These results, of course, apply only to this instrument and in the general case are qualitative only. The size of the bead and thermocouple junction, the diameter and length of the lead wires, and the capacity of the source of vacuum used, all affect the accuracy of the temperature reading, as does the main source of radiation error, the difference in temperatures of the gas and the radiant solid surfaces to which the pyrometer element is exposed. Since any method of accurate gas temperature measurement involves only the reduction of radiant heat transfer to the measuring element and increase of flow by conduction and convection, the temperature indicated will approach the true temperature asymptotically, but will never entirely reach it. The accuracy of the method depends, finally, on how far it is possible or convenient to carry this convergence toward the asymptote. Structurally, the accuracy of the aradiant convection pyrometer depends primarily on the design of the head. The pyrometer may be made as sensitive as desired by decreasing
VOL. 8, NO. 5
the size of the thermocouple bead while maintaining the necessary linear gas velocity. Too great sensitivity is in many cases undesirable, however, the averaging effect of the less sensitive head making the use of the pyrometer data much less complex. Thus, in the experimental gas-fired furnace used in this work, a sensitive head showed irregular temperature oscillations with amplitudes of more than 50" F. and periods of the order of l second, a t temperatures about 700" F. Furthermore, displacement of the head by less than 0.5 inch produced a variation of 50" F. in an otherwise essentially constant temperature. Such sensitivity would be excellent in an instrument designed to study drafts and eddy currents in a furnace, but the automatic averaging effect of the larger bead is more desirable in making heat balances and heat-transfer studies. The pyrometer may be very effectively used to take gas samples while making the temperature readings. While a high linear gas velocity is desirable for the purpose of increasing the rate of heat transfer between the measuring element and the gas whose temperature is to be measured, this same high velocity will also tend to promote heat transfer between the gas and portions of the head of the instrument, the temperatures of which are affected by radiation, thus introducing error. This was found to be the case in instruments in which the thermocouple bead was placed in an alundum capillary tube of any length. The temperature of the gas passing a t high velocity through the capillary would tend to approach that of the refractory material, which in turn was greatly affected by radiation. This was shown by the high
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FIGURE 3. TYPICAL CALIBRATION CURVE
lag in some of the instruments. However, the substitution of a refractory disk drilled to give the proper gas velocity, with the thermocouple bead placed directly behind the hole, gave excellent results. Constant temperature was attained after a lag of only 1 to 2 seconds in this type of instrument. Upon removal from the furnace the reading would immediately drop to room temperature, although the head of the instrument remained hot. Shutting off the gas flow resulted in an immediate increase in reading, caused by conduction from the heated head. Resumption of the gas flow caused the reading to drop to room temperature again. The aradiant convection pyrometer was tested for radiation errors by pointing the instrument in a direction such that the element was exposed to radiation from the back of a muffle furnace a t bright red heat. No change in reading was perceptible when the source of radiation was covered and uncovered. The terms "high-velocity" thermocouple and "suction" pyrometer have previously been applied to this type of instru-
SEPTEMBER 15, 1936
AZALYTICAL EDITION (2) (3) (4) (5) (6) (7) (8)
ment. Neither of these appellations is satisfactory; neither is sufficiently descriptive of the purpose and the first is in some ways actually misleading. Since the instrument is expected to minimize the effect of radiant energy absorption and increase the transfer of heat by convection, and since the principle is applicable to a.t least two different types of measuring elements, it is suggested that such a piece of equipment be termed an aradiant convection pyrometer.
(9)
(10) (11) (12)
Literature Cited (1) Forrest, Special Report, Mass. Inst. Tech., Dept. Chem. Eng., 1923.
393
Guillon, Chaleur & Ind., 7, 395, 472 (1920). Haslam and Chappell, Bull. Mass. Inst. Tech., 60, No. 80 (1925) Hildebrand, Arch. Wtirmezuirt., 7, 319 (1926). Kreisinger and Barkley, Bur. Mines Bull. 145 (1918). Laoey and Woods, IND.ENQ.CHIM.,27, 379 (1935). Mattocks, Ind. Gas, 13, No. 2, 15 (1934). Monrad, IND.ENQ.CHEM., 24, 505 (1932). Mulliken, Power, 78, 565 (1934). Nusselt, 2. Ver. deut. Ing., 53, 1750, 1808 (1909). Parkin and Winks, J. SOC.Glass Tech., 26, 315-26T (1932). Robinson, J. IND.ENQ.CHEM.,13, 820 (1921).
RncEIvEn November 21, 1935. Presented before the Midwest Regional Meeting, Louisville, Ky., October 31 to November 2, 1935.
Laboratory Gas-Absorption Vessels WILLIAM MCKINLEY MARTIN,l Montana Agricultural Experiment Station, Bozeman, Mont.
T
HE gas-absorption vessels shown in Figure 1 were de-
gas wherever needed in the laboratory. It has been found especially useful in preparing large volumes of carbon dioxide-free distilled water by the aspiration method. When used for this purpose, the stream of carbon dioxide-free air supplied from the scrubber is dispersed in the water by means of a sintered-glass distributing disk which may be prepared by the methods described by Kirk, Craig, and Rosenfels ( 5 ) , and Cool and Graham (9). The carboy aspiration assembly is shown in C. The air may be forced through the system by either compression or suction, the latter usually being preferable when the aspiration is allowed to run overnight, or when compressed air is not readily available. The efficiency with which carbon dioxide is removed from water by this method
veloped after considerable experimentation with various types of absorbers. They operate on the same principle as the absorbers designed by Weaver and Edwards (8), Milligan (7), Beaumont, Willaman, and De Long ( I ) , and Harvey and Regeumbal (S),but in certain respects are perhaps better adapted for general laboratory use.
Gas Scrubber Apparatus -4is a convenient and efficient gas scrubber designed to supply a continuous stream of purified air or other 1 Present address. Research Maywood. Ill.
Department, American Can Company,
STANDARD ACID
------a
COMPRESSED AIR,
7
D
c
C02 FREE
1
C . -
_ _
1
I A
B
FIGURE1. A. B.
Laboratory gas scrubber. Quantitative gas-absorption vessel in which absorbing solution is titrated directly; a current of carbon dioxide-free air being used to stir and circulate the solution in the vessel during titration.
D
APP.4RATUS
C.
Removal of carbon dioxide from a carboy of distilled water by aspirating with a current of carbon dioxide-free air.
D. .issernbly for stirring solution with a current of carbon dioxide-free air during titration.
