A DESIGN OF EXPERIMENTAL VACUUM OVEN FOR TEMPERATURES LESS THAN 300 DEGREES CENTIGRADE G. FRBDERICK SMITHAND V. R. HARDY, UNIVERSITY OP ILLINOIS,
URBANA,
ILLINOIS
Commercially available electrically heated vacuum ovens which provide for a working temperature above one hundred and fifty degrees centigrade are not sold. I n addition, stock equipment is expensive. A n electrically heated tube furnace which provides for small-scale vacuum drying at temperatures including three hundred degrees centigrade i s herein described. The parts are constructed of standard &e and fittings, and heat i s provided by standard electric s@ce heaters. The deteils of construction provide for the maintenance of a vacuum of a fraction of a millimeter and the oven has been shown to hold the reduced pressure practically undiminished for periods of twelve hours without pumping. Time-temperature heating curves under diferent power consumption are giva. Modifications in wiring to economize in power consumption are described. This upfiratus can be manufactured at a fraction of the investment in stock equipment of more limited range.
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Experimental vacuum ovens from the usual commercial sources are both expensive and limited in working temperature range. For work demanding temperatures between 200 and 300°C. a t pressures not greater than a few millimeters of mercury, i t is found to be a troublesome problem to provide satisfactory apparatus. For the solution of certain problems dealing with the preparation of various alkaline earth metal perchlorates by dry reactions a t temperatures of approximately 250°C., and under pressures of 2-5 mm., the vacuum oven about to he described has proved very satisfactory. General Characteristics The vacuum chamber consists of a section of standard six-inch galvanized iron pipe threaded a t each end to receive standard pipe caps. The pipe cap on the exit end is brazed to the vacuum chamber and is provided with a a/l-inch section of pipe threaded and brazed into the center of the exit pipe cap to connect to the vacuum line. The pipe cap used a t the open end of the vacuum chamber had its closed side machined away to one-third of its width and served as a collar to support the door to the vacuum chamber. The outside edge of the vacuum chamber a t the open end was so machined as to provide an inverted " W contact surface for the metal door. The top middle portion of the vacuum chamber is drilled and threaded to receive a S/s-inch galvanized iron pipe nipple to serve as a support for the thermometer used. The door is made of a disk of iron I/, inch thick faced with a disk of lead I/, inch thick soldered in place. The door was provided with a series of equally placed 7/ls-inchholes to provide for bolting the door to the collar of 548
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the vacuum chamber. The bolts used were bent a t right angles and threaded a t one end to provide for the use of thumb nuts for fasteners. The vacuum chamber is heated by being placed within a transite board housing with octagonal ends and side pieces to match. Each rectangular side piece of the vacuum chamber housing has mounted inside an electric space heater. The vacuum chamber and transite board housing is placed within a heat-insulating housing made in the form of a cylinder split in half along the vertical axis and dividing the chamber into two equal parts. The transite board housing and sheet metal insulating jacket are supported on a base constructed of strap iron. The sheet metal housing for the heating chamber of transite board provides a two-inch air space between these parts for heat insulation and is coated outside with asbestos paper. Variation in heating value of this system is governed by the use of a lowresistance, high-capacity rheostat connected in series with the lieating elements with an A.C. ammeter covering the range 0-15 amperes. Dimensions and Description of Individual Parts The vacuum chamber tube with the door and attachments is shown in Figure 1. The dimensions of the individual parts as indicated in Figure 1 are as follows: A. Standard six-inch black iron pipe cap with all but 6/s inch of the open end machined off. This collar serves to hold the catch bolts E of door C.
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FIGURE ~ . - ~ I E A T I N GCHAMBER AND ITSMETALINSULATING JACKET
B. Standard six-inch galvanized pipe 25'/2 inches long threaded for two inches a t each end. The entrance end contact surface.is machined to give an inverted "W" shaped contact with the groove '/la inch deep. The vacuum chamber is 61/2 inches outside diameter with 1/2-inch wall thickness. inches in diameter by '/4 inch thick C. The vacuum chamber door, faced with l/e-inch sheet lead. There are ten equally spaced holes ?/M inches in diameter placed lJ/sinches inside the outer edge of the door. D. The thermometer tube. A standard 3/8-in~hpipe nipple 5 inches long by 1/2-inch outside diameter and a/8-inchinside diameter. E. Vacuum door thumb nut clamps. These are made of 3'/%-inch lengths of 7/la-inch round iron threaded a t one end to receive '/winch thumb nuts. The 31/2-inch bolts were bent a t right angles a t a point two inches from the threaded end, giving a 1-inch contact to collar A for each bolt. F. A standard six-inch black iron pipe cap used as closure for the exit end of the vacuum chamber. It is brazed to the vacuum chamber and the center of the closed end is drilled and tapped to receive the 3/4-inchexit tube G. G. The vacuum pump connection consisting of a 31/2-inch length of '/4-inch pipe nipple threaded and brazed into end plug F and tapered a t the outside end to receive the rubber tube connection to the vacuum pump. The details of the heating chamber and its metal insulating jacket are
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shown in Figure 2. The dimensions of the individual parts shown in Figure 2 are as follows: C. The octagonal end piece of the heating chamber. Outside edges 4l/% inches in length. The hole is 7 inches in diameter. The transite board is inch thick. E. The octagonal side pieces of the heating chamber are '/, inch thick, 4'/% inches wide, and 15 inches long. B. Corner brackets, by inch strap iron, each leg 2 inches long fastened with '/*-inch by 1/2-inchround head brass machine screws.
D. Electrical space heaters, 110 volts, 220 watts, 12 inches long and 2 inches wide (Westinghouse number 299425). Eight space heaters are employed, one for each side of the octagonal heating chamber of Figure 2. The various connecting wires to the space heaters are asbestos insulated. Mounted as shown, no di5culty was observed in keeping the insulation coating unimpaired after several years operation of the heating elements, none of which have had t o be changed. A. The nature of the outer jacket cylinder is obvious from Figure 2. A two-inch air space is provided all around. Twenty-two-gage galvanized iron serves as the metal for the outer jacket.
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F and G. Line switch and contacts respectively. The wiring details are shown in Figure 3. The space heaters are connected in parallel and a rheostat R and ammeter A placed in a portion of the circuit outside the heatmg chamber as indicated. The general arrangement of the apparatus assembled is shown in Figure 4. Excessive losses by radiation a t each end of the protruding vacuum chamber are prevented by placing a metal shield in the form of a section of a cone from the circular ends back to the metal jacket. These have been removed in Figure 4, giving a more detailed view of the assembled apparatus. It is to he understood in the following description of the performance of this vacuum oven that such heat-insulating caps for each end of the vacuum chamber are employed. Heat Variation and Control The heating value of the equipment described is shown graphically in Figure 5. On the horizontal axis is plotted the time in hours during which the vacuum oven is heated and the temperature attained plotted on the vertical axis. Curves A and B are those for the use of 9 and 11 amperes, respectively. Curve C consists of two portions, the almost vertical portion obtained with the use of 12 amperes and the horizontal portion obtained using 11 amperes after the first Z1/, hours application of the higher am-
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perage. By the scheme shown in w e C the temperature 250°C. is obtained in three hours corresponding to nine hours using 11 amperes according to curve B. Curve D is obtained if the units plotted on the horizontal axis are
4 5 8 7 8 9 10 11 12 13 14 Amperes, and time in hours. F ~ o m ~ . - ~ R A P%OWING H HEATING VALUE OF THE EQUIPMENT
1
2
3
15
changed to read amperes instead of time in hours. From an examination of the table the following observations are of note: Curve D being so nearly a straight line, the efficiency of the apparatus described in preventing heat losses is demonstrated. Maximum tempera-
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FIGURE
MARCH, 1931
~.-VACWM OVEN TRAYS AND SUPPORTS
tures of 175, 215, 250, and 285 degrees centigrade are obtained using 9, 10, 11, and 12 amperes. A summary of the power consumption under varying conditions is given in Table I. TABLE I Summary of Vacuum Oven Power Consumption under Varying Conditions Rheostat resistance = 5.35 ohms total resistance, capacity 15 amperes Vacuum chamber heating units in parallel = 6.87 ohms Poser Con.wmOlion
Told Currcnl
Told
Rhrariol
Used,
Woilr
Sattins Ohms
AmOercs
Consumcd
i n Walls
rn
Rheaslot
In
Oacn
Oocn Tcmperolurr. Dezrrcr C.
From an examination of Table I it will be seen that by plotting oven temperatures as abscissa and watts consumed by the oven as ordinates and extrapolation to 150°C., the watts consumed would be 440. At this temperature the wasted energy given off by the rheostat would be equal to that consumed by the oven. At this point it would, therefore, be appropriate to change the space heater wiring. By joining the space heaters by pairs in series, and wiring the four pairs in parallel into the 110-volt circuit, the power realized with the rheostat withdrawn from the circuit would be 440 watts as required to give a temperature of 150°C. For temperatures below this value the rheostat is again employed. Of course, for many
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temperatures special wiring of the space heaters would be supplied to avoid wasted energy in the rheostat. The two wirings discussed above have the distinct advantage that they provide a symmetrical distribution of the heat within the oven which is a distinct advantage. Discussion of General Operating Conditions By the use of the apparatus described temperatures constant to *3'C. can be easily maintained over long periods of time even with the average fluctuation in line voltage of the commonly employed power line. With sources of power controlled to avoid changes in voltage, temperatures of smaller variation could be easily obtained. The vacuum realized using a moderate capacity vacuum pump (such as the Mega-Vac) was found to be from 1-3 mm. Shutting off the vacuum pump from the oven a t such pressures showed a maximum leakage of 0.5 mm. per hour and oftentimes less. Opening and closing the vacuum chamber repeatedly does not alter the possibility of attaining an equally low vacuum. The estimated cost of the equipment designed as described is thought to be less than one hundred dollars including material and workmanship.
