A Simple Laboratory Hot-Air Dryer I. R. C . BICK University of Queensland, Brisbane, Australia
A
MEANS of drying rapidly glassware and other apparatus is a virtual necessity in the chemical laboratory, particularly for organic work. An apparatus for this purpose has been described by Dunbar in THIS JOURNAL [20, 130 (1943)], which consists essentially of a gas-heated copper tube wound into the form of a helix through which a current of air is passed. This form of apparatus, while being extremely convenient, has certain disadvantages; in particular, after using it for some time, the tube becomes coated internally with a loose scale of copper oxide which is readily detached and carried over into the vessel that is being dried. In addition, the use of a gas flame involves the risk of a "flash-back," or minor explosion, if the vessel has been rinsed with alcohol or some other inflammable organic solvent to assist the drying. I t is believed that the apparatns herein described possesses advantages in being simple, robust, and easily and cheaolv , constructed from materials a t hand in anv laboratory; in addition, it is free from the disadvantages mentioned above. An electrically heated element is used which consists of about 350 cm. of 26 S.W.G. nichrome wire wound loosely around a pyrex tube 45 cm. in length and having a 2-mm. bore, leaving about 5 cm. free of wire at one end. Connections are made a t each end with20 S.W.G. copper wire; a t the end left free of nichrome wire, the winding is continued with copper wire to the end of the tube. The copper wire a t the other end is threaded through the tube, and the nichrome wire is drawn through almost the whole length of the tube until the connection with the copper wire is within 5 cm. of the end. The heating element so formed is placed inside a silica tube about 46 cm. long, of 15 mm. external diameter and 10 mm. bore. The silica tube used in this laboratory was taken from a broken thermocouple; if desired, a pyrex tube could be substituted. The elec-
.
trical connections, which are taken out from the same end, but a t diametrically opposite positions, serve to keep the heating element in place. A wide-bore rubber tube is slipped over the end of the silica tube and the connecting wires to a distance of about 3 cm., as the latter are held f i d y in position and are insulated from one another; a t the same time an airtight seal is formed, particularly when a clamp is placed over the rubber tubing and connecting wires. The rubber is connected to a foot-bellows or some other supply of forced air. The silica tube is mounted to a retort stand over that part of i t which is covered by the rubber tubing. It was found most convenient to incline it at an angle of about 30" to the horizontal with the lower end about 8 cm. above bench level. The element, with a pilot lamp in parallel, is connected to a 240-v. main supply through a switch mounted on the base of the stand. In usinc the " the drver it is found nnnecessarv to k e e ~ heater on continuously, since an instantaneous supply of hot air is available whenever it is switched on. The apparatns to be dried, which has been rinsed with alcohol, is held at an angle above the end of the tube; this permits draining of the excess alcohol and promotes rapid drying. In cases where it is desired to dry apparatus with a narrow opening, an adapter is slipped over the end of the silica tube. This consists of a pyrex test tube just wide enough to fit easily over the silica tube, with a narrow-bore pyrex tube sealed into the bottom. A dryer of this type has been in constant use by stndents in our laboratories for over a year and has proved itself to be thoroughly foolproof and reliable despite its simple construction. Since the hot element is always surrounded by an upward stream of air, the possibility of a "flash-back" is negligible even when ether is used to assist the drying.
PYREX TUBE
.
