JOURNAL OF CHEMICAL EDUCATION
258
A N ELECTRIC FLASK HEATER FREDERICK MATHEWS Beloit College, Beloit, Wisconsin
Tm
purpose of this paper is to describe the constmction of a very inexpensive but extremely reliable and convenient electrical flask heater, made from common and readily obtainable materials. The original idea of this heater is not the author's, and this article is merely a complete description of a composite, with refinements, of several types observed in various laboratories in the last ten years. The actual constmction of the heater demands no mechanical skill or special tools, and, in fact, most of the heaters in our laboratory were made with student help or were made entirely by stu-
rigvre 1
The flspli hss just been farred into the deprepsion Note thnt ihe emergent wire ia coiled and not r ~ m i z h t .
for
the first tirne
dents. This type of heater can be used only in conjunction with a variable-power transformer. The performance of these heaters has been extremely satisfactory in every way. They are safe, durable, and very flexible in all applications, and with reasonable care they will last for years. Temperatures obtainable with liquids in the flask can be varied from 40' to 400°C.or more depending upon the size of the heater. The materials for the construction of a 300-ml. flask heater include a one-pound coffee can with two holes drilled, glass insulators, coiled resistance wire (22 ft. of B&S Nichrome or chromela wire), and sufficient asbestos cement (Johns-Manville No. 352.1.' The mandrel upon which the resistance wire is coiled is made of a piece of heavy wire about 18 in. long and ' 1 s in. or slightly more in diameter with one end bent in the form of a crank handle. A small hole will serve to fix the end of the resistance wire during the winding. The wire coil is slipped off the mandrel and then pulled out to about 3'/< ft. in length. Straighten the ends for about 5 in. The volume of dry cement needed is difficult to estimate but usually a volume equal t o about three or four containers will suffice. Add water to the cement with kneading until the cement reaches a puttylike consistency; when no water can be brought to the surface of it when it is squeezed hard, enough water has been added.
' Johns-Manville Corporation, 22 East 40th St., New York, can supply the address of the noarest dealer. Local heatingequipment concerns can supply asbestos cement that will often he suitable for use in heaters.
MAY, 1951
Figure 2
The coil has been forced into the side of the cavity and has been partly oovered aith cement.
Insert one glass insulator (a glass tube in. in length with a flange a t one end) in the lower hole of the can and spread an inch layer of cement, working it well into the corners and around the glass insulator. Put the resistance wire in place, insert the other glass insulator, and place cement all around the sides of the heater, retaining a depression about the shape of the flask bottom. Build up to within about ' / a in. of the top of the container. When the cavity is just slightly smaller than the size of the flask force the flask into the cavity (see Figure 1). The resistance wire is now pushed into the cavity side (Figure 2). The wire will not stay in place very well a t this stage until it is covered with fresh cement. To do this, take a small amount of the cement and force it into the coiled wire with a finger. The covered coiled wire should just faintly show (Figure 3). The heater can be dried with the flask in place. This will prevent cavity deformation due to the shrinkage of the cement. Heating at 100' to 120°C. overnight will drive off enough water to permit further drying with the flask removed. Electricity should not be passed through the wire until the cement is quite dry, because of possible wire corrosion by the wet cement. After the heater appears to be dry pass 40 to 70 volts (depending on the size of the heater) for several hours with no flask to drive out any residual moisture. The heater is then ready for use. The considerable shrinkage that is noted with some cements is not undesirable for it permits more rapid drying at the edge of the heating block. However, a little fresh cement should be forced down along the
sides when the cement of the heater is dry. This will take out the looseness between the heating block and the container. Cracks, if any develop, can be patched in this manner. The wires emerging from the insulators must be fmed so they can easily be connected to the power supply and yet will not break off with heavy use. To do this, cut the wire so only ?/a in. protrudes from t,he end of the glass insulator. With long-nose pliers bend the end around and force about in. of the end back into the glass tube. This will form a strong '/&-in.loop that can easily be clamped with the small battery-clamp connectors, but cannot be broken off. The small 1'/2-in. battery clips of the "Pee-Wee" type, when covered with a 2-in. length of 3/8-in. rubber tubing, make safe and convenient connectors. A heat-resistant aluminum paint applied before or after fabrication will dress up the metal-can heaters and prevent rusting. For a heater larger than 500 ml. an enameled pan makes a good container. However, because of the slanting sides of these pans it, is necessary to anchor the cement with at least three nails placed through the sides near the bottom of the pan. Heaters may be designed to fit other vessels such as a beaker, Florence flask, or a funnel.
Figun, 3
All af the wire has been oovered.
Suitable wire lengths for various sized heaters are: Flask size, ml.
B&S
50 50 100-300 300-5000
2fi 28 24 22
Length,
P.
Approzimate wattage
12 12 16 22
450 300 600 650
