Laboratory Uses of Heat-insulating Blocks DAVID LYMAN DAVIDSON Middlesex Uniuersity, Waltham. Massachusetts
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EAT-insulating materials in the form of blocks are of value in teaching students of general science, physics, and physical chemistry the principles of heat flow and of its efficient retardation. The simple beaker jacket described is also useful in the control of temperatures in experiments in organic, quantitative, and other chemistry courses. ROR TEACHING THE BEHAVIOR OF HEAT INSULATION
There are now available commercially several types of heat insulation in the form of blocks, recommended espedally for the building of cold-storage vaults, rooms for creating artificial climatic conditions, and so forth. It is desirable that students should learn early in their science courses something about the way such materials serve their purpose as retarders of the flow of heat. These blocks of heat-insulating material come in rectangular shapes 12 by 18 inches or larger, and 2 inches or more in thickness. Figure 1shows how
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FIGURE 2.-Coo~1No Cunv~ssonA 250-ML. Bsmsn oa WATER. (A) IN 'TOAMOLAS" JACKET. (B) WITHOUT JACKET.
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I.-lNsrmTmo JAmrT A 2EO-ML BEAnR ONE 12"X18'X2" BLOCKOR INSULATION
such a block of "black glass suds," Armstrong Cork Company's "Foamglas," may be cut S t h a fine saw or strip of metal into six square pieces. The vent in square No. 1 is made with a cork borer. In squares No. 2 and No. 3 the hole and rim-seat are easily trefined with the rims of a condensed soup can and a number 2 vegetable can, respectively. "Foamglas" is easily worked, as i t is quite fragile. The six pieces may then be assembled to form a complete jacket for a 250-ml.pyrex Griffinbeaker, as shown a t the bottom of Figure 1. The beaker is suspended by its rim in square No. 2, and the cavity in squares No. 2 and No. 3 is deep enough so that the bottom of the beaker is about onehalf &ch above the top face of square No. 4. The vent accommodates the stem of a thermometer. or wires to a thermocouple, or the shaft of a stirrer.
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Figure 2 shows the cooling curves obtained simultaneously with 250-ml. beakers of very hot water, with and without an insulating jacket as described. Readings of temperature are taken a t 5-minute intervals after approximately 200 ml. of boiling water are poured into the two beakers, and matched thermometers are inserted. The thermometer in the jacketed beaker passes through the vent in square No. 1, and thus rests on the bottom of the beaker. The thermometer in the uninsulated beaker should be held by a clamp in an identical position. No stirring was done in making the readings. Figure 3 shows warming curves obtained in a similar way, but beginning with ice water in the two beakers. Any pieces of ice floating in the water should he picked out just before the temperature readings are begun. From curves plotted as in Figures 2 and 3 it will become apparent to the student how the insulation slows down the passage of heat to or from the beaker, so that a linear temperature-time function is exhibited. The curves also show how the effect of the insulation is greatest at those temperatures furthest removed from room temperature. It will be noted that the first part of curve A in Figure 2 is nonlinear. This portion represents the period during which the insulation was becoming "saturated," until the slow and steady flow of heat through it could be established. This initial saturation can be proved by starting with the water not as hot, say a t 70°C., whereupon the graph has an initial dip at this lower range before becoming linear. In Figure 2 curve A is linear in this region. Numerous other experiments are possible with this simple outfit: 1. Convection effects can be shown: (a) by making the cooling curves whiie square No. 1 is removed, thereby largely nullifying the heat retardation, "with the cover off" ; and (b) by making the warming curves while square 4 is removed, and square 3 rests on the edges of squares 5 and 6 laid flat. This latter arrangement lets the cool air fall away rapidly from the bottom of the beaker. 2. The effect of the side walls can be removed by taking away squares 2, 3, 5, and 6, and letting No. 1 rest on the rim of the beaker. Conversely, the side walls can be increased in thickness with more blocks. 3. Similar jackets can be cut from blocks of corkboard, cinderbrick, wood, fiber-board, or mineral-wool board, and the insulating value of these materials compared. 4. Sampling effects may be studied by stirring the contents of the beakers and by varying the position of the thermometers in the body of the fluid in successive experiments. 5. By using various sizes of jackets and beakers, and different amounts of water, volume effects can be measured. 6. Initial temperatures can be greatly varied through the use of liquids other than pure water, such as brine, lubricating oil, or antifreeze mixtures. There are so many interesting variations possible,
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40 60 80 1M) Time in Minutes momE 3.-WARMING CURVES FOR A 250-ML. BEAKEROWWATER. (A) IN "FOAWGLA~" JACKET. (B)wrmour JACKR.~. 0
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that in a class small groups of students can be assigned a slightly different setup or technique, learn the same fundamentals about heat gradients, and then get additional instruction by comparing results with the other members of the class. When the more fragile blocks, such as "Foamglas," are used, i t will probably be best for each user to make his own insulating jacket, for this is easily and quickly done. For other materials like wood and brick, breakage is less likely, and sets can he expected to last well. In cutting large holes in some of the other materials, tin cans may be used which have been made into trefines by cutting teeth in their circular rims with tin-snips, but "Foamglas" is friable enough so that it is cut by the smooth rim of the can. IN lMPROVING TECHNIQUES
Particularly in warm weather, insulating jackets are useful in maintaining low temperatures in the lahoratory, when methyl orange or some other dye is being prepared by diazotization. Less ice is required to keep receivers cold in collect in^ - distillates such as diethyl ether. On the other hand, such a method of insulation will assure the slow cooling of a hot solution from which large crystals are desGed, and will promote the production of coarse precipitates by the hot digestion method, unattended, in quantitative analysis. "Foamglas" was tried first for these uses because of its chemical stability, noncombustibility, and impermeability to vapors and liquids. However, due t o its fragdity, i t does not withstand physical shocks very well. Where greater ruggedness is required, the insulating blocks should be made of a tougher insulator such as cork or fiber, properly wrapped or coated with a water-repelling layer such as sodium silicate, synthetic resin, or asphalt. The author is grateful to the Boston officeof the Armstrong Cork Company for furnishing him with the "Foamglas" used in this work, and for giving him courteous advice.
