Guard heating

Experimental data on heat transfer from spheres to an air stream are shown in Fig. 5.20. Despite the large number of studies over the years, the amount of reliable data is limited. The data plotted correspond to a turbulence intensity less than 3%, negligible effect of natural convection (i.e., Gr/Re <0.1 see Chapter 10), rear support or freefloating, wind tunnel area blockage less than 10%, and either a guard heater on the support or a correction for conduction down the support. Only recently has the effect of support position and guard heating been appreciated a side support causes about a 10% increase in Nu... 

A semi-automatic heat meter apparatus was described by Howard et al35 and a guarded heat flow meter by Foreman36. Calibration of heat flow meter apparatus has been discussed by Lackey et al37 and Scott and Bell38. 

Guarded heat flow meters are variants on the guarded hot-plate technique, which utilize a heat flow meter to measure the heat flux. These techniques are comparative in nature because a reference material is needed to calibrate the heat flux meter. They utilize smaller samples and are thus able to equilibrate faster, improving the versatility of the guarded hot-plate method. 

Figure 2 Schematic diagram of the guarded heat flow meter. (Courtesy of Holometrix Corp.)...

Hickman Still-Rotary Evaporation Apparatus. A simple microscale rotary evaporator for use in the instructional laboratory consists of a 10-mL round-bottom flask connected to a capped Hickman stOl (side-arm type), which in turn is attached to a water aspirator (with trap) The procedure involves transferring the solution to be concentrated to the preweighed 10-mL flask. The flask is then attached to a Hickman stiH with its top joint sealed with a rubber septum and threaded compression cap. The apparatus is connected by the stiU side arm to the trap-vacuum source with a vacuum hose. lAdth the aspirator on, one shakes the apparatus while warming the flask in the palm of the hand. In this manner, bumping is avoided and evaporation is expedited. The stiU acts as a splash guard. Heat transfer is very effective, and once the flask reaches ambient temperature, the vacuum is released by venting through the trap stopcock. 

Guarded Heat-flow-meter Method. The previous two methods are absolute determinations of thermal conductivity, that is, all the quantities required for the application of equation 9 are measured directly. A widely used comparative method is illustrated schematically in Figure 6. This design is the basis of an ASTM procedure (22) it is characterized by a heat-flux transducer, a sensitive device that produces an electrical output proportional to heat flux. A number of configurations are available, each having certain advantages. 

Fig. 6. Schematic diagram of a guarded heat flow meter device, the single-transducer, one-face configuration.

In a 200 ml. distilling flask place 64 g. (50 ml.) of dry n-butyl bromide and 80 g. of dry silver nitrite (1). Insert a reflux condenser, carrying a cotton wool (or calcium chloride) guard tube, into the mouth of the flask and close the side arm with a small stopper. Allow the mixture to stand for 2 hours heat on a steam bath for 4 hours (some brown fumes are evolved), followed by 8 hours in an oil bath at 110°. Distil the mixture and collect the fraction of b.p. 149-151° as pure 1-nitro-n-butane (18 g.). A further small quantity may be obtained by distilling the fractions of low boihng point from a Widmer flask. 

Method 1. a-Naphthonitrile. Place 80 g. (54 ml.) of redistilled a-bromonaphthalene (Section IV.20), 43 g. of dry powdered cuprous cyanide (Section II,50,J) and 36 g. (37 ml.) of dry pure pyridine (1) (Section 11,47.22) in a 250 ml. round-bottomed flask fitted with a ground-in reflux condenser carrying a calcium chloride (or cotton wool) guard tube, and heat the mixture in a metal bath at 215-225° for... 

Thionyl chloride method. Mix 100 g. of pure p-nitrobenzoic acid and 125 g. (77 ml.) (1) of redistilled thionyl chloride in a 500 ml. round-bottomed flask. Fit the flask with a double surface reflux condenser carrying a calcium chloride (or cottou wool) guard tube and connect the latter to an absorption device e.g.. Fig. II, 8, c). Heat the flask on a... 

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