Controlling Thermostated Water Baths
Norman A. deBruyne
661 Brunswick Pike Princeton, New Jersey 08540
Near Room Temperature
It is sometimes necessary t o operate a thermostatically controlled ~vaterbath a t or somewhat below room temperature: this can be done by circulating cold tap water through a cooling coil in the bath. However (as Goldberg and Bailey' have stated) there are places in the USA where during the months of May through October tap mater temperature rises t o levels above 30°C. I n many communities supply companies impose restrictions on consumption of water during the summer. With this in mind the author attempted to make a small water cooler similar i11 principle to the large water coolers used in power stations. Tliese depend upon evaporative cooling of water by air blowing across the surface of wood slats on to which the water trickles. A small counter current cooler ~ v a sproduced capable of cooling a 4-gal water bath down t o the wet bulb temperature of the atmosphere. However, it was subsequently found that equally good results >yereobtained by directing the air stream from a cross flow centrifugal air blower onto the surface of the vater bath. The efficiency of the device is surprisingly high and exceeds that of small refrigerabors working under comparable conditions as the following comparison shorn
so that by plotting loglo (8, - Oh) against t a straight line will be obtained. Figure I shows experimental results vhen BE = 20.0°C. A suitable cross flow centrifugal blower is the Grainger 4C267 (W. W. Grainger Inc. 335 Mulberry Street, Newarli, X. J.). This supplies air a t SO cu ft/min a t zero back pressure; it is a skeleton blorer and needs an earthed protective case. Figure 2 shows a complete protected unit hanging on the side of a water bath (available from Techne, Inc. 661 Brunswick Pike, Princeton, N. J.). It is a pleasure t o acknowledge the help received from Mr. Tony Coolce and Mr. Patrick H. Summers.
Eficiency o f eonvenlional water bath refrigerator
Ext,raction rate s t 2 0 T = 1000 Btu/hr = 283 watts Motor '/a hp = 149 watts Extraction Rate 293 -Efficiency = Power Input 149 = 2 ' 0 Efieiencu o j maporalive cooler
Figure 1. Graph of 0, ogoind time, 1. Volume of woter, 12,900 ml; initial rate in the drop in temperature, O.lA°C/min; initial rate of heat removol, 1 8 0 0 cal/min.
Extraction rate s t 2 0 T = 30 cal/see = 125.4 watts Matar 33 watts Efficiency = 125.4/33 = 4 . 5
The rate of temperature decrease is proportional to the difference between the ~vaterbath temperature 0, and the wet bulb temperature Oa (as measured by a psychrometer) of the ambient air, or - dO/dt = k(0, - Ba)
Let On gives
=
room temperature. Integration of (1) then log(0, - 0s)
' GOLDBERG, STANLEY I., (1970). E ~ u c ,47,783 ,
416
/
- log(8n - Ba) AND
=
-kt
B.LILEY, W I L L I . D., ~ J. CHEM.
lournol of Chemical Education
Figure 2.
A complete protected cooling unit dtached to o woter bath.
