Heat transfer and scale resisting properties of silicone resins
The effects of silicone coated heat-exchanger tubes on the water film coefficient of heat transfer and on the formation of gypsum seals were studied. The heat-exchanger tubes consisted of 74 ½-in. sections of 1-in. Nominal diameter standard iron pipe into whose surface were installed six copper-constantan thermocouples for measuring the temperature of the tube well.
Silicone resins DC 804 and DC 802 were used for coating the tubes. Silicone resin DC 804 was cured for 16 hours under infra-red heat at 440-450°F to a pencil hardness of 7H; however, due to the fact that it would not withstand the thermal stresses encountered in the heat-exchanger, its use was abandoned. Silicone resin DC 802 was cured for 12 hours under infra-red heat at 440-450°F to a pencil hardness of 2H.
The horizontal tube double-pipe heat-exchanger consisted of a 5-ft. section of 2-in. I.D. Pyrex pipe fitted with a reducing tee and a reducing cross as the end-sections. Heat was applied to the heat-exchanger by steam from 15 to 40 psig.
In the "Heat Transfer Tests", which were conducted with water at 60°F and at Reynolds numbers from 2500 to 11,000, it was found that the water film coefficients on the silicon surface varied from 810 to 1590 BTu/(hr.)(sq.ft.)(°F); while those on non-coated iron tubes varied from 244 to 670 under the same conditions.
In "Accelerated scaling tests", which were conducted at Reynolds numbers of 15,000 and 4,100 using a saturated gypsum solution at l55°F as the scaling medium, it was found that scale formation on the coated and non-coated tubes reduced the film coefficient approximately 50 percent in 3 ½ to 4 ½ hours. A coating of 0.0006-in. Thickness caused the initial water film coefficient to be 70 percent of the coefficient for a non-coated tube at a Reynolds number of 15,000. A coating of 0.0018-in. thickness caused the initial. water film coefficient to be 83 per eent of the initial coefficient for a non-coated tube at a Reynolds number of 4,100.
The thermal conductivity of silicone resin DC 802 was found to be 0.096-0.103 Btu/(hr.)(aq.ft.)(°F per ft.) at 150°F.