Document wqdL81OM113QwX1EqoYrE0Ep6

546 CHAPTER 23 1952 Guide Convection Transfer Convection values of heat transfer are not easily established. Con vection in a panel heated space is usually considered-to be of the natural type, that is, air motion is generated by the warming (by conduction) of the boundary layer of air which starts moving as soon as its tempera ture exceeds that of the surrounding air. In practice, however, there are many factors interfering with natural convection. Infiltration, localized drafts, ventilation, and movement of persons are all likely to disturb this process so that it becomes difficult to determine the exact convection effect; Until results from current research become available, an interim evaluation of convection heat exchange can be determined for ordinary panel heating applications from available theoretical relationships. As investigated34-6 by Nusselt and Henky, Griffiths and Davis, Wilkes and Peterson, McAdams and. others, natural convection is found to be affected by only two factors: a. Temperature difference between the heat emitting surface and the surrounding air. b. The position of the surface. The basic general equation for natural convection from a flat surface is of the following form: ?o=/c(<.-tO" (4) where q,, = heat transfer by convection, Btu per (square foot) (hour). / = a coefficient (surface conductance) representing the heat transfer from a unit area per unit difference in temperature, Btu per (square foot) (hour) (Fahrenheit degree temperature difference between surface and air), n = an exponent depending on surface position and temperature difference be tween the surface and the surrounding air. t, -- temperature of the surface, F. i. = temperature of the air, F. The value of n is usually taken as 1.25 regardless of the magnitude of the temperature difference and the position of the surface. However, Wilkes and Pfeterson state that a value of 1.12 for n is more appropriate for low temperature differences, with heat flow upward from horizontal surfaces, and that a value of 1.00 is best in the case of heat flow downward from horizontal surfaces. The various investigators mentioned also indi cate that values of fc vary from 0.2 to 0.38 for downward heat flow from ceilings, and from 0.38 to 0.81 for upward heat flow from floors. The variation in the value of /,,, as reported by the various investigators, is dependent upon the temperature difference and the value of the exponent n. More recent experimental work indicates a provisional correlation be tween measured convection outputs in actual floor panel heated rooms and the laboratory values obtained by the previous investigators. This correlation indicates that the values obtained from Wilkes and Peterson's equation 3. = 0.81 (i. - l.)lu (5) for heat flow upward from horizontal surfaces and small temperature differences, give results that check within reasonable precision the. con- Panel Heating 547 vection 'output from floor panels, with temperature differences as recom mended. Curve A in Fig. 10 is based on the equation of Wilkes and Peterson, and gives values which may be used for convection outputs from floor panels. More experimental data are required on convection outputs of ceiling panels. The warming of the air by a ceiling panel does not in itself create convection currents. Instead, it is the action of cooling along the outside wall or walls which causes the air movement. In lieu of actual test data which would take this factor into account, it is recommended that the values obtained from Curve B in Fig. 10, based on the Nusselt and Henky equa tion 3o = 0.22 (U - L)'-k ; . (6) / Ar / / 3L /# A JZ. vA Ar f /A ir y kI- 1 -J 1 1 1 1 1-1 1 1 O 5 IO 15 20 29 39 35 40 49 50 55 00. Ai,0-tD pAHR OEG, . U = panel surface temperature U -- inside air temperature Fig. 10. Heat Output bt Convection from Floob and Ceiling Panels should be used for determining convection outputs from ceiling panels for various surface temperatures. Combined Heat Transfer The sum of the radiant heat transfer from Fig. 9 and the convective transfer from curve A or B of Fig. 10, gives, the combined useful heat transfer to the room for any combination of panel surface temperature, room air temperature, unheated mean radiant temperature (UMRT), and panel location. Example 1: Find the combined total useful heat transfer from a square foot of floor panel having a surface temperature of 85 F, when the average room air tempera ture is 70 F and the mean radiant temperature UMRT of the unheated room sur faces is 60 F. Solution: Radiation Btn/ftrXaq <0 (Fig. 9 for UMRT of 60 F and t. of 85 F) 21.4 Convection (Fig. 10, Curve A for a temperature difference of 15 deg F) 16.9 Total useful heat transfer 38.3