Document YreGkOmr6aQbq2ewnKr5MGG2k

610 CHAPTER 23 1958 Guide if The actual radiation transfer in a room may be somewhat different from that given by Equation 3 because of non-uniform..temperatures, irregular room surfaces, variations in erhissivity of materials, etc. It is. generally agreed, however, that the equation is accurate to within iO percent when used in conventional heating and cooling calculations. Tests4 conducted in the ASHAE Environment Laboratory6 showed that the value of the constant of Equation' 3 was 0.152 in the test room. The design informa tion in this Chapter is based on that value of the constant. Panel Heating 611 the natural convection from floors and ceilings can be calculated from Equa tions 4 to 9 which follow. Natural convection from heated ceiling q. = 0.041 (ip - <) Z>. Natural convection from heated floor 9,, = 0.39 P - f.)1-31 D.o.ra (4) (5) Radiation exchange calculated from Equation 3 is given in Fig. 9. The values apply to either ceiling, floor, or wall panel radiation output. Convection Transfer Convection values of heat transfer are not easily established. Convec tion 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 temperature exceeds that of the surrounding air. In practice, however, there are many, factors which-interfere with or affect natural convection. The configura^ tion of the room and the space connected to it and the variation in tern-, perature throughout the two spaces determine the natural convection^ Infiltration, ventilation, and the movement of persons may serve to intro-, duce some forced convection which can disturb the natural process. The laboratory approach has been to measure the natural convection in a bare, box-like room which is sealed against infiltration and in which all surfaces, except the heated panel, are at one uniform temperature. Con:, vection is also measured with varying amounts of infiltration and a cor rection determined for use in practical problems.6- Tests4 conducted in the ASHAE Environment Laboratory indicated that-j TEMP. OT PANEL SURFACE MINUS TEMP. Of AtP ~F Fig. 10. Heat Output by Convection from Floor and Ceiling Panels Natural convection from heated wall where ? = 0.29 (tp - Q" H- (6) 7C ~ heat transfer by convection, BTU per (hour) (square foot) tp = temperature of panel surface, Fahrenheit. ** = temperature of the air, Fahrenheit. = equivalent diameter of panel (area X 4 t perimeter), feet. -= height of wall panel, feet l - easi)rements of panel performance in furnished test rooms which did not ,, e TM*>rm temperature surfaces showed variations that are not large the pff i r significant in heating practice.8 Other tests9 established that eniio*- Ct ` room size was also usually insignificant. The convection f0nSiCan therefre be simplified to: a, ural convection from heated ceiling <? = 0.021 (tp - !.) (7)