Book - School of Science and Technology

Metal radiant panelsRadiant heating 179Fabricated steel panels, of the type shown in Figure 7.1, are particularly suited to largefactory spaces when supplied with high or medium temperature hot water or even withsteam. With systems able to provide surface temperatures in the range 100±150 C, flat metalplates such as these produce a powerful radiation output which may be felt at some distance.As may be seen from the diagram, the panels consist of a 15 mm sinuous pipe coilwelded to a heavy gauge mild steel front plate. The rear of the panel may be treated in anumber of ways as shown in section, i.e. (a) exposed coil, (b) double sided, (c) insulatedback, and (d) shielded back. In (e), an alternative form of exposed coil is shown where theplate is welded as a diaphragm on the centre line of the pipe loops. Panel sizes arecommonly 2500 mm 1200 mm or 1800 mm 900 mm and approximate outputs forone size are given in Table 7.3. It will be noted that, although the rear is uninsulated inthis case, the radiant output is 55±60% of the total quoted output.It is usual to suspend such panels vertically along walls, or between intermediatecolumns in a large workshop, at a height of three or four metres above floor level. Theymay, as an alternative, be fixed horizontally or be inclined at an angle. In designing asystem using such panels, it should be borne in mind that the convective component ofoutput serves to counteract the cooling effect of the roof or roof glass but that anunnecessarily large allowance here will lead to an excessive temperature gradient. Panelsfixed to external walls and those suspended horizontally are usually provided withinsulated or shielded backs.Another form of radiant panel, used with water at a lower temperature, may be fitted asa heated skirting or dado. Construction is from an all-welded waterway grid formed from`D' section tubes at 150 mm centres with square section headers, the tubes being heldagainst a substantial front panel by means of a profiled plate spot welded to it, as shownin Figure 7.2. This product, which is not mass produced but bespoke-made for particularapplications, may also have expanded metal as a substitute for the front panel and be usedas the base for a floor finish at an exposed room perimeter. In the last application, onlyabout 50% of output will be by radiation.Past practice used another form of lower temperature panels, consisting of cast ironwaterways to which a sheet steel front plate was fixed, for space heating in a variety ofopen areas such as department stores, lecture theatres and the like. Some of theseapplications were successful as a result of rule-of-thumb appreciation of the balancebetween radiant and convective effects.Table 7.3 Emission from a radiant panel in free air:proportions radiant and convectiveHot waterRadiation Convectionmean temperature( C) kW % kW %100 2.87 0.55 2.32 0.45150 5.78 0.57 4.30 0.43200 9.90 0.61 6.53 0.39Notes1. Values quoted are derived from Imperial data.2. Panel was 2500 mm 1200 mm without rear insulation.3. Water temperature drop ˆ 30 K. Air at 15 C.