Book - School of Science and Technology

194 Heat emitting equipmentConvective heatingWhile radiators and other similar surfaces are now used only with warm and lowtemperature hot water systems, earlier practice with steam and higher temperature waterwas to use such equipment but in a protective enclosure of some type. Billington notes theuse of box and syphon coils and of finned pipes in the middle of the nineteenth century,enclosed in decorated openwork cases called calorifeÁres. These may be thought of asancestors of modern natural convectors (`indirect radiators' was the disparaging descriptionused in the past by other manufacturers!).Natural convectorsAlthough a wide variety of equipment falls within this general category, it is used here toidentify the cabinet type only, other patterns being dealt with separately. The principalcomponents are a finned tubular element mounted near the bottom of a sheet metal casingsuch that a chimney effect is created and a rising column of warm air flows from thetop, inducing an inlet of room air at or near the base. Figure 7.17 shows the generalarrangement.The tubular element will normally span between two headers which accept the externalpipe connections and, depending upon the required duty and the particular manufacturer,take a variety of forms. The relationships are complex since the tubes can vary as tonumber, size, and shape (round or oval) and the fins as to area, spacing, thickness,material and method of bonding to the tubes (solder or mechanical expansion). Thecasing height has an important effect upon output as may be seen from Table 7.8 whichprovides a digest of ratings for a temperature difference, water to air, as stated. Table 7.9provides correction factors which enable these ratings to be adjusted for a narrow rangeof other conditions, the effect of water velocity in the tubes being the factor limitingfurther extrapolation.Control of output may be either by adjustment of the temperature of the heatingmedium or, locally, by use of a damper fitted within the casing. Ideally, the dampershould be mounted just above the element but it is more usually fitted at a convenienthand level as shown in Figure 7.17. Emission with the damper closed is by radiation fromDamperPipe connectionsFinnedelementFigure 7.17 Cabinet type natural convector