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.470 CHAPTER 25 , 1946 Guide . able data and does not represent definite results of tests. For'such coiis the heat emission varies as the height of the coil. The heat emission of each pipe of ceiling coils, placed horizontally, is about 126 Btu, 156 Btu, and .175 Btu per linear foot of pipe, respectively, for 1-in., lj^-in., and 13^-in. coils. ' 4 CONVECTORS Cast-iron radiators may be concealed in a cabinet or other enclosure for appearance. In such cases a greater percentage of heat is conveyed to the room by convection thereby resulting in a form of gravity convector. A typical recessed convector is shown in Fig. 1. The heating element consisting of a large percentage of fin surface is usually shallow in depth and placed low in the enclosure in order to produce maximum chimney effect in the enclosure. The air enters the enclosure' near the floor line just below the heating element, is moderately heated in passing through the core and delivered to the room through an opening hear the top of enclosure. Since the air can only enter the enclosure at the floor line, the . cooler air in the room, which always lies at this level, is constantly being withdrawn and replaced by the warmer air. This air movement accom plishes the-desired reduction in temperature differentials and assures , maximum comfort in the living zone. Concealed heaters or convectors are generally available as completely built-in .units. The enclosing cabinet'should be designed with suitable air inlet and outlet grilles to give the heating element its best performance. Tables of capacities are cataloged for various lengths, depths and heights, and combinations are available in several styles for installations, such as the wall-hung type, free-standing floor type, recess type set flush with wall ' or offset, and the completely concealed type. Most of these types may be arranged with a topi outlet grille in a plane parallel with the floor, although the front outlet is practically standard. In cases where enclosures are to ; . be used but are not furnished by the heater manufacturer, it is important , that the proportions of the cabinet and the grilles be so designed that they0 Will not impair the performance of the assembled convector. It is desirable that the enclosure or housing for the convector fit as snugly as possible Radiators and Convectors________ ,- ' 471. so that the air to be heated must pass through the convector and cannot be by-passed in .the enclosure. The output of a convector, for any given length and depth, is a function of the height. Published ratings are generally given in terms of equiva lent square feet, corrected for heating effect. However, an extended surface heating unit is entirely different structurally and physically- from a direct radiator and, since it has no area measurement corresponding to the heating surface of a radiator, many engineers believe that the per formance of convectors should be stated in Btu. For steam convectors, as for radiators, 240 Btu per hour may be taken as an equivalent square foot of radiation. When more than one heating unit is used, one mounted above the other in the same cabinet, the output of the upper unit or units will be materially less than that of the bottom unit. RADIATOR AND CONVECTOR RATINGS A standard method of testing radiators was adopted by the A.S.H.V.E. in 1927 *. This Code provides for a standard test room, the' temperature of which is to be maintained at 70 F, measured in the center of the room at an elevation of 5 ft above the floor. The steam temperature in the radi-; . ator is to.be 215 F, which corresponds to 15.6 ib per square inch absolute. The weight of condensate per hour, under, these standard conditions, multiplied by the difference in the enthalpy of the steam entering the' radiator and that of the condensate leaving the radiator, gives the radiator output in Btu per hour. This output divided by 240 gives the steam rating of the radiator in square feet. ,j Similar test methods for convectors are the A.S.H.V.E. Codes for. Testing and Rating Concealed Gravity Type Radiation 2, (Steam Code 1932 and Hot Water Code 1933). These Codes recognize a different type of test booth, and the air temperature used is that of the air entering the convector casing instead of the temperature in the center of the room.. The entering air temperature for standard test conditions is 65 F. For hot water the'standard test conditions call for a mean temperature of the. water in the convector of 170 F. The Convector Manufacturers Association has adopted the A.S.H.V.E- standard in the formulation of its ratings and. has compiled a tentative standard of heating effect allowances for various enclosure heights to be included in the ratings by its members. All published ratings bearing the title C.M.C. Ratings (Convector Manu facturers Certified Ratings) indicate that the convectors have been tested in accordance with the A.S.H.V.E. Code by an impartial and disinterested laboratory and that the ratings have been approved by the Standardiza tion Committee of the Convector Manufacturers Association. Effect of Operating Conditions The heat output of a radiator is proportional to the 1.3 power of the temperature difference between the air in the room at the 60 in. level and the heating medium in the radiator. The heat output of a convector is. proportional to. the 1.5 power of the temperature difference between the . air entering the convector and the heating medium, steam or hot water, within the convector3. For hot water the arithmetical average between entering and leaving water temperatures is used. These laws may be expressed as correction factors to change from output under standard