Document KJraZrRy7od0EYYRv8vxDQxQK

HEATING VENTILATING AIR CONDITIONING GUIDE 1942 accomplishes the desired reduction in temperature differentials and assures maximum comfort in the living zone. The Convector Manufacturers Association has adopted the A.S.H.V.E. Standard11 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. Concealed heaters or convectors are generally sold 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 catalogued 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 top 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 arid the grilles be so designed that they will not impair the performance, of the assembled convector. It is impor tant that the enclosure or housing for the convector fit as snugly as pos sible 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 variable .. Standard Code for Testing and Rating Concealed Gravity Type Radiation (Steam). Teans*ctions Vol 371931. p. 367); (Hot Water), (A.S.H.V.E. Transactions. Vol. 39. 1933. p. 237). (See also A.S.H.V.E. Transactions. Vol. 41, 1935, p. 38. and Vol. 42, 1936, p. 29). 262 CHAPTER 13. RADIATORS AND CONVECTORS 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 SELECTION The capacity of a radiator varies as the 1.3 power, and that of a con vector12 as the 1.5 power of the temperature difference between the heating medium and the surrounding air in the case of the radiators, and the entering air in the case of the convector. It is obvious that for conditions other than the basic ones with the heating medium at a temperature of 215 F, and the room temperature at 70 F in the case of a radiator, and the inlet air temperature at 65 F in the case of a convector, the heat emission will be other than 240 Btu per square foot of rating. Table 5 shows factors by which radiation requirements, as determined by dividing heat load by 240, shall be multiplied to obtain proper radiator or convector sizes from published rating tables for room temperatures ranging between 50 and 80 F as well as for steam or water temperatures from 150 to 300 F. For other room and heating medium temperatures the factor is determined by the following formulae: For radiators: C. /215 - 70\ l l.-lr / For convectors: where CS = correction factor. = steam temperature, degrees Fahrenheit. tx = room temperature, degrees Fahrenheit. (i -- average inlet air temperature, degrees Fahrenheit. a) As previously indicated, the output of radiators and convectors is still designated by the terms of older practice, but this is gradually giving place to an engineering method of designating heat emission. The A.S.H.V.E. has adopted the following standards: Code for Testing Radiators (1927); Codes for Testing and Rating Concealed Gravity Type Radiation (Steam), 1931, and (Hot Water), 1933; (see also A.S.H.V.E. Transactions, Vol. 41, 1935, p. 38 and Vol. 42, 1936, p. 29). For steam services the actual condensation weight is taken without any allowance for heating effect; for hot water services the weight of circu lated water is used without allowance for heating effect. In all cases the total heat transmission varies as the 1.3 power for radiators13 and the 1.5 power for convectors14 of the temperature difference between that inside siia H V.E. Research Report No. 998--Factors Affecting the Heat Output of Convectors, by A. P. Kratz. M. K. Fahnestock, and E. L. Broderick (A.S.H.V.E. Transactions, Vol. 40. 1934. p. 443). Loc. Cit. Note 11. !<Loc. Cit. Notes II and 12. 263