Document 0gxMdGQ8002JV6VNr5e4rVm1b

T a b l e 1. C onstants fo r D e t e r m in in g , t h e Ca p a c it y of U n it H eaters for V ar io u s Ste a m Pressures . And T emperatures of E ntering A ir ( Based on Steam Pressure of 8-lb Gage and Entering A ir Temperature o f 60 F) 1.538 1.585 1.640 1.730 1.799 1.861 1.966 . 2.058 2.134 2.196 2.256 2.283 2.312 2.361 2.409 1.446 1.495 1.550 1.639 1.708 1.769 1.871 959l i 2.035 2.094 2.157 2.183 2.211 2.258 2.307 1.369 1.405 1.456 1.545 1.614 1.675 1.775 . 1.862 - ' 1.936 1.997 2.057 2.085 2.112 2.159 2.204 0.671 0.713 0.760 0.838 0.897 0.952 1.042 1.119 1.187 1.239 1.293 1.316 1.342 1.383 1.424 0.739 0.782 0.829 0.908 0.970 1.024 1.115 1.194 1.262 1.314 1,368 1.392 1.418 1.461 1.502 1.663 1.581 1.541 1.577 1.497 1.552 1.472 1.527 1.447 1.472 1.393 1.416 1.338 1.349 1.270 1.266 1.190 1.174 1.097 1.117 1.043 1.056 0.982 0.974 0.901 0.926 0.853 0.881 0.809 1.621 . Heating Ventilating Air Conditioning Guide 1938 8 & eOoQ "s 8 S o li 0.956 1.000 1.050 1.131 1.194 1.251 1.346 1.430 1.498 1.555 1.610 1.635 1.660 1.705 1.749 1.034 1.078 1.127 1.211 1.275 1.333 1.429 1.511 1.582 1.640 1.696 1.721 1.748 1.792 1.836 2.015 1.927 1.968 1.880 1.925 1.836 1.896 1.808 1.782 1.725 1.755 1.666 1.683 1.596 1.597 1.509 1.498 1.416 1.441 - 1.335 1.375 1.290 1.289 1.206 1.237 1.155 1.191 1.110 ' 1.872 1.811 1.. 273 1.320 1.370 1.460 1.525 1.584 1.684 1.771 1.845 1.902 :96ll 1.990 2.015 2.063 . 2.108 fe T 1 SE geo al P OfNUDO^lOOOOOOJOOOO 440 Note.__To determine capacity at any steam pressure and entering temperature, multiply constant from table by rated capacity at 60 F entering and 2 lb pressure. Chapter 23. Unit Heaters, Ventilators, Air Conditioning. Cooling Units OUTPUT OF HEATERS It is standard practice to rate unit heaters in Btu per hour at a given temperature of air entering the heater and at a given steam pressure main tained in the coil. Steam at 2 lb pressure and air entering at 60 F are used as the standard basis of rating4. The capacity of a heater increases as the steam pressure increases, and decreases as the entering air temperature increases. The heat capacity for any condition of steam pressure and entering air temperature may be calculated approximately from any given rating by the use of factors in Table 1. This table is accurate within 5 per cent. Unit heaters are customarily rated as free delivery type units. If outside air intakes, filters, or ducts on the discharge side are used with the heater, proper consideration should be given to the reduction in air and heat capacity that will'result because of this added resistance. The percentage of this reduction in capacity will depend upon the characteristics of the heater and on the type, design, and speed of the fans employed, so that no specific percentage of reduction can be assigned for all heaters for a given added resistance. In general, however, disc or propeller fan units will have a larger reduction in capacity than housed fan units for a given.added resistance, and a given heater will have a larger reduction in capacity as the fan speed is lowered. When confronted with this problem the ratings under the conditions expected should be secured from the manufacturer. DIRECTION OF DISCHARGE Heaters may be distributed through the central portions of a room discharging toward exposed surfaces, or may be spaced around the walls, discharging along the walls and inward as well, especially when there are considerable roof losses. In general, it is better to direct the discharge from the unit heaters in such fashion that rotational circulation of the entire room content is set up by-the system rather than to have the heaters discharge at random and in counter-directions. Various types and makes of unit heaters are illustrated in the Catalog Section of this edition. Usually hot blasts of air in working zones are objectionable, so heaters mounted on the floor should have their discharge outlets above the head line and suspended heaters should be placed in such manner and turned in such direction that the heated air stream will not be objectionable in the working zone. In the interest of economy, however, the elevation of the heater outlet and the direction of discharge should be so arranged that the heated air shall be brought as close to the head line as possible, yet not into the working zone. In general, the higher the elevation of the unit, the greater the volume and velocity required to bring the warm air down to the working zone, and conse quently, the lower the required temperature of the air leaving the unit. Vol'li'6'193(jEp S165)dard Colle for Testing and Steam Unit Heaters (A.S.H.V.E. Transactions, 441