Document jm0qq3ajyyo3KDq6E58b3QKGR

American Society of Heating and Ventilating Engineers Guide, 1937 from this cause is negligible for superheat up to 50 deg and will not exceed 3% per cent for any degree of superheat. 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. BOILER CAPACITY The capacity of the boiler should be based on the rated capacity of the heaters at the lowest entering air temperature that will occur, plus an allowance for line losses. Ordinarily for recirculating heaters the lowest n entering temperature will occur at the beginning of the heating period and is usually taken as 40 F, while for ventilators taking air from outdoors the lowest entering temperature will be the extreme outdoor temperature expected in the district. No greater allowance in boiler capacity beyond the calculated heat demand need be added in order to supply unit heaters than for any other type of system. It is unwise to install a single unit heater as the sole load on any boiler, particularly if the unit heater motor is started and stopped by thermostatic control. The wide and sudden fluctuations of load that occur under such conditions would require closer attendance to the boiler than is usually possible in a small installation. Where oil or gas is used to fire the boiler, it is possible by means of a pressurestat to control the boiler, in response to this rapid fluctuation. In most cases, however, and particularly where the boiler is coal-fired, it is advisable to use two or more smaller heating units instead of one large unit. Steam pressures below 5 lb can be used with safety for recirculating unit heaters when their coils are designed for the purpose and when proper provision is made for returning the condensate. If ventilators are to take in air that may be at a temperature below freezing, however, a steam pressure of not less than 5 lb should be maintained on the convector 244 --''"'TTT'i__ IJnit Heaters, Ventilators, Air Conditioning, Cooling Units r.HAPTER ------- ---------------------------:----------------------------------------------------------------------------- orresponding differential in pressure between the supply and returns be maintained by means of a vacuum. QUIETNESS Fan speed alone is not a measure of relative quietness of fans having a fferent designs and proportions. Quietness is a function of type, j? ter, blade form and other variables besides speed, and all these must be considered. In general for a given design, the higher the fan speed, the greater the noise, and centrifugal fans are more quiet than disc or propeller fans. PIPING CONNECTIONS Piping connections for unit heaters are similar to those for other types of fan-blast heaters. Typical connections are shown in Figs. 4 and 5. One-pipe gravity and vapor systems qre not recommended for unit heater Fig. 4. Unit Heater Connections Where Condensation Is Returned to Vacuum Pump or to an Open Vented Receiver Fig. 5. Unit Heater Connections Where .Condensation Is Returned to Boiler Through Wet Return work. For two-pipe closed gravity return systems the return from each unit should be fitted with a heavy-duty or blast trap, and an automatic air valve should be connected into the return header of each unit. Pres sure-drop must be compensated for by elevation of the heater above the water line of the boiler or of the receiver. In pump and receiver systems the air may be eliminated by individual air valves on'the heaters, or it may be carried into the returns the same as for vacuum systems and the entire return syfetem be free-vented to the atmosphere, provided all units, drip points, and radiation are properly trapped to prevent steam entering the return's. On vacuum or open vented systems the return from each unit shpuldTje fitted with a large capacity trap to discharge the water of condensation and with a thermostatic air valve for eliminating-the air, or with a heavyduty trap for handling both the condensation and the air, provided the .air finally can be eliminated at some other point in the return system. For high pressure systems the same kind of traps maybe used as with 245