Document ga4X33Ezp14VoRXVmJ2J10dQQ

398_____________________________________ Chapter 21 _________ ;_______________ 1945 Guide 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 heater. Provisions must be made for compensating for the pressure drop by eleva ting the unit heater above the water liije 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 as in vacuum systems and the entire return system be free-vented to the atmosphere, provided all units, drip points, and radiation are properly trapped to prevent steam entering the returns. On vacuum or open vent systems the return from each unit should be fitted with a large capacity trap to discharge the wateV 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. Fig. 9- Typical Unit.Ventilator Showing One of Many Arrangements of Dampers and Heating Coils For high pressure systems the same kind of traps may be used as with ? vacuum systems, except that they must be constructed' for the pressure Re used. If the air is to be eliminated at the return header of the unit, a! '' high pressure air valve can be used; otherwise the air may be passed with.-:? the condensate through the high-pressure return trap, with its elimination at -some other point in the system. ' Boiler Capacity for Steam Unit Heaters The capacity of the boiler should be based on the rated capacity of the-*? unit heaters at the lowest entering air temperature and highest fan speed ?, that will occur, plus an allowance for pipe line losses. It is unwise to?-, install a single unit heater as the sole load on any boiler, particularly ife? the unit heater motor is started arid stopped by thermostatic control;*?. The wide and sudden fluctuations of load that occur under such con-?; ditions would require closer attention 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 tfe? this rapid fluctuation. In most cases, and particularly where the boileCS Unit Heaters, Unit Ventilators, Unit Humidifiers,-399 is coal-fired, it is advisable to use two or more smaller unit heaters instead of onela^ge unit. Steam pressures below. 5 lb can be used with safety for recirculating unit heaters when their heating surfaces are designed for those pressures, and when proper provision is made for returning the condensate. If units admit air that may be at a temperature below freezing, a steam pressure of not less than 5 lb should be maintained on the heating element, or a corresponding differential in pressure between the supply and returns should be maintained by means of a vacuum. UNIT VENTILATORS Unit ventilators are similar in principle to unit, heaters, except that they are provided with an arrangement of dampers for introducing out door and recirculated air in varying quantities. In general, they are obtainable only in the steam type but unit heaters of all types may be adapted to the unit ventilator principle by the addition of outdoor air and Table 4. Typical Capacities of Unit Ventilators " for an Entering Air Temperature of Zero Cubic Feet of Air per Minute Anemometer Rating Condensate Rating Total, Capacity in Square Feet, Equiv alent Direct Radiation Capacity Available for Heating the Room, Square Feet Equivalent Direct Radiation Final Air Temperature Deg F 750 1000 1260 1560 . 500 750 1000 1250 214 320 427 534 . 56 84 112 141 95 95 QA 95 recirculating dampers. Unit ventilators are used to supply air with a discharge temperature at or slightly higher than room temperature. Also they are provided with an arrangement for introducing outdoor and recirculated air in varying quantities.' If the unit is only used for circu lating air, then radiators or some other equipment must be provided for' heating the room. Unit ventilators are intended primarily for schools, offices, and semi-commercial establishments. A typical unit ventilator is illustrated in Fig. 9. A roof ventilator used for exhausting air is some times termed a unit ventilator. For information on roof ventilators, see Chapter 42. Hatings Unit ventilators of the steam type are customarily cataloged with two ratings, one the input arid the other the output capacity. The first is the input to the unit which is determined by measuring the temperature and quantify of condensate and the pressure and quality of the steam. . "e second is the heat output of the unit which is determined by measur ing the quantity of air delivered and the temperature of the air to and rom the unit. Table 4 shows the air handling capacities by the two .et"0(ls f rating4 and also the approximate heating data. In accordance th the A.S.H.V.E. Standard Code for Testing and Rating Steam Vo].*3^ '^25)"^2rt* Code for Testing and Rating Steam Unit Ventilators (A.S.H.V.E. Transactions.