Document QMgEdrvvJ3K553wREN9q7R9nv

504 iCHAPTER 26 . 1948- Guide 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, and then eliminated' at some qther. point in the system. Fig. 8 represents the connections to a hot water heating system. . The ah' vent is not required if the main is above the heater and air can be eliminated through the piping 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 if the unit heater motor is started and stopped by thermostatic control. The wide and sudden fluctuations of load that occur under such -con ditions would require closer attention to the bqiler 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, and particularly .where the boiler is coal-fired, it is advisable to use two or more smaller:unit heaters instead of one large unit. Steam pressures below 51b 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 A unit ventilator is essentially a unit heater equipped with dampers for introducing outdoor and recirculating air in varying' quantities, arid arranged with a system of control that permits the heating effect to be varied with the .fans operating continuously. Either steam or forced hot water may be employed as the. heating medium, .although steam is the more, common. ."Unit ventilators are intended primarily, for; use in schools, meeting rooms,' offices or other applications where the density of occu pancy indicates the need; for ventilation'. In normal operation,'.the dis charge air temperature frorii a unit ventilator is varied in accordance with the room demands'. Where a heating effect is required, the air delivered is above room temperature. Where the heat generated within the"room by occupants, sun, etc., is sufficient-to cause overheating, then the air delivery: temperature must be below that of the rooms. - It is cutomary to equip unit ventilators with control devices that prevent the delivery of air at a temperature that will cause cold drafts. Unit ventilators may be of the radiator or damper controlled type constructed on the blow-through or draw-through principle as illustrated in Fig. 9. Ratings of Unit Ventilators Unit ventilators of the steam type are customarily cataloged with two ratings, one the input and the other the output capacity. The first is the :Vnit Heaters, Unit Ventilators, Unit-Humidifiers 505 heat input to the unit which is determined by measuring the temperature arid quantity of'condensate arid the pressure and quality of the steam; The 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 from the unit. Table 3,shows the.air:handling capacities by the two methods of.rating* and also the approximate heating data, In accordance with the A.S.H.V.E. Standard Code for Testing and Rating Steam Unit Ventilators the information to be supplied regarding ratings arid the basis of rating is the following:... Rating Factors to Be Specified. The rating of the unit ventilator shall specify the following: a. Final temperature at different entering air temperatures. b. Total EDR at different entering air temperatures. Fig. 9. Typical Unit Ventilators Showing Two.of-Many Arrangements of Dampers and Heating Coils c. Air delivered, by the unit in cubic feet per minute at the standard basis of rating with the fans operated at rated speed, with all air being blown through the heating unit and with the standard louver and grillC on the outlet. The Standard Basis of Rating shall be. as follows: a. Dry saturated steam at a temperature at the unit corresponding to an absolute pressure of. 16.7 psi (218.5 F)., b. Entering air temperature of zero Fahrenheit degrees. c. Volume delivered in cubic feet per minute converted to standard air at 70 F. . . Rating Tables for unit .ventilators shall contain the following data in addition to the standard rating, for entering air temperatures from --30 F to +60 F: a. Inlet temperature, Fahrenheit degrees. b. Final temperature, Fahrenheit degrees. c. Total EDR at the specified entering temperature. d. Surplus or heating EDR at the specified entering temperature.