Document mmewzx2kmMB8N9prn52RGYbkb

540 CHAPTER 24 7 :1950 Guide 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 arid stopped by thermostatic control. The wide-and sudden fluctuations of load that occur under such conditions 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 to thk rapid fluctuation. In most cases, and particularly where the boiler is coal-fired, Unit Heaters and Unit Ventilators 1 541 ing rooms, offices or other applications where the density of Occupancy indicates the need for ventilation. In normal Operation, the discharge air temperature from 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 occu pants, sun, etc., is sufficient to cause overheating, then the air delivery teinperature must be below that of the rooms. It is-customary, to equip unit ventilators with control, devices that, prevent the 'delivery of air at a tem perature that will cause cold drafts. Unit ventilators may be of the radia tor 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 heat input to the unit which is determined by measuring the temperature and quantity of condensate and the-pressure and quality of the steam. The second is the heat output of the unit which is determined by measuring A .- Table 3. Typical Capacities of Unit Ventilators for an Entering Air Temperature of Zero Cubic Feet or iiia peb Minute Total Capacity in Squabs Feet, Anemometer Rating Condensate Rating < Equivalent Di rect Radiation Capacity Available pob Heating the Room, Square : Feet Equivalent Direct Radiation 750 500 1000 750 1260 1000 1560 1250 214 320 427 534 56 84 112 141 Final Air , Temperature F Deo 95 95 95 95 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 rating6 and also the approximate heating data. In accordance with the I DAMPERS DAMPERS A.S.H.V.E. Standard Code for Testingand Rating Steam Unit Ventilators the rating information to be supplied and the basis of rating are: ; Draw-through Type , Blow-through Type .... Fio. 9. Typical Unit.Ventilators Showing Two of Many Arrangements. : .of Dampers, and Heating Coils Rating Factors to Be Specified. The rating of the unit ventilator shall specify: a. Final temperature at different entering air temperatures. b. Total EDR at `different entering air temperatures. ! it,.is advisable, to use two or more smaller units instead of one large 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 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 grille on the outlet. The Standard Basis of Raling shall be as follows: when , proper provision is made for returning the condensate. If units . Dry saturated steam at a temperature at the unit corresponding to an absolute admit,air that may be at a temperature below freezing, a steam pressure of , pressure of 16.7 psi (218.5 F). not. less than 5 lb should be maintained on the heating element, or a corre . Entering air temperature of zero Fahrenheit degrees. sponding differential in pressure,between tiie supply and returns should be c. Volume delivered in cubic feet per minute converted to standard air at 70 F. maintained by means of a vacuum.: 1 . - . . 7. Rating Tables for unit ventilators shall contain the following data in addition to ;:;'7 1 ' UNIT VENTILATORS the standard rating, for entering air temperatures from --30 F to +60 F: a. Inlet temperature, Fahrenheit degrees. .\S,A unit.ventilator is essentially a unit.heater equipped with dampers for introducing .outdoor and recirculating " air,in .varying quantities, and arranged,with,a system, of control that permits the heating, effect to be varied with the fans operating cohtinyousty^'dSitbef steam or. forced.hot.water may be employed as.the heating medium, although steam is the m.6re common. Unit ventilators are intended'primarily for use in schools, ineet- b. Final temperature, Fahrenheit degrees. c. Total EDR at the specified entering temperature. d. Surplus or heating EDR at the specified entering temperature. Surplus or Beating Equivalent Direct Radiation for the purposes of this code shall be construed to mean difference between the total EDR at a specified inlet temper ature and the EDR required to heat the air from that temperature to 70 F.