Document yR66wX3qQqVVmajbL4KqV9K4

536 CHAPTER 24 . 1950 Guide Stoker-Fired.. The stoker-fired type of unit heater can be obtained in ranges, of from 300,000 to 6,000,000 (dr more) Btu per "hr output ca pacity. Ratings are based upon delivered output at heater'outlet. Effect of Resistance Upon Capacity Unit heaters are customarily, rated as free delivery type units. . If out: side air intakes, air filters, or ducts on the discharge side are used with the unit, a reduction in air and heating capacity 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, so that no specific percentage reduction can be assigned for all heaters at a given added resistance. In general, however, propeller fan type units wifi experience a larger reduction in capacity than housed cen trifugal,fan units for a given added' resistance, and a given heater will have a larger reduction in capacity as the fan speed is lowered. The heat output to be expected under other than free delivery conditions should be secured from the manufacturer. ' Determining Unit Heater Requirements The formulas given in the section on:Unit Ventilators may be used to determine unit heater capacity requirements. Location of Unit Heaters . Care should be taken in the location of unit heaters to insure free air circulation to the intake: The best arrangement is to locate units so that they discharge air nearly, parallel to exterior,walls, and in a direction which will produce a rotational circulation around the room. This is preferable to directing the discharge against the outside walls. Various types and makes of unit heaters are illustrated in the Catalog Data Section of this edition. As hot blasts of air in working zones are usually objectionable, heaters mounted on the .floor should have.their dis charge outlets above the head line, and suspended heaters should he 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, how ever, the elevation of the heater outlet and the direction of discharge should be so arranged that the heated air is brought as close above the head line as possible, yet not into the working zone. . . .. In connection with the use of vertical type unit heaters, care must be exercised in the selection of the heater. It has been found that, the higher the unit is placed above the floor, the'lower must be the outlet temperature of the air leaving the heater in order that the heated air may be forced into the occupied zone. Inlet, Outlet and Space Temperatures with Unit Heaters ' In the selection of unit heaters for any. particular design, consideration should be given to the temperature of air entering , the heaters, as well as the temperature to be maintained in the working zone of the space. In general, the temperature differences per foot of elevation, when using unit heaters, are less than corresponding variations when using direct radiation.? High velocity units will maintain slightly lower temperature differences than low discharge velocity units. : Correspondingly, unit3 with lower dis charge air temperature wUTmaintain'.lower temperature differences thdn Unit Heaters and Unit Ventilators 532 units with higher, discharge temperatures. , Directional eontrol ;of the dis: charged air from a unit heater can be an important factor, added.to.qualities of reasonably good' outlet-velocity and outlet temperature,, in; effecting satisfactory, distribution of heat and reducing floor-to-ceiling temperature difference.-.,' . ,:iv . Since the outlet temperature of air from a unit heater increases with:the temperature of the heating medium, such as high pressure steam, heaters can be obtained Withlheating elementS having less, than the regular: amount of heating surface in order to obtain, with the-high temperature heating medium, approximately the same leaving air temperature as would-,be'bb4 tainedfrom-a;lower, temperature heating medium.- a-.-.;.:t .When some outside air is introduced, the temperature of-the"mixture of outside and recirculating air.must be calculated and used as the entering air temperature at the heater. Unit heaters connected in this rnn.nner.per- form. the function ,of unit, ventilators. For a. discussion of-this,function see the section of this.chapter.entitled Unit Ventilators. ' ' For-recirculating heaters located-at the floor or with intakes.at the floor;: thetemperature of air entering the heater should be assumed to be.the same as that.to-be maintainedin the room itself. .: ; - : -b .in. "J-v: - " Automatic Control of Unit Heaters " ' v\ ' .. -. 0 -Thermostatic control, of unit heaters may be accomplished, either- by starting; and stopping: the fan, or by controlling the flow of the-heating:, medium to the: heating element: If the fan is controlled; it is.advisableto. provideia temperature-operated switch .to prevent,the fan-from: starting , until the, heating element .is heated throughout.. Unit heaters, may,rbe