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Handbook of air conditioning and refrigeration / Shan K

Handbook of air conditioning and refrigeration / Shan K

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8.16 CHAPTER EIGHT<br />

FIGURE 8.4 Typical electric heater wiring diagram. (a) Single-phase line voltage control with thermal cutout <strong>and</strong> fan interlock;<br />

(b) three-phase line voltage control with fan interlock <strong>and</strong> backup contactor.<br />

Electric Furnaces, Electric Heaters, <strong>and</strong> Duct Heaters<br />

Electric furnaces installed in ro<strong>of</strong>top packaged units, electric heaters equipped in <strong>air</strong>-h<strong>and</strong>ling units<br />

<strong>and</strong> packaged units, electric duct heaters mounted in <strong>air</strong> systems, <strong>and</strong> electric unit heaters installed<br />

directly above the conditioned space are all electric resistance warm <strong>air</strong> heaters. Air is forced<br />

through the electric heating element by the supply fan in a ro<strong>of</strong>top packaged unit or in an <strong>air</strong>h<strong>and</strong>ling<br />

unit, or by the separately installed propeller fan in an electric unit heater.<br />

Because the mixture <strong>of</strong> outdoor <strong>air</strong> <strong>and</strong> recirculating <strong>air</strong> or the outdoor <strong>air</strong> alone is clean <strong>and</strong><br />

nonhazardous after passing through the <strong>air</strong> filter, most electric resistance heaters use heating

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