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Heating Ventilating Air Conditioning Guide 1938 nected to one or more sections of the heater, depending upon the con densation rate of the engine. The recirculation duct connected with the opening in the suction duct should be extended to a point as near the floor as possible. When ventilation is not a requirement or is considered relatively unim portant, as in shop and factory heating, and the number of persons vitiat ing the air is small compared with the cubical contents of the building, or the process does not generate obnoxious gas or vapors, the air may be recirculated, sufficient outside air for ventilation being supplied by infiltra- 8y*pass Damper Fig. 1. Arrangement of a Central Fan Heating System (Draw-Through) Canvas Connection Heater Fig. 2. Arrangement for Heating Unit (Blow-Through) tion. The amount of heat to be supplied the heating unit in this case is the same as would be required for a direct radiation installation. When ventilation is a requirement to be met, an arrangement similar to that shown by Fig. 1 may be employed. Since the amount of air necessary for heating is generally in excess of the amount required for ventilation, considerable fuel economy may be effected by, recirculating a portion of the air. In this case only sufficient outside air is drawn into the system to meet the ventilation requirement and the remainder of the air, required for heating, is recirculated. This may be readily effected by an arrange ment of ducts and dampers on the suction side of the fan as previously mentioned. If the outside air, introduced is to be washed or conditioned the washer or humidifier and tempering coil may be added between the inlet for the recirculated air and the fresh air intake. 410 Chapter 21. Central Systems for Heating and Humidifying Blow-Through, Draw-Through When the heating unit is located on the suction side of the fan, the system is known as draw-through. (See Fig. 1.) When the heating unit is located in the discharge from the fan, the system is known as blowthrough. (See Fig. 2.) The draw-through combination is used for factory and toilet room installations. because a more compact arrangement of the apparatus usually is possible. In addition, air leakage will be inward. The blow-through combination is used principally in schools and public buildings, and for all booster coil arrangements where different tempera tures and independent temperature regulation are required for different heated spaces. In public building, installations, the fan frequently blows the heated air into a plenum chamber from which the air ducts radiate to the various rooms of the building; this arrangement is sometimes called the plenum system. HEATING UNITS The heating units for central fan systems using steam as the heating medium may be classified as (1) tempering coils, (2) preheater coils, (3) reheater coils, (4) booster coils, and (5) water heaters, either open or closed. Tempering coils are used with ventilating and air conditioning systems for raising the temperature of the outside cold air to above freez ing, or 32 F. They are not required for heating systems where all of the air is recirculated, since the temperature of the recirculated air will be above freezing. Preheater coils are used with air conditioning systems to raise the temperature of the air from that leaving the tempering coils to such a temperature that in passing through the water sprays of the washer (without water heater) the air will become partially saturated (adiabatically) having a moisture content corresponding to the required dew point temperature. Preheater coils therefore supply heat as necessary to control the dew-point temperature. The reheater coils are used to raise the temperature of the air leaving the tempering coils (in the case of a heating or ventilating system) or the air leaving the washer (in the case of an air conditioning system) to that necessary to maintain the desired tempera ture in the rooms or spaces to be heated or conditioned, except where booster coils are used, in which case the reheater coils raise the air tem perature to approximately room temperature, or slightly higher. Booster coils are installed in the duct branches to control the temperature of the air entering the rooms or spaces for which it is intended. Water heaters are used on an air conditioning system to control the dew-point temperature. They are used mainly for industrial work, seldom for comfort conditioning. They are not used where preheater coils are employed. The open type supplies steam directly to the spray water, while the closed type utilizes a heat interchanger by which the steam imparts its heat to the spray water. Where water heaters are required for comfort conditioning, the closed type is used. The heating units for central fan systems in use at the present time con sist either of pipe coils, finned tubes of steel, copper, brass or other metal, cast-iron sections with extended surfaces, or the cellular type. Steam is passed through these heating units and the air to be heated is passed over their exterior surfaces. 411