Document N2jjjG6dNVpJQ64a74qzYovbg

798 CHAPTER 43 : . V ' 1948 Guide delivery to the room may permit a reduction in fan capacity and the use of smaller-ducts. In some cases a by-pass may be desirable in order to maintain the primary air volume and to provide additional control. This system can provide a degree of zoning that is usually-difficult with con ventional design since the air delivered by each unit can be controlled individually. Selection of induction convectors should be made with due regard to noise level. The inductive capacity of; the device, increases with the jet velocity, but high jet velocities may result in objectionable noise. INDUCTION CONVECTORS--HIGH PRESSURE TYPE Another type of induction convector `Fig. 5 employs'nozzles which produce a high velocity air jet without objectionable noise. The term. Fig. 5. Induction Unit (High Pressure Type) high pressure, is to some extent inaccurate, since-the air-pressure at the nozzles, while several times that used with-a1 low pressure induction convector, is-still less than the total resistance pressure of a conventional central system. 5 The high velocity jet of primary air induces a flow of air from the'rooni'-through coils located in the secondary air stream and supplied ' with chilled water in summer and with hot water, in winter. The chilled water removes .a large portion of the sensible heat in summer and the hot water supplies the sensible heat loss in winter.: The primary air: is delivered at a sufficiently low dew-point < to- compensate for the latent heat gain in summer. In winter the primary air is supplied at a sufficiently high dew-point to take care of latent heat losses. Control of temperature is obtained by throttling the water quantity supplied to the secondary coil. The required flow of-primary air is greatly reduced due to the fact that a portion of-the-sensible heat load is carried by the secondary air stream. Since the primary quantity is:small, very high velocities can be maintained in the supply ducts without requiring fan power in-excess of- that for a conventional system.- This means that the supply ducts or pipes can be very small and can be run in chases, or Central Systems for Comfort Air Conditioning 799 furred in at columns along with the water pipes. The primary air is treated in the usual manner to reach the required dew-point and a surface or spray dehumidifier or a dehydrator may be used. The primary air quantity-is sufficient for ventilation purposes and frequently consists entirely of outdoor air. The water piping for the coils can be so valved that hot water will be supplied to one zone that may require heating while cold water may be supplied at the same time to a zone that requires cooling. This system usually is limited in application to hotels, apartments, office buildings and other multi-room installations having a large per imeter with relation to the floor area. The units usually are installed beneath the windows, replacing direct radiation or thermally-circulating enclosed convectors. Where the spaces to be conditioned extend a con siderable distance from the outer wall into the interior of the building, a separate system or zone for the conditioning of the interior portions may be required. EVAPORATIVE COOLING In climates where, on the hottest days, the outdoor wet-bulb depression is relatively great, it may be possible to dispense with refrigeration or .other cooling sources by use of the evaporative cooling effect. A well designed air washer using recirculating sprays will reduce the entering dry-bulb temperature to within a degree or two of the entering wet-bulb condition. Thus, it may be possible that, with air entering at 100 F dry-bulb, 60 F wet-bulb, a leaving condition of 62 F dry-bulb, nearly saturated, can be obtained. Under some conditions of latent and sensible heat load, evaporative cooling may be adequate. At times when the outdoor, wet-bulb temperature is not quite low enough to permit the use of straight evaporative cooling it is possible to use precooling convectors with refrigeration, well water or a cooling tower as the basic source of sensible heat removal to reduce the wet-bulb tem perature of the air before it enters the spray chamber. Where internal heat loads are high, this scheme may be more.economical than.one using return air. Where the required supply air dew-point is too low to permit straight evaporative cooling and where the sensible heat load is not too great, intentional partial saturation may be employed. That is, the low dew-point of the outdoor air is utilized by permitting some of this air to pass through the humidifying sprays untreated, or to by-pass the humi difier. All of these remarks with regard to evaporative cooling are based on the assuriiption that all of the supply air will be taken from outside. Provision should be made in most cases for the return of some air from the conditioned spaces for control purposes as well as for economy of fuel in winter. PRECOOLING Where sufficiently cold water from wells or streams is available, a saving in refrigeration may be obtained by the use, in location ahead of the dehumidifier, of precooling coils through which the cold water is circulated. The resultant cooling of the air decreases the load to be carried by the dehumidifier and refrigeration plant. In normal practice the water, after passing through the precooling coils may be further utilized in the refrigeration plant condenser. The economic advantages of this scheme are apparent and it is frequently used.