Document Oz0w9Mvw987gEvDvx2NOOGKL1

26 CHAPTER 2 1962 Guide And Data Book normally designed on the basis of toi* criterion. Drains may be nmifojyj in hospitals except in special areas where excessive latent loads may exist. In the case of hotels and apartments, where high latent loads occur frequently condensate drains are required- Insulation Economics normally govern the thickness and qualify of insulation. The following are offered as guides for apparatus, ductwork, and piping. Apparatus. Normal practice is to insulate from the outdoor air intake up to the dehumidifier with 1 in. of insulation and from the dehumidifier to the fan with 2 in. of insulation. A good grade of insulation should be used and it should be vapor-sealed and plaster-coated. Ductwork. When headers are installed in any non-conditioned space, they should be insulated and vapor-sealed. Long headers, passing through a conditioned space, should be insulated to avoid excessive heat loss or gain. Risers are normally furred with metal lath and plaster, gypsum block and plaster, or as required by local codes. No insulation is required, provided all openings in the floors are sealed to eliminate stack effect and retard air circulation within the furred space. The' material used for sealing the floor openings must be fireproof. It is sometimes desirable to insulate the last sections of risers. For ncmchangeoter system* the temperature difference, be tween the air in the riser and in the chaae may be'considerable. Insulation requirements of the risers should be carefully analysed. Excessive heat loss can materially affect the per formance of the system. Piping. All chilled water piping other than the secondary water supply and return risers and run-outs should be insu lated and vapor-sealed. Most installations should be designed so that the secondary water temperature is not more than 3 F deg below the design room dew point If this principle is followed, it is not necessary to insulate the supply and return riser group. However, the risers must be sealed from uncon ditioned spaces, such as basement and attic. If the 3 F deg limitation is exceeded, supply and return risers should have a minimum of % in. of insulation and be vapor-sealed, and run outs to the induction units should also be insulated.- All insulation applied to the secondary water circuit should be checked for temperature resistance and life at both chilled and hot water temperatures. Controls The central apparatus of the air-water induction unit sys tem provides each induction unit with primary air at & pre- fig. 28 .... Method of Intermediate Season Cooling determined. design temperature and dew point during toe summer. As the weather becomes cooler, the primary air is reheated to a temperature which is keyed to the heat trans mission component of the room load. Cooling capacity at the induction unit in this case is controlled by varying the second ary coil capacity. Fig. 27 illustrates the variation of the secondary water and primary air temperatures throughout the hating and cooling season for a typical nonchangeover system. In a large building with multiple machines it may be ex pedient to run the refrigeration equipment to provide the secondary water cooling. But in most systems, this can be by circulating the secondary water through the primary air dehumidifier (and toe interior cone dehumidifier if more capacity is required) where it is cooled by the primary air passing over the coil. Hence, the refrigeration machine can be shut off as soon as the dehumidifier capacity is adequate to provide the necessary cooling. Fig. 28 shows a typical ar rangement. Although the system can be operated throughout the heat ing season with warm primary air for heating and cold second ary water for cooling, installations may operate more economi cally if changed over. Fig. 29 illustrates variation of the Dual-Duct/ Induction Unit, and Fan-Coif Conditioner Systems 27 secondary water and primary air temperatures throughout the heating and cooling season for a changeover system. If the dehumidifier is used to provide cooling for the second ary water, toe changeover process can be simplified. The re frigeration machine is turned off as soon as the outdoor tem perature reaches the. point at which the dehumidifier can provide .the secondary water cooling required. The system, however, need cot be changed over from summer to winter operation until toe primary air temperature required to meet heating requirements becomes excessive. (Heat losses become a problem at toe higher primary air temperatures. Hence they should be evaluated so that insulation can be added as required.) Figs. 30 and 31 show control systems which can be used. The system operation is as represented in Figs. 27 and 29, re spectively. Tables 5 and 6 outline the general function for non-changeover and changeover systems, respectively. Two items that appear on Tables 5 and 6 should be em phasised. 1. A non-changeover system requires normally-open room unit controls for gravity or emergency heating. 2. Changeover and operation of refrigeration equipment are not necessarily related or coincidental on many systems. Psychrometries The results that can be obtained with the air-and-water induction unit system can best be illustrated by an analysis of the psychrometrics involved. Fig. 32 shows the peak and partial load psychrometric processes for a typical system. The letters A, B, C, etc. shown in toe system schematic diagram correspond to the letters on the skeleton psychrometric charts on which these processes are shown, and reflect the condition of the air in the system. Peak load. The psychrometrics of thia process show that reheat is not required at design. As in the all-air induction unit toe mixture of the induced room air and primary air results in a fairly high discharge temperature (see point I). Note that the allowance has been made for fan heat gain (3 F deg) and duct heat gain (4 F deg) to the supply air after it leaves the debumidifying coil. Although these values will vary some what with system design, the variation is minor. Partial load. The psychrometrics of this process show toe conditions that can be maintained by the air-water induction unit system. Note that reheat is added to offset negative transmission on toe basis of a 71 F room at no load, and con trol on the secondary coil capacity is imposed to keep the room from going above 75 F at full internal load. Design Procedure The procedure for designing an air-water system includes the following items: 1. Survey. 2. Preliminary layout. 3. Unit Selection: (a) Room load calculations, including cooling loads as well as heating loads for forttd or gravity heating. - (6) Preliminary selection of unite. (c) Primary air requirements for ventilation and humidity. cvtsjdc v*i - was - P Fig. 27 .... Temperature Variations for Nonchangeover System fig. 29 .... Temperature Variations for Changeover System fig. 30 .... Schematic Control Diagram for Nonchangeover System fig. 31 .... Schematic Control Diagram for Changeover System