Document Eq13gYEGQ55J01R28ENrRZvV4

24 CHAPTER 2 1962 Guide And Data Book Gravity heating at 180 F entering water **= 2600-9500 Btuh ABC A-AUI IMi) OAMJt UMJT OafTMX (MHT C-LO HOBKT WtTU CMTMOL UNIT Rg. 25 .... Typical Air-Water Induction Units their following numbers on Fig. 25 are: primary air plenum (2), damper (not shown), nozzles (3), secondary coil (8), mixing chamber (6), cabinet or enclosure (7). Plenum. The conditioned primary air is introduced to the plenum which insures good air distribution to the nozzles. The plwinm is baffled and lined with sound absorbing material to belp attenuate the noise generated in the high-velocity duct system. Balancing Damper. A dmjw (external or integrated with the unit) is used to adjust the primary air quantity to the unit. : Nozzle*. The nozzle is an outlet for the primary air which pro vides the motivating force to induce flow of the secondary air through the unit coil. Secondary Coil. Room temperature is controlled by modulating the flow of hot or cold water through the unit coil. Equipment'is also available for maintaining desired room temperatures by by passing air around the secondary coil as room conditions dictate. Mixing Chamber. This chamber serves as a mixer for the pri mary ana secondary air. It also provides the stack effect re quired for gravity heating. .. Enclosure. The enclosure can be a factory furnished cabinet or furring to suit on the job. It must be designed to allow proper flow of secondary air through the coil and so unobstructed' dis charge. Unit Selection Criteria Four factors govern the selection of air-end-water induction unite: 1. The unit must have sufficient capacity to satisfy maximum room sensible cooling requirements. 2. The unit must provide sufficient primary air at a dew point that will satisfy latent load requirements. 3. The unit primary air quantity must include sufficient out door air for ventilation purposes. 4. The unit must satisfy the room heating requirements. These can be met by providing the proper quantity of heated primary air to each room. (See Unit Selection Procedure for discussion of A/T ratio) or by supplying warm water to the unit coil. The ratio A/T is the ratio of primary air (efm) to the heat transmis sion per degree of space being conditioned. Range of Available Equipment The following summary shows the available general range of sizes, air quantity, heating and cooling capacities, and pressure drops of current equipment. Air-Water Induction Unit Summary Number of available sizes -- 5 TM 20 to 62 in. S= 8 to 12 in. = 12 to 26 in. Air quantities " 20 to 150 cfm Nozzles pressure ~ 1.0 to 3.5 in. of water Water flow rate * 1.0-2.0 gpm Cooling capacity at 52 F entering water = 2000-13,000 Btuh ^ Unit Application Unite can be mounted either vertically or horizontally in recesses or in cabinets. The preferred arrangement in the northern climates (when outdoor design temperature is below 20 F) is vertically underneath windows to minimize downdraft. There are, few limitations in unit location in very temperate climates other than the convenience of physical layout and economics. Unite are available for free standing, wall hung, or ceiling hung applications. Standard unite are suitable for average window sill heights or all glass buildings. In addition to dimensional differences, units are available in varying induction ratios (ratios of secondary coil air to pri mary air capacities). The wide selection is required due to the wide variation in sensible load that exists. For example, all-glass office buildings with high lighting loads require units with very high induction ratio; existing hotels with small windows and very light internal loads require a unit with low induction ratio. . System Description The air-water induction unit system differs from the all-air system in that both fluids are capable of beating or cooling. By supplementing each other during the cooling season, the 1 air quantity required can be greatly reduced. 8 Outdoor air for ventilation is drawn into the central primary 5 air apparatus where it is conditioned and filtered. In summer 8 and intermediate seasons, the dehumidifier chilled water coil B removes excess moisture from the air. During intermediate seasons, a reheater warms the air sufficiently to offset building transmission losses. During the summer most of the cooling toad, and in inter mediate seasons all of the cooling, is accomplished by circu lating cold secondary water through the induction unit coil to cool the secondary air. This type of operation may continue through the heating * season or the system can be changed over to operate with cool primary air and warm water for heating during cold winter weather. In either case, in freezing weather the pre heater will temper the air so that it has the capacity to ab- , sorb moisture when pasting through the humidifier sprays. Nonchangeover systems can be applied on any building, but are normally confined to the following circumstances. 1. In areas which have a mild winter climate where periods of - cold weather are of short duration and are comparatively mild, ] as for example, Florida, Texas and California. 2. In buildings where a changeover system would increase the primary air quantity above ventilation and cooling load require^ ments. In a severe winter climate where temperatures of 0 F or below are prevalent for weeks at a time, there is no reason why - this prohibits operation on the winter cycle during these periods. Chilled water from a central refrigeration plant is circulated : through coils in the central dehumidifier. A secondary water < pump circulates warm or cold water to the coils of the indue- ' tion unite. A high pressure centrifugal fan delivers the con ditioned air through high velocity air conduits to the indue- ' tion units- A sound absorber reduces the noise level as the . air leaves the fan. The basic system components are the central station condi- i turning equipment, the air transmission system, primary chilled water piping, secondary water piping, control system, ] and room induction unite. Fig. 26 is a schematic diagram of the j system showing the various components which are discussed < in the following sections. > Dual-Duct, Induction Unit, and Fan-Coil Conditioner Systems 25 Central Station Apparatus The central station air-conditioning equipment is con ventional in design as generally described in Chapter 1. How ever, there are certain qualifications which should be made. This apparatus can be designed to supply air of one tem perature to every unit in a building. The supply air temper ature in this case is coordinated with the outdoor temperature. A reheater as well as a preheater is required to achieve this control. The. reheater should be sized for approximately 25 percent excess capacity to allow for duct heat losses and burning pickup requirements. In cases of low calculated changeover temperatures and where it is desired to shut off the refrigeration nw.hirw at higher temperatures, the primary air dehumidifier can be used to provide a source of cooling from outdoor air for the second ary water during intermediate seasons. If the capacity of the dehumidifier for this service is not adequate, supplementary cooling may usually be obtained from the interior zone dohumidifiers. In those instances where the refrigeration machine must*be operated to obtain the secondary water cooling, steps must be taken to make the cooling tower suitable for winter operation. If zoning is required with a single apparatus it may be accomplished by providing a reheater for each zone on the dis charge side of the fan. If the size of the building is such that multiple conditioning stations are required, it may be desir able to arrange these stations as different zones. Air Transmission Systems The air transmission system should be designed for high velocity and high pressure, and therefore, sound absorbers are normally required on tire discharge side of the fan. The duct layout should be as symmetrical as possible and ducts should be sized by static regain methods. This minimiypa the pressure variation between unite, reduces fan horsepower and keeps balancing tima at a. minimum. Piping Conventional piping practices as diweiMwad in Chapters 8 and 74 will apply to the air-and-water induction unit system. However, there are certain qualifications: Balancing. The use of a reverse return system generally eliminates the need for balancing. In new buildings it usually costs no more to design a complete reverse return system. In Rg. 26 .... Typical Air-Water Induction Unit System existing buildings, where this may be difficult the system can be designed with reverae return headers. Properly Hasignad direct return risers result in only minor penalties. Zoning. Usually with systems including automatic room control, zoning is not necessary except where it is desired to change over only pari of the building from summer to winter operation and vise versa (a discussion of change-over is in cluded under controls). Also zoning is not recommended for designs using manual room control if such buildings are lo cated in metropolitan areas where shadows from adjacent structures materially influence the room load. Since it is im possible to zone effectively for these conditions, there is little to be gained by incurring the added expense. If the buildings are located where a good delineation of zones results then room regulation of temperature can be improved for manual room control applications by use of judicious zoning. This permits such control of zone water temperature that a minimum of adjustement is required at the room valve. Diversity. When more than one exposure is served by a common secondary water circuit and automatic valves are used to control water flow, both the headers and the pump may be designed for lower flow rates because of diversity of load. A brief analysis of peak and coincidental off-peak loads (sun load is the major variable) indicate the magnitude of re ductions that can be made. Note, however, that no diversity should be used when an air bypass control is used on the room induction unit secondary coil. Flushing. It is desirable to provide a shut-off valve on both the supply and return run-oute between the riser and the in duction unit or group of units, to permit servicing. If *.h?a is done, it will be possible to drain a tingle unit or a separate group of units to service an induction unit coil. Flushing a new installation is imperative. Since the second ary coil is received in a clean condition, it is desirable to fluah the piping system without flushing through the coiL Accessories. Particular attention should be paid to Htign of air venting, dirt legs, expansion tank- shut-off, and drains. Secondary Water Pump Selection. Pumps should be selected for continuous operation. The flow through the secondary pump is subject to considerable reduction, because of the throttling action of valves at the units. It is desirable to select these pumps on a fiat part of the head vs flow curve, so ex cessive pressures do not result during throttling. In extensive systems where pump bead requirements are high, it may be necessary to relieve the excess pressure across the control valves during throttled conditions to avoid loss of control, noise, and chattering valves. A differential pressure controller may be used as a sensing device to operate a pump bypass or the main throttling valves. New piping results in less friction than indicated by current data. Hence, in a new installation the pump usually operates at greater than design flow and brake horsepower. Thus, in selecting a centrifugal pump, the addition of a safety factor to the calculated pump head increases the possibility of motor overload. It is recommended, therefore, that pump and pump motor selection be made from manufacturers' data, to insure that motor overload does not occur under any normal oper ating condition. - Condensate Drainsfrom Inductions Units. Condensate drain piping may be omitted with proper system designrTf the secondary watei;temperature is maintained at no more than 3 F deg below the room dew point, dry secondary coil operation results. Usually this can be accomplished by designing the the system with a 48 F apparatus dew point and 53 to 55 F secondary water temperature. If there is any question, a check of the final room dew point should be mad*. Office buildings where abnormal conditions seldom occur are