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216 CHAPTER 15 1959 Guide Cycle Y. A minimum amount of outdoor air (normally 25 to 50 percent) is admitted during the heating and ventilating stags This percentage is gradually increased to 100 percent, if needed, during the cooling and ventilating stage. Cycle Z. Except during the warm-up stage a variable amount of outdoor air is admitted as needed to maintain a fixed tem perature of the air entering the heating element. This is con trolled by the air-stream thermostat Which is set tow enough (often 557) to provide cooling when needed. Night Control . For maximum economy it is general practice to maintain the building at reduced temperature at night and over weekends and vacations. Several control arrangements to tuor>mpiih this are in common use. One uses the unit-as a convector at night. Another turns the fans on and off as in the case of a unit heater whenever the unit convective capacity is insufficient. Still another arrangement uses'the convective capacity of the unit supplemented by the con vective capacity of auxiliary equipment installed within the . room. More elaborate arrangements are available, permitting operation at daytime temperatures.in some spaces and low ered temperatures in others for maximum economy during night occupancy of part of the building. For general infor mation on controls, see Chapter 43. Location of Unit Ventilators The location of the unit is important. Wherever possible it should be placed, against the outside wall and near the centerline of the room. It is difficult to obtain proper airdistribution if the unit is installed either on an inside wall or in the corner of the room. Standard unit models- are set against the wail, or may be recessed into the wall; and dis charge the air stream upward. Ceiling models discharge the air horizontally. Air Exhaust Vents The location and size of exhaust vents' are not of partic ular importance on systems employing unit ventilators. However, many states have regulations which require vents in public buildings. When vents are used, it is important that they be sized and located properly. Vents should be fitted with maniiaj dampers so that the exhaust can be regulated for proper air distribution. Back-draft dampers should be installed in the vent: opening to prevent cold air from enter ing through the vent. It has become common practice, where regulations per mit, to exhaust into the corridor through louvers in the classroom door, with powered or gravity exhaust from the corridor to the outdoors. Thus, the slight pressurizing of the building by the unit ventilators provides the force to Thnri. the entire building. It is. common practice also to use the exhaust from the classroom as secondary ventilation by passing the exhaust air through wardrobes and lockers. Many state and local codes make this arrangement manda tory. Window Downdraft Prevention Increasing use of large window areas has presented a win dow downdraft problem caused by chilled air flowing down the windows and into the occupied areas of the room. Three - basic means of combatting these downdrafts have been de- veloped for use with unit ventilators. These are shown in Fig. 5 and operate as follows: Window riff heating employs finned radiation of moderate capacity installed along the wall under the window area. Heated air rises by convection upward and counteracts the downdraft by tempering it and diverting it upward to elimi- nate the cold draft. -. Window sill recirculation is obtained by placing the retiirn air intake along the window sill. Room or return air to the unit includes the cold downdrafts, takes them out of the occupied area of the room and thus eliminates theproblem. Window stU discharge directs a portion of the unit venti lator discharge air into a delivery duct along the sill of the window. The unit discharge air, delivered vertically at"the window sill, provides room-wide air distribution and com bats downdraft by upwardly-directed streams of air; **'` Unit Ventilators For Mechanical Cooling With increased use of school buildings during the summer months, some school building programs incorporate mechan ical cooling or provisions for including it in.the future. In either instance, the unit ventilators provide.the same func tion during the heating season as the unit ventilators previ ously discussed in this chapter, using hot water as the. heat ing medium. Chilled water is used to provide summer air fig. 5.... Three Methods of Preventing Window Downdraft Unit-Ventilators and Unit Heaters 217 conditioning. Units designed only for heating and ventilating Unit heaters are used principally for heating commercial are not suitable for use with chilled water. When provision and industrial structures such as garages, factories, ware for future mechanical cooling is made, the supply and re houses, showrooms, stores and laboratories, and, for heating turn piping, piping insulation, unit ventilators and controls corridors, lobbies, vestibules, and similar auxiliary spaces in are usually installed initially as required for complete air all types of buildings. conditioning. Water chillers, cooling towers, or evaporative . Unit heaters may be applied to a number of industrial condensers, and associated accessories are installed' later processes, such as drying and curing, in which the use of when mechanical cooling is required. heated air in rapid circulation with uniform distribution is UNIT HEATERS of particular advantage. They may be used for moisture ab sorption, such as fog removal in dye houses, or for the pre The term unit heater denotes an assembly of elements, the principal function of which is heating. The essential elements of a unit heater are a fan and motor, a heating element,1 an enclosure, and a directional outlet. Filters, dampers, duct collars, combustion chambers, and flues may also be in cluded. Some types are shown in Figs. 6 to 13. vention of condensation on ceilings or other cold surfaces of buildings in which process moisture is released. When such conditions are severe, it is necessary that the units be equipped to draw air from outdoors in sufficient volume to provide a rapid air change, and that they operate in con junction with exhaust ventilators or fans for exhausting the moisture-laden air. (See discussion of condensation in Chap ; Classification............ .. ter 10.) ' ] i The'various types*of unit heaters in present use can usu Factors Affecting Application jj ally be classified according to one or more of the following i methods: There are four major factors to be considered in the ap j ! ; 1. By heating medium: Under this classification there are five heating media to be considered: (o) steam, (6) hot water, (c) gas, (d) oil, (e) electric. ~2. By type of fan: Under this classification there are* two plication of unit heaters: (1) the heating medium to be employed, (2) the type of unit, (3) the location of the unit for proper heat distribution and (4) the permissible sound level. types of fans to be considered: (a) propeller and (b) centrif ugal. Propeller fan units. may be of the horizonlaUblow or Heating Medium down-blow type. Centrifugal fan units may be of the smaller cabinet type, or larger industrial type. Either - may be ar The selection of the proper heating medium is usually de > ranged for the delivery of air horizontally, or vertically up or down. 3. By arrangement of elements: Under this-classification there are two types of units to be considered: (a) the drato- termined on an economic basis, and requires an examination of the first cost, the operating cost, and the conditions of use. Steam and hot water unit heaters are relatively inexpensive, through type, in which'the fan draws air through, and (b) the but require a boiler plant and piping system, tbe per-unit blow-through type, in which the fan blows air through the heat cost of which generally decreases as the number of units in ing element. Direct-fired unit heaters are always of the blowthrough type. stalled increases. Gas- and oil-fired units are more costly, but the'fuel lines and vent flues required are comparatively Application inexpensive, and the overall insts.Hflt.irm cost will usually be lower where a small number of units is used. Electric units - Unit heaters have three principal characteristics, as dis tinguished from gravity heating units: (1) relatively large are relatively inexpensive to purchase and install, but are usually'associated with a high operating cost per Btu output heating capacities in compact casings, (2) the ability to pro unless electric rates are very favorable. ject heated air in a controlled manner over a considerable Steam or hot water units are most frequently used in new distance, and (3) a relatively low installed cost per Btu output. They are therefore usually employed in applications where the heating capacity requirements, or the physical volume of the heated space, or both, are larger than those installations in which the number of units is large enough to justify the expense of a new boiler and piping system, and in existing installations or additions in which the system is of sufficient capacity to handle the additional load. High- that can be. adequately or economically handled by gravity heating units. pressure steam or high-temperature hot water units are usu ally employed in larger installations or where a high tem Where the heating capacity requirement is large, the in perature medium is also required for - process work. stallation of a small number of unit heaters will usually Low-pressure steam and conventional hot water units are prove more economical than will the use of a multiplicity of usually installed in smaller installations and in those pri gravity heating units of the same total capacity. Where the marily concerned with comfort hating volume of the space is large, and especially where ceilings are high, unit-heaters can provide comfort through uniform heat distribution, and economy of operation by confining of Gas-fired and oil-fired unit heaters are usually preferred in installations in which the number of units required does not justify the expense of a new boiler and piping system, the heat to the occupied zone and eliminating stratification where the space or the time required to construct a new 'in a manner which is difficult to attain with gravity radiation. boiler plant is not available, or where individual metering Unit heaters located overhead permit 100 percent utiliza of the fuel supply is required, as in a shopping center. Gas- tion of floor space. They may often be used to advantage in fired units are usually of the horizontal propeller type or the specialized applications requiring spot heating, or intermit industrial centrifugal type. Oil-fired units are largely of the tent heating, such as the blanketing of outside doors in in dustrial plants, or in corridors and vestibules. Unit heaters industrial centrifugal type! Some codes limit the use of di rect-fired unit heaters in certain applications. are also employed where filtration of the heated air is re-`. . Electric unit heaters are used where low cost electric power quired, and they may be modified to provide ventilation where the introduction of outdoor air is required. is available, or for isolated locations, intermittent use, sup plementary heating or temporary service requirements. Typ- ' . IP' t hf