Document 4vOoEJeM8vEza80VB9XGzQXXV

226 CHAPTER 15 1960 Guide Cycle Y. A minimum amount of outdoor air (normally 25 to 50 percent) is admitted during the heating and ventilating stage. 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 low enough (often 55 F) 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 accomplish 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 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 ma-rimum 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 air distribution if the unit is installed either on an inside wall or in the comer of the room. Standard unit models are set against the wall, 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 property. Vents should be fitted with manual 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 exhaust 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 riU 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 till recirculation, is obtained by placing the return air intake along the window all. Room or return air to the unit includes the cold downdrafts, takes them out of the occupied area of the room and thus eliminates the problem. Window sill discharge directs a portion of the unit venti lator discharge air into a delivery duct along the sill of the window. Hie unit discharge air, delivered vertically at the window all, 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 conditioning. Units designed only for beating and ventilating are not suitable for use with chilled water. When provision for future mechanical cooling is made, the supply and re turn piping, piping insulation, unit ventilators and controls are usually installed initially as required for complete air conditioning. Water chillers, cooling towers, or evaporative condensers, and associated accessories are installed later when cooling is required. UNIT HEATERS 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, 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. Gassifkation The various types of unit heaters in present use can usu ally be classified according to one or more of the following methods: 1. By heating medium: Under this classification there are five heating media to be considered: (a) steam, (6) hot water, (c) gas, (d) oil, (e) electric. 2. By type of fan: Under this classification there are two types of fans to be considered: (a) pro-oeUer and (b) centrif ugal. Propeller fan units may be of the horizontal-blow or down-blow type. Centrifugal fan units may be of the smaller cabinet type, or larger industrial type. Either may be ar 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 drawthrough type, tn which the fan draws air through, and (b) the blow-through type, in which the fan blows air through the heat ing element. Direct-fired unit heaters are always of the blow: through type. Application Unit heaters have three principal characteristics, as dis tinguished from gravity heating units: (1) relatively large heating capacities in compact casings, (2) the ability to pro ject heated air in a controlled manner over a considerable 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 that can be adequately or economically handled by gravity heating units. Where the heating capacity requirement is large, the in stallation of a small number of unit heaters will' usually prove more economical than will the use of a multiplicity of gravity heating units of the same total capacity. Where the 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 the heat to the occupied zone and eliminating stratification in a manner which is difficult to attain with gravity radiation. Unit heaters located overhead permit 100 percent utiliza tion of floor space. They may often be used to advantage in specialized applications requiring spot heating, or intermit tent heating, such as the blanketing of outside doors in in dustrial plants, or in corridors and vestibules. Unit heaters are also employed where filtration of the heated sir is re quired, and they may be modified to provide ventilation ' where the introduction of outdoor air is required. 227 Unit heaters are used principally for heating commercial and industrial structures such as garages, factories, ware houses, showrooms, stores and laboratories, and, for heating corridors, lobbies, vestibules, and similar auxiliary spaces in all types of buildings. Unit heaters may be applied to a number of industrial processes, such as drying and curing, in which the use of heated air in rapid circulation with uniform distribution is of particular advantage. They may be used for moisture ab sorption, such as fog removal in dye houses, or for the pre 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 ter 10.) Factors Affecting Application There are four major factors to be considered in the ap 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. Hearing Medium The selection of the proper heating medium is usually de 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, but require a boiler plant and piping-system, the per-unit cost of which generally decreases as the number of units in stalled increases. Gas- and oil-fired units are more costly, but the fuel lines and vent flues required are comparatively inexpensive, and the overall installation cost will usually be lower where a small number of units is used. Electric units are relatively inexpensive to purchase and install, but are usually associated with a high operating cost per Btu output unless electric rates are very favorable. Steam or hot water units are most frequently used in new installations in which the number of units is large enough to justify the expense of a new boiler and piping system, and in gristing installations or additions in which the system is of sufficient capacity to handle the additional load. Highpressure steam or high-temperature hot water units are usu ally employed in larger installations or where a high tem perature medium is also required for process work. Low-pressure steam and conventional hot water units are usually installed in smaller installations and in those pri marily concerned with comfort heating. 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, where the space or the time required to construct a new boiler plant is not available, or where individual metering of the fuel supply is required, as in a shopping center. Gasfired units are usually of the horizontal propeller type or the industrial centrifugal type. Oil-fired units are largely of the industrial centrifugal type. Some codes limit the use of di rect-fired unit heaters in certain applications. Electric unit heaters are used where low cost electric power is available, or for isolated locations, intermittent use, sup plementary heating or temporary service requirements. Typ- ,