Document mmjnMRL7eZKG5LvX8o87VYXwO

American Society of Heating and Ventilating Engineers Guide, 1937 meshed wire. Where outside air is used for ventilation, more complete filtering of dust particles is necessary and for this purpose, there are a ' large number of filters available on the market. Some of these filters are of the so-called throw-away type, constructed of inexpensive material so ; that when they become dirty or clogged they may be thrown away and replaced with new ones. All of these filtering methods are described in detail in Chapter 16. A number of air conditioning units are being offered for the relief of hay t fever sufferers and render relief in this disease both by maintaining proper; air conditions and by removing most of the air borne pollens from the ventilated air. Such filters, however, must be designed for this purpose? and must have an exceptionally high efficiency. The degree of filtration and its efficiency depends entirely upon the design of the unit or the unitary system. For high efficiency of air cleaning, large and expensive filtering area is required. Small filters mean either low efficiency of cleaning or else high resistance with increased fan power or decreased air capacity. Ventilation Inasmuch as air purity is one of the factors that constitute true air con ditioning, ventilation or the introduction of outside air is an essential part of any air conditioning unit or system. While a unit that recirculates all its air capacity is still considered an air conditioning unit, the better type system provides for the introduction of a certain proportion of outdoor air. In some instances one of several units may operate entirely on out side air, while in other cases only a portion of the air handled by the unit is drawn from out of doors. In such cases a damper is provided either in the unit or in the duct connections for controlling the proportion of outdoor air. Location of Aix Conditioning Units The characteristics of the conditioned space, the building construction, the type of system employed, the duct connections as well as the source of power, piping and refrigeration influence directly the location of air conditioning units. Primarily a unit is either of the portable or fixed type. The portable unit is usually a simple self-contained air conditioning or cooling unit either with or without ventilation, but includes the condensing unit. If the condensing unit is of the air-cooled type, the unit must be located adjacent to a window or other source of outside air. On the other hand, if it is of the water-cooled type, its location should be convenient to sources of water and drainage. Portable units are invariably located within the conditioned space. Non-portable units, or units of fixed.location may or may not be located within the occupied space. Naturally units that are located in such spaces must be built with decorative cases in order to harmonize with the surroundings. Such units are normally of comparatively small capacity varying from a fraction of a ton up to as much as five or six tons. Many conditioning units and particularly those of the larger sizes are located 260 13--Unit Heaters, Ventilators, Air Conditioning, Cooling Units ternally to the occupied and conditioned space and are connected thereto by means of delivery and return ducts. Such an arrangement 01nnits the location of the conditioning unit convenient to either the ^urces of refrigeration or outside air. It frequently permits the use of the hasement or of space less valuable than that on the level or floor of the occupied zone. Oftentimes the same type of unit may find application in exposed position for one job and in a concealed location for another. Thus it can be seen that it is not possible to define a unit merely on the basis of its location. Frequently conditioning units are built into the structure or into the architectural design of a room so that they are entirely concealed except for the discharge and return grilles or openings which are designed so as to correspond to the decorative scheme of the room. Air Distribution With portable units or units exposed within the conditioned space, the distribution is usually through grilles or louvres built entirely into the equipment. The discharge of the air from this unit, in general, should be upward immediately at the unit, with sufficient horizontal component to carry it to the most remote point in the room. Such a distribution permits the cool air to drop slowly over the entire zone and return to the inlet of the unit below the breathing line and along the floor. The location of doorways, air vents, and heat exposed walls should be carefully observed, as they have a marked effect on the direction of the air flow and on its uniformity of temperature. Velocities below the breathing line should be kept low, normally not over 40 to 70 fpm. Suchr velocities are usually best checked by a Kata thermometer rather than by means of an anemo meter. With the suspended type of unit, located within the conditioned space, sufficient outlet air velocity should be provided to give adequate induction and mixing with the room air thereby preventing the immediate dropping of the air stream and resulting objectionable cold drafts. This is one of the limitations to the size of such suspended units inasmuch as it is difficult to distribute from one point a large refrigeration tonnage without cold drafts. Where the units are located outside the conditioned space, air dis tribution is more frequently provided through multiple outlets located in ducts from the conditioning unit. The location of these outlets is quite critical and is influenced both by the building construction, economies of connections and by the distribution of load. Normally with a cooling system, it is desirable to blow the conditioned air toward the exposed walls or windows. This delivers the cool air to the point where it is most needed. These problems are discussed in considerable detail in Chapter 19. There are a wide variety of outlet types used, and most of these have fixed delivery characteristics, thus requiring careful consideration in their location. Some types of outlets are now available with adjustable vanes thereby permitting some alteration in the delivery of the air stream after installation. This frequently eliminates objectionable down drafts re sulting from the impingement of the air stream against posts, pillars, fighting fixtures, and beams. 261