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HEATING VENTILATING AIR CONDITIONING GUIDE 1943 6. A Pressure Type Unit is for use with one or more external elements which impose air resistance. 7. A Forced-Circulation Air Cooler is a factory encased assembly of elements by which heat is transferred from air to evaporating refrigerant3. UNIT AIR CONDITIONERS This equipment takes the form of an encased assembly including the apparatus necessary to perform either some or all of the functions of cooling, dehumidifying, filtering, ventilation, air circulation, heating, and humidifying. Control of air conditions is provided by manual switches, manual dampers, and automatic devices, in combination'. The controls are either mounted on the units, or in some suitable location in the conditioned area. See Chapter 34 for a discussion of controls. The various, conditioning elements and their functions, which produce the required effects on air, are discussed under separate headings. Heating Heating is usually accomplished by means of a heating coil in the unit, supplied with steam or hot water from an external source. Electric strip heaters may also be considered, where installation and operating costs justify their use. They are often used in special control applications. Reverse cycle heating as described in Chapter 25 has been developed as a feature in some unit air conditioning equipment, but this form of heating has not yet been generally adopted. Humidifying Adding moisture to the air involves the absorption of heat, by the humidifying water, for conversion to water vapor. Heat may be supplied by heating the humidifying water, or supplied from the air to be humidi fied. In the latter case the air may be warmed or the water finely divided to present a large evaporating surface to the existing, air. The source of heat may be-from electricity, steam or hot water coils, or from, a heat transfer surface as in a direct-fired unit. Occasionally vapor is added directly by means of steam jets but this is usually confined to industrial applications because of the presence of some odor from the steam. Methods and types of humidifying apparatus are dealt with in detail in Chapter 27. ' For. unit application, some of the available methods are the spray nozzle, or the atomizing nozzle, the impact-jet, the drip screen and the evaporating pan. The first method is usually employed where humidi fication on a large scale is desired, as-with large remote industrial units. Eliminator plates are necessary and the water supply may be recirculated with a pump or wasted to the drain. The drip screen, impact-jet, evapo rator pan or small atomizing jets are used when humidification is desired in the smaller remote and self-contained units. Wetted surfaces exposed to the air stream and utilizing the capillary action of water on porous substances such as fabrics and ceramics, are used for adding moisture to air. . Frequent cleaning or replacement is 'Defined in Proposed A .S.R.E. Standard Methods of Rating and Testing Forced-Circulation Air Coolers for Commercial and Industrial Refrigeration. . CHAPTER 23. UNIT AIR CONDITIONERS, UNIT AIR COOLERS, ATTIC FANS necessary to avoid closing of the pores and to maintain freedom from odors or growths on the humidifying element. Cooling and Dehumidifying These two functions of air conditioning are usually performed simul taneously, although both may be done separately. For example, air may be dehumidified or dehydrated without sensible cooling by the process of adsorption. Sensible cooling of air may be accomplished without dehu midifying by maintaining the cooling surface temperature above: the dew-point temperature of the air to be treated. Chapter 24 explains these fundamentals in detail. Unit equipment commonly utilizes heat transfer surface such as pipes or coils, through which a cooling medium as cold water or. a refrigerant, is circulated. Types and methods of generating cooling mediums are covered in Chapter 25. Brine or water sprays may be used if desirable, either separately or in combination with coil or pipe surface.' However, these methods find their best application in the larger, remote type units. Surface temperature and area of the coil or pipe, air volume and velocity, and spray temperature and volume are some of the controlling factors in unit air conditioner, design and application. Ice as a cooling medium is practical but is.seldom used in connection with units. Filtering Cleaning of outside or recirculated air discharged to. the conditioned area is one of the important functions of air conditioning, and air filters should be, included on all units which condition air for comfort. Pro tection is also afforded the cooling and heating coils as well as the con denser coil in the case of the air cooled type. Means of filtering may vary from the lint screen to the electrostatic " filter, with the degree of efficiency covering a wide range. Inexpensive. throwaway type filters lend themselves well to the compact design of the unit conditioner. Air cleaning devices form the subject of Chapter 29.: Ventilating : Provision for the introduction of outside air should be an essential part of unit conditioner design. Odors and air vitiation are avoided and better load control is possible when a positive means of introducing outside air is available. " ' On the small, air cooled room units, it has been found practical to con trol the air in such a way as to permit variation from all recirculated air. to 100 per cent outside air. An added feature is a dampening arrangement whereby it is possible to exhaust air from the room to remove smoke a.nd generally ventilate the space.. A code4 sponsored by a Joint Committee of the A.S.H.V.E. and the American Society of Refrigerating Engineers may be consulted, although it should be realized .that individual applications may often show a need for ventilation in excess of these minimum requirements. `Code of Minimum Requirements for Comfort Air Conditioning (A.S.H.V.E. Transactions, Vol. 44, 1938, p. 27). Reprints of this code are available at *0.10 a copy. 443