Document o8NBgLK36BLMdzag6dG70qNE

938 CHAPTER 65 1965 Guide And Data Book given capacity at a stated operating'condition can'usually be accomplished by a wide' variety of component combinations. Thus, a high capacity.compressor used with a m*ll evapora tor^and condenser may balance outsat the same capacity performance point as.an oversize.coil and condenser, linked toa low capacity compressor.' This balancing out ofthe evapo rator, condenser, and compressor is largely an economic choice toyield the lowest overall system cost compatible with other system performance ^requirements. "Although capacities will Rg. 8 .... Roof-Top Installation of Air-cooled SinglePackage Unit* be equal at the balance point, other performance items such as Btu per watt and sensible heat factor will be affected by the choice. For this reason, it is impossible to state the ca pacity rating of a compressor or coil without relating it to the operating condition limits imposed upon it by the other com ponents of the system. The following paragraphs are intended to serve as a general index of details which inust be considered in the design of unitary air conditioners. Comments are limited to general industry practice, with no attempt to explain in detail each of these design'considerations. Wherever possible, reference is made to other chapters in this book dealing with the various components of unitary air-conditioning equipment. Refrigeration System . The refrigeration' system in a unitary air conditioner includes the following: 1. Compressor. Compressors are usually of this reciprocating type, and most unitary equipment is now designed for use with compressors of either welded shell or-semi-hermetic construction. The use of open compressors with belt or direct drive is generally restricted to the largest units. For detailed discussion of compres sor design,,see Chapter 38. . . 2. Cooling Cod. Cooling coils are either direct-expansion type evaporators or chilled-water coils for use with a water chiller. For details on evaporator coil design, see Chapter 34. Zi'`Condenser. Condensers are of threetypes: water-cooled; air cooled, and evaporative-cooled. Design of condensers is dismissed in Chapter 42. v v.. Unitary Air Conditioners 939 Water-cooled condensers, although least expensive from a first cost standpoint, have rapidly given way to air-cooled condensers due to water shortages. Other advantages of air-cooled condensing include ability to operate at all seasons of the year without danger of frecze-up, and freedom from water treatment problems. Overall unit cost for evaporativo-cooled condensers is about equal to air-cooled condensers. If a cooling tower and circulating pump are used in conjunction with water-cooled unitary equipment, the relative first cost of ail three condenser types is approximately equal. ' Operating costs for water-cooled condensing equipment are generally lower than for air-cooled condensing. (See Btu per watt, ratioe in Table 1.) However, when a cooling tower is used with, water-cooled equipment, the relative operating costs are about equal. In some cases, depending upon geographic location, air cooled equipment may actually be less expensive to operate.* - Operating costs of evaporative condensing equipment are equal to, and can be less than, the cost of water-cooled units. 4. Expansion Device. Current design uses capillary tubes, con stant pressure valves and thermostatic expansion valves (ad justable or non-adjustable superheat). No specific expansion de vice is particularly favored, each being used as design atuation warrants (See Chapter 45 for details of flow control devices). 5. Refrigerants. Unitary equipment is currently designed, for' use of Refrigerants 12, 22, 500, and 502. . '. > 6. Water Chillers. Unitary air conditioners designed for use ~ with a remote chilled-water cooling coil must include a .water chiller. For design details, see Chapters 43 and 61. ; 7. Refrigerant Piping. Equipment in the category of factory- assembled refrigeration cycles includes complete factory-as sembled refrigerant piping. This permits the unit to be evacuated,' dehydrated and shipped from the factory with the correct re frigerant charge. The unit requires no further work on its re frigeration system at installation. This is also true of unitary air conditioners for use with remote water chillers, in which the refrigerant cycle ts completely factory-assembled and charged. The two remaining categories of unitary air conditioners are designed for use with ether remote condensers or remote con densing units. They require inter-connecting refrigerant piping installed at the job site. In split systems of this type, it is indus try practice to ship the low ride evacuated and dehydrated and with a gas holding charge sealed by means of caps.or quick coupling type connectors. .. The high side is likewise evacuated and dehydrated and usually shipped with the correct refrigerant charge. The inter-connecting refrigerant piping (liquid line and hot-gas line or suction line) must be installed, purged and leak-tested on the job by the in stalling contractor. Some of the smaller split systems are now available-withsealed flexible inter-connecting piping. Quick-coupling type, connectors are used with this system in order to eliminate the need for field-fabricated joints, purging, and leak-testing. Air-Handling System The air-handling system includes the followmg: 1. Propeller Fans. Because of their performance characteristics^ propeller fans are used for low-static pressure application. Hence,' most air-cooled condensers (not designed for use with duct work) are equipped with propeller fans. (Refer to Chapter 32-for design details on both propeller fans and centrifugal fans.) 2. Centrifugal Fans. Evaporator air systems, in general, and some condenser air systems, are designed for use with extensive duct work. Therefore, evaporator air fans are almost always of - the centrifugal type. Air-cooled condensers for use with duct work nhnnlH generally be used with centrifugal fans. Again, this choice is based on performance characteristics of housed fans. (See Chapter 32.) 3. Fan Motors. Fan motors used for unitary air conditioners' must be quiet, efficient, and dependable. Quiet operation is essential because the evaporator fan motor is generally located in' the circulating air stream and contributes directly to the sound level of the unit. This location also makes, efficiency im portant because all power to the fan motor reappears in the form of heat which must, in turn, be removed by the refrigeration' system. Motors must frequently be life-time.lubricated because1 of difficult or impossible service access after installation. (Refer to Chapter 57 for a discussion of the types of motors used for] fan duty purposes.) '; 4. Dvrect os. Belt Drive. Propeller fans are generally direct- driven up to speeds of 1500 rpm, depending upon size and sound' - -* Table 1 .... Engineering Design Details Rem Detail Roogo of Current Vdovs Cttamenf* 1 Evaporator Cfm/Ton 300-500 ARI certified capac am ity rating at -450 . 2 Air-Cooled Conden 600-1200 Median 900 ser Cfm/Ton 3 Evap. Cooled Con 200-500 denser Cfm/Ton 4 Btu/Watt--AirCooled Condenser 6-11 An efficiency rating expressed as the ra tio of net cooling ef fect to total power input. - 5-` Btu/Watt--i-WaterCooled and Evap. Cooled 9-15 ' An - efficiency 'rating expressed as the ra tio of net v>ling ef fect to total -power input. *6 Sensible Heat Factor 0.65-0.85 Ratio of sensible to total cooling effect at 80FDB-67FWB entering'air - ... 7 ;Maximum Operating As Specified See ARI 210-64 Conditions By ARI Section. 5, Sub-Sec tion 5.20 8 Low-Temp. Operat As Specified See ARI 210-64 ing Conditions By ARI , Section 5,. Sub-Sec tion 5.30 T External 8tatic ' Press. Evaporator As Specified See ARI 210-64 . By'ARI Section 4, Para. 4.18 10 Sound Level.- No Standard -Receiving increasing industry - attention. 'Restricted by local codes. level requirements. In the larger sizes (30-in. diameter and up) some propeller fans are belt driven to. reduce the speed below t-ha-t. of the driving-motor and meet sound-level requirements."* Centrifugal fans in the smaller sizes (up to 10 in.), are either direct-driven or belt-driven in accordance with sound-level and other application requirements. Centrifugal- fans-in'the huger sixes areLalmost exclusively belt driven... ' Adjustable pitch pulleys are usuallyprovided with belt-driven centrifugal-fans to permit balancing air delivery against system resistance!--v 5. Filters. Low-Velocity. filters (300 fpm) are still used in. many unitary air conditioners. However, the trend to reduced physical sue has led to the more, general use of high-velocity filters (400 fpm end-up) for more compact design. These are satisfactory'where'the filter is -specifically designed for the higher velocity service. Both disposable and cleanable types of filters are in use.'See Chapter 38.......... The filter-is always located on the inlet side of the evaporator coil to prevent dirt accumulation on the wetted coil-surface. Controls'System ... n. . ^The.'.controlfiTBystera includes safety devices, protective controls and capacity control devices. .... -.- >* "l.'Safety'Devices. Knee the unitary air conditioner is a pres sure'system,'protection from excessive pressures must1be pro vided. The several codes pertaining to this item differ,' depend ing upon the exact type of unit, its location in-the bmlding, and the size of the refrigerant charge. - * "See AmeriacnStandard Safety Codefor Mechanical Refrigeration (ASA B9.1-1964) and Underwriters'; Laboratories. Air. Con ditioners,.Central Cooling (UL No. 465).- -w ii.