Document B86YYd8MKn6JabdXk7mKMEqKX

712 CHAPTER 44 1965 Guide And Data Book Component Balancing in Refrigeration Systems 713 Air Rva 3200 cfra. Sub-cooGags 10 F deg. Fig. 3 . . Condenser Performance Diagram been assumed for`both, the evaporator and condenser. If either or both of the air quantities are varied, the procedure can be repeated, with each variation finally resulting in a new similar to Fig. 6. These diagrams ran be com pared to determine the effects of air flow variations, as a guide to selection of optimum air flow rates for both evaporator and condenser. Heat Transfer Surface If desirable, performance diagrams such as Figs. 1 to 6 may be plotted for several different amounts of heat transfer sur face in the evaporator and condenser. This procedure is often useful in determining the optimum amount of heat transfer surface that should be provided. Hg. 2.........Compressor Performance Diagram The points of intersection of Knes on Fig. 5 are re-plotted, omitting the internal variables.of refrigerant.temperatures, to obtain Fig. 6, thus showing clearly, the effect of the two major external variables on the system capacity. . .: "By adding lines of constant evaporator refrigerant tempera-* tore, either Fig. 5 or Fig. 6'can be used todetermine th'eratio of sensible beat capacity to total capacity, from the known characteristics of the evaporator and various evaporator inlet dry-bulbair temperatures. --------- ^ - After the balance point has been determined for any given set of external variables, the values of the internal variables can be'determined from the individual diagrams of compo nent performance! From these values, other operating 'char acteristics, such as power input, refrigerant pressures, and refrigerant flow rates, can be determined.' Although tiie preparation of performance diagrams may be quite extensive:in the. case of-a large-pr complex.system, several simplifying assumptions usually are made to obtain a broad view of thesystem behavior. The resultant diagram can be used to determine which assumptions should'be re considered, aAd to indicate the procedure which should be followed for a more detailed study.' .^ EFFECTS OF OTHER VARiABlES Air Flow .1 ... It hold be noted that, in the development of- the graphical illustrated by Figs. 1 to 6, a constant air flow has, Pressure Drop The effects of pressure drop in the refrigerant piping have been neglected in order to amplify the discussion of the graphical 'method of component balancing. However, in actual design practice these effects must be considered. AP. though the effects of pressure drop on overall system re frigerating capacity may be relatively slight, they may have an important influence on the ability of the system to continue operating under extreme conditions. ' The pressure drop from the compressor to the condenser can be estimated for the refrigerant flow at approximately the design conditions. This pressure drop can be expressed as an equivalent temperature drop by use of the refrigerant satura tion tables in Chapter 20. ' If tiie' equivalent temperature drop were 2 F deg, for ex ample, Fig.'3 would not.be superimposed directly on Fig. 2,: but would be moved to the right by 2 F deg. Thus, when the saturation temperature corresponding to the condenser inlet pressure is 110 F, the saturation temperature corresponding to the compressor discharge pressure.is 112 F. ' A similar procedure is used to include the effect of pressure drop in the piping between the evaporator and the compressor. By:using 'the refrigerant flow at the design conditions'and knowing'the. length of connecting line, the pressure drop in the suction line can be estimated. This- pressure drop also can be expressed as an equivalent temperature drop. If this equivalent temperature drop were 1 F deg, for exam ple,Fig. 1 would not besuperimposed directly on to Fig. 5, but would-be moved to the left by 1 F deg. Thus, when the saturation temperature corresponding to the evaporator out let is 40 F, the' saturation temperature''corresponding to .the . compressor inlet pressure is 39 F. Fig. 6 .... Performance Diagram for Overall System ! and entering air dry-bulb temperatures for' dry operation. Superposing one diagram on the other will permit determina tion, for any..evaporator inlet conditions, as to whether or 'not there will be dry operation, or the proportion of sensible and latent capacity if there is to be wet operation. Refrigerant Flow Control Device Fig. 4------Combined Performance Diagram for Compressor and Condenser Other assumptions used in developing the graphical method were that the refrigerant flow control device would maintain the stated evaporator superheat and that the condenser sub- ' cooling would remain constant. These assumptions are acceptable, at least within a reasonable operating range, for a thermostatic expansion valve. ; If a capillary tube restrictor were used as the refrigerant flow control, it would be necessary to make other assumptions and perhaps to include a diagram of the performance of the : irestrictor in the superposition procedure. .. Other Considerations Dry Coil Operation In developing the graphical method, it was assumed that the evaporator capacity could be expressed as a function of the entering-air wet-bulb temperature. This aasnmptinp is permissible only if the evaporator is both cooling and de- humidifying the air (wetted-coil operation). If the coil is dry, the variable'used in plotting Fig. 1 is entering air dry-bulb temperature. ' If there is any doubt as to whether the evaporator' will be operating wet or dry under some conditions, it may be de sirable to prepare two evaporator performance diagrams; using entering' air wet-bulb temperatures`for wet operation Depending on the objectives of tire analysis, the perform-' ance diagrams may be prepared, nsing the refrigerant flow rate instead of the refrigerating capacity. - If the system uses water or brine as an intermediate fluid between.the refrigerant and the air or other. fluid being cooled, the performance diagrams must include the characteristics :of the additional heat exchangers involved. TROUBLE DIAGNOSIS The use of performance diagrams may often be helpful in diagnosing operating difficulties in a system. Comparison of. actual performance and actual values of internal, variables such as refrigerant temperatures and presures with: the values predicted by the diagrams, will often lead to an, ex planation of the difficulty. It may only be necessary to obtainan indication of the direction in which certain measurable, variables should move with changes in operating conditions!For' this purpose', free-hand sketches' often may be used,' without plotting the performance characteristics quanti tatively to scale. f*9- 5 .... Combined Performance Diagram for Evaporator, Compressor, and Condenser SYSTEM PERFORMANCE Although this chapter has been limited to a discussion of the hftlanHng of refrigeration system components, the basic haUnring process may be applied to other component systems or may be enlarged to include the characteristics of the load on the refrigeration system. An example of the use of the procedure in other component systems is the combination of f, motor, and system resist ance to air flow, as digrnggftd in Chapter 32. In sir-conditioning system design, superposition of the beat