Document 2jDxzQZ1pZMwN75Qkqa0gYY6R

650 CHAPTER 38 1965 Guide And Data Book fig. 11 .... Vector Diagrams Leaving Wheel of, a Centrifugal Compressor purge systems to -continually remove nancondensables and impurities from the system. The purge system draws a small flow of vapor from a quiet zone near the top of the condenser, where the noncondensable gases have been concentrated. The refrigerant' and any water vapors which may be present are condensed. Hie noncondensable gases are blown off through a relief valve, and any water accumulating in the purge is periodically drained'off, or is collected by a system of weirs. The condensed refrigerant is returned to the evaporator through a float valve. PERFORMANCE Hie centrifugal compressor is basically a relatively constant head) variable capacity machine. The work input per unit magq flow be derived as a direct function of C/jCbj -- t/jCi where V is the peripheral-velocity of the impeller and C,, is the tangential velocity of the gas. Subscript 1 is for the conditions entering the impeller, and subscript 2 is for the conditions leaving the impeller. In the simple case where no inlet prewhiri is applied,' C*i is zero, and the work input is ' Fig. ll chows the vector diagrams at the tip of two impel lers, (a) radial bladed and (b) with backward-curved blades. Note that because C*s is smaller for the backward-curved impeller, must be larger in order to achieve the same work input. Cma is the through-flow velocity and is approximately proportional to capacity. Thus, at reduced capacity, the dotted vector diagiWs will result. By inspection it can be seen that the value of C* changes only slightly with change in capacity in either case, but the change is more pronounced for the backward-curvedj for the radial impeller. At shutoff or zero flow, must be equal to U in either case. In Pig. 12, theoretical work input per unit mass flow is shown as a func tion of flow or capacity. Hie output of the compressor is a function of the work input and the losses. Fig. 13 shows input and output curves for the two impellers. Generally, the radial- bladed impeller has an output characteristic that is not as broad and flat as the backward-curved type, and surge tends to occur at a higher capacity. However, it should be noted that there is no definite line of demarcation between radial and backward-curved impellers, since any degree of backward curvature may be selected. The illustrated comparisons are between a radial and a highly backward-curved impeller in order to show the extremes. Another point to be noted is that the slope of the work input curve for the backward-curved impeller is steeper than that for the radial impeller. The result of this characteristic,-as shown in Fig. 13, is that the horsepower for the backward- curved impplW reaches a peak at a capacity somewhat higher than dftgjgrn capacity, whereas the horsepower of the radial- bladed impeller continues to rise because of the slight slope of. the work-input curve. This characteristic of the backward- curved impeller is important when electric motor, drive is to be used, since it is possible to use this characteristic tq prevent overloading of the motor. For the radial-bladed im peller, other wmiw of control must be used to prevent motor overload. This feature is not of particular significance for a turbine drive. All of the previous discussion has been based on the utiliza tion of constant speed drive and fired inlet prerotation. Control of capacity and head is normally obtained in this case by throttling (suction damper). However, since both head and capacity are affected by throttling, it is sometimes neccs1 sary to use a bypass in order to maintain head at very low loads. Other common means of control are the utilization of variable inlet guide vanes and variable speed. Any or all of these could also be used in combination. ' As far as the compressor is concerned, variable speed is a most efficient means of control. However, it has much the camp characteristic as'throttling and.cannot maintain high head at low flow. Also, the efficiency of the driver must be into account before any conclusions can be drawn con cerning system power consumption. Fig. 14 shows perform ance with variable speed. Note the significant reduction in horsepower with reduced capacity. Surge is'a characteristic of centrifugal compressors which occurs at reduced capacity. At this flow condition, a sudden breakdown of flow occurs, similar to stalling of an airplane wing. When this happens, the compressor can no longer main tain the condenser pressure and'a momentary reversal of flow occurs with an attendant lowering of condenser pressure. At lower condenser pressure, the compressor is again able to function, the gas flows in the normal direction, and at an in creased'rate, which again allows stable compressor flow. Condenser pressure is again built up, and the flow rate reduces until the stalling point is again reached, at which time the cycle repeats! Hie frequency of the cycle is determined by;, the flow characteristics of the entire refrigeration system.;. \ Compressors 651 A steady-surge may occur as often' as one or more timpq a second, or with several seconds between flow reversals. An iniermiUeni surge may sometimes be noted, with several minutes, between surges. . ' . ` Surging does not result in an appreciable loss of refrigera tion, or in loss of economy, nor is the mechanical function of the machine necessarily affected. The repeated flow reversals will, however, impose higher than, normal loads on thrust bearing surfaces. Refrigerant and bearing temperatures can sometimes build up to a dangerous point if a severe degree of surging is maintained for a long period of' time. Surge is detected primarily by a significant intermittent change in noise level of the.machine. Hie reduced capacity at which surge occurs varies widely, depending largely on'the inVpplW tip peripheral Mach number. It may be'as low as 50 percent of design capacity for low Mach number, and may increase to'ashigh'as 100 percent of capacity for very highrMach numbers.".' V !; Variable inlet guide vanes in use today perform a suctionthroitling function in combination with.a variation of the amount of prerotation imparted .to the incoming gas. Fig. 15 shows the' vector diagrams at the inlet to.`an impeller.' Hie design or peak efficiency occurs at that inlet flow velocity Cmi which results in a relative flow angle into the impeller such that the relative* vector W, lines up approximately with the leading edge of the impeller blades.'If inlet guide vanes are set so as to impart a swirl to the inlet gas in .the. direction of rotation,, the vector diagram (b) ` results, \dierr angle a, is obtained with'a'lower value of C*U in diagram (a). If the inlet guide vanes are set to impart a swirl Against rotation; vector diagram (e) results where Ci to obtains is larger. As'mentioned,'work done on ,the gas is proportional' to bj -- UiCuuwhere C in the direction'of rotation is positive/ Thus, prewhiri with the impeller rotation mil result in'some reduction of head as compared to that' for ho whir!,' and pre^ whirl'against rotation will result in some inriW-<a inrbead. Since the velocities at the impeller inlet are' low in comparison with the discharge velocities, the change in Hoad is not large even for fairly substantial'amounts of prewhiri. Fig. 10 shows the compressor characteristic at constant'speed for varying prewhiri Note that this method of control aIba effectively lowers horsepower with capacity, but hot as rapidly as with variable speed. However, the head is maintained quite well at lowered capacity, in comparison toJeither simple throttling or speed variation. If variable speed, and prewhirl are com bined, the compressor will-have this-liype of characteristic at each speed. PflCWHIRL fig. 15 .... Met Vector Diagrams for a Centrifugal Compressor with Met Guide Vanes CENTRIFUGAL REFRIGERATION CONTROLS Capacity Control In. operation,, centrifugal .refrigeration mnyhinA must accommodate two basic variables, refrigeration load and con denser water inlet temperature. These variables are impressed upon'the compressor,as flow and head, respectively. The ex tent and relationship of variations are functions of the application. The load can be relatively constant in one appli cation or may vary from zero to full load in Another applica tion. Condenser water temperature be virtually constant, vary with the season, or vary with the wetrbulb temperature, depending upon whether well river, lake, or cooling tower water is used. The head-flow characteristic required from the compressor for a typical water chilling application using cooling towers is shown in Fig. 17. Here, every conceivable means of reducing the head is desirable in order to attain the maximum ability to unload the compressor. Although the centrifugal compressor is basically a constant bead, variable flow device, there remains the problem. of matching the compressor output exactly to the load required. An early method of capacity control was to vary,the condenser water:flow so as to make the load curve coincide with .the compressor characteristic. This method limits the stable control range to the fixed speed characteristic of- the' com pressor. Other capacity control m^Ans, already described in tiie section on performance, are. variable speed, suction throttling, variable inlet guide vanes,' And hot gas bypass. Suction throttling modifies the compressor characteristic in a manner quite similar to that of variable speed, but at'some what lower efficiency. Fig. 14 indicates that these two methods will not fulfill the complete requirements of Fig. 17. Fig. 16,- the. characteristic for.variable inlet-guide vanes, illustrates that this method comes very close to fulfilling the complete fig. 16 .... Performance Characteristics of a Centrifugal Compressor-with'Variable Met Guide Vanes-