The following are the various methods of capacity control.
1. OFF AND ON CONTROL
2. HOT GASBY PASS CONTROL
3. CONDENSER, WATER REGULATION
4. BUTTERFLYS DAMPERS
5. SPEED CONTROL 1. Off and On Control A simple solution would be to have the machine operate at its maximum output until the load was stratified then shut itself off and automatically continue on and off operation. Even through the motor of a centrifugal compressor starts under load; starting and stopping a large motor of say five hundred horsepower six or eight an hour would be prohibitive satisfactory motor of life. 2. Hot Gas By Pass Control A bypass can be installed between the discharge and suction side of the compressor, when the load is reduced to almost the surge point, a value beings to open the by pass and permits high pressure vapor form the discharge side of the compressor to enter the evaporator, as the load is reduce further the bypass opens wider so as to maintain a fixed minimum volume flow rate to the compressor; Theoretically the state range is extended to zero load although in practice the compressor may become too hot - when the entire vapor is reticulated through the by pass. 3. Condenser Water Regulator A curve of heat vs. capacity for a constant impeller speed is quite flat for a centrifugal compressor. This means that a small in crease in load will be accomplished by a large decrease in capacity. One the means available to increase the discharge head is to increase the pressure in the condenser. This is easily accomplished by reducing the amount of condenser water.
To reduce the amount C condense water a thermostatic control is used that Senses the chilled water temperature change leaving the evaporator. A decrease in chilled water temperature signifies a drop in load and this information is used to reduce the amount of water being delivered to the condenser. As the water quantity supplied to the condenser drops the temperature of leaving condenser water will rise and cause the condenser pressure to rise, This method of capacity control is attractive if city water is purchased for condensing because the operating charge for condenses water is reduced under partial condensing because the operating charge for condenses water is reduced under partial load. There may be a disadvantage in using this control method for certain types of condensing water. With a rise in temperature certain dissolved compound referred to, as temporary hardness will deposition the side of the condenser tubes at an accelerated rate. This will materially reduce the heat transfer coefficient tubes walls and condensing water. Control of condenser water as a means of capacity control is not in general use because of high operating costs. The accelerated of the water side of the tubes increase maintenances cost because mare frequently cleaning is required. Power costs at high also as the increase in head pressure require more power for given load then other control system. The stable range of the compressor is artificially shorthand by raising head pressure, which is also undesirable. 4. Butterfly Damper A butterfly damper is a simple device for reducing load. If locate din the discharge line of the compressor it will not charge the surge point. On the other .hard if locate din the suction line as customary the surge point will be lowered. As been explained for a given set of operating conditions a certain minimum volume flow, through the impeller is required to prevent surging. With a butterfly damper at the suction of the compressor there is a cause the specific volume of refrigerant gas entering the impeller to increase.
This is reflected in an increase in the volume flow. This increase in volume flow means that the required minimum volume how to prevent Surging can be attained with the refrigerant of less mass or density. Since a lower mass flow rate means less refrigerant less refrigerant is vaporized in the compressor is actually operating at a lower load. The butterfly damper thus permits stable operating range down to about 40 to 50 percent full load capacity.
The butterfly damper is readily adapted to automatic control devices but it is not however a particularly economical means of capacity control. The decrease in horse power for a butter fly damper control system follows a falling load reasonable well but because of the resistance the damper introduces in the suction line the power required does not fall off as fast as the capacity is reduced. A brake horsepower of approximately 50 percent at a capacity of 40 percent is about the best that can be expected form a butterfly damper. 5. Speed Control Speed control is another means of regulating the capacity of a centrifugal compressor, this is based on reducing the impeller, speed to lower the volume rate of flow at the surge point there by extending the stable range of the compressor to a lower load, However the inherent characteristics of the compressor are such that the head drops quite rapidly with small changes in speed. In the usual air conditioning application load caries over such a wide range that the surge point is reached before a sufficient decrease in capacity is attained the head Vs capacity characteristic is such that until selected form performance curves with a high efficiency at full load may only have a 20 to 30 per cent capacity variation by means of speed control. If the original selection of the machine were poor, that is a larger machine operating at lower efficiency, then the variation of capacity with speed control might be much as 50 percent before surge. While this technique improves that first cost because if the larger compressor and drive and, higher operating cost because of the lower efficiency. Despite the above difficulties speed control of open centrifugal compressor is fairly common.
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