Engine : Refrigeration AC - 897/610
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| The fluid used as a source of actuating power against the underside of the unloader power element piston of the refrigeration compressor capacity control mechanism illustrated is obtained from where? Illustration GS-RA-13 |
| A) discharge of the compressor lube oil pump |
| B) gas discharge from the compressor |
| C) high side liquid receiver |
| D) discharge of a secondary hydraulic pump specifically installed for this operation |
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| among01 - 2022-06-17 23:43:48 Registered (42) |
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| harpreets175@gmail.com - 2022-06-16 02:09:33 Registered (1) |
Capacity Control of refrigeration system Any refrigeration or air-conditioning system must be designed to handle the maximum refrigeration load anticipated. In refrigeration and Air conditioning system only during 10 to 15% of the operation time, this maximum load occurs. 85 to 90% of the time there is part load operation and less than about 50% of refrigeration capacity is needed for over 60% of the operational time. During operating periods of a multi-box refrigeration system using a capacity-controlled compressor, when all of the evaporators of a plant are actively being fed with liquid refrigerant, the control oil pressure acting on the hydraulic relay piston will be at the highest value. In the capacity control system, the fluid used as a source of actuating power against the underside of the unloader piston is obtained from the discharge of the compressor lube oil pump. cylinder unloading by holding suction valves open A common method of varying the capacity of multi-cylinder compressors is to vary the number of active cylinders by holding the suction valves open. The capacity control system unloads cylinders (that is, cuts cylinders out of operation) in response to changes in refrigeration load based on monitoring the suction pressure. Compressor lubricating oil is used to operate the valve lifting mechanism. Under high loads (high-suction pressures), oil is admitted to the actuating cylinders and the suction valves are allowed to operate normally. As the load decreases (and the suction pressure drops), the oil pressure is cut off to the actuating cylinder and the suction valves are held open, stopping the compression of the controlled cylinders. Typically, compressor cylinders are unloaded in pairs, or in banks, by having different set points on the hydraulic mechanism, thus maintaining an even load on the crankshaft. If the suction pressure continues to fall off after all controlled cylinders are cutout, the compressor will stop on the low pressure switch. Since oil pressure is required to load the cylinder, the compressor will start with all controlled cylinders unloaded, thus reducing the starting load on the compressor motor Cylinder unloader Cylinder unloader is a device used to hold open the refrigeration compressor suction valve during starting to reduce the compression load, working as a safety device, is also used for the purpose of capacity control. Most large reciprocating compressors (above 10 tons [35 kW]) are fitted with cylinder unloader that are used to match the compressor’s refrigerant-pumping capacity with the falling evaporator load, by progressively deactivating piston–cylinder pairs. The cylinder unloader shown in this example reciprocating compressor uses an electrically actuated unloader valve to close the suction passage to the cylinder that is being unloaded. In response to a decreasing load, an electronic controller sends a signal to open a solenoid valve. This solenoid valve diverts pressurized refrigerant vapor from the compressor discharge to the top of the unloader valve, causing the unloader valve to close and shut off the flow of refrigerant vapor into the cylinder. Even though the piston continues to travel back and forth inside this cylinder, it is no longer performing compression since it cannot take in any refrigerant vapor. In response to an increasing load, the controller sends a signal to close the solenoid valve. This closes the port that allows the pressurized refrigerant vapor to travel to the top of the unloader valve. A controlled leakage rate around the unloader valve relieves the pressure, allowing the valve to open and refrigerant vapor to once again flow to the cylinder to be compressed. Another type of cylinder unloader uses either pressure or electrically-actuated valving mechanisms to hold open the suction valve of the piston–cylinder pair. Since the suction valve is prevented from closing, no compression occurs in that cylinder and the discharge valve does not open. Still other types of cylinder unloaders divert the compressed refrigerant vapor back to the suction side of the compressor. Unloading devices, assisting in reducing the system load at the time of start, also acts like a safety device. |