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Figure  6-6.Unloader  mechanism  in  unloaded  position.
Evacuating the Compressor

Engineman 1 & C - Advanced engine mechanics training manual
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Chapter  6—REFRIGERATION  AND  AIR  CONDITIONING manufacturer’s technical manual or the directions enclosed  with  every  new  seal. CAPACITY  CONTROL Controlling the capacity of the compressor is accomplished  by  unloading  and  loading  the cylinders. This is a very desirable design feature of the unit, because if the compressor is to be started under a load (all cylinders are working), there is a much greater amount of torque required and it is necessary to have a much larger drive motor. Also, if the compressor is running at a con- stant  capacity  or  output,  it  will  reach  the  low temperature or pressure limits and will be con- stantly  starting  and  stopping,  thereby  putting  ex- cessive work on the unit. Unloading of the cylinders in the compressor is  accomplished  by  lifting  the  suction  valves  off their seats and holding them open. This method of capacity control unloads the cylinders com- pletely and allows the compressor to work at as little as 25% of its rated capacity. Unloader Mechanism When the compressor is not in operation, the unloader  power  element  mechanism,  which  is operated by oil pressure from the capacity con- trol valve, is in the unloaded position (figure 6-6). The  unloader  spring  pushes  against  the  unloader piston. This action moves the unloader rod to the left, thereby rotating the cam rings. As the cam rings are rotated, the lifting pins are forced up- ward, raising the suction valve off its seat. The suction valve is held in this position until the com- pressor  is  started  and  oil  pressure  of  approxi- mately 30 psi is reached. At this time, the oil pressure from the capacity control valve pushes the unloader piston back to the right against the unloader  spring.  The  motion  transmitted  through the push rod rotates the cam ring, thus lowering the lifting pins and allowing the suction valve to close  or  operate  normally  and  the  cylinder  to become loaded (figure 6-7). On most compressors unloaders are connected to the cylinders in pairs. Figure  6-7.—Unloader  mechanism  in  loaded  position. 6-5







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