DOUBLE RISERS

  On system equipped with capacity control compressor, or where tandem or multiple compressors are used with one or more compressors cycle off for capacity control, single suction line risers may result in either unacceptably high or low gas velocities. A line properly sized for light load conditions may have too high a pressure drop at maximum load, and if the lines is sized on the basis of full load conditions, then velocities may be adequate at light load conditions to move oil through the tubing. On air conditioning application where somewhat higher pressure drops at maximum load conditions can be tolerated without any major penalty in overall system performance, it is usually preferable to the additional pressure drop imposed by a single vertical riser. But on medium or low temperature application where pressure drop is more critical and where separate risers from individual evaporators are not desirable or possible, a double riser may be necessary to avoid on excessive loss of capacity.

  A typical double riser configuration is shown in figure. The two lines should be sized so that the total cross-sectional area is equivalent to the cross-sectional area of a single riser that would have both satisfactory gas velocity and acceptable pressure drop at maximum load conditions. The two lines normally are different in size, and the smaller lines must be sized to provide adequate velocities and acceptable pressure drop when the entire minimum load is carried in the smaller riser

  In operation, at maximum load conditions gas and entrained oil will be flowing through both riser. At minimum load conditions, the gas velocity will not be high enough to carry oil up both risers. The entrained oil will drop out of the refrigerant gas flow, and accumulate in the¨P¨ trap forming a liquid seal. This will force all of the flow up the smaller riser, thereby raising the velocity and assuring oil circulation through the system.