Refrigeration Piping Charging Residential AirConditioning R

11
drop is more critical and where separate risers from individual evaporators are not
possible, a double riser may be necessary to avoid an excessive loss of capacity.
A typical double riser has a small and large riser. The two should be sized so that the
total cross-sectional area is equivalent to the cross-section area of a single riser that
would have both satisfactory gas velocity and acceptable pressure drop at maximum
load conditions. The larger is trapped and the smaller line must be sized to provide
adequate velocities and acceptable pressure drop when the entire minimum load is
carried in the smaller riser.
Another method of suction oil return is the use of a double riser, as shown
in Figure 1. Oil return is accomplished with this method at minimum loads. In addition,
excessive pressure drop at full load is avoided. The small riser “A” is sized to return
oil under minimum capacity conditions. Riser “B” which, may be larger, is sized so
pressure drop through both risers during full load conditions is adequate. Traps with
minimum oil holding capacity are recommended.
11