Product Manual AGR-3
AGR 070A through 100A
43
The vertical length can be sized smaller to accommodate the lower circuit tons at reduced load.
Minimum compressor capacity can be increased by eliminating the lowest step of compressor capacity.
Hot gas bypass can be introduced at the distributor to the evaporator, increasing the volume of gas
available in the suction line to entrain the oil.
An oil separator may be installed in the discharge line.
With reciprocating compressor units only, and only as a last resort, double suction risers can be utilized.
Since a double suction riser works by providing an oil trap to assist the return of some oil, with
refrigerant, up the smaller diameter line,
a)
the trap must be as small as possible,
b)
there must not be multiple traps, and
c)
whenever double risers are used in a suction line, a suction accumulator with a controlled oil
return must be installed in the line ahead of the compressor.
In horizontal refrigerant gas lines, oil return to compressors is aided by sizing lines at a velocity above
the minimum recommended and pitching the lines in the direction of refrigerant flow.
SIZING A DOUBLE RISER
At maximum circuit tons, the line size should be selected from the table based upon the recommended
maximum line loss.
With the minimum tons known, a smaller line size should be selected from ASHRAE Chapter 2, Table
13 or 14 capable of entraining oil at the reduced tons. The smaller sized line should be the one installed
to always be active.
The net internal area of this smaller sized line (see Table 13 or 14) should be deducted from the area of
the size selected in paragraph 1) immediately above. The remainder represents the area of the other
riser. From Table 13 or 14, select a line size with an area equal, or close, to the calculated net area.
The combination of these two risers will provide the required performance at full circuit tons. The line
selected for the minimum load should always be active; and both lines should enter the overhead
horizontal line in a manner to prevent spillage of oil back down the other riser.
LONG VERTICAL RISER INSTALLATION
Where job conditions require refrigerant gas lifts of more than 25 feet, McQuay recommends the
installation of a short trap half-way up the riser or at not more than 20 feet intervals. These traps are
required to capture and hold small quantities of oil during off cycles.
Liquid Lines
Liquid lines are generally sized for 1 to 2 degrees F line losses or their equivalent in pressure drop.
Actual selection can vary based upon the pressure drop expected from refrigerant specialties such as
solenoids, refrigerant driers, valves, etc. piping lifts or risers and the amount of condenser sub-cooling
expected.
The principal concern in sizing and routing liquid lines is assurance that liquid is present in the line at
start-up of the compressor, and that liquid, not vapor, is available at the inlet to the expansion valve
during system operation.
Liquid may not be available in a liquid line at start-up if:
1.
The solenoid valve is located adjacent to the condenser or condensing unit; remote from the
expansion valve.
2.
An excessive length of liquid line is located in a heated ambient and the application permits
migration of the refrigerant to a cold air cooled condenser.
3.
Liquid refrigerant is permitted to gravitate from the liquid line to the condenser because of the
relative location of components.