Handbook of air conditioning and refrigeration / Shan K

REFRIGERATION SYSTEMS: RECIPROCATING, ROTARY, SCROLL, AND SCREW 11.17
FIGURE 11.9 (a) Single riser, (b). double riser, and (c) additional oil trap.
velocity drop so that oil is no longer entrained in the large riser at part-load operation because
of cylinder unloading or on/off control, oil accumulates in the oil trap. This oil blocks the vapor
passage to the large riser. The vapor velocity in the small riser therefore increases to a velocity high
enough to carry the entrained oil to the compressor.
If the vertical riser has a rise greater than 25 ft (7.5 m), an additional oil trap for each 25-ft (7.5m)
rise is required. Thus, the upper and lower sections are drained separately, and oil leaves the
traps in quantities that will not damage the compressor.
Maximum Pressure Drop. In a suction line using HCFC-22 as the refrigerant, a pressure drop
equivalent to 2°F (1.1°C) change in saturation temperature between 40 and 50°F (4.4 and 10°C) is
about 3 psi (21 kPa). When HFC-134a is used as the refrigerant, a 2°F (1.1°C) equivalent is about 2
psi (14 kPa); and if ammonia R-717 is used as the refrigerant, a 1°F (0.56°C) equivalent is about
1.5 psi (7 kPa).
Suction Line Sizing Chart. Figure 11.10 shows the Q rl-L eq suction line sizing chart. The abscissa
of this chart is the refrigeration load Q rl that the evaporated refrigerant in the evaporator can provide
at a corresponding mass flow rate, in tons of refrigeration, at 40°F (4.4°C) suction and 105°F
(40.6°C) condensing temperature. The ordinate is the equivalent pipe length L eq, ft (m). Table 11.4
indicates the minimum refrigeration load in the suction riser so that the suction vapor velocity v suc
inside the copper tubing can still carry the oil.
Suction temperature is the saturation temperature of the refrigerant at the inlet of the compressor.
It is usually 2°F (1.1°C) lower than T x in the evaporator or the DX coil, depending on the pressure
drop of the suction line. When the suction temperature T suc � 40°F (4.4°C) and the condensing temperature
T con � 105°F (40.6°C), the corrected refrigeration load Q rl,c, in tons, for sizing the suction
line can be found by dividing Q rl by correction factors F suc,s and F suc,c, as listed in Table 11.1, that is,
Q rl,c �
Qrl Fsuc,sFsuc,c (11.2)