42
AGR 070A through 100A
Product Manual AGR-3
Extreme care should be exercised to assure that both piping and controls are suitable for the
application such that displaced oil is returned to the compressor moderately.
Note, too, that oil
loss to the system can be due to a hang up in the evaporator, as well as in the piping.
SUCTION LINES
McQuay recommends the use of ASHRAE for guidelines in sizing and routing piping with one
exception. See the current ASHRAE Handbook Refrigeration Edition, Chapter 2 for tables and
guidelines. The single exception is to the piping of direct expansion cooling coils located above the
compressors. In all cases, regardless of whether the equipment has pumpdown control or not, a trap in
the suction line equal to the height of the coil section is recommended. In its absence, upon a power
failure, all of the liquid in the coil will fall by gravity to the compressor below.
Suction line gas velocities may range between 900 and 4000 feet per minute. Consideration should be
given to the possibility of objectionable noise in or adjacent to occupied space. Where this is a
concern, gas velocities on the low side are recommended.
Routing must also take into account the requirement established in the latest ANSI/ASHRAE 15.
To size the suction line, determine:
a.
The maximum tons for the circuit
b.
The actual length in feet
c.
The equivalent length contributed by elbows, fittings, valves or other refrigerant specialties.
ASHRAE Tables 2-10, 11 & 12
d.
If a vertical riser exists including the trap at the coil, determine the minimum tons for the
circuit.
Add b and c above to obtain the total equivalent feet. Use ASHRAE Table 3 (for R22) or Table 4 (for
R134a). Suction line selections are based upon the pressure equivalent of a 2ºF loss per 100 equivalent
feet.
Select a line size that displays an equal or slightly larger tons than that determined in a) above.
To determine the actual line loss:
1.
Modify the table tons by the value in Note 4 of Table 3 or 4 for the design condensing
temperature.
2.
Use the formula in Note 3 to calculate the line loss in terms of the saturation temperature.
3.
Convert the saturation temperature loss calculated to a pressure drop equivalent using the
(Delta) listed in the table for the comparable delta temperature.
Caution
: Excessive pressure drop is undesirable because:
•
It reduces available compressor capacity.
•
It increases power consumed from the net tons realized.
•
It may affect the performance of both the evaporator and the expansion valve previously
selected for the application.
Note
: The line loss calculated, expressed in temperature, or PSIG pressure drop will be used to
establish the temperature required at the evaporator to produce the required cooling, as well as, the
suction pressure that the compressor must operate at to deliver the required capacity.
Having selected the suction line size, based upon total equivalent length and maximum tons, verify the
line size selected will maintain entrainment of the lubricating oil up any vertical risers at the minimum
tons for the circuit. See d above, and ASHRAE Table 2-13.
Note:
If the line size selected will not maintain satisfactory oil return in a suction riser, the following
options are available: