SS-APG008H-EN
5
Overview
Trane’s TTA and TWA, 6 – 25 ton condensing unit product
line (specific model numbers are listed on the cover) has
been designed for use only with R-410A and POE oil. R-
410A is a higher pressure refrigerant that requires the other
components of the system to be rated for the higher
pressures. For compressor lubrication, the refrigerant
requires POE oil.
Traditionally, refrigerant piping practices were guided by
four principles:
•
Return the oil to the compressor.
•
Maintain a column of liquid at the expansion valve.
•
Minimize the loss of capacity.
•
Minimize the refrigerant charge in the system.
These piping practices remain the same. However,
because of the different mass flows and pressures, the line
diameter required to carry the oil and refrigerant may not
be the same as a similar tonnage R-22 unit. Also, the
allowable pressure drop may be greater for R-410A than R-
22.
Evidence accumulated over years of observation
demonstrates that the lower the refrigerant charge, the
more reliably a split air-conditioning system performs. Any
amount of refrigerant in excess of the minimum design
charge becomes difficult to manage. The excess refrigerant
tends to collect in areas that can interfere with proper
operation and eventually shortens the service life of the
system. To successfully minimize the system refrigerant
charge, the correct line size should be used and the line
length must be kept to a minimum.
Background
In a split air-conditioning system, the four major
components of the refrigeration system are connected by
field-assembled refrigerant piping. A vapor or gas line
connects the evaporator to the compressor, the discharge
line connects the compressor to the condenser, and the
liquid line connects the condenser to the expansion device,
which is located near the evaporator inlet. Operational
problems can occur if these refrigerant lines are designed
or installed improperly.
Figure 1.
Interconnecting refrigerant lines in a typical
split air-conditioning system
vapor or gas
line
The origin of the requirements for equivalent line lengths of
components, line pressure drop, and minimum and
maximum refrigerant velocities is uncertain. It appears
likely that at least some of the supporting data was derived
from measurements and/or equations involving water.
Some resource materials even show water components
when illustrating refrigerant piping requirements.
Subsequent reviews of analytical and empirical data for
refrigerant piping resulted in the publication of two research
papers:
Pressure Losses in Tubing, Pipe, and Fittings
by R.
J.S. Pigott and
Refrigerant Piping Systems — Refrigerants
12, 22, 500
by the American Society of Refrigeration
Engineers (ASRE). In his paper, Pigott described his use of
refrigerant as the fluid and his direct measurement of
pressure drops. His findings indicated that the pressure
drop of many line components is small and difficult to
measure. For these components, he used experimental
data to derive a formula relating the geometry of the
component to its pressure drop. Overall, his calculated
pressure loss of the components was less than originally
determined.
The conclusion of the ASRE research paper stated that the
minimum required velocity to maintain oil entrainment in
vertical risers and horizontal lines will vary with the
diameter of the tube and with the saturation temperature of
the suction gas. In other words, the minimum required
velocity for oil entrainment is not constant.
Updated Guidelines
Liquid Lines
Historically, liquid lines were sized to minimize the pressure
losses within the piping circuit. Oil movement through the
piping wasn’t a concern (nor is it today) because oil is
miscible in liquid refrigerant at normal liquid-line
temperatures. The historic and traditional 6 psid liquid line
