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SS-APG008-EN
Overview
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
pressure drop had the unintended consequence of requiring line sizes with large internal
refrigerant volumes. Since our objective is also to minimize the refrigerant charge to make the
most reliable systems, we increased the allowable liquid pressure drop to 35 psid (R-22), which
allows for the selection of a smaller liquid line while still maintaining refrigeration operation.
With R-410A refrigerant and POE oil, this pressure drop can be as high as 50 psid. Within these
guidelines, refrigeration operation is maintained while minimizing the refrigerant charge. It is
still required to limit the liquid line velocity to 600 ft/min to help avoid issues with water hammer.
Suction Lines
R-410A is a high-pressure refrigerant and allows higher-pressure drops in the suction lines. With
R-22, a 2°F loss in the suction line means a pressure drop of 3 psi. With R-410A refrigerant, that
same 2°F loss is a 5 psi drop. Additional pressure drop may be tolerated in certain applications.
R-410A refrigerant suction lines must be sized to maintain oil-entrainment velocities in both the
horizontal lines and vertical risers. Oil entrainment for R-410A is based on suction temperature as
well as tube diameter. At the time of this writing, no known direct oil-entrainment tests have been
published. Trane has used ASHRAE data to create equation-based formulas to predict the
entrainment velocities of R-410A refrigerant and POE oil. These minimum velocities are reflected
in the line sizes listed in the component selection summary tables (
, and
).
Summary of Contents for TTA240F Series
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