Updated Guidelines; Liquid Lines - Trane BCHC Application Manual

Overview
Figure 1. Interconnecting refrigerant lines in a typical split air-conditioning system
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.
2
vapor or gas
line
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