Borrowing from a previous publication with equations for the pressure drop of two phase flow, a method of calculating critical flow was included in the article. All designers should become familiar with this because this is what causes reverse flow, such as happened at Millard Refrigerated and several other plants that experienced low temperature evaporators being exposed to the piping system, that inadvertently reduced pressure because of cold evaporators and causes critical reverse flow. This is from the article referenced below:
DESIGN AND SAFE OPERATION
AMMONIA REFRIGERATION SYSTEMS
(With Emphasis on Recirculation Pipelines)
By: Henry B. Bonar, II
Second Edition 02/15/1997
Originally Given At:
1996 RETA NATIONAL CONVENTION
VALLEY FORGE, PENNSYLVANIA
OCTOBER 24, 1996
The calculation for critical flow from the article is as follows.
P2 – Downstream (lower) pressure
P1 – Upstream (higher) pressure
5 Chaddock, Werner, Papachristou, “Pressure Drop in the Suction Lines of Refrigerant Recirculation Systems,” Paper presented at ASHRAE Annual Meeting, Nassau, Bahamas, June 25-29, 1972. RR
To read the rest of the articles from the Spring 2015 Refrigeration Review, click the links below.