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Partitioning of noncondensable gases in refrigerant liquid and vapor phases.

Article Excerpt
INTRODUCTION

The entrapment of noncondensable gases (NCGs) in factory- or field-assembled air-conditioning and refrigeration systems is an issue most frequently ignored by many engineers and HVAC contracting organizations. The makeup of NCGs are widely varied and can range from factory air to dry nitrogen.

More recently, the rules to minimize NCGs are being violated as an excuse to improve productivity of a line item. The violation comes when manufacturers minimize evacuation times to improve production rates. Entrapment of air and water vapor is probably the worst case, whereas entrapment of nitrogen is less of a problem since nitrogen is sometimes used as a purge gas during manufacturing at some locations. The negative result of entrapped air during manufacturing was extensively studied in a selected contaminants project wherein 160 hermetic systems using both mineral oil and polyolester lubricants and both hydrochloroflurocarbon and hydrofluorocarbon (HFC) refrigerants were studied (Cavestri and Schafer 2000).

This research project has a much wider scope and eliminates the complications associated with lubricants and other system contaminants in studying the effects of NCGs on saturated gas pressures. The project does not take into account their chemical reactivity. Dissolved NCGs can create pressures that are not relevant to the physical properties of condensed, saturated gases. Often, systems with low levels of NCGs (mostly air) do not exhibit noticeable pressure temperature responses but do produce a NCG response due to oxidation and other oil breakdown byproducts when air is present.

This project utilizes a stirred equilibrium cell similar to the rocking cells described by Parmelee (1964) and Williams (1959), but at much lower temperatures. The phase equilibria for carbon dioxide, ethanol, and water (Lim et al. 1994); 1,1-Difluoroethane and acetylene (Lim et al. 1996); and vapor liquid equilibria of binary nitrogen and various halon substitutes (Lim and Kim 1997) studies were all conducted in a temperature-controlled air bath with a pumped circulation equilibrium cell and were sampled using a fixed volume loop.

This research uses a vertical constant-temperature liquid vapor cell that is highly agitated. Liquid and vapor are forced downward through the pump impeller to intimately mix NCGs and liquid. The pump then forces the mixture back to the top of the cell. The vertical cell is thermostated from 0[degrees]F to 130[degrees]F (-17.7[degrees]C to 54.4[degrees]C). Partial pressures of air and nitrogen are added up to 60 psig. The NCG content of both liquid and vapor samples of an agitated system is determined by assessing a minimum of two or three evaluations per test point.

Room temperature refrigerants are introduced as liquids into the cooled vertical cell (Figures 1 and 2) that has an approximate 0.25 L volume, nominally filled to 70%-80% full. The entire circulating...

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