3 Refrigerant Fundamentals and Practices

Refrigerant pressure and temperature

A refrigerant absorbs heat as it evaporates and releases heat as it condenses. Its boiling temperature depends on pressure: for a given refrigerant, each saturation pressure corresponds to a saturation temperature. A pressure–temperature (P–T) chart provides the relationship between those values, and the chart or data must match the specific refrigerant in the system because refrigerants have different thermodynamic properties.

Reading saturation and operating conditions

In a two-phase system, liquid and vapor coexist, and pressure indicates the saturation temperature. For example, if the measured pressure corresponds to a saturation temperature of $40^\circ\mathrm{F}$ on the refrigerant’s P–T chart, the refrigerant is boiling or condensing near $40^\circ\mathrm{F}$ wherever both phases are in equilibrium. This does not necessarily mean that the pipe or the entire appliance is at that temperature.

Gauge pressure and absolute pressure use different reference points. Gauge pressure (psig) is measured relative to atmospheric pressure, while absolute pressure (psia) is measured from a perfect vacuum. Near sea level, psia is approximately psig plus $14.7\,\mathrm{psi}$, but atmospheric pressure varies with conditions and elevation.

P–T charts generally give saturation relationships; by themselves, they do not show system charge or diagnose a fault. Interpret pressure together with temperatures, operating conditions, airflow, and other measurements.

For superheated vapor, compare its actual temperature with the saturation temperature at the same pressure. For a subcooled liquid, compare its actual temperature with the saturation temperature at that pressure.

Zeotropic blends can change composition as they boil or condense, producing temperature glide. Use the blend’s dew-point value for saturated vapor and superheat calculations, and its bubble-point value for saturated liquid and subcooling calculations. Follow the refrigerant and equipment manufacturer’s guidance.

Recovering, recycling, and reclaiming

Recover, recycle, and reclaim describe different levels of refrigerant handling:

• Recover: Remove refrigerant from an appliance and store it in an external container. The refrigerant need not be tested or cleaned.

• Recycle: Clean recovered refrigerant for reuse, generally in equipment belonging to the same owner, without meeting reclamation specifications. Typical processes separate oil and use filter-driers to reduce moisture, acidity, and particles.

• Reclaim: Reprocess recovered refrigerant to applicable purity specifications and verify it through the prescribed analysis. EPA requires used refrigerant to be reclaimed by an EPA-certified reclaimer before it is sold to a new owner.

Recover refrigerant rather than intentionally releasing it. Use recovery equipment suited to the refrigerant and appliance, follow the equipment instructions, and put recovered refrigerant in an approved, correctly identified recovery cylinder. Do not mix refrigerants: mixed or unidentified refrigerant can be difficult to reuse and may require special handling. Prevent overfilling, keep cylinders within their rated limits, and use a scale to monitor cylinder contents.

Regulatory recovery evacuation levels

Recovery evacuation and system evacuation are separate procedures with different purposes. Recovery evacuation removes refrigerant to the applicable regulatory level before equipment is opened. System evacuation occurs after repair: a vacuum pump removes air and moisture from the opened circuit. Evacuation does not replace refrigerant recovery.

For appliances containing more than $5$ pounds of refrigerant, the following EPA levels apply to recovery equipment manufactured after November 15, 1993. The inches-of-mercury values are vacuum readings relative to standard atmospheric pressure, except that the low-pressure-appliance value is absolute pressure:

• Very-high-pressure appliance: $0$ in. Hg vacuum.

• High-pressure appliance or isolated component with a charge under $200$ lb: $0$ in. Hg vacuum.

• High-pressure appliance or isolated component with a charge of $200$ lb or more: $10$ in. Hg vacuum.

• Medium-pressure appliance or isolated component with a charge under $200$ lb: $10$ in. Hg vacuum.

• Medium-pressure appliance or isolated component with a charge of $200$ lb or more: $15$ in. Hg vacuum.

• Low-pressure appliance: $25$ mm Hg absolute.

Different levels apply when recovery equipment was manufactured before November 15, 1993. For small appliances containing $5$ pounds or less, EPA generally requires recovery of $80\%$ of the refrigerant if the compressor is not operating or qualifying pre-1993 equipment is used. The general requirement is $90\%$ if the compressor operates and qualifying post-1993 equipment is used. Evacuating the appliance to $4$ inches of mercury vacuum is an alternative. Follow the applicable EPA rule and equipment instructions.

Deep system evacuation

A deep vacuum is measured in microns of mercury ($\mu\mathrm{m\,Hg}$), an absolute-pressure unit. An electronic vacuum gauge is designed to measure this low pressure; an ordinary manifold gauge cannot reliably confirm a deep evacuation.

Pulling a vacuum lowers water’s boiling point so moisture can vaporize and be removed. Trapped moisture, leaks, restrictions, or inadequate equipment can prevent effective evacuation. Follow the equipment manufacturer’s specified target and verification procedure.

The key distinction is the goal and measurement: regulatory recovery evacuation removes refrigerant before an appliance is opened, while deep system evacuation after repair removes air and moisture. Use the correct refrigerant P–T data, distinguish gauge from absolute pressure, and handle recovered refrigerant in a suitable cylinder for recycling or reclamation as appropriate.