Triple Point In Water and CO2 w/ Andrew G. and Matthew T.

Key takeaways

The triple point of water is a universally reliable constant used to calibrate temperature measurements, and HVAC technicians encounter this during every standard vacuum evacuation.
Ice formation during deep vacuum pulls is slow and requires adequate heat input; misunderstanding this can lead to false readings and incomplete evacuation.
Micron gauges measure thermal conductivity, not pressure, which can cause erratic readings if refrigerant is entrained or contaminants are present in the system.
A decay test is the most reliable method to confirm if a system is truly dry, as persistent micron readings indicate residual moisture or leaks.
The triple point of CO₂ occurs around 70°F and 75 PSI; rapid pressure drops or improper venting can cause dry ice plugs, especially in transcritical CO₂ systems.
Above the critical point, CO₂ behaves as neither liquid nor vapor, complicating system operation and requiring precise management of pressure and temperature.
Large hoses and fast vacuum pumps do not eliminate moisture problems; they can sometimes mask the presence of residual moisture, reinforcing the need for thorough procedures.
Emerging CO₂ systems, especially smaller transcritical units, are expanding the industry, but high-pressure tooling remains a challenge for service technicians.

Summary

Introduction and Episode Overview

The podcast begins with host Bryan Orr introducing the theme of the episode, which focuses on the phase change boundaries known as the triple point, specifically comparing water and CO₂. It highlights the importance of understanding these phenomena for HVAC and refrigeration technicians, especially when working with vacuum systems or transcritical CO₂ systems. The episode features discussions with experts Matthew Taylor, a supermarket refrigeration specialist focused on CO₂ systems, and Andrew Greaves, NAVAC's Director of User Experience, who brings insights into vacuum technology and metrology.

The Significance of the Triple Point of Water and Kelvin Definition

This section discusses the triple point of water at approximately 4,579 microns, correlating to just above 32°F, and its critical role in the definition of the Kelvin temperature scale. Andrew Greaves explains how the Kelvin was historically defined based on the triple point of water, making it a highly reproducible and universal constant. This constant is the basis for calibration and ensures consistency in temperature measurement worldwide. The discussion emphasizes how HVAC technicians encounter the triple point during standard vacuum evacuations, though they are often unaware of its significance.

Understanding the Triple Point of CO₂ and Its Practical Implications

Matthew Taylor explains that the triple point of CO₂ occurs around 70°F and 75 PSI, conditions that are easily encountered during service work. The triple point is crucial because if pressure is relieved too quickly in a system containing CO₂, dry ice forms instantly, creating blockages. The section details the behavior of CO₂ above and below the critical point at 87°F and 1,055 PSI, where the refrigerant becomes neither a true liquid nor vapor, complicating system operation. The importance of controlling pressure and temperature to avoid dry ice formation, especially in transcritical CO₂ systems, is emphasized.

Vacuum and Ice Formation: Practical Challenges in HVAC Service

This part covers how moisture and refrigerant oil can cause ice formation during deep vacuum pulls. Andrew explains that ice formation during evacuation is often misinterpreted and can persist if heat input is insufficient, leading to false readings on micron gauges. Techniques like nitrogen sweeping, turbulence creation, and even heating nitrogen are discussed as methods to improve vacuum quality and prevent false positives. The conversation illustrates that ice sublimation is a slow process needing adequate heat, and that large systems with extensive volume can delay total evacuation.

Metering, Measurement, and Recognizing Moisture or Refrigerant in Oil

This section clarifies how micron gauges work based on thermal conductivity rather than pressure, which can lead to erratic readings if refrigerant is entrained in oil or other contaminants. The importance of decay tests to distinguish between residual moisture, refrigerant in oil, and leaks is highlighted. It stresses that a continuous rise in micron readings indicates a leak, whereas stabilization after thorough evacuation suggests moisture removal. The group underscores that proper sweeping with nitrogen and understanding gauge limitations are vital for accurate diagnosis.

The Behavior of CO₂ at the Critical and Triple Points in Service Situations

Matthew describes the operational challenges of CO₂ systems, especially during venting or power outages. The danger of rapid pressure drops causing dry ice formation inside pipes is explained. The episode covers how CO₂ behaves as a supercritical fluid above 87°F and 1,055 PSI, where traditional PT charts don't work, and how to manage system pressures via high-pressure valves and flash tanks. The concept of 'burping' in pop-off valves during hot ambient conditions to prevent overpressure is discussed, along with innovations like battery-powered subcooling solutions for grocery stores.

Key Takeaways and Practical Lessons for HVAC Technicians

The episode concludes with crucial lessons: larger hoses and faster vacuum pumps don't eliminate moisture concerns and may create false confidence. The decay test remains the definitive way to verify dryness. Nitrogen sweeping serves multiple functions, including turbulence, moisture displacement, and temporarily raising system conditions above the triple point to assist moisture removal. Technicians must understand phase boundaries, system behaviors during evacuation, and the importance of proper diagnostic procedures, especially when working with emerging refrigerants like CO₂. The discussion also touches on the growth of CO₂-based systems and the tooling challenges that come with high-pressure refrigeration.

Show notes

In this episode, Bryan is joined by Matthew Taylor, a supermarket refrigeration specialist whose expertise centers on CO₂ rack systems, and Andrew Greaves, Director of User Experience at NAVAC Tools, for the third installment of their series on triple-point evacuation. The conversation expands beyond water to bring CO₂ into the picture, exploring where the two refrigerants behave similarly around the triple point and "more importantly" where the differences can create serious field problems. Whether you're pulling vacuums on residential equipment or servicing transcritical CO₂ racks at a grocery store, understanding what actually happens at these phase boundaries will change how you approach the work.

The triple point of water sits at 4,579 microns, corresponding to just above 32°F (273 Kelvin). Andrew shares a fascinating piece of metrology history: for decades, the Kelvin was formally defined as one two-hundred-and-seventy-third of the triple point of water, making it the most reproducible temperature constant ever established. In practical HVAC terms, this means that any time a technician hits industry-standard evacuation targets, they pass right through the triple point of water "every single time." The danger isn't the crossing itself but what happens when moisture is present in a cold ambient: the vacuum pump can create ice, and because sublimation requires enormous energy input from the surrounding environment, that ice can persist and deliver a falsely passing vacuum reading. Andrew explains why a decay test is the real proof of a dry system, since sublimating ice will continue to raise the micron reading after the pump is isolated. The conversation also covers how micron gauges actually work: they measure thermal conductivity, not pressure, which is why refrigerant vapor entrained in compressor oil can cause wild, erratic gauge behavior that mimics both moisture and a leak simultaneously.

For CO₂, the triple point arrives at roughly 70°F and 75 PSI "conditions that are easy to stumble into in the field." Matthew explains that in an operating transcritical CO₂ system, the triple point itself isn't the daily concern; the danger comes when a technician relieves pressure too quickly and liquid CO₂ flashes right through the triple point, instantly forming dry ice inside the lines and creating a hard plug. On the opposite end of the scale, CO₂ faces a critical point at just 87°F and 1,055 PSI, something that is essentially unreachable for most conventional refrigerants but is a routine operating reality in warm climates. Matthew walks through exactly what happens in a transcritical system: above the critical point, the CO₂ behaves as neither liquid nor vapor, the PT chart no longer applies, and a conventional condenser becomes a "gas cooler" that rejects heat but cannot condense the refrigerant. The refrigerant must be forced through a high-pressure valve to drop it back below the critical point, where it flashes instantly into liquid inside a flash tank. The episode also covers the "burping" behavior of CO₂ pop-off valves during high-ambient conditions, the dangers of isolating liquid CO₂, and how grocery stores have evolved from keeping spare CO₂ cylinders on hand to battery-powered fractional compressor systems that keep the flash tank subcritical through power outages.

The episode wraps up with takeaways that apply across both refrigerant worlds. Big hoses and fast pumps don't eliminate moisture problems; they can actually create a false-confidence scenario where ice forms, the system still pulls deep, and the technician believes the job is done. The decay test remains the only reliable proof of dryness, and nitrogen sweeps serve multiple purposes: adding turbulence to help release refrigerant from oil, displacing refrigerant from the micron gauge sensor, and temporarily raising the system back above the triple point so ice converts to liquid before being removed as vapor. The group also briefly touches on the emerging CO₂ carbon capture industry, where the triple point is used intentionally to separate and harvest CO₂ from industrial flue gases. The world of CO₂ service is growing fast, from niche grocery racks to mini-split-sized transcritical units, and the tooling needed to work on the high-pressure side hasn't fully caught up yet.

Topics Covered

Triple point of water: 4,579 microns / ~32°F / 273 Kelvin, and its historic role defining the Kelvin scale
Why technicians pass through the triple point of water during every standard evacuation
How ice forms during deep vacuum pulls and why sublimation is slow without adequate heat input
How micron gauges measure thermal conductivity, not pressure, and what causes erratic readings
Distinguishing moisture, refrigerant-in-oil, and actual leaks during a decay test
Triple point of CO₂: ~70°F / 75 PSI, and how dry ice forms when liquid CO₂ is vented through it
Why liquid CO₂ must never be isolated and the dangerous pressure implications if it is
The critical point of CO₂ at 87°F / 1,055 PSI and why transcritical operation is unavoidable in warm climates
How transcritical CO₂ systems work: gas coolers, high-pressure valves, and flash tanks
CO₂ system operating pressure ranges across the suction, liquid, and discharge sides
Pop-off valves and the "burping" phenomenon in transcritical CO₂ systems during high-ambient conditions
Grocery store power outage scenarios and battery-powered CO₂ subcooling solutions
The role of crankcase heaters when pulling a vacuum on systems with oil-logged refrigerant
Nitrogen sweeps: turbulence, sensor displacement, and temporary triple-point recovery
Heated nitrogen as an emerging technique for improving vacuum quality
CO₂ carbon capture: using the triple point industrially to harvest CO₂ from flue gases
The rapid growth of smaller transcritical CO₂ systems and the current gap in high-side service tooling

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