Fast Evacuation Series Overview

Introduction

Evacuation is the most critical and time-consuming step when installing, repairing, and servicing refrigeration systems. Although vacuum is an exact science, it can sometimes be difficult to understand and explain. In this post, we will demystify the science behind vacuum, assisting you in performing quality evacuations faster than you previously thought possible.

Vacuum and Pressure

To start, it’s important to understand vacuum. At sea level, atmospheric pressure is 14.7 psia, or absolute, created by the gravitational effect on the earth's atmosphere. Atmospheric pressure will reduce as altitude increases. Sometimes, vacuum is described as a negative pressure, which isn't entirely true. Vacuum is still a positive pressure above zero psia, but we conceive it as negative because it is below atmospheric pressure.

Altitude vs. Pressure

Importance of Vacuum and Evacuation

Why is vacuum so important when servicing refrigeration systems? Refrigeration systems are very complex, using mechanical and electronic components to move a heat transferring medium (refrigerant) to achieve many different outcomes. Whether it is for comfort, cooling and heating process, food preservation, or another role, they all rely on the same process. Heat is absorbed on the low side of the system and rejected on the high side. Regardless of the intended outcome, this process's efficiency and the components' longevity rely on the initial installation and further servicing being carried out to the highest standards. The most critical process is evacuation. Evacuation of a refrigeration system is the degassing and dehydration of a hermetic system designed to operate with only refrigerant and compressor oil. When other elements exist in the system, it can no longer operate as designed, and inefficiencies and failure will ensue.

Evacuation: Degassing

The most critical process is evacuation. Evacuation of a refrigeration system is the degassing of a system. Degassing is the removal of the air that has entered the system during insulation or repairs. This air is considered to be a non-condensable. If not removed, it results in high head pressures and reduced unit efficiency. The reduced efficiency and additional strain on the components will result in excess energy consumption and premature failure. Degassing is relatively easy and happens in the early stages of the evacuation, but what's left behind can pose more of an issue.

Degassing and Dehydration

Moisture and Oil in the System

Moisture can be present in both a visible and invisible form. The air we breathe contains moisture. Depending on the relative humidity of the day, varying degrees of moisture will be left in the system after the degassing process has been completed. In more extreme cases, water may have entered the system during installation or service or due to the failure of refrigerant to water heat exchangers on either the high or low side of the system. Both refrigerant and compressor oil are hygroscopic, meaning moisture is absorbed quickly as refrigerant circulates through the system. The moisture will freeze in components, such as the expansion valve, causing blockages. It will also reduce the heat exchanger's capacity to absorb and reject heat, reducing system efficiency.

As the refrigerant enters the compressor, it carries moisture, which mixes with the compressor oil. This will negatively affect the oil, altering its viscosity and lubrication capabilities. This turns the oil into a sludge that can build up and block strainers, valves, capillary tubes, and other components.

Moisture Freezing on Components

Hydrolysis and System Failure

Hydrolysis (A chemical reaction in which a molecule of water breaks one or more chemical bonds) causes an acid to begin forming. This process accelerates when the acid is introduced to the heat of compression. Now, the acid is traveling through the system, causing corrosion and stripping copper from the components and tubing. The copper acid and sludge make their way back to the compressor, where copper plating on the crankshaft journals occurs, causing the compressor to draw more power. The motor will run hotter, and in the case of hermetic and semi-hermetic compressors, the acid will start eating away at the varnish on the motor windings. Now, the race is on to see how the unit will fail first. From a refrigerant leak caused by copper etching? or will the compressor fail mechanically or electronically? These issues are why the dehydration process of evacuation is so important.

Hydrolysis Stripping Copper

Evacuation: Dehydration

Dehydration is the removal of moisture from a refrigeration system. This process is why pulling a system into a vacuum is necessary as we pull a deeper and deeper vacuum. We're actually lowering the system's internal pressure, thus lowering the boiling point of water. The goal of a deep vacuum is to pull it deep enough that the boiling point inside the system is the same or lower than the ambient temperature outside of the system. The ambient temperature will then be hot enough to evaporate any liquid moisture, which will be evacuated through the pump. If there is a high level of moisture in the system it will freeze before it can be removed as vapor. Fortunately, ice will sublimate, which means it can change state from a solid directly to a vapor, increasing the vacuum in the system will further reduce the boiling point, aiding in this process.

Internal Boiling Point vs. Ambient Temperature

Conclusion

With a proper understanding of degassing and dehydration, you are ready to learn how to increase the speed and quality of your evacuation. Read more here.