Last updated on May 25th, 2024 at 10:36 am
Introduction
Refrigeration systems play a crucial role in various industries, ranging from food storage to medical preservation. To ensure optimal performance, proper evacuation of these systems is vital. In this article, we will delve into the fundamentals of refrigeration system evacuation, exploring its importance, key steps, and potential pitfalls.
Why Is Evacuation Important?
The evacuation process removes non-condensable gases, moisture, and contaminants from the system. These substances, if left within the system, can negatively impact its performance, efficiency, and lifespan. The primary goal of evacuation is to achieve a vacuum within the refrigeration system, enabling it to function at its maximum potential.
Understanding Vacuum Levels
An Overview of vacuum levels
The system’s vacuum level is measured in microns, with a lower value indicating a higher level of vacuum. The widely accepted industry standard for refrigeration evacuation is below 500 microns. Achieving a deep vacuum minimizes the potential for refrigerant leaks, moisture condensation, and system contamination, thereby maximizing system efficiency.
The role of atmospheric pressure on the moisture content in a system
It is a commonly held misconception in the HVAC industry that a vacuum pump removes moisture in its liquid form from the system during the evacuation process. This simply isn’t true, as any moisture present in the system would first need to be converted to its vapour state before it could be removed. This then leads us to the primary role of evacuation in refrigeration systems, which is to lower the atmospheric pressure in the system, thereby lowering the boiling point of the liquid to below the ambient temperature. This allows the moisture to ‘boil off’ and then be removed by the vacuum pump in its vapour state. A system that has been evacuated down to a vacuum level of 500 microns, for example, would have a moisture boiling point of about -25 degrees Celsius or -13 degrees Fahrenheit. This would mean that most of the moisture in the system would have been boiled off and removed by the time the system was commissioned.
Hydrogen Co-Valent bonds and their effect on the evacuation process
If you put a drop of water on a table or similar surface and bring a page close enough to it, you will be able to draw the water droplet closer to the page edge without touching it. This is due to the covalent bond that exists between the two materials. These bonds draw their atoms together, and they then share electrons in the area around their respective nuclei. The strength of these bonds is a determining factor when it comes to the speed at which the evacuation process happens, as the water molecules would first have to be separated from the internal walls of the closed loop system before their state can be changed from a liquid to a vapour. For this reason, technicians often purge the system with dry nitrogen before evacuation in order to break these bonds and lower the moisture content in the system before commencing with evacuation.
Understanding triple evacuation
Triple evacuation is the process where the system is evacuated 3 times instead of a single deep vacuum, which is often used. A dry nitrogen holding charge is introduced to the system between evacuation runs, which helps absorb any traces of moisture that may still be present in the system. It should be mentioned, though, that in order for the nitrogen to absorb any significant amount of moisture from the system, it would need to be left there for a considerable amount of time, and even then, the amount of moisture being absorbed is negligable. The weakening of the covalent bonds between the moisture and the inner walls of the system is a much more efficient outcome of dry nitrogen triple evacuation.
A typical order of function for a triple evacuation is as follows :
- Evacuate the system down to 1000 microns
- Break the vacuum with a dry nitrogen charge of 5 PSIG for 5 minutes
- Evacuate the system down to a vacuum level of 500 microns
- Break the vacuum with a dry nitrogen charge of 5 PSIG for 5 minutes
- Evacuate the system down to a vacuum level of 200 microns
- Isolate the vacuum pump from the system and shut it off
- Perform a 15 minute vacuum decay test and make sure the vacuum level doesn’t rise above 500 microns
How to perform a vacuum decay test
A vacuum decay test is a method used to test a system for leaks by observing the vacuum level over a predetermined period of time. The test is performed by pulling the system down to a vacuum level of about 300 microns before isolating the vacuum pump from the system and shutting it off. The vacuum level is then observed over a 15-minute period, where an increase to above 500 microns indicates a failed test, but the test is considered successful if the system holds the vacuum level to below 500 microns. In the case of a failed vacuum decay test, the system should be pressure and leak tested with dry nitrogen.
How vacuum hose permeation effect evacuation levels
Every material on earth has a degree of permeation, which determines how quickly a medium can pass through it. The various hoses used for refigeration applications, for example, each have their own rate of permeation, with standard refrigeration hoses having a higher rate of permeation than vacuum-rated hoses. The standard hose set also only holds a vacuum level equal to its rating; hence, it is of utmost importance that you use a properly rated vacuum hose for your evacuation procedures. In a test conducted in our workshop, we found that standard refrigeration hoses could not hold a vacuum level below 1800 micron. This is because, even though the system was successfully pulled down to 100 micron, the hose then permeated at that low pressure until it equalised at the 1800 mark. Interestingly, the test was repeated numerous times over a period of a few days and the exact point at which it leveled out was exactly 1800 micron. This is obviously much higher than the industry standard of 500 microns for a successful vacuum decay test.
Pre-Evacuation Preparation
Before initiating the evacuation process, several preparatory steps should be taken:
- Gather necessary equipment: Use a quality vacuum pump that has been tested to go down to 50 microns, a micron gauge, a refrigerant recovery machine, large-diameter vacuum-rated hoses, and a vacuum-rated valve core removal tool.
- Confirm system integrity: Ensure all valves, service ports, and connections in the system are secure and free from leaks.
- Remove contaminants: Clean and nitrogen purge the system of any debris, such as oil or particles, to prevent obstruction during evacuation.
The Evacuation Process
Now that you have prepared the system, let’s explore the step-by-step evacuation process:
Mistakes to Avoid
During the evacuation process, it’s crucial to avoid common mistakes that could compromise the system’s performance:
- Insufficient evacuation time: Rushing through the process may result in incomplete removal of contaminants and non-condensable gases.
- Inaccurate pressure monitoring: Failing to monitor pressure levels with a reliable micron gauge may lead to an insufficient vacuum.
- Incorrect placement of micron gauge: Installing the micron gauge too close to the vacuum pump can result in an inaccurate measure of the system’s vacuum level.
- Improper equipment usage: Using subpar or mismatched evacuation equipment can result in less effective evacuation and potential system damage.
- Neglecting system integrity checks: Overlooking leaks and faulty connections can lead to compromised system performance.
Post-Evacuation Steps
Once the desired vacuum level is achieved, there are a few additional steps to complete:
- Isolate the system: Close the valves and disconnect the vacuum pump, ensuring no air or moisture enters the system.
- Perform a vacuum decay test: Perform a vacuum test to ensure that there are no leaks in the system. A vacuum level of 500 microns or lower is required for a successful test.
- Recharge the refrigerant: If a refrigerant recovery process was necessary, recharge the system with the appropriate refrigerant.
Conclusion
Proper evacuation of refrigeration systems is essential to ensuring optimal performance, efficiency, and component longevity. By following the fundamental steps outlined in this article, you can maintain a high vacuum level, free from contaminants and non-condensable gases. Attention to detail and adherence to industry guidelines will ultimately result in a reliable, efficient refrigeration system that meets your needs.
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