BY ARCHIE NALBANDIAN
As a senior field service engineer, I am regularly called upon to assist in troubleshooting a problem job. As I discuss the issues with the contractor or technician, I hear all too often those simple words, "Yes, I evacuated the system. I ran my vacuum pump for an hour. The system can't possibly have any moisture in it." Well, how long is long enough? We will attempt to answer the question, and illustrate that time is not as important as quality of the vacuum.
Clean Installation Prevents Trouble Later
A refrigeration system is designed to operate with oil and refrigerant flowing internally. Anything else is considered foreign to the system and must to be removed. This includes moisture, noncondensible gases (such as air), oxides from brazing without a nitrogen purge, field debris, etc.
Good piping and installation practices are the first steps in keeping a system clean during installation.
Working smart is the second step. You can do this by planning your scope of work in a manner that allows you to complete other tasks — such as system wiring or insulating piping — while the system is being evacuated properly. Good piping and installation practices will reduce evacuation time, extend the service life of the equipment, and reduce warranty-related costs.
Good piping and installation practices will actually reduce evacuation time, extend the service life of the equipment, and reduce warrantyrelated costs.
Evacuation and Dehydration
At sea level and 68F (normal temperature and pressure), one atmosphere exerts a pressure of 14.696 pounds per square inch (psi), or 29.921 in. Hg. (inches of mercury). At this pressure, a calibrated compound pressure gauge will read 0 pounds per square inch gauge (psig). This pressure is referred to as Standard Pressure and is equal to one atmosphere (psia).
At this pressure, pure water boils at 212F. If we were to move our container of pure water to an altitude higher than sea level (say Denver, CO at 5,000 feet above sea level) our same container of water will now boil at 203.4F since the atmosphere exerts less pressure (approximately 25 in. Hg, 12.278 psi or 0.835 psia). If we increase the pressure on our container of pure water to 15 psi (30 psia), the boiling point ofthe water will rise to 250F. This is known as the pressure–temperature relationship. The boiling point can be changed and controlled by adjusting the vapor pressure above the water. By lowering the vapor pressure into a vacuum, any water in the system will boil into vapor for removal by the vacuum pump.
There are two pressure scales on the face of the gauge. Above zero is the scale for measuring positive pressure, or pressure above one atmosphere, in pounds per square inch. The scale below zero is used to measure negative pressure, or pressure below one atmosphere, in inches of mercury. This scale is accurate up to 29 in. of mercury. Vacuum levels between 29 in. Hg and 30 in. Hg (absolute vacuum) are measured in microns. One thousand microns is equal to one millimeter of mercury and are measured with an electronic vacuum gauge.
If we reduce the internal pressure of a sealed refrigeration system to 29 in. Hg., we can say that we have evacuated the system. We have removed air and noncondensible gases. In doing so, we have lowered the boiling point of any remaining moisture in the system, but we have not necessarily dehydrated the system. To do this, we need to attach an electronic micron gauge to the system to measure the vacuum level between 29 to 30 in. of mercury.
The scale of the electronic gauge ranges from 0 to 5,000 microns. As we continue to extract vapor from the system with a vacuum pump, the decrease in pressure will register on the micron gauge. Conversely, if we were to stop extracting vapor and allow the system to remain static, no rise in pressure will be noted on the micron gauge provided the system is completely sealed and dehydrated. If moisture is present, a notable rise in pressure will be seen on the micron gauge as liquid moisture boils off into a vapor.
Quality of Equipment Counts
The vacuum pump must be in good operating condition, clean, and leak-free. You should change the oil prior to every use. Besides providing pump lubrication, the oil also provides an internal seal. Clean, fresh oil is a must. Follow the pump manufacturer's maintenance instructions. Use good quality hoses and be sure they're free of cuts and blemishes, and do not leak. If you plan on pulling a vacuum below 500 microns, consider using copper tubing instead of hoses whenever possible.
Never use a vacuum to test for leaks. You will not be able to find a leak under vacuum, and you will contaminate the system with moisture and noncondensible gases.
Pressure test
Most service trucks are equipped with vacuum pumps capable of pumping 5 or 6 cu.ft. per minute. On installations larger than 20 hp, you may need to use a larger capacity pump or more than one pump in order to work efficiently.
Pressure test the system to be evacuated with dry nitrogen and a trace charge of refrigerant. Be sure not to exceed the manufacturer's test pressure limits. (Most evaporators have a 150 PSI limit). Allow the system to stand under pressure. If no drop in pressure is noted over a 12-hour period, you can rest assured the system is sealed.
Never use a vacuum to test for leaks. You will not be able to find a leak under vacuum, and you will contaminate the system with moisture and noncondensible gases. Never pull a vacuum through a Schrader valve. This practice will only increase your evacuation time due to the high restriction it will cause. Always remove the Schrader valve and reinstall it when the system is under a slight positive pressure.
If you plan on running your vacuum pump overnight, it's highly recommended to use a full port solenoid valve inline with the vacuum line to the pump, so you won't lose the vacuum during a power failure.
Systems with high moisture contamination — such as water-cooled equipment or chillers that have suffered a tube failure — should be purged with dry nitrogen. This is done by connecting a nitrogen cylinder to the equipment in a manner that will permit a low volume purge to remove as much moisture as possible by evaporation. Drain the compressor oil and do not recharge with oil until the system is completely dry.
Drain as much water as possible, paying close attention to all the low points. One pound of water (approximately one pint) will produce2,948 cu.ft. of water vapor. Using a 5 cfm vacuum pump, it will take a considerable amount of time to boil off this much moisture. Consider using a higher capacity pump or add additional pumps to assist in the pulling out of the vapor.
A cold trap may be required to prevent contamination of the vacuum pump oil sump. As the pump pulls high volumes of water vapor out of the system these vapors can condense inside the pump eventually displacing the oil and ruining the pump. Moist air is drawn through the cold trap where the moisture will condense and freeze to the cylinder walls and baffles.
Apply as much heat as possible to the system, keeping it as uniform as possible. If it's cold outside, moisture could condense at the condensing unit if heat is only applied to the evaporator. You can prevent this by covering the condensing unit with a tarp and using a heat lamp to prevent condensation. Use caution, you don't want to start a fire.
Check the vacuum pump frequently. It may be necessary to change the vacuum pump oil several times due to moisture contamination even if a cold trap is used. Allow the vacuum pump to run overnight without supervision only after a visible inspection of the drained vacuum pump oil reveals little or no moisture.
The Time Factor
We discussed different methods of measuring a vacuum, issues related to actually pulling a vacuum and dehydrating an extremely wet system. But, have we run the vacuum equipment long enough?
The micron gauge will determine when sufficient time has elapsed. Systems that are designed to run at 10F to 45F saturated suction temperature should be evacuated down to 400 to 500 microns. Systems designed to operate between 20F to 10F should be pulled down lower. Follow the manufacturer's installation and operating instructions.
When using a micron gauge, always allow the system to sit at rest and watch for a rise in pressure. If the system has no leaks, the rise in pressure will indicate the presence of moisture boiling off, which will require more evacuation time. Only when there has been no rise in pressure can we say with a degree of certainty that the system is dry.
Archie Nalbandian is a factory field service engineer for Heatcraft Refrigeration Products. He can be reached at 770/465-5600.