Many vacuum pumps in the field are not working well enough to do their intended job. Vacuum pumps can easily be underappreciated. Their contribution to the reliability of air conditioning equipment is not easily observed. However, moisture in an air conditioning or heat pump system can wreak havoc.
Moisture enters a refrigeration system anytime it is opened up for repair, or when the low side of a system with a leak runs at a vacuum. Mixed with refrigerant, moisture creates acid that attacks motor windings and compressor bearing surfaces. It also mixes with compressor oil, creating a sludge that can restrict strainers and expansion valves and cause a reversing valve to stick.
Your vacuum pump must be working properly to prevent all of these malfunctions.
When using your vacuum pump to evacuate a system, some acid and moisture pulled from the system is trapped in the vacuum pump oil. This contamination has the same destructive effect on your vacuum pump as it does on refrigeration systems. This, in conjunction with inadequate maintenance, slowly degrades pump efficiency, rendering it increasingly ineffective.
There are so many ineffective vacuum pumps in the field today that your pump’s condition could make all the difference for a customer. If the system has been opened one or more times by others, it may be in terrible condition in terms of its moisture content.
If your pump is working properly when you service that system, you may be able to completely arrest any damage that was taking place prior to your arrival. If your pump is working poorly, you may not be able to stop the damage in the system — or worse, you may contribute to it.
If you aren’t sure that your pump is working properly, check its performance with an electronic vacuum gauge. If you don’t have one, check with your local parts suppliers. Often they have vacuum pump clinics for testing.
PUMP POINTERSBelow are some tips regarding vacuum pumps and moisture in systems.
Determining when you should have your vacuum pump rebuilt is somewhat arbitrary. Your pump must pull a vacuum deep enough to boil any moisture that may be in the system, but the boiling point of moisture changes with its temperature.
In 86 degree F weather, for example, your pump need only pump to 32,000 microns to begin boiling water (remove moisture from the system). In 21 degree weather, your pump must be able to reach 2,500 microns to begin to remove moisture. A lower micron reading means a deeper vacuum. Zero microns, if it were possible to create a pressure this low, would be a perfect vacuum.
A pump rebuilder I talked to recently said that if your pump will achieve a 50-micron vacuum, it’s in great shape. However, he was unable to give me a definite micron level at which a pump must be rebuilt.
The primary cause of poor pump performance is failure to change the oil on a regular basis.
Your manifold gauge set and hoses can prevent you from attaining adequate vacuum even if your pump is working properly. Your refrigerant hoses and manifold can appear not to leak when they are under pressure, but may leak when under a vacuum.
Vacuum through both the high and low sides of the system to minimize evacuation time.
Consider buying an electronic vacuum gauge for your own use. Using this tool during an evacuation is the only way to ensure an effective job. It tells you if you have removed all moisture from the system, as well as if you have a leak. It also speeds up your work by preventing you from leaving the pump on too long.
When using an electronic vacuum gauge, pump the system down to 500 microns. Valve off the system from your hoses and manifold and allow the system to sit for about 5 min:
Many technicians leave the old liquid-line drier in the system when they repair it. I have seen systems that were opened three or four times without replacing the drier.
When a drier is left in the system, if it does not become restricted, it eventually absorbs too much moisture and will come apart.
If you have ever worked on a system where a drier came apart, you know that it is one of the worst things that can happen to an air conditioner. The drier’s desiccant circulates through the system, lodging in all the refrigerant system components and causing myriad malfunctions.
Always replace the liquid-line drier when you open a system. The modest cost of a drier to the customer is a small price to pay for the reliability it provides.
Never add a liquid-line drier to a system without ensuring that any other liquid driers already in the system get removed. Two driers in series with each other will cause a partial liquid-line restriction and degrade the system’s performance in hot weather.
Howard Leonard is president of Total Tech HVACR Training, Phoenix, AZ. His firm specializes in training for service technicians. He can be reached at 602-943-2517.
Publication date: 06/17/2002