The saying “nature abhors a vacuum” is attributed to Aristotle who mused on the nature of voids in Book IV of his lectures on physics. Unlike Aristotle and Mother Nature, refrigeration technicians ought to have a healthy love of vacua because they offer security, safety and hassle-free operation.
The process of “pulling a vacuum” should be an essential step in commissioning or reinstating a refrigeration system whether it is after initial installation or some piece of intrusive service work such as changing a component or even cleaning a filter. By pumping air out of the system before the refrigerant is reintroduced several benefits can be gained, both for the commissioning engineers or service technicians and for those who will look after the plant in future.
Air (which would otherwise act as a non-condensable gas) that blankets the inside surface of the condenser and makes the system run inefficiently is removed before refrigerant is added to the system. This is so much easier than purging the non-condensable gas, which is tricky and at best only ever partially successful. When there is excessive air contamination in a system, particularly if it is on the low pressure side of the system, there is a danger of compression ignition of any organic refrigerant which contains hydrogen, even the supposedly non-flammable ones.
This is even more likely with A2L refrigerants—another good reason for paying attention to the vacuum.
Other contaminants which come in with the air, the most notable being moisture, are also removed by this process. Leaving moisture in the system can lead to acid formation with fluorocarbons and carbon dioxide but it can also freeze at the expansion orifice causing intermittent blockages and unreliable operation. If the system has been contaminated by free water—not uncommon in larger industrial systems where pressure vessels and heat exchangers have been hydraulically pressure tested—the removal of the water is a slow and laborious process. Pulling a vacuum, particularly while keeping the system relatively warm, allows the water to evaporate to steam and be removed from the system through the vacuum pump.
There are some handy tips to remember when working with vacuum pumps. Always use a connecting hose that is as large a diameter and as short as possible and connect it directly onto the system using the biggest connection you can. Connect the vacuum gauge onto a different part of the system which is as far from the vacuum connection as possible. Remember that the objective is to persuade individual air molecules that they want to head through the connecting valve and down the hose to the vacuum pump. Make it easy for them!
The pump itself must be kept in good condition if it is to be effective. It’s really a compressor, not a pump, with the discharge of the pump at atmospheric pressure. How good a vacuum can be pulled depends on how high a pressure ratio can be developed by the compressor. These are often two-stage rotary vane compressors and it is essential that the oil is kept in good, clean, dry condition. If the oil is contaminated with moisture then the moisture will tend to gas off as the pressure falls, preventing a proper vacuum from being achieved. Bear in mind that the oil acts as a seal between the vane and the housing, so good condition is essential. During the early stages of the process the gas ballast valve on the vacuum pump should be opened to allow some air from the atmosphere to bleed into the pump suction. This might seem like a crazy thing to do when trying to pump air from the system to atmosphere, like half-filling your bucket with seawater while trying to bail out your boat. However, the gas ballast valve allows most of the moisture to be drawn out of the system without pulling the pressure down to the point where water would condense into the oil.
Special thanks to Barrie Morgan of Hamilton, New Zealand, for suggesting this topic.