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Cold Rooms: Filter Driers and Sight Glasses

Date: 28 March 2017

When moisture enters a cold room’s refrigeration system, it interacts with the refrigerant and oil to create acid, which can corrode the copper piping and break down other components. Additionally, if debris also gets into the system, any moisture present will make the debris stick together; if enough debris builds up, it can slow down or even completely block the flow of refrigerant. If this happens, repairs will be needed in order for the cold room to once again operate efficiently. The best way to avoid this is by using a filter drier and a sight glass.

Filter drier
As their name suggests, filter driers absorb moisture and filter debris. Two of the more widespread filter drier technologies are loose desiccant with spun copper tubing and solid core molecular sieve with steel housing, though these two types provide very different kinds of protection against moisture, acid, and debris. Spun copper filter driers use loose desiccant beads enclosed in a thin copper casing tube and with a built-in wire mesh or fiberglass mat for solid particulate filtration, while solid core filter driers use an integrated process when the refrigerant passes through molecular sieve material and polyester felt mats. Spun copper technology uses small beads coated with a binder necessary to prevent the beads from disintegrating during daily operation. As a result, this coating slows down the moisture adsorption rate and makes the filter less efficient. Conversely, the solid core filter drier consists of a molecular sieve which retains the highest amount of water possible due to the strong bond between the molecular sieve and the water. The solid core selectively sorts molecules primarily based on a size exclusion and polarity process due to a dimensionally consistent molecular pore structure: the micro-pore opening is large enough for water molecules to enter, but not refrigerant molecules, meaning that water gets trapped by the sieve, while refrigerant passes through. Experiments have shown that the initial moisture content of solid core and the spun copper technology is significantly different. Solid core filter driers can be delivered with a moisture level up to three times lower than the spun copper. In some cases, the moisture content of the spun copper filter drier has been as high as 10%, compared to only about 2% for the solid core filter drier. In other words, solid core filter driers start with lower moisture content than spun copper filter driers, which can extend the lifetime and reduce the maintenance costs of the system.

Sight glass
While filter driers are great for preventing moisture from building up in the first place, sometimes moisture still slips in, often during maintenance. A sight glass is a great way to detect moisture present in any refrigeration system and an important part of any cold room. The sight glasses should be installed directly after the filter drier, ensuring that the filter drier is functioning properly. Refrigerant flows normally through the sight glass, which contains a small, glass-covered chamber. Here, there is an easy-to-see indicator that changes color when moisture levels build up too high and threaten the health of the system.
Due to the glass cover, it is possible to see the refrigerant as it flows through the system, providing an opportunity to watch for bubbles. Bubbles in the refrigerant may indicate a pressure drop after passing through the filter drier, which can usually be fixed by replacing the filter drier. Bubbles may also mean that not enough refrigerant is present—likely caused by a leak—which will the lower the efficiency of the cold room and may even be dangerous, depending on the refrigerant used. Finally, bubbles may be a signal that subcooling is not being achieved, which is a critical step in the refrigeration cycle. Being able to catch these problems early will keep the cold room operating at peak efficiency and prevent costly repairs later on down the line. So with two fairly inexpensive parts, a lot of damage can be avoided, saving money, both in terms of energy usage and repairs. Join us next time as we continue to explore the components that make up a cold room’s refrigeration system and how to choose the best ones for your needs.  

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