Introduction to Critical Refrigeration Components
Two of the most common and important Refrigeration and AC System Accessories are Driers and Sight Glasses. We will cover Driers first, and Sight Glasses later in the article. These components play crucial roles in the overall refrigeration cycle, which you can learn more about in this detailed explanation.
Understanding Filter-Driers
“Filter-Driers” are more commonly referred to as just “Driers” as their trade name. Filter-Driers get their name from encompassing both a Filter for grabbing small particles and debris, and a Drier, which is the Desiccant element (loves moisture).
Filter-Driers are most commonly installed in the Liquid Line. This allows the moisture absorbing element to catch any H₂O in its liquid state, as in Low-Temperature Systems, where it could be frozen in the Suction Line. This location also affords proximity to the Metering Device inlet, allowing the Filter element to catch any particles before a main system component, which may be prone to blocking/clogging.
Filter-Driers are replaceable, and this is a common Service Practice. Reasons for this may include that the system has been opened for major repair work, or that the Filter-Drier has absorbed moisture and/or debris. For the latter, the Pressure Drop or Temperature Change can be measured across a Filter-Drier to confirm this condition.
Filter-Driers are directional components which are marked with an arrow for flow as seen in the first two images. Filter-Driers come in Brazed, Flared, and other connection styles, and they can be sized for selection from Equipment Selection documents like this one from Sporlan.
Construction Types of Filter-Driers
“Throw-Away” Style
The two above images use this Construction Style. The Filter and Desiccant are contained in a welded vessel that comes in various lengths and sizes. These are the most popular due to their cost and simplicity – they’re easy to source, and the Drier is fully disposed of once used.
The Refrigerant connections come in various sizes, and this type can be Brazed, Flared, or other newer (to North America) connection styles such as “ZoomLock”, referenced later. Their application is mainly Domestic, or Light/Heavy Commercial.
When brazing a Filter-Drier of this Construction Style, it is important not to overheat the internal components. Use of a wet rag to keep the Drier cool is best practice. For technicians looking to explore alternatives to traditional brazing methods, check out our article on Brazing Alternatives for the Progressive HVACR Technician, which covers newer connection technologies for components like filter driers.
Note: When removing or replacing a Brazed Filter-Drier, you do not want to “Sweat-Out” the Drier by Brazing – heating the Drier causes its moisture/contaminants to boil and be re-released into the system. Cutting this style of Drier out is the preferred removal method.
Replaceable Core Style
The image below is a Replaceable Core style Filter-Drier. These are more expensive and can be harder to source, but offer better serviceability. The end of the Replaceable Core’s shell has a flange which can be removed, so that the internal Filter/Desiccant material can be cleaned/replaced.
This comes in various larger connection sizes with Brazed connections. They are installed without the core inside, so overheating is not a concern. Their applications range from Heavy Commercial to Industrial.
The Replaceable Core style will have an access port on its Flange (see Shrader in image below on the right), where refrigerant can be drained for service once it is isolated. When working on systems requiring refrigerant removal, our guide on Refrigerant Recovery provides essential techniques for safely and efficiently managing this process.
Note: When installing the Filter-Drier Core into the Shell, tighten the bolts with a Torque Wrench to the Manufacturer’s Specifications. If over-torqued, the aluminum flange will warp and not seal properly again.
Desiccant Considerations
Moisture Vs. Contaminants
The moisture-loving element of a Filter-Drier has some different considerations for both Throw-Away and Replaceable Core types. There are compositions for new system install that are 100% geared towards absorbing moisture. There are also compositions for replacement which focus 70-80% on moisture absorbing, and the remaining 20-30% of the Desiccant solution is there to deal with potentially formed system contaminants.
The combination of desiccants may be more suitable for a replacement, where system contamination is of concern. Each manufacturer has a unique product nomenclature to describe what the desiccant is composed of. Asking your supplier or manufacturer representative for a breakdown is a good way to get knowledge to assist with the selection of desiccant type.
Burnouts and System Contamination
A Compressor Burnout occurs when a Hermetic or Semi-Hermetic Compressor has failed Electrical Windings and is due to Corrosive Oil/Moisture/Contaminants which have eaten away the Windings’ Insulation, and caused the Motor to “Arc Out”/fail.
There are specific Desiccants of both Throw-Away and Replaceable Core types for this application. Often, a new Filter location is added in the Suction Line just before the Compressor. This allows for any remaining harmful materials to be absorbed by the Filter-Drier right before the newly replaced Compressor.
This would be in addition to replacing the Liquid Line Filter-Drier, which is good/required practice when opening the system for any major work. The Liquid Line Filter Drier may be replaced by a Burnout Drier, or one of the two standard types mentioned above, depending on the situation.
Sight Glasses and Their Functions
A view into the system to see Refrigerant Flow/Level, or Oil Level, is provided by a sight glass. There are different construction methods like the permanent/sealed type shown above, or a threaded or flanged type as seen in the two images below. When brazing a Sight Glass as shown above, it is important to protect the Component by using a wet rag to prevent overheating.
An additional component that is incorporated into Sight Glasses is the Moisture Indicating Element shown above, from this Sporlan Equipment Selection guide. Yellow is “Wet”, and Green is “Dry”. This image also shows connection styles of Brazed, Flared, or “ZoomLock”.
Note: A “ball” may float in the sight glass for easier viewing of levels. This can be seen in the two images below.
Refrigerant Sight Glasses
This is the most common application for Sight Glasses. When installed in a Refrigerant Line, a clear/full sight glass may be an indication of good system operation or Refrigerant charge. A bubbled sight glass may indicate a system undercharge, restriction, or just that at the time viewed, the refrigerant is elsewhere in the system.
The most common location for a Sight Glass is in the Liquid Line, immediately after the Filter-Drier. This allows view of any potential blockage, and if equipped with a moisture indicator: potential passing of H₂O through the Drier if it is saturated with moisture. Wondering why your sight glass might be showing yellow? Check out our podcast episode Why Is The Sight Glass Yellow On My Refrigeration System? for a detailed explanation of moisture indicators and what they tell us about system condition.
This product from United Refrigeration offers a combo Drier with Sight Glass, for simple install. This combo part is a good candidate for a Liquid Line Filter-Drier with Sight Glass immediately following.
The other purpose of a Refrigerant Sight Glass is to indicate the operating level of a vessel, like in the image below. This High Pressure Receiver has three Sight Glasses for low, medium, and high levels visually indicated on the vessel.
Oil Sight Glasses
Not all Refrigeration Compressors have Oil Sight Glasses, but this is common for Semi-Hermetic or Open-Type Construction. Another Oil Sight Glass location may be in an Oil Separator, Oil Pot, or other System Component housing Oil.
The Oil Sight Glass on a Compressor may be installed directly into the oil sump or affixed to its Oil Management Device, like in the image below. The purpose of a Sight Glass for Oil is to visually ensure that the sufficient/expected Oil level is maintained within the System Component it is installed.
Summary
Filter-Driers and Sight Glasses are two of the most common Refrigeration and AC Components. Understanding their purpose can allow a Technician further ability to diagnose system issues or confirm standard operation. Installing or replacing Driers or Sight Glasses is common practice in the HVAC/R trade, and being comfortable with these procedures is a great skill to have.