Headspace management requires a focus on stabilizing the cleanliness and dryness of the environment inside a component's casing. Although this seems like a simple task on the surface, it can be difficult to accomplish on many systems and in many operating environments. To effectively manage a component's headspace, there are three areas to address:
Excluding - Keeping contaminants from entering the system at all
Removing - Having a system in place for removing contaminants that do enter the system or are generated from within
Monitoring - Employing tools to monitor levels of contaminants in order to drive maintenance
Proper execution in these three areas will translate to big gains in machine reliability and extended component life. Though monitoring and removal are important facets of headspace management, I want to focus on exclusion for this Machinery Lubrication article.
Expansion Chambers and Desiccant Canisters
Exclusion of all things not wanted in your system is the least expensive and most effective way to manage the headspace. Certainly, we need to be focused on excluding solid contaminants and moisture. As our systems breathe, the air transfer occurs through the headspace. Therefore, exclusion starts with either an air filter or an expansion chamber.
In my opinion, expansion chambers offer an excellent barrier to protect the system from invading contamination. However, if you are using an expansion chamber, you must also use a desiccant canister. Expansion chambers offer a "closed" system environment for your component. When sized properly, these chambers will expand when the system wants to breathe out and contract when the systems want to breathe in. No air is passed from the outside atmosphere to the inside of the system. What this means is that anything already existing in the headspace will remain trapped in the component unless a system is in place to remove it. This is an extremely important consideration for moisture.
Dry headspace translates into dry oil. Dry air above the oil will act as a desiccating blanket and pull moisture from the oil. Often, this moisture can be expelled from the system when the system exhales. When you eliminate the possibility of the system breathing to the atmosphere, you also eliminate the possible exhaustion of humid air. To ensure this humid air is removed, install a simple, non-venting desiccant canister.
Comparing Breather Options
Modern air filters offer a gamut of features. All breathers pale in comparison to the features and functionality of the hybrid-style breather. The major disadvantage to using standard disposable desiccant breathers is that they are an always-opened fixture on the system. Regardless of the state of the system (breathing, static, cycling), these breathers are interacting with both the component headspace and the surrounding atmosphere. Because desiccating media is hygroscopic (it naturally attracts moisture), these breathers will always absorb moisture - if not from the system headspace, then from the surrounding environment. A desiccant breather installed on a machine will always absorb moisture, whether the system is operating or not. This means that depending on the level of humidity surrounding the system and the installed desiccant breather, the desiccant may not last very long at all. In extreme cases of high humidity or heavy washdown environments, the life of an always-opened disposable desiccant breather can be measured in days. The result in many cases may be a low up-front cost with a low life cycle.
Hybrid-style desiccant breathers combine several elements that offer a normally closed system, external breathing capabilities, fine solid-particle filtration and moisture stripping. Obviously, these breathers incorporate desiccating media to capture moisture. The way these breathers differ from their disposable cousins is that they are normally closed. So when the system is static or not breathing, the desiccant is only exposed to the headspace of the component and not the surrounding environment. This enables the breather a comparatively longer life cycle than the disposables. This design allows for moisture stripping as the system breathes and moisture capture from the component headspace while it is not breathing.
To allow the breather to maintain a normally closed state, an expansion chamber has been incorporated into the design to allow for small changes in total volume. If the system breathes beyond the capacity of the expansion chamber, pressure and vacuum relief valves are there to pop open and allow adequate air flow.
Make Decisions Based on Knowledge and Total Cost
So, the question then leads to how you want to spend your money. Many will tend to opt for the low up-front cost and deal with the short life cycle. I have seen this happen more times than I care to remember. A client, new to desiccant breathers, decides to try out a couple on some critical systems. They decide to bring in some of the disposables because: 1) they're about half the cost of the hybrid style, and 2) they don't have the depth of knowledge required to make appropriate decisions about what their systems actually require in the area of air filtration.
What happens next is that the breathers (for reasons like high ambient humidity, frequent washdowns or a reservoir already full of free and emulsified water) don't last very long. As a result, the client decides that desiccant breathers won't work in their facility for one of the reasons previously listed.
Headspace management is one of the keys to extended equipment life and improved reliability. Like any minor modification, you should always research the options to make educated decisions on what is most effective for your systems, location, environment or process.