How Do I Control Contaminants In My Compressed Air System?
Contaminants in compressed air are bad for business. They degrade product quality, impair the safety and efficiency of manufacturing equipment, and can sicken customers and end users. Fortunately, there are proven methods and techniques for taking them out before they affect anyone’s health or the financial bottom line. Before detailing these though, it is important to understand what these contaminants are, the problems they can cause, and where they come from.
Contaminants in Compressed Air
Compressed air can be contaminated by:
- Solid particles
Water can be present as a liquid, a vapor or as an aerosol, (a suspension of microscopic particles in air.) Oil can also appear as a liquid or aerosol. Oil vapor is uncommon but not impossible.
Solid particles can range from flakes of rust and pipe scale to grains of sand and general dirt. Micro-organisms include fungi, (like mold and yeast,) as well as viruses and bacteria.
In some industries such as medical devices and pharmaceutical manufacturing, it is vital that compressed air be cleaned to a known level. ISO 8573 defines purity classes for such applications.
Consequences of Compressed Air Contamination
Micro-organisms are arguably the biggest concern because of their potential health impact. Customers or end-users could suffer through exposure to contaminated products. Within the factory air-powered equipment could expose workers to micro-organisms spread through the air from equipment exhausts.
Oil is also bad for health, but where it shows up most is in product quality problems. Especially when compressed air is used as a transport medium, (paint spraying, powder coating and powder conveying for example,) oil can contaminate the product and affect finish and performance.
Water can have similar adverse impacts on product quality. Where water really causes problems though is in its ability to cause rust and promote the growth of microorganisms.
Besides affecting product quality, solid particles like rust flakes can block filters and damage seals on air-powered devices. This can lead to slower and unpredictable movements that increase cycle times and put nearby workers at risk of injury. It will also shorten equipment life and increase the amount of maintenance needed.
Contaminant Entry Points
Ambient air drawn into the air compressor contains water vapor. Raising the pressure of the air reduces its ability to hold water vapor, which condenses out as liquid. Cooling the air after compression can cause more water to condense.
In polluted industrial environments ambient air may contain some oil vapor, but most oil enters the compressed air at the compressor. Most compressor designs inject oil into the compression chamber where it lubricates, seals and cools. Inevitably, some is entrained in the air delivered into the compressed air system.
Ambient air will also contain dirt and micro-organisms. However, these can both be generated within the compressed air system, with water being the culprit. Water promotes both rust and growth of microorganisms, which is why it’s important to remove as much as possible.
There are two ways to reduce the level of contaminants drawn in with ambient air. First, site the air intake in a location where it is not exposed to dust, fumes or humidity. Second, install a high-quality filter on the intake.
Note that “high quality” does not equate with small particle size or high efficiency. Filtering incoming air to a very high level creates a large pressure drop across the filter and makes the compressor work harder. As a rule of thumb, attempting to remove particles smaller than 25 µm will drive up operating costs.
Filters should be replaced on a regular basis, and before dirt becomes apparent on the surface, as by then performance is seriously degraded. If practical, use pressure drop as an indicator of when replacement is needed.
Liquid water can be taken out at the compressor with a condensation separator. (A two-stage compressor will have a separator after each intercooler.) Be aware that this water will contain some oil and so must be disposed of appropriately.
Install a refrigerated dryer after the compressor to chill compressed air sufficiently to extract most of the water vapor. Refrigeration may be supplemented with a coalescing filter that pulls out any water in aerosol form. (A coalescing filter works by making the minute aerosol particles clump together into larger, heavier drops that are pulled out by gravity.)
Oil-injected compressors have a separator in the air path that pulls out entrained oil and returns it to the sump or tank. For higher levels of oil removal, add an adsorption filter that uses a desiccant to extract vapor from the air.
Both coalescing filters and dry filters are effective for pulling particulates, (including microorganisms,) out of the compressed air.
Get Expert Advice on Contaminant Control
The compressed air used in every factory or process plant must be clean or problems will arise. These range from reduced efficiencies, shortened equipment life and higher maintenance costs to health risks for workers and customers/end users.
Selecting the appropriate equipment for removing contaminants requires detailed knowledge of the application and industry requirements. For businesses in the medical, pharmaceutical and food and beverage sectors ISO 8573 is a useful tool for defining cleanliness levels.
JHFoster has specialists who understand the details of contaminant removal. Contact us to schedule a consultation.