Oil Management and Oil Separators for Large Refrigeration
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Oil management is the way we control oil on parallel racks or other multi-compressor refrigeration systems. We need to think about controlling the oil level, separation, and pathway through the equipment, and we can do that via high-pressure or low-pressure oil control systems. Both types have a separator, reservoir, and regulator, but low-pressure control systems have a filter while high-pressure components have a pressure-reducing valve or high-pressure float. Most applications in our Florida service area tend to use low-pressure oil control systems.
The reservoir stores oil, and the oil separator is what removes the oil from the refrigerant and sends it back to the reservoir. Three main types of separators include impingement, centrifugal, and coalescing separators.
The impingement separator is the oldest of the three types, and it has a baffle between screen socks. It separates the oil from the refrigerant via impingement on the screen sock and a change in velocity inside the separator. Impingement separators are less efficient than the other types, but oversizing has less of an impact on performance than it does for the other types, and these separators are reliable and relatively inexpensive. Sizing is based on the tonnage and should have a pressure drop of 1-2 PSI.
The centrifugal separator relies on changes in velocity to create a centrifugal force and fling the oil against the inner sides of the separator, where it then drains out. The separator also has a screen mesh to prevent particulates from getting into the oil reservoir. It’s a highly efficient model that is relatively inexpensive, but it must be sized correctly; floating the head pressure will also affect the mass flow and make it more difficult to separate the oil from the refrigerant. Tonnage is a good starting point for sizing, but the mass flow rate (discharge CFM or DCFM) and velocity matter a lot more.
The coalescing separator uses a borosilicate glass fiber filter that works from the inside out. Oil collides with itself on the filter fibers and becomes heavy enough to drain downward and into the reservoir. Sizing is critical for these separators. Although coalescing separators are very efficient and remove other system contaminants, they require somewhat regular filter changes and can be expensive as a result. Like centrifugal separators, tonnage and DCFM are required to size coalescing separators properly, and the DCFM must represent the actual CFM at rated conditions.
A DCFM chart shows the varying DCFM values based on the evaporating temperature and condensing temperature of the refrigerant in question. Once you have the DCFM for the rated conditions, you have to multiply it by the tonnage to size centrifugal and coalescing separators properly.
Oil separators also have some replacement parts, including the oil float assembly. Oil assemblies are found at the bottoms of separators, and they are mechanical joints with a ball that rises. As the ball rises, they lift a pin out of a brass seat and allow the oil to drain out of the separator and into the reservoir. Sometimes, debris could clog orifices and cause brass seats to prematurely wear out. To address that issue, Westermeyer added a magnet to the float assembly to collect metallic debris.
Coalescing oil separators have replaceable filter elements. The filter can be inserted or removed through a top flange on the separator, and you can use a differential pressure gauge to measure the pressure drop across the filter and determine whether the filter needs replacement. Filters are typically made of 4-micron pleated paper with or without desiccant for POE oils. Coalescing oil separators may also have a differential pressure switch that indicates when the filters are clogged. Newer serviceable filters can be mounted vertically or horizontally, and it comes with a tapped and slotted flange for easy access.
High-pressure systems may have a liquid level switch, which indicates whether oil is present at a certain point of the system or not.
To troubleshoot oil management components, it’s best to start thinking about sizing, whether the problem is new or recurring, system age, load conditions, oil line return temperature, reservoir pressure compared to suction pressure, what happens after defrost, whether the head pressure is allowed to float, oil float or filter condition, screen ring condition, and oil regulator function. If the oil regulators don’t control the oil level, then they should be valved off.
Westermeyer has been developing electrical products as well, including an electronic level gauge.
Find out more about Westermeyer at http://www.westermeyerind.com/