Application Notes Flooded Compressor

One consideration for the flooded compressor is the recovery of the liquid. In the conventional arrangement, the lubricating oil is separated at the compressor outlet, cooled, filtered, and returned to the compressor. Figure 4-11 is a diagram of a typical oil-flooded system. This is fine for air service where oil in the stream is not a major problem, but when oil-free air is needed, the separation problem becomes more complex. Because the machine is flooded and the discharge temperature is not high, separation is much easier than with compressors that send small amounts of fluid at high temperature downstream. Usually part of the lubricant is in a vaporized form and is difficult to condense except where it isn't wanted. To achieve quality oil free air, such as that suitable for a desiccant type dryer, separators to the tertiary level should be considered (see Figure 4-12). Here, the operator must be dedicated to separator maintenance, because these units require more than casual attention. Separation in refrigeration is not as critical if direct expansion chillers are used. In these applications, the oil moves through the tubes with the

Figure 4-11. Lubrication diagram for a flooded screw compressor. {Courtesy of SuHait)

Suction

Suction

Figure 4-12. Oil and gas separation system for a flooded screw compressor. {Courtesy of Sullaii)

refrigerant and comes back to the compressor with no problem, if the temperature is not too low for the lubricant. In the case of kettle type chillers, things tend to be more complex. Oil can be removed from a kettle with a skimmer arrangement, but getting the skimmer working on a wildly boiling and foaming fluid tends to make the refrigerant maintenance people somewhat testy. Again, it is necessary to match the lubricant to the temperature to get it back at all. No lubricant will return if it's frozen to the walls of an exchanger. If the evaporator is not field-located and can be made a part of the refrigeration package, some of these problems of oil handling can be deferred to the compressor vendor.

A nice feature of the flooded or dry screw compressor is its ability to achieve the required outlet pressure regardless of the molecular weight. The compressor process that starts on nitrogen and then gradually brings in a hydrogen-rich gas mixture does not change in performance as a centrifugal compressor would.

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