Liquid Ring Compressor

ROTARY VANE vs LIQUID RING

COMPRESSORS AND VACUUM PUMPS

PRINCIPLES OF OPERATION

Both the liquid ring and rotary vane depend on an eccentrically mounted rotor.  In the case of the vane, blades slide in and out during rotation forming a seal with the housing bore.  The liquid ring depends on a motive liquid, typically water, to form a seal during rotation.  Both technologies reduce the displacement during rotation by virtue of the eccentric rotor.  Compressed gas is discharged at the point of smaller displacement yielding compression.  Rotary vanes wear by design and maintain their efficiency by sliding vanes always mating with the bore.  Liquid ring vanes wear and result in continuously decreasing efficiency.

COMPRESSION CAPABILITY

• Ro-Flo vanes are capable of inlet pressure to 28" Hg, discharge pressures up to 150 psig and pressure ratios up to 6:1.
• Liquid ring compressors are vacuum capable with varying discharge pressure and pressure ratios up to 4:1.
• Rotary vanes are not sensitive to high inlet pressure.  Liquid rings are not tolerant of high inlet pressure.

EFFICIENCY

• In similar service, rotary vane compressors require 1/2 the BHP of liquid ring machines, often less than 1/2.
• The compression seal of a liquid ring could present slightly less "blow-by" of process gas.
• Liquid rings require filtration of motive liquid, often in several stages, and require considerable make-up liquid.

OPERATIONAL RELIABILITY

• Liquid rings can tolerate a "slug" of liquid ingestion without severe damage.
• Rotary vanes can tolerate droplets or a continuous mist of liquid, but blades will tend to break with excessive ingestion.
• Rotary vanes have an established reputation for high run time and are field repairable with hand tools in a few hours.
• Liquid rings have been known to "trip out" systems and are prone to cavitation.  Repairs are costly and specialized.

COOLING

• Rotary vanes typically use a small amount of recirculated coolant to maintain operating temperature up to a maximum of 350F.
• Liquid rings use the motive liquid for cooling and circulate liquid at 5 to 10 times the rate of vanes.
• Flashing and temperature increases of the coolant will reduce the efficiency of liquid rings.
• Liquid rings must be kept cool to avoid cavitation.
• Coolant is isolated from process gas in Ro-Flo's and retains its integrity.
• Coolant is exposed to process gas in liquid rings and is prone to contamination, which can create a HAZMAT.
• Liquid ring's employing water as the coolant/sealant require special metallurgy in the presence of acid or sour gas streams.
• Liquid ring coolant continually absorbs the heat of compression which is lost to downstream use.

LUBRICATION

• Rotary vanes use a small quantity of lubricant injected directly to critical points during operation.
• Rotary vane lubricant is usually coalesced and captured downstream of the compressor.
• Liquid rings do not require lubricant injection as the motive liquid seal keeps vanes separated from the housing.

CONCLUSION

Liquid ring technology present a clear advantage in tolerance for liquid ingestion.  Rotary vane technology presents significant advantages in reduced capital cost, reduced operational cost, lower maintenance cost and higher operational reliability.  Rotary vanes present a very small amount of lubricant waste while liquid rings can present increased waste water treatment expense and makeup water expense.  Compared to a good inlet scrubber, liquid rings present an expensive, complex alternative and rotary vanes are the lower cost, more reliable choice within their performance range.