Non-stop pumps keep coffee production flowing 19 May 2010

Star Refrigeration, which provides freeze-drying plant to major food producers, says that Nikkiso non seal canned motor centrifugal pumps, supplied to an instant coffee plant, have been in near constant operation, without problems, for nearly a decade.

Commissioned in June 2001, half of the pumps have operated almost continuously 24 hours a day, 7 days a week without any servicing, repair or breakdown, according to Angus Gillies, technical director of Star Refrigeration.

All six pumps, supplied by Michael Smith Engineers, are pumping liquid carbon dioxide, which is non-lubricating and has a viscosity of around 0.1 cP, at a temperature of -54oC. Four of the pumps are pumping 50m3/hr at 25 metres differential head, while two are pumping 44m3/hr at 35 metres differential head. All the pumps are operating with a hydraulic efficiency of around 60%.

The Nikkiso BQ reverse circulation pump were recommended by Michael Smith Engineers for their vertical mounting design, with the motor over the pump ensuring quick and efficient venting of vapour/gas in the rotor chamber.

Gillies points out that this design uses the process fluid to lubricate the sleeve bearings. Liquid carbon dioxide from behind the pump impeller passes through the rotor chamber, lubricating between the rotating pump shaft/rotor and the stationary sleeve bearings.

This process fluid flow also dissipates any heat building up in the rotor chamber. However, as the liquid carbon dioxide will have warmed up in the rotor chamber, there is a chance that it could flash off or boil, and so the lubrication stream is taken from the rear bearing back to the vapour zone of the carbon dioxide storage vessel. The reverse circulation flow is around 1m3/hr, less than 2% of the total process flow.

"These are the most reliable refrigerant pumps we have ever used," says Gillies. "Carbon dioxide often causes refrigerant pump problems due to bearing wear but the Nikkiso design has operated very well from commissioning. This is our first choice of pump for this challenging application."

Brian Tinham

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