Problem: A freeze-drying plant used in the manufacture of instant coffee provided by Star Refrigeration required pumps to operate around the clock. Pumping refrigerants at extreme temperatures places pumps and their key components under significant pressure.
Solution: Michael Smith Engineers supplied Nikkiso Non Seal canned motor centrifugal pumps. Since June 2001 half of the pumps have operated almost continuously 24 hours a day, seven days a week without any servicing, repair or breakdown. All six pump liquid carbon dioxide, which is non-lubricating and has a viscosity of around 0.1cP, at a temperature of -54°C.
Four of the pumps are pumping 50m3/hr at 25 metres differential head while two of them are pumping 44m3/hr at 35 metres differential head. All are operating with a hydraulic efficiency of around 60 per cent.
The Nikkiso BQ Reverse Circulation design was recommended for these duties – a pump designed to be mounted vertically with the motor over the pump ensuring quick and efficient venting of any vapour or gas in the rotor chamber.
Angus Gillies, Technical Director of Star Refrigeration, comments: “These are the most reliable refrigerant pumps we have ever used. 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.”
This design uses the process fluid to lubricate 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. As the liquid CO2 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.