The purpose of the CDM Smith report was to evaluate current dewatering centrifuge technology for the replacement of 13 existing centrifuges at Wards Island, and potentially adding three additional centrifuges, for a total of 16 machines.
Five different centrifuges were evaluated for replacement of the existing Humboldt CP4-1centrifuges:
- Alfa Laval, Model G2-115
- Andritz, Model CP4-1.2 (a retrofit, using same frame)
- Andritz Model D6LX
- Centrisys Model CS26-4
- Westfalia, Model CF 7000
Per the CDM report, “the capacity of the current centrifuges was originally specified at 250 GPM with a range of 1% to 3% solids, and a 24.5% sludge cake solids requirement. The basis of sizing new centrifuges is to use the design capacity of the existing centrifuges, based on the premise that the ancillary components (piping, conveyors, hoppers, pumps, etc.) were designed to support the combined centrifuge throughput.”
The report noted that “all models evaluated are mid-feed, counter-current design using AC variable frequency drives (VFDs) for the main drive motors. Solids conveyor (scroll) gearbox/speed reducer input shafts are driven by VFD controlled AC motors as well, with the exception of Centrisys, which uses a radial piston hydraulic motor with a VFD driven oil pump.”
Summary of Findings: Using an evaluation matrix, the centrifuge manufacturers / proposed products were ultimately ranked as follows, from highest score to lowest:
1) Centrisys CS26-4, receiving the highest score due to:
- Second highest G volume
- Highest torque capacity
- Second lowest operating costs
- Minimal structural and mechanical modifications needed for installation.
2) Alfa Laval G2-115, receiving the second highest score due to:
- Comparatively smaller G volume.
3) Westfalia CS 7000, ranked lower due to:
- Proposed new model had no operating track record
- Installation requires centrifuge opening structural modifications.
4) Andritz D6LX, ranked lower due to:
- It carries the second highest power consumption estimate
- It has the second highest capital cost
- Installation requires centrifuge opening structural modifications.
5) Andritz CP4-1.2.
The re-fitted Andritz CP4-1.2 has the largest capacity of all the machines evaluated. It was not recommended, however, for the following reasons:
- The design capacity is 250 GPM, not 300 GPM as Andritz indicates the re-fitted unit is capable.
- Accommodating 300 GPM unit design flow would include upsizing polymer pumps, sludge feed pumps and centrate piping.
- Existing carbon steel frames are to remain in service, cost estimate includes new SS cladding, however the frames are over 20 years old, and the extent of metal fatigue is not known.
- Highest estimated capital cost
- Highest estimated maintenance cost
- Largest connected electrical load
- Highest energy consumption
- Carbon steel bowl, vs. duplex SS for all other units considered
Background on the Centrisys Corporation Model CS26-4
The CS 26-4 energy efficient design preceded all models in USA installations:
- The beach angle is 15 degrees
- The bowl diameter/solids discharge diameter/bowl cylinder length is 26/15.3/89.6 inches.
- The CS 26 bowl length is the longest of the units evaluated.
- The scroll is a closed flight design near the feed section to maximize solids transport and open near the centrate end to promote settling/capture, as indicated by Centrisys.
- The scroll is cast duplex and 316 SS to protect against corrosion and wear protection is provided by tungsten on the full length of the flights, using tiles in the feed zone and spray applied tungsten in the effluent zone. Replaceable tungsten carbide inserts are provided at the feed and discharge ports.
- The scroll design does not use a separator disc to raise the pond depth but incorporates a solids
baffle on the beach which Centrisys claims affects the driest solids to be discharged from the
machine. - The main drive system consists of a VFD controlled main motor that rotates the bowl via an in-line belt and pulley arrangement. The scroll drive is a radial piston motor, manufactured by Viscotherm AG. This hydraulic conveyor drive has several differences from the hydraulic drives installed on the original NYCDEP centrifuges. The new piston motor and oil pump unit are more compact than the
components of the original drives. The radial piston motor is of smaller diameter and weighs less than the planetary and cyclo boxes utilized by other manufacturers, while producing more torque at the scroll. - The scroll speed is controlled by a gear pump remotely located in an oil reservoir driven by a variable speed AC motor. Changing the motor speed changes the oil flow, directly affecting scroll speed. This provides for an extremely controllable differential speed not affected by variations in load on the conveyor. In addition, unlike a sealed, oil filled speed reducer, the hydraulic oil flowing to the motor is constantly filtered to remove contamination from machine wear or condensation.
This prolongs the life of the drive system. An external cooling water flow of 3 GPM is required to cool the hydraulic oil. Air cooling is also an option. Centrisys indicates the water supply can be plant effluent.
- The radial piston hydraulic motor is mounted outboard of the main bearings. The motor is bolted to the scroll and the output shaft is connected to the scroll shaft, rotating the scroll at a low rpm independently of the bowl speed. This arrangement allows the motor to drive the scroll whether the bowl is rotating or not, and has advantages of cleaning solids out of the bowl.
- The bearings are oil lubricated using an air conveyed single pass oil system. This system is simple in that oil drips from a reservoir into an air stream and sprays into the bearing. This is a favored method of lubricating high speed bearings from a bearing life standpoint, in that the oil is not recycled, there is no potential for contamination and viscosity breakdown.
The Centrisys CS26-4 utilizes a hydraulic motor scroll drive motor manufactured by Viscotherm AG. The radial piston motor (Rotodiff) is directly bolted to the bowl, and performs the work of a gearbox and electric motor. The Rotodiff rotates at very low speed, and the output shaft is directly coupled to the scroll shaft, controlling scroll speed independently of the bowl speed.
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