What Is a Bag Pulling Centrifuge?

The bag pulling centrifuge is a vertical batch filter centrifuge built on the scraper-discharge principle, with one addition that changes what the machine leaves behind. A conventional scraper removes the bulk of the cake from the bowl wall but always leaves a residual layer - the heel cake - which then compromises the purity of the next batch and causes batch-to-batch variation. The PLD carries a liftable filter bag inside the bowl. After the scraper has discharged the bulk through the blanking holes in the base, the bag pulling centrifuge mechanism pulls the bag down and oscillates it, detaching the residual layer completely, and then resets the bag for the next cycle. The result is genuinely automatic, residue-free discharge, with no operator ever touching the product.

Working Principle

1) Feed. Slurry is fed at a controlled rate into the rotating basket and distributed across the filter cloth. 2) Filtration and washing. Liquid passes through the cloth while solids build up as a cake; wash liquid is then applied to displace mother liquor and remove soluble impurities. 3) Spin-dry. The basket runs at full speed until the residual moisture target is reached. 4) Scraper discharge. The scraper cuts the bulk of the cake off the bowl wall and it leaves through the blanking holes in the base of the bowl. 5) Bag pulling and oscillation. The liftable filter bag is pulled down inside the bowl and moves reciprocally, generating an oscillating action that detaches the residual heel cake layer cleanly and completely. 6) Automatic reset. The bag returns to its working position on its own, so the next batch starts from a genuinely clean filter medium with no operator intervention anywhere in the cycle.

Key Features

No heel cake, every cycle - the residual layer that a scraper alone leaves on the bowl wall is cleared by the bag pulling mechanism before the next batch begins.

No cross-contamination between batches - because nothing carries over, the purity of the next batch is not compromised by the previous one, and the batch-to-batch variation caused by heel cake disappears.

Crystal breakage kept low - the oscillating bag strips the residue away instead of digging at it, so fragile and needle-shaped crystals survive better than under a scraper alone.

Fully automatic and unattended - variable-frequency speed control with PLC program control; bag lowering and resetting are automatic and require no operator intervention.

Easy internal access - large-flip or fully flip-open housing versions expose the bowl, the filter bag and the cloth for inspection, cleaning and filter cloth replacement.

Safe on solvent duty - optional nitrogen protection with online oxygen detection, plus explosion-proof motor and control panel, for flammable solvent-wet products.

Typical Applications

Because residue-free discharge and low crystal breakage are now core purchasing criteria for pharmaceutical-grade filtration, a PLD bag pulling centrifuge is specified wherever a heel cake is unacceptable:

• cephalosporin and penicillin APIs· high-potency APIs (HPAPI)· pharmaceutical intermediates· fine chemicals· food-grade crystalline salts· food additives· high-value dyes and pigments· pesticide intermediates

Selection Guide

The real difference between these machines is what is left in the bowl when the batch ends - and that is what decides your product purity and your yield:

Specify the PLD bag pulling centrifuge when a heel cake is unacceptable - which means API crystallization, high-potency compounds and high-value materials where carry-over costs either purity or yield. A plain scraper machine is adequate when the product is not purity-critical; a bag lifting machine is the right choice when the cake has to leave the bowl as an intact block.

Technical Specifications

Customization & Options

Product contact parts in SUS304, 316L, 904L, duplex steel, titanium or Hastelloy according to the liquor; filter cloth in nylon, polypropylene, polyester or PTFE selected from your particle size distribution and chemical compatibility; GMP-compliant design with CIP clean-in-place connections; large-flip or fully flip-open housing; variable-frequency drive with adjustable separation factor; nitrogen protection with online oxygen detection and explosion-proof motor for solvent duty; PLC with stored recipes for multiple products; bench-scale testing of your actual material in our laboratory before the model is fixed.

FAQ

A heel cake is the layer of solids left on the bowl wall after a scraper has discharged the bulk of the cake. It matters for two reasons: it carries over into the next batch and compromises product purity, and its thickness varies from cycle to cycle, which introduces batch-to-batch variation. In API and other high-value production both consequences are unacceptable, and that is precisely the problem the bag pulling mechanism is designed to eliminate.

The bowl carries a liftable filter bag. Once the scraper has discharged the bulk of the cake through the blanking holes in the base, the bag is pulled down inside the bowl and moves reciprocally, generating an oscillating action that detaches the remaining heel cake cleanly. The bag then resets to its working position automatically, so the next cycle starts from a clean filter medium.

It is gentler than a scraper alone. A scraper has to cut the cake off the bowl wall and, on the residual layer, is effectively digging at it, which breaks fragile crystals. The oscillating bag strips the residue away instead, so needle-shaped and fragile crystals survive better and the manual digging that operators often resort to is no longer necessary.

Yes. The machine uses variable-frequency speed control with PLC program control, and bag lowering and resetting are completed automatically. No operator enters the bowl, digs out residue or handles the bag, so the cycle can run unattended. Filter cloth replacement is straightforward because the housing is available in large-flip or fully flip-open versions, giving direct access to the bowl and the bag.

Yes, with the optional nitrogen protection and online oxygen detection, together with an explosion-proof motor and control panel. Those are the same provisions a solvent-wet product needs on any filter centrifuge: an inert atmosphere inside the process zone, and verified oxygen content before feed is permitted.

Why Jiangsu Shengli Centrifuge

Jiangsu Shengli Centrifuge Manufacturing Co., Ltd. builds this equipment in a 158,905 m2 plant with in-house machining, hard-facing and rotor dynamic balancing. The company holds ISO 9001, ISO 14001 and ISO 45001 certification, CE marking and more than 30 patents, and has supplied machines to customers in over 30 countries. Every unit is balanced and run-tested before shipment, and the balancing report is supplied with the machine.

Request a Quote or a Free Separation Test

Send us a sample of your material and we will run it on a pilot machine, then report the measured moisture, throughput and clarity before you order. Tell us your material and your target, and our engineers will recommend the model, the material of construction and the options that fit.