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What is a Filter Press?

In some batch filtration processes, highly permeable suspensions dewater fast compared to the rest of the process. This work explores the impact of fast-filtering compressible materials on the throughput of fixed-chamber filter presses. The dewatering properties for a compressible yet highly permeable minerals processing slurry are used as inputs to a standard filter press model to explore the effects of operating and design parameters. For fast-filtering materials, maximum throughput is achieved with wide cavities and minimal handling time, while membrane resistance can be significant. Pressure affects the maximum achievable concentration, as given by the strength of the material. Overall, this work demonstrates the combined use of material characterisation and device modelling for filter press optimisation.

This article answers three common inquiries. What is a filter press? How does a filter press work? What is a filter press used for?

We’ll also give you some advice on sizing your equipment (including your feed pump). Our Sales and Service Team is looking forward to answering any other questions you might have.

WHAT IS A FILTER PRESS?

A pressure leaf filter is a batch operation, fixed volume machine that separates liquids and solids using pressure filtration. A slurry is pumped into the filter press and dewatered under pressure. It is used for water and wastewater treatment in a variety of different applications ranging from industrial to municipal.

M.W. Watermark manufacturers filter presses ranging from .06-600 cubic feet.

Slurry is pumped into the filter press. The solids are distributed evenly during the feed (fill) cycle.

Solids begin to build on the filter cloth. Most of the solid/liquid separation is done by the filter cake building on the cloths. At first some fines may pass through the cloth (1), but eventually the solids begin to form a layer on the filter cloth (2) much like a pre-coat. That layer traps the fine particles and forms a filter cake (3).

As the vertical pressure leaf filter builds pressure, the solids build within the chambers until they are completely full of filter cake. When the chambers are full, the fill cycle is complete. The filtrate (liquid) exits the filter pack (plates) through the corner ports into the manifold; when the correct valves in the manifold are open, the filtrate exits the press through one single point, the filtrate outlet.

HOW LONG DOES A FILTER PRESS CYCLE TAKE?

The Total Cycle time is the Fill Cycle time plus a constant. For presses of 125 cubic feet and under this constant is usually around 45 minutes. This is the time required to close/open the press, perform the Air Blow Down and discharge the filter cake. If the particular application requires operations such as Core Blow or Cake Wash, for example, this constant is longer.

HOW LONG DOES A FILL CYCLE TAKE?

The Fill Cycle is dependent on many parameters. The most important parameter is the nature of material to be dewatered. A sand slurry releases its water readily and dewaters quickly. On the other hand, an Aluminum Hydroxide waste slurry from beverage can manufacture does not readily release its water and dewaters slowly.

The next most important parameter is the concentration of the solids by weight in the slurry. The Fill Cycle for a 5% solids slurry is about twice as long as a 10% solids slurry (with all other parameters being equal). This is because the press has to process half of the water to fill with solids.

Other parameters include the thickness of the filter cake, the maximum feed pressure which the slurry is fed to the press, and the filter cloth selection. These parameters are typically fixed during the proposal process.

CAN YOU GIVE SOME EXAMPLES OF FILL CYCLES?

With 32mm (1.25”) cake chamber thickness, 100 psi max feed to the press and a 3-5 SCFM filter cloth, a 5% sand slurry would be expected to dewater in 20-30 minutes and a 10% sand slurry in 10-15 minutes.

Conversely, the 5% Aluminum Hydroxide slurry may take 4-6 hours to dewater, while the 10% slurry would dewater in 2-3 hours.

We have an in-house laboratory where we can test a sample of your slurry to determine the Fill Cycle time as well as the other outputs from pressure filtration testing. Give us a call.

WILL A SMALL PRESS FILL FASTER THAN A LARGER PRESS?

The Fill Cycle times for a 1 cubic foot, 10 cubic foot and 100 cubic foot press are approximately the same. Press volume is the ability to remove solids. Associated with this volume is the square feet of surface area in the press.

Square footage is the ability to process fluid. As volume is added, square feet of surface area is proportionally added and the cubic foot to square foot ratio remains (roughly) constant. Therefore, the Fill Cycle Time is basically the same.

Example

A 1 cubic foot press at 32mm cake thickness has 22 square feet of surface area for a 0.045 cf/sf ratio. A 10 cubic foot press at 32mm cake has 211 square feet of surface area for a 0.047 cf/sf ratio.

WHAT TYPE OF PUMP SHOULD I USE TO FEED MY PRESS?

For press capacities of 125 cubic feet or less, the double Air Operated Diaphragm pump (AOD) is uniquely suited for vibrating filter operations. The filter press is not a constant flow device. As the solids build up within the press, the resistance to flow increases and the flow rate through the press decreases. At a given air pressure supply, the time between pump strokes for an AOD pump, then, constantly increases with no harm done to the pump.

On presses larger than 125 cubic feet in capacity, AOD pumps become impractical because three or more large (3”) pumps would be required.

On large presses pumps such as progressive cavity, centrifugal and piston membrane pumps are often used. Control of these constant flow pumps is through a PLC that requires input from a pressure transducer and a flow meter to control a VFD (Variable Frequency Drive) on the pump motor. For a small press the control system for a constant flow pump is generally more costly than the press itself.

SO, ARE THERE ANY RULES OF THUMB ON PRESS FEED FLOW RATES THAT CAN BE USED TO SELECT A PUMP SIZE?

For presses 125 cubic feet and less, an AOD pump that can deliver 0.1 gallons per minute per square foot of surface area after the initial fill should be selected.

Examples

800mm, 20 cubic foot press has about 420 square feet of surface area. A pump that can deliver 42 gpm should be selected. This can be either a 1.5” or 2” AOD pump.

A 1200mm, 100 cubic foot press has 2030 square feet of surface area. While one 3” AOD pump can deliver 203 gpm, the operating envelope would be on the edge of the curves. Here, two 3” AOD pumps in parallel would be recommended.

For the very large presses a constant flow pump would be sized to be able to fill the void of the press in about 4-6 minutes. For example, a 300 cubic foot press is about 2,250 gallons. To initially fill this press in 5 minutes requires a pump that can deliver 450 gpm.

IS THERE ANY WAY TO QUICKLY RELATE THIS RULE OF THUMB INFORMATION TO PRESS CAPACITY?

The vast majority of filter presses sold are 125 cubic feet in capacity and below. The table below gives quick guidelines. However, there are always overlaps at the ends of the range. A 15 (or 16) cubic foot press can be fed with a 1.5” or 2” pump.

For feed pumping purposes, the fully automatic filter press can be viewed as an open system. There can always be one more stroke of the AOD pump. It may take days to get it, but there will be one more stroke. A practical end of the dewatering cycle has to be determined.

For all practical purposes, the Fill Cycle is finished when the flow rate through the press at terminal pressure is 0.01 gpm per square foot of surface area. For an 800mm, 20 cf press with 420 square feet of surface area, this is 4.2 gpm. There is a correlation between this terminal flow rate and the time between pump strokes at terminal feed pressure, typically 100 psi. Depending on the nature of the slurry being dewatered, the interval between pump strokes at terminal pressure is 30-60 seconds.

DO I GIVE THE FEED PUMP 100 PSI OF AIR PRESSURE FROM THE START AND STAND THERE TIMING THE INTERVAL BETWEEN PUMP STROKES OR IS THERE SOME WAY TO AUTOMATE THIS?

The press Fill Cycle can be started at full blast by giving the AOD pump a 100 psi air supply, walking away and coming back in a couple of hours to check on the interval between pump strokes. However, to save on wear and tear on the AOD feed pump and prolong the useful life of the filter cloths, it would be preferable to ramp up the feed pressure to the press.

The feed pressure can be manually ramped up by installing a pressure regulator in the air supply line to the AOD pump and ramping up in 25 psi increments for example. The ramp point for the 25, 50 and 75 psi stages would be when there is a 5-10 second interval between pump strokes. On the 100 psi stage the termination point is 30-60 seconds between pump strokes. This ramp up and termination can be done automatically with the M.W. Watermark Automatic Feed Pump Control System (AFPCS).

A filter press is one of the oldest and most trusted pieces of dewatering equipment. It’s used for wastewater treatment across a variety of industries and applications. A filter press works by separating out solids from liquids, removing impurities, and suspended solids from industrial wastewater. This allows plant managers to easily handle and dispose of waste while returning clean water to their systems.

Filter presses separate liquids and solids. Specifically, the filter press separates the liquids and solids using pressure filtration across a filter media. Afterward, the slurry is pumped into the filter press and then dewaters under pressure.

What are the Four Main Components of a Filter Press?

Frame

Filter Plates

Manifold (piping and valves)

Filter Cloth (This is key for optimizing filter press operations.

Basically, the filter press design is based on the dewatering volume and type of slurry. ChemREADY is an expert in liquid and solid separation and offers a wide range of filter press types and capacities to suit specific application needs for trouble-free, economical dewatering.

The origin of the filter press dates to around the mid-19th century in the United Kingdom, where a rudimentary form of the press was used to obtain vegetable oil from seeds. However, it wasn’t until major developments in the mid-20th century that engineers were able to develop the world’s first automatic horizontal-type filter press.

It’s this long history of advancements that’s allowed the filter presses of today to achieve significantly lower energy and maintenance costs compared to their belt press and centrifuge counterparts. In fact, the total operating filtration cost for a filter press can easily be 1/6 the cost of what it would be for a belt press or centrifuge.

While there are many different styles of modern edible oil filter press, the plate and frame filter press are one of the oldest and most tested types of dewatering equipment available. You can read more on this type of filter press, along with a more detailed comparison between different types of dewatering equipment, in our Water Facts blog on How Industrial Wastewater Pretreatment Works.

Filter presses are especially useful as the leftover solids are cheaper and easier to move than the entire slurry. With the clean water that filter presses return, plant managers can discharge that to their local municipalities, watersheds or use the water in their own closed-loop systems, creating highly efficient processes.