Frequently Asked Questions at Filtration Services Ltd
Welcome to the Filtration Services FAQ page, your one-stop resource for answers to all your questions about our specialised solid/liquid separation and filtration solutions. Whether you’re interested in our bespoke rotary drum vacuum filters, need expert process consultancy, or are looking for high-quality filter spares and consumables, you’ll find the information you need right here.
What is a rotary drum vacuum filter?
Quite simply, a rotary drum vacuum filter is a multi-panel, horizontal axis, rotating drum, which is covered by a filter medium, and partially immersed into a solid-liquid suspension. When vacuum is applied to the drum, the liquid [filtrate] is drawn through the filter medium, whilst the solid [filter cake] is retained on the surface. The filtrate is removed from the drum via a special valve arrangement [rotary valve]. By slowly rotating the drum, the filtered solids on each panel can be continuously removed from the drum surface.
Are rotary drum vacuum filters suitable for all filtration applications?
No, but they are used in a lot of industries and on a wide variety of applications. There are many different types of filters used in today’s industries, and rotary drum vacuum filters are just one style of continuous, bulk filtration equipment. Filters are selected using different criteria, and the filtration characteristics of the suspension. Contact us to discuss your application.
How big are rotary drum vacuum filters?
Rotary drum vacuum filters range in size from, typically, 1m² to 60m² filtration area – although both smaller and larger are available.
Do rotary drum vacuum filters require a lot of labour to operate them?
No, the labour requirement to operate a rotary drum vacuum filter is very low. As a steady-state filtration unit, once set-up, the filter is usually unattended and can operate 24/7.
Do rotary drum vacuum filters require a lot of maintenance?
No, the maintenance requirement for a rotary drum vacuum filter is minimal. Due to the slow rotation of the drum and oscillation of the cradle agitator, and the robust construction of these types of filter, the wear and tear on mechanical components in almost imperceptible. Periodic lubrication of rotating and pivoting joints ensures the filter’s longevity.
How is the size of a rotary drum vacuum filter calculated?
The size of a rotary drum vacuum filter, i.e. its filtration area, is determined by the filtration flux rate and the overall mass balance at the filter. The filtration flux rate represents the speed at which the filtration takes place, and is a function of the solid-liquid suspension. The flux is expressed as mass (or volume) per unit area per unit time. By combining the flux data with a mass balance, the required filtration area can be determined.
What is the filtration flux rate?
The filtration flux rate is, effectively, the speed of filtration and is directly linked to the overall permeability of the filter cake. The flux rate can be expressed as mass (or volume) per unit area per unit time – for example, kg/m²/hr or m³/m²/hr. The flux rate can be applied to the feed, filtrate or filter cake. And when used in conjunction with a mass balance, can be used to determine the required filtration area. The filtration flux rate can only be determined by test work.
How fast does a rotary drum vacuum filter rotate?
In general, rotary drum vacuum filters operate within the range of 0.1 and 1.0 rpm. However, there are exceptions to this, and the drum rotational speed reflects the filtration characteristics of the suspension.
What is the filtrate?
The filtrate is the liquid phase from the solid-liquid suspension – after it has been filtered out.
What is the filter cake?
The filter cake is the solid component from the solid-liquid suspension – after it has been filtered out?
What is the moisture content of the filter cake?
The moisture content of the filter cake is the amount of retained moisture within it. The amount of moisture is a function of the filter cake and how is dewaters. The filter cake moisture content can only be determined by test work.
What are rotary drum vacuum filters made out of?
In general, rotary drum vacuum filters are constructed in stainless steel due to enhanced chemical compatibility. The most common materials are 304/304L stainless steel and 316/316L stainless steel. There are also a number of polymeric and elastomeric components on a rotary drum and these materials are selected, again, based on chemical compatibility. Other materials of construction are also available for specialist applications.
What are the filter cake discharge methods for a rotary drum vacuum filter?
There are several filter cake discharge methods employed on rotary drum vacuum filters. These include scraper, precoat, belt, roller, string and heel. The selection of the appropriate discharge method is based on the characteristics of the filter cake.
What is special about a precoat discharge rotary drum vacuum filter?
A precoat discharge rotary drum vacuum filter uses a sacrificial, porous filter medium – a precoat bed. These filters tend to be used for the clarification of the liquid phase, especially when the solids are fine, gelatinous and often at low concentration; conventional filters tend to blind off prematurely. A precoat filter, however, keeps refreshing the filter medium by shaving off the blinded layer. Precoat filters are often used in the food and pharmaceutical industries, or where liquid purity is required. They are generally unsuitable where the solid is the desired product due to the solid being contaminated with small amounts of precoat.
What types of precoat materials are available?
The most common types of precoat materials used on precoat discharge rotary drum vacuum filters are perlite, diatomaceous earth, wood flour, cellulose and starch. The selection of the correct type and grade of precoat is a function of the solid-liquid suspension and its filtration characteristics.
How do precoat discharge rotary drum vacuum filters work?
Precoat discharge rotary drum vacuum filters work by the application of a precoat bed – an outer layer of precoat on the surface of the drum. This layer is, typically, up to 150mm thick when first applied. The solid-liquid suspension is then filtered through this precoat bed, allowing the clarified filtrate to pass through. The fine solids are retained on the surface of the precoat and peeled off on each drum rotation (along with a small amount of precoat) using a very slow advancing precoat knife blade, thereby ‘refreshing’ the precoat surface. The precoat bed can last many hours, sometimes days, before being replenished.
How can a solid-liquid suspension be tested for use on a rotary drum vacuum filter?
The simplest test method to check whether a solid-liquid suspension can be used on a rotary drum vacuum filters is by using an inverted filter leaf. This mimics the operation of a rotary drum vacuum filter, and can be used to test small, representative suspension volumes of around 5 to 10 litres. The Pocket Guide to Filter Leaf Tests provides guidance on this.
Are there any test versions of rotary drum vacuum filters available for hire or purchase?
Filtration Services Ltd have both lab-scale and pilot-scale rotary drum vacuum filters for hire or purchase. The lab-scale, the Rotavac Lab, is 0.1m² filtration area, whereas the pilot-scale, the Rotavac One, is 1.3m² filtration area. Please contact us for more information.
What is the AKAS on a precoat discharge rotary drum vacuum filter?
The AKAS is the acronym for the automatic knife advance system. It controls the movement of the precoat knife blade on each drum rotation, and can be adjusted in increments of 5 microns to ensure optimum precoat usage. The AKAS is standard on all of Filtration Services’ precoat discharge rotary drum vacuum filters, and can be retrofitted on many existing legacy filters.
Is there a maximum operating temperature for a rotary drum vacuum filter?
The maximum operating temperature of a rotary drum vacuum filter is limited by the polymeric components and the vapour pressure of the liquid phase being filtered. In the majority of instances, the liquid phase is aqueous based, which limits the maximum temperature to around 65°C. This is because the applied vacuum can induce ‘boiling’ of the liquid phase at reduced temperatures. The polymeric components, mainly polypropylene and polyethylene, are generally stable up to about 85°C. Nylon, polyester, PTFE and polyacetal can also be used on certain applications.
What are the typical suspension properties for use on a rotary drum vacuum filter?
There are no typical suspension properties for use on a rotary drum vacuum filter. The suitability of a rotary drum vacuum filter is determined by test work on representative samples. As a general guide, suspensions with particles of between 5 and 200 microns, and solid concentrations between 5 and 20% by weight tend to be the most suitable. However, some suspensions within these criteria are unsuitable, and some suspensions outside these criteria are suitable. Test work is the only method for validation.
Are rotary drum vacuum filters continuous or batch?
In general, rotary drum vacuum filters can be considered as continuous filtration equipment. This is particularly applicable to the scraper, belt, string and roller discharge versions. The precoat and heel discharge versions of the rotary drum vacuum filter are batch, insofar as they will need periodic re-setting. A precoat discharge rotary drum vacuum filter, for example, will slowly deplete the precoat bed over several hours or days, and will need to be recoated with another bed of precoat.
How thick is a filter cake on a rotary drum vacuum filter?
With the exception of a precoat discharge rotary drum vacuum filter, the filter cake should be at least 5mm thick for successful discharge from the drum. Thin filter cakes do not, in general, release well from the drum surface. Thick filter cakes have better mechanical properties for discharge. It is also beneficial to have the interface between the filter cake and the filter medium as dry as possible, since this also aides discharge. For a precoat discharge rotary drum vacuum filter, the filter cake is generally around 1mm thick.
Do rotary drum vacuum filters use a lot of electrical power to operate?
The rotary drum vacuum filters do not use a lot of electrical power, but the associated ancillary equipment usually does. The drum and agitator gearmotor drive units on some of the largest rotary drum vacuum filters are, combined, still less than 5kW. The main power consumer on a rotary drum vacuum filter installation is the vacuum pump.