Picking the Best Filter
by P. Palmer Kimball,
Founder, Valve and Filter Corporation, Denver, CO
Whatever filter choice is best suited for a particular application, always obey the cardinal rule of filtration – design the system with at least 10 percent more filter capacity than the system is expected to require. It is always better to have a little more capacity than not quite enough.
Always use the largest size micron opening screen possible. For example, it just does not make sense to specify a 50 micron screen when using the filter on nozzles. A smaller screen than needed costs more initially, costs more in flush water and costs more in maintenance.
Most manufacturers assist with filter system design. Some will even do water testing using your sample to help determine the size of filter to best fit particular needs. Take advantage of these no cost services whenever they are available.
It is critically important to select the correct type and size of filter – as well as to design the system conservatively to accept overload and upset conditions. A wide variety of filter types are available for general filter applications. All have advantages and disadvantages.
FILTER OPTIONSGravity Coanda Effect Screen Filters
Offer a low cost option. They are easy to install and maintain, but require
a large footprint and periodic cleaning. Its low efficiency is usually limited
to 200 micron and above; and it cannot be used on pressure systems.
Bag and Cartridge Filters
Range from medium to high cost. Replacement cartridges can be costly.
These high efficiency filters work to the sub-micron level. They are labor
intensive and require bag or cartridge change outs. This type of filter does
have a small footprint.
Media (Sand) Filters
Also range from medium to high cost. They offer high efficiency with effective
filtration to 5 micron. A media filter requires a large footprint and periodic
media changes. Twin systems are required for uninterrupted automatic operation.
Flush water volume is large and flush time can be very high- as much as 10
minutes or more.
Centrifugal Filers
Come a t a low cost. They are suitable for full-time automatic operation.
Efficiency is high. Operation can be batch or continuous low flush water.
Centrifugal filters capture down to 75-micron level but they cannot filter
particulate below about 1.1 specific gravity. They also cannot filter algae
and are flow sensitive. Footprint ranges from small to medium in size.
Tangential Screen Filters
Also offer a low cost solution. They can be adapted to automatic operation.
An advantage is the small footprint. A disadvantage is the low efficiency
flush. Tangential screen filters usually require periodic screen removal and
pressure wash cleaning, which can be labor intensive.
Automatic Screen Filters
Range from medium to high cost. They offer a high efficiency flush with
flush water volume very low. Flush time is fast – usually 8 to 20 seconds.
A big advantage is the completely automatic operation that does not interrupt
system flow during the rinse cycle.
CHOOSING THE RIGHT FILTER
Making a good filter selection requires investigating the reliability required by the system, available equipment and installation costs, operating and maintenance cost, fluid makeup costs (wastewater replacement) and the exact level of filtration (micron size required. For example, if the particulate has a specific gravity of 1.1 or higher and the filtration level is 200 micron or higher, a low cost centrifugal filter might be the most economical and reliable filter to select.
Automatic Screen Filters
For a number of reasons, the automatic screen filter is becoming the filter
of choice for general water applications in industry and irrigation. It’s
reliable, requires low maintenance, flushes quickly, wastes very little flush
water and is easy to install in new or existing systems.
The principal features tat make for high efficiency flushing in an automatic screen filter are the dirt collector and design of the wastewater flow path through the filter. The dirt is collected on the inside of a cylindrically shaped stainless steel screen (woven wire weave or wedge wire). As the dirt layer builds up on the screen, it will cause a pressure loss on the downstream side of the screen.
Collapsing Dirt Cake
The difference between the filter inlet pressure and the outlet pressure
is called the differential pressure (DP). When the dirt cake is thick enough
to cause an 8 to 10 psi DP the flush cycle is started.
The dirt cake itself will act as a filer and collect much smaller particles than the stainless screen. For example, a 100-micron screen will usually capture some particles smaller than 25 micron before the DP is high enough to start a flush cycle. As the DP builds up, the dirt cake itself is subject to increased pressure.
There will be many voids in the cake, just as there are in a pile of rubble on the ground. As the pressure increases it will get high enough to cause the cake to partially collapse in on itself because of these voids. This is not a good thing since it will allow some of the smaller particles (in the example under 100 micron in size) to pass through the screen. More of the ‘dirt’ reenters the system and results in lower filter efficiency.
Flushing and Rinsing
The solution is to initiate a flush cycle just before the collapse pressure
of the dirt cake occurs. The collapse pressure varies with the type and size
of particulate, but in general occurs at a DP of over 10 psi. It’s generally
accepted to use a set point DP of 8 psi or lower on the filter controller
to start a flush cycle.
The stainless steel screen should be large enough to allow a smooth, low velocity flow of the fluid being filtered. A high velocity will tend to impact some irregularly shaped particles in the screen, which are then difficult to remove by back-flushing.
The most efficient flush occurs when the particulate is removed 180 degrees from the direction it was collected on the screen. The dirt collector nozzles are positioned to remove the dirt from the screen by suction at this angle. The passage of the dirt-laden fluid from the nozzle through the dirt collection system must create an aggressive flow velocity at the dirt collection nozzle for efficient cleaning.
Suring the rinse cycle, the nozzle(s) sweep over the screen area to collect the dirt. Either an outboard electric motor or an internal hydraulic reaction motor drives the collector rotation system. In some filer models, the entire dirt collection systems is mounted on a hydraulic piston or worm driven shaft to create back and forth movement of the dirt collector at the same time it is rotating. The nozzles sweep the entire screen in an overlapping spiral fashion.
The internal hydraulic reaction motor and piston system is usually preferred because it is simple, has not rotating shaft that pierces the pressure boundary of the filter, has only two moving parts and requires no outside source of power for automatic operation.
Staging Filters
Since reliability is such an important consideration, several small filters
in a bank are often installed in place of one large filter. This offers flexibility
and allows one filter to be taken offline for maintenance without stopping
filtration on the fluid stream.
In one application a three-filter system operates at 9000 gpm in a gold mining operation. The filters remove gold bearing carbon particles from leach pad water. They not only keep the drip tubing from plugging but also salvage gold. The carbon particles removed are smelted and the gold reclaimed.
In another application an 800 gpm two-filter packaged system filters water for cooling tower service in a paper mill. The system also includes filter isolation valves and an automatic bypass valve that opens if a system upset occurs.
CONTROL SYSTEMS
The filter control system of an automatic screen filter is an important part of the package. It should be very reliable, easy to operate and capable e of initiating a flush cycle either by high differential pressure or on a timed basis. It should also provide an alarm function to warn an operator of a filter malfunction. An LCD display screen to count flushes, to show the time since last flush and to show the operating condition of the filter is also desirable.
Controllers are sized to fit each particular application and range from large industrial controllers to small battery powered controllers. An industrial controller is capable of flushing up to four filters in parallel. The small, battery powered controller features a three-year battery life and is ideal for controlling filters located where a power source in not available.
More Information: VAF2000 - Our Automatic Self Cleaning Industrial Filter
Please contact us if you need help choosing a filter.
Copyright ©2002 Valve and Filter Corporation.
All Rights Reserved. Do not reproduce in any form without prior written consent.
P. Palmer Kimball is an electrical engineer by education and the founder of
Valve & Filter Corporation in Denver Colorado. (visit www.valveandfilter.com).
Valve & Filter Corporation is a custom manufacturer of automatic backwash
(screen) filter systems.