Researchers at Purdue University have...
The following is part one of a two-part article.
Whether a filter system is installed in a residential,
commercial or industrial application, the cartridge often is thought of as the
heart of the system because the cartridge's chemical and physical properties
determine the filter's effectiveness in removing specific materials
suspended in fluids. Flow rates, the fluid's contact time with the
filtering medium and pressure drops, as well as environmental characteristics
such as exposure to extreme temperatures, pressures and corrosive materials
will affect your choice of a cartridge. However, the cartridge operates within
a housing and even a cartridge that is perfectly matched to the application
won't perform adequately if it's not also matched to a housing that
meets the same application requirements. Specifying them together as an
integrated system is the best way to assure they provide optimal performance.
There are many sizes and styles of housings that are made
from a variety of materials and come equipped with features to enhance
performance in a range of applications. Selecting the most appropriate housing
involves both science and art, but it's a relatively simple process of
matching the housing to the cartridge and the application requirements using a
number of key factors.
To select the correct cartridge and housing you will need to
consider the following factors.
contaminant needing to be filtered.
quantity of the contaminant needing to be filtered.
size of the filters available to filter the contaminant.
temperature of the fluid to be filtered.
and peak operating pressure.
of the fluid with the housing components and cartridge.
drop across the housing(s) and cartridge(s).
of the inlet and outlet connections with the piping.
(residential, industrial or commercial).
Because it's almost impossible to find a filter
housing that optimally addresses each factor, your ultimate choice may be the
result of making compromises. In the case of large scale applications featuring
many housings it often is advisable to test the filter on a more limited scale
to make sure it performs to expectations before rolling it out across the
Filter cartridges vary in size to accommodate the amount of
material necessary to reduce contaminants to specified levels and to achieve
other performance requirements such as service life, flow rates and pressure
drop. Because cartridges often are designed to filter a limited number of
contaminants, you can narrow your search of the many cartridges available by
identifying the contaminants you want to filter. This will simplify your task
of selecting the most appropriate cartridge and it should be your first step.
Application parameters will have an impact on the service
life you need from your filter cartridges. For example, if you are filtering
sediment you will want to make sure that your cartridge has sufficient
dirt-holding capacity to last between service visits. If customers are going to
change the cartridge, you need to consider what is most convenient for them. If
you are unsure as to how long a cartridge is likely to last, you may want to
choose the largest size possible to maximize the time intervals between
cartridge changes. In those situations where multiple cartridges are connected
in parallel to increase capacity and service life, such as in large industrial
applications, performance can be tested with a single 93/4-inch. standard
cartridge at the desired flow rate per cartridge. These results then can be
extrapolated to a larger multiple cartridge housing with the same flow rate per
cartridge to achieve the desired service life. You also can contact the
cartridge manufacturer for information.
When filtering chlorine or volatile organic chemicals
(VOCs), the percent reduction required and the number of gallons that need to
be filtered between cartridge changes may dictate cartridge and housing size.
Longer service life generally correlates to larger cartridges that require
larger housings to accommodate them. In some applications, fluids are filtered
in batches and cartridge change-outs are scheduled periodically. In other
applications, physical space constraints may restrict the size of cartridges
and housings that can be applied. In this case you may need to install a
cartridge and housing that provides the longest life possible given the space
Temperature can have an effect on cartridge service life.
This is especially true when filtering chlorine. Most carbon cartridges are
tested between 63° F and 73° F. Lower water temperatures will reduce
service life and the percent reduction of chlorine, while higher water
temperatures will increase the life of the cartridge and increase the percent
reduction of chlorine.
Once you've determined the contaminant you will be
filtering out and how much of it must be removed between cartridge change-outs,
it is time to select the proper cartridge size, which will determine the
housing size. Manufacturers in the water treatment industry make cartridges and
housings in common sizes. The most common double open end (DOE) cartridge
lengths come in 47/8-, 93/4-, 20-, 30- and 40-inch. lengths and
standard 23/8- to 27/8-inch. outside diameters. Cartridges also are
available with 41/2-inch. outside diameters and lengths of 93/4-
and 20-inch. lengths. Single open end (SOE) cartridges are commonly available
to fit 12-inch. or specially modified 20-inch. housings. Consult the
manufacturer's literature for the types of contaminants and life of their
cartridges as well as the recommended flow rates, maximum temperatures,
pressure drops and other related information on the cartridges and housings you
plan to use.
Flow rate can be critical in determining the inlet and
outlet size of the housing as well as the number of housings that may need to
be connected in parallel to achieve the desired pressure drop. Most filtration
processes improve with lower flow rates because contact time between the fluid
and the filtering medium increases. At lower flow rates, pressure drops are
reduced, filtration efficiency is increased and contaminant loading is
maximized. Lower flow rates are achieved with larger cartridges or more
housings. The savings in cartridge change out labor and better utilization of
the filter cartridges can be significant. This is especially important in
industrial or food service applications where down time is costly. In some
applications there are space restrictions where more or larger housings are not
possible. If space restrictions do not allow you to optimize the housing size
to meet the requirements of the application, it's advisable to let your
customer know about your concerns.
Housings are made of various materials to maximize
performance in different applications and environments. The temperature of the
fluid being filtered is important in determining whether or not the material
from which a housing is constructed is appropriate for a given application.
Talc-filled polypropylene housings can be used with temperatures up to 125°
F in general water filtration. Temperatures up to 160° F can be handled by
glass reinforced nylon housings. Temperatures up to 300° F typically are
handled by stainless steel housings.
Peak operating pressures determine the materials of which a
housing should be made and, therefore, its cost. Talc-filled polypropylene
works with pressures from 90 to 125 psi depending on the size of the housing.
Some stainless steel housings can operate at pressures up to 250 psi. For
higher pressures and temperatures, specialty housings will be required.
Chemical compatibility will be a factor primarily in the
commercial and industrial markets where different liquids, strong bases or
acids may be encountered. Care must be taken to specify housings that maintain
their performance characteristics in these environments. Installing housings
that will be exposed to chemicals to which they are incompatible with them may
cause damage and result in catastrophic failure.
The types of materials generally used in the housing market
today include Lexan, Talc-filled polypropylene, styrene-acrylonitrile (SAN),
glass-reinforced nylon (hot/high pressure), stainless steel and aluminum. Most
manufacturers have chemical compatibility tables for quick reference with
commonly encountered chemicals. If you cannot find the answer or want a second
opinion, do not hesitate to call the manufacturer for technical support. It is
their job to help you.
Part II of this article will appear in the February issue.