Review of Filter Types

The types of filters are generally known as the bed-type, the spool-type and the pre-coat type. The first two are inexpensive units for industrial use. The pre-coat us the type used by the vending industry.

The bed-type filter employs a bed of granular material usually graded in size from fine on top to coarse at the bottom. The bed may be in a replaceable canister or permanently placed in the filter's pressure vessel. The water comes in the top and out the bottom. Since only the surface area at the top of the bed is exposed, the entire filter area is determined by the formula π × r².

Since the area is small, any given amount of water flows through the surface area of the filter with relatively great velocity (stated in gallons per minute per square foot). High velocity per square foot is very inefficient and detrimental to the end product as we shall see later.

The spool-type filter can be compared to a spool of twine or a roll of cloth. Water flows from the outside of the spool, through the material, and out the center core. This configuration provides more filter area than the bed type since the entire area of the cylinder walls receive water. The formula for this filter area is the same as for the exterior of a cylinder, π × D × H. The velocity through each square foot of surface area is reduced somewhat, but still not enough to accomplish the needed level of water quality control.

The pre-coat type consists of filter cloth folder over a separator (like a sandwich) in the center. The Separator is called the grid of drainage and the entire assembly is called the element. water flows from the outside of the element into the space provided by the separator and then to an outlet. Both sides of the element are functional an this the filter area is determined by H × W × 2. Since the area is so much larger, the velocity of the same amount off water through any square foot area is considerably less than either of the two previous types.

The element is not the active filtering material. It serves only as a support for a filter coating (filter media) that is deposited uniformly on the element by the flow of water in an initial activation procedure. A special filter media developed for the vending industry is of a talcum-powder consistency for increased capability of fine-filtration and adsorptive capacity.

Comparing the amount of filter area in units with equal outside physical dimensions, the bed-type provides 10 square inches of filter area, the spool-type has 90 and the pre-coat has 388 square inches.

The pre-coat type therefore typically provides 40 times the filter area of the bed-type and over 4 times the area of spool-types in units that appear to be the same size.

So if we consider the same number of gallons of water flowing through the three filter areas, we find that the bed and spool types must have extremely large pores to accommodate this amount of water. Conversely, the pores of the pre-coat are very small-and yet because there are so many of them, it can handle the same amount of water as the bed and spool types. The pre-coat pores measure 1 or 2 microns in size.

Flow Rate and Pore Size

So, knowing the maximum rated flow of the filter, it is easy to see that the velocity per square foot of filter area will vary like this:

Bed Type:
Velocity ÷ 10 square inches (0.07 square foot)

Spool Type:
Velocity ÷ 90 square inches (0.60 square foot)

Pre-Coat Type:
Velocity ÷ 388 square inches (2.70 square feet)

We find that the rush through the pores in the bed and spool types is very fast, while the same amount of water through the pre-coat type is almost mist-like in velocity. So as the filter area is increased, the flow rate of the water through each pore can be reduced.

This the effectiveness of mechanical filtration depends on the inter-relationship between filter area, the size of the pores in teat area, and the velocity of water flow through those pores.

As a rule of thumb, any "filter" with a maximum rated flow of more than 2 gallons per minute per square foot of filtering area will be inadequate.

Another important consideration is what the filter does with its collected dirt. The structure of the filtering material must be designed to resist the water pressure stress that builds up as the pores are blocked by collected dirt. This force is indicated by the pressure differential between inlet and outlet faces of the filter area. As the filter plugs up, the pressure differential can approach full line pressure. The pressures can be even higher when subjected to the solenoid-controlled piping which produces water hammer.

When this pressure differential occurs, the bed-type units release their accumulated dirt directly into the machine and drink. The result is called channeling and leaves a permanent channel in the filter bed.

Spool-types vary widely in structure, but none now being made are sufficiently inflexible to prevent the release of collected dirt. After this dumping or unloading, some spool types have the ability to spring back, collect dirt again, and then dump with the next water pressure change. All this goes on and no one may ever notice it-except the customer who gets the collected dirt and the maintenance man who has to clean up a dirt-fouled machine. Laboratory tests show the the water from channeling or dumping of the household-type filters can be dirtier than the water being fed into the filter during test. This these types of filters can be worse at times than not having a filter at all.

The pre-coat type of water purifier uses a "dynamic tension" approach that will not allow the release of collected impurities. This type consistently maintains dependable output, will not dump or channel, and will not "secretly fail." The quality never varies-just the amount of water is reduced as the small pores are filled as they take the impurities out of the water.

Taste and Odor Removal

Another requirement of water quality control is taste and odor removal. The chlorine added by the city for purification will ruin the delicate aroma of the coffee and actually attack some constituents of soft drink flavors. Other tastes and odors, such as fishy or musty smells also play havoc with the drink.

Taste and odor removal is accomplished by adsoprtion — attraction of the offensive molecules to the surface of the adsorptive material.

Since this is neither a chemical reaction, nor is it mechanical, the adsorptive surface area must be exposed to the raw water. The tastes and dissolved gas molecules are then attracted to and held by the surface area. The action is similar to dust that has been attracted to a comb rubbed on a wool suit. Activated carbon is the type of material used for this. The amount of surface area exposed determines the capacity of adsorption.

One water purifier used by the vending industry has activated carbon in its filtering material that is ground talcum-powder fine so that each purifier has over a million square yards of particles surface area for adsorption.

This provides a tremendous capacity for absorbing dissolved gases and odors, and the small holes between the particles mean that every molecule of water must rub against an adsorptive surface. The bed and spool types use large hunks of charcoal that quickly run out of available surface area, this allowing the odors and tastes to enter the drink.

Most spool-type filters designed for home use also use a cellulose material that under certain water conditions can actually add an objectionable "rotten egg" smell to the drink. In certain cities, the problem is much greater than others.

The EVERPURE^® pre-coat water purifier combines over a million square yards of adsorptive particle surface area with ultra-fine mechanical filtration of 1-2 microns. The effectiveness of mechanical filtration depends on the inter-relationship between the filter area, the sizes of pores in that area and the velocity of the water through those pores.

The quality of filtration is related to the amount of surface area that is exposed to the raw water. The formulas for the computation of this area are given.

In units with equal outside dimensions, the pre-coat type provides 40 times the filter area of bed-types and over 4 times the area of spool-types.

With the water flow needed for beverage dispensing equipment, the bed-type must have extremely large holes to accommodate the water. Conversely, the pores of the pre-coat are extremely small and excellent for filtration.

The net result is that the water gushes through the head and spool-types, preventing adequate removal of impurities from the water. Only the EVERPURE® pre-coat is designed to meet the exacting needs of the beverage-making industry.

We have the largest selection of official Everpure replacement cartridges and filters. We also offer reverse osmosis, whole house, and commercial water systems and replacement filters.