Process Gas

Process gas filters are sized dependent upon the application, the flow rate and the pressure rating of the system. The user must determine if the filtration objective is to remove particulate contaminant, coalesce liquid from gas, or remove vapor from the gas stream, or a combination of these.

A particulate filter is generally used in any process gas filter circuit. The particulate filter removes particles that can clog a coalescer, or make it less effective. Particulate filters use a variety of filter media. Cellulose, fiberglass and stainless steel media are popular choices. Particulate filters are selected based on the desired micron removal rating, and the flow rate of the system.

A coalescer filter removes water and oil aerosols and solid particles to a specified level. Many process gas coalescers are rated at a liquid removal efficiency of 0.001 ppm by weight and by liquid aerosols down to 0.01 ?m. A coalescer stops particulate contamination through direct interception. Its main function is to remove water and oil aerosols by coalescing the droplets. This happens partially because of torturous path and membrane characteristics and partially due to pressure drop. Coalescer's remove both water and oil from the gas stream. Coalescers are sized based upon the amount of water or oil contained in the gas, and on the flow rate of the system.

It is important to realize a coalescer is excellent at removing moisture but does not remove vapors. Vapors are organics like hydrocarbons and odors. A vapor filter removes vapors.

A vapor filter removes organics from the gas stream. Organics like tastes and odors need to be removed from breathing air systems. In general industry applications, vapor filters remove hydrocarbons and other chemical vapors from the gas system. An activated charcoal 5 ?m filter element is a standard vapor filter. Depending on flow rate, vapor filters need to be replaced every few months because the effectiveness of the activated carbon degrades as it adsorbs. Vapor filters are sized based upon the flow rate of the system and the concentration of the chemical vapor to be removed.

If a process gas needs to have moisture removed to a -10ºF dew point level or lower, a desiccant dryer is used. This filter uses water absorbing desiccant material that chemically binds the water and removes it from the gas. Desiccant filters are sized based upon the dew point requirements of the final gas, the incoming moisture level and whether this is a continuous or intermittent system flow.

Typically a particulate filter in the 1 to 3 um range is used downstream of the desiccant to stop desiccant fines from migrating downstream in the process gas system. The particulate filter protects piping and downstream equipment from particulate damage by removing rust, pipe scale, metal oxides, and dirt particles.

Air enters an HVAC system through the return air filter. This filter is sized according to the cubic feet per minute (cfm) of air that the system processes. If the filter is smaller than optimum, it will clog rapidly causing a reduction in air flow.

Another critical sizing parameter is air velocity. If the velocity is too high for the existing filter area, dirt will pass through the filter. When filters are not working properly dirt builds up on the blower wheel or the evaporator coil affecting the systems efficiency. Dirt on the blower wheel or the evaporator reduces the surface area and restricts airflow. Also, if the filter and filter housing are too small for the air velocity, they will vibrate and whistle. An HVAC system having a large surface area filter will reduce the effect of any cracks in the system.

Two design criteria to keep in mind are filter surface area and maximum filter velocity. In order to reduce the effect of cracks, the filter surface area is recommended to be a minimum of 2.50 cubic feet per minute (cfm) per square inch of filter area. For disposable filters, the maximum allowable filter velocity is 300 feet per minute (fpm). Filter manufacturer recommend the maximum allowable filter velocity for their products.

For example, given a 1600 cfm air flow and 2.50 cubic feet per minute (cfm) per square inch of filter area, the minimum required filter area is 640 in2.

(1600 ft3/min)/2.50 ft3/min/in2) = 640 in2
Use two (2) filters 20 in X 20 in for an actual filter area of 800 in2.

The filter velocity is calculated by dividing the airflow by the actual filter area in ft2 to obtain ft/min. For this example, this would be:

(1600 ft3/min)/(5.5556 ft2) = 288 ft/min

This value is less than the maximum allowable filter velocity of 300 feet per minute. Therefore, a disposable filter may be used.

Intake air filters are sized based primarily on the cfm or airflow per square inch. For cartridge type intake air filters, the minimum filter area required for a specific application is calculated using the following equation:

A = (CID x RPM)/V

Where:
A is the effective filtering area = (p x height of the air filter) - 0.75 in. (0.75 in. is subtracted to account for the area of the filter that is partially or completely obstructed by the seals on the ends of the element)
CID is the cubic inch displacement
RPM is the revolutions per minute at maximum power
V is a constant that is provided by the specific manufacturer

For square or rectangular panel type intake air filters, the length of the filter is multiplied by the width of the filter to obtain the area.

The computed area can then be compared to the filter area given in the manufacturer's specifications, in order to obtain the correct filter for the application.

The size, or number, of filter elements required for a Liquid/Water application is influenced by many factors. The most critical factor is the flow rate of the process. The filter must be sized to accommodate the flow rate.

Factors that affect the flow rate an element can accommodate are:

• Viscosity
• Acceptable system pressure drop
• Retention (micron rating)
• Type of contaminant
• Acceptable element life

In general, filters are sized based on the pressure drop the filter produces in the system. The pressure drop, or differential pressure, is measured by subtracting the system pressure at the filter outlet from the system pressure at the filter inlet. A clean pressure drop of below 3 psid is usually sufficient. The higher the initial differential pressure, the shorter the filter life.

Most filter manufacturers provide information indicating the initial pressure drop at a variety of flow rates. This information can be found in the eFilter manufacturer's product information pages.

Some generalities to consider when sizing a filter:

• The lower the micron rating of the filter the higher the initial pressure drop and the shorter the filter life.
• Higher viscosities produce higher initial pressure drops. Often multiple filter elements are required.
• The system temperature may affect the viscosity of the fluid. Many chemicals are more viscous at lower temperatures.
• The more usable filter surface area the lower the initial pressure drop. For example, pleated filter cartridges often produce a lower initial pressure drop than depth filter cartridges of the same micron rating.

The string of questions filter users can ask themselves when sizing a filter is: What is my system's flow rate?
What pressure drop can I accept?
What is the most open filter I require?
What material of construction is compatible with my fluid and my system temperature?
How often can I service this filter?

To size a hydraulic or lubrication filter, you need to know the application. For example pressure line, return line, kidney loop or other application. The following specifics are required to finish sizing a lubricating filter:

• Maximum system flow rate the filter could see
• Fluid pressure the filter will operate under
• Viscosity of the fluid being filtered
• Desired fluid cleanliness

A filter is sized based upon the maximum allowable pressure drop for the clean filter at the rated flow. This clean flow rate pressure drop standard ensures that there will be sufficient filter life before the filter element requires replacement, and limits the likelihood of the assembly going into bypass during surge flows.

Every filter element has a collapse pressure. In order to prevent a catastrophic element collapse, filters contain bypass valves that allow fluid bypass after reaching the pre-set valve cracking pressure. Standard filter assemblies are normally manufactured with a bypass valve. This bypass valve has a cracking pressure between 25 ands 100 psid. As the filter element gets blocked with contaminant, the differential pressure across the filter element rises until it reaches the bypass valve cracking pressure. At this point, part of the flow through the filter assembly begins to bypass the filter element through the valve. This action limits the maximum differential pressure across the filter element. Proper sizing of the filter can eliminate the assembly going into bypass prematurely.

For example, when selecting a return line filter, maximum flow volume including surge flows must be considered. If the system incorporates accumulators or cylinders, then the return flow can significantly exceed the pump flow when the accumulators discharge and the cylinders contract. The maximum surge flow should be the flow used to calculate the pressure drop through the filter.

Most filter companies provide flow rate vs. pressure drop curves that show the pressure drop for an empty housing and the pressure drop through various filter elements of different micron ratings. For simplicity, these curves are constructed for a given fluid of a specific viscosity. If the viscosity of the fluid you are using is different from the viscosity of the fluid used in calculating the flow vs. pressure drop curves, you must determine a viscosity correction factor. This correction factor is found by the relationship:

K= viscosity1 new/viscosity1 old

Multiply the old pressure loss though the element by k to determine the new element pressure loss.

1Note that viscosity units must be consistent, SUS or cSt.

For calculating housing pressure loss with a new fluid, the pressure loss is proportional to the ratio of the new fluid's specific gravity divided by the old fluid's specific gravity.

As a general rule of thumb, the clean flow rate pressure drop should be one-half to one third of the bypass valve setting to ensure adequate filter life. For example, if a filter is fitted with a 50 psid bypass valve setting, the initial clean pressure differential should be no greater than 16.7 - 25 psid.

Pressure line filters- Pressure line filters need to be sized based upon the system pressure, the maximum flow rate of the pump and the cleanliness requirements of the system. Pressure filters are designed to protect sensitive components, such as servo valves downstream of the filter. Some system components are extremely sensitive to contamination. In those cases, filters without bypass valves would be used to ensure that no unfiltered fluid could bypass the filter element and potentially damage the downstream components. When no bypass valve is used, it is strongly recommended that a high collapse pressure filter element is used in conjunction with a differential pressure indicator. This indicator device could be visual, electrical or a combination of both.

Most filter companies provide flow rate vs. pressure drop curves that show the pressure drop for an empty housing and the pressure drop through various filter elements of different micron ratings. For simplicity, these curves are constructed for a given fluid of a specific viscosity. If the viscosity of the fluid you are using is different from the viscosity of the fluid used in calculating the flow vs. pressure drop curves, you must determine a viscosity correction factor. This correction factor is found by the relationship:

K= viscosity1 new/viscosity1 old

Multiply the old pressure loss though the element by k to determine the new element pressure loss.

1Note that viscosity units must be consistent, SUS or cSt.

For calculating housing pressure loss with a new fluid, the pressure loss is proportional to the ratio of the new fluid's specific gravity divided by the old fluid's specific gravity.

As a general rule of thumb, the clean flow rate pressure drop should be one-half to one third of the bypass valve setting to ensure adequate filter life. For example, if a filter is fitted with a 50 psid bypass valve setting, the initial clean pressure differential should be no greater than 16.7 - 25 psid.

In bag houses designed for dust collection, the air-to-cloth ratio is the most common design parameter. This value is typically calculated for the maximum possible gas flow rate. For a given flow, the lower the air-to-cloth ratio, the larger the unit. A basic form of this parameter is:

air-to-cloth ratio in ft3/min/ft2 = gas flow rate in ft3/min (actual conditions) / fabric area in ft2

The air-to-cloth ratio can be calculated on a gross or net basis. The gross air-to-cloth ratio calculation uses the total amount of filter media that the bag house contains. The net air-to-cloth ratio calculation accounts for one or more bag house compartments being out of service. This computation uses a reduced filter media area to compute the air-to-cloth ratio.

The quality of the filter media and its utilization are also important. When sizing a dust collection system the following must be considered:

• The gas approach velocity
• Bag spacing and length
• Bag reach and accessibility

The gas approach velocity takes into account the settling of the dust cake and its potential to return to the dust cake surface after a cleaning pulse. The gas approach velocity is directly proportional to the air-to-cloth ratio. For bag spacing and length, the bag arrangement in the bag house and the length of the bag affect the potential settling problems and abrasion problems (pulse jet systems). In general, for a given air-to-cloth ratio, the longer the bag, the greater the gas approach velocity and the greater the potential for settling related design problems.

When designing a reverse air or shaker system, the bag reach and accessibility are important for installation and maintenance. There should be enough space between each bag that they can individually be checked, capped off, or replaced. Also, the maintenance personnel must be able to "reach" each bag. Therefore, there is a maximum distance that is practical between the walkway and the farthest bag.