What is a housing?

A housing is a vessel manufactured from any of a variety of materials (stainless steel, carbon steel, plastic, etc) and is designed to hold a specific type of filter(s). A housing holds a certain type and number of filters of a particular length that use a specific sealing mechanism. The housing needs to be able to accommodate the pressure of the fluid and type of fluid being filtered.

The purpose of a housing is to secure the filters in place and seal them as required by the fluid and application. The material must be consistent with the fluid being filtered and must not corrode or affect the quality of the fluid. The housing is designed to direct the flow through the filters for maximum filtration efficiency.

Air Sizing
In order to properly size a housing for an air or gas application, the following items must be known:

• Filter type to be used in the housing
• Filter media to be used in the housing
• Type of gas
• Flow rate
• Pressure
• Temperature
• Application
• Filter seal material
• Housing seal material
• Continuous or batch operation
• Direction of fluid flow

If the application is for high purity, the materials of construction are also critical. Finally, the connections in and out of the housing are sized to accommodate the flow rate.

Liquid/Water Sizing
In order to properly size a housing for a liquid/water application, the following items must be known:

• Filter type to be used in the housing
• Filter media to be used in the housing
• Type of gas
• Flow rate
• Pressure
• Temperature
• Application
• Filter seal material
• Housing seal material
• Continuous or batch operation
• Direction of fluid flow

If the application is for high purity, the materials of construction are also critical. Finally, the connections in and out of the housing are sized to accommodate the flow rate.

If errors are made in the sizing of the housing, a myriad of issues may result. Here are some common errors and the resulting issues.

Housing Material of Construction Compatibility
If the housing material is not suitable for the application, corrosion, over-pressurization, and/or contamination of the fluid may occur.

Seal Material Compatibility
The seal materials of both the filters and the housing are based on the application. Should the seal materials be unsuitable, they can break down. This could cause contamination of the fluid, incomplete filtration as the fluid passes through the open gaps where the seal was, and leaking of fluid out of the housing.

Use the Correct Filter Type
If the filter type used in the housing is incorrect, the sealing between the filter and the housing may be incomplete, allowing the fluid to pass to the filtrate without going through the filter.

Connection Size
If the connection sizes are too small the desired fluid volume may not be able to move through the piping and connections to the filters themselves. In addition, water hammer and rapid flow through the filter elements could be issues if the demand for water downstream of the housing exceeds the ability of the housing.

There are advantages to oversizing a filter housing. Building in this safety cushion can save you if the actual flow rate ends up higher than that used to size the housing. And, should the demand for the fluid increase (and therefore the system has an increase in flow rate), the housing can accommodate this without adding equipment. An oversized housing often has a longer service life and requires less maintenance than one that just meets the demands of the system.

The cost of a housing depends upon the materials of construction and if it needs to be ASME coded. But, the initial cost of the housing is not usually the primary decision-making parameter for choosing a housing for a particular application.

In many instances the higher initial cost of a housing is more than offset by the longer life, lower maintenance, and safety of a high quality housing. In addition, there are often regulatory reasons for choosing one housing over another.

For example, in some high purity and sterile applications a 316L electro-polished stainless steel is required by industry regulations. For many standard industrial applications, a 304 stainless steel housing is adequate. Plastic housings are preferred for applications that require a low potential for a reaction between the fluid and the housing material.

Flow rate is defined as the rate at which a product is passed through a vessel or system and is generally expressed in gallons per minute, barrels per hour, barrels per day, actual or standard cubic feet per minute, actual or standard cubic feet per hour, actual or standard cubic feet per day, etc.

The flow rate of the fluid being filtered is critical in selecting the proper housing. After the filter type, sealing mechanism, and filter media are selected, the proper number and length of filters is determined. The volume of a particular fluid a filter can process depends on the type of fluid being filtered. Based upon the results of the filter determination, a housing that can hold the determined number of filters and can accommodate the filter lengths and sealing mechanism is then chosen.

The inlet and outlet connections on a housing are sized to accommodate the type of fluid, the pressure of the fluid, the temperature of the fluid, the material of construction, and the volume of fluid flow per unit of time. Typical dimensions utilized for the diameter of the inlet and outlet connections are inches, millimeters, and centimeters.

The inlet and outlet connection sizes of a housing can sometimes determine which housing will be chosen for an application. If all of the other requirements are met (the housing can accommodate the type of fluid, fluid pressure, fluid temperature, the materials of construction, and fluid flow rater) the required connection size is then considered to determine which housing or housings will be best. If the calculated connection size is not available, the next highest connection size should be used.

When determining connection size, the actual or next larger connection size is typically the optimal one for a housing design. If the connection size is too small, the desired fluid volume may not be able to move through the piping and connections to the filters. The same problem may result if the inlet pipe is bushed down to the inlet connection dimension of the housing. In addition, water hammer and rapid flow through the filter elements could be issues if the demand for water downstream of the housing exceeds the capacity of the housing. Choosing a larger diameter connection is acceptable in most instances.

The flow design of a housing is the path through which the fluid must flow in order to pass from the inlet of the housing through the filter and to the outlet. The type of housing flow design selected is determined by the flow path that best suits the application and the type of filter utilized in the housing. The most common flow designs are in-line and T-type.

In-Line
A housing with an in-line flow design has two different descriptions, depending on the industry.

In most industries, an in-line housing is one with the inlet and outlet connections directly across from one another. The fluid flows through the inlet connection and to the outside diameter of the filter. The fluid is then filtered and enters the inside diameter of the filter and passes out of the filter housing via the outlet connection. The seal prevents crossover from the inlet to the outlet.

In the hydraulics/lubrication industry an in-line housing is connected horizontally to the piping and generally accepts one porous stainless steel element. The inlet connection is on the upstream side end of the filter housing and the outlet connection is on the downstream side.

T-Type
T-Type is generally used in the hydraulics/lubrication industry to refer to a housing with the inlet and outlet connections directly across from one another. The fluid flows through the inlet connection and to the outside diameter of the filter. The fluid is then filtered and enters the inside diameter of the filter and passes out of the filter housing via the outlet connection. The seal prevents crossover from the inlet to the outlet. This set-up is often called in-line in other industries.

The pressure drop across the housing must be considered in the design of a filtration system as the pressure profile at all points in the system is important to system operation. The pressure drop is the difference between the pressure at the inlet of the filter and the pressure at the outlet of the filter. This value will increase as the filter becomes dirty. Flow is restricted as the filters trap dirt and become clogged. In order to restore the filtration system to its initial conditions, the filters require replacement or filter backflushing at a predetermined pressure drop. These methods restore the filtration system to it original condition and the filtration cycle continues in this manner.

Aluminum
Aluminum is a white metal that is readily available in rod and bar form for machining of filter heads and bowls. It is used for impact extruded bowls and is cast as an aluminum alloy for filter heads. It is used in low to medium pressure applications (generally up to 3000 psi) and is compatible with oils and many industrial chemicals. Aluminum is not used in the presence of caustic fluids.

Brass
Brass is an alloy of copper and zinc. A typical brass alloy, yellow or cartridge brass is 70% copper and 30% zinc. Brass is machined and used for heads, or is cast for the same application. Brass it typically used in low-pressure applications. Brass is used with stainless steel bowls in applications to prevent galling. Brass housings are used in water applications, but are being replaced with stainless steel in most applications.

Carbon Steel
Carbon steel is an alloy of iron and carbon, with other alloying elements. Carbon steel heads are mostly cast steel heads. Carbon steel bowls are typically forged steel bowls. Steel is a cost effective and strong material. It is used extensively with oils. If uncoated, steel is susceptible to rusting when used in the presence of water.

Cast Iron
Cast iron refers to a family of iron and carbon alloys that contain more than 2% carbon. There are many forms of cast iron. The most popular type used as castings in the filter industry is nodular, or ductile iron. Ductile iron castings are used for filter heads in the hydraulics and lubrication industry. These heads are generally rated up to 6000 psi.

Stainless Steel
Stainless steel is mainly an alloy containing iron, chromium and nickel. There are many stainless steel alloys used in the filtration industry. The most common is 304 stainless steel. This grade contains about 18% chromium, 8% nickel and is used in applications requiring corrosion resistance. For greater corrosion resistance, 316 stainless steel is specified. This grade contains 17% chromium and 12% nickel. For ultra-high pressure applications, 17-4 PH stainless steel is specified. This grade is a precipitation-hardened material that must be heat treated to gain its maximum tensile properties. Filter heads and bowls made from 17-4 PH combine the corrosion resistance of stainless steel with the strength of a precipitation hardenable alloy. Filter housings made from 17-4 PH are rated at pressures up to 15,000 psi.

Plastic
Plastic is a generic term that refers to a man-made, hydrocarbon-based polymer. Plastics used most commonly in the filtration industry as housing materials include polypropylene, polytetrafluoroethylene, styrene, and glass-filled polypropylene. Plastics are easily molded and are relatively inexpensive. Plastics are also compatible with many industrial fluids, including several corrosive fluids that are corrosive to steel and iron. The disadvantage to plastics is the relative low strength when compared to metal housings and their low impact strength, especially at low temperatures.

The housing connection type determines how the housing will be plumbed into the process stream. Connections are determined by:

• Process flow
• Pressure conditions
• Piping materials of construction
• Housing material of construction

For example, for low-pressure applications in the process water industries (<125 psi), PVC plastic piping may be used and an NPT connection to a plastic housing may be the most appropriate. For ultra high-pressure hydraulic systems (>10,000 psi) carbon steel piping and SAE connections that can handle the pressure would be applied.

BSP
British Standard Pipe, or BSP connections are used mostly in Europe for applications typically less than 5000 psi. The thread design is tapered and utililizes a 55º included angle. This is not compatible with the popular National Pipe Thread (NPT) connection. These connections do not utilize a gasket or o-ring seal. An appropriate thread sealant may be used.

Flanged
Flanged connections are typically used for connections larger than 2". Flange connections take the place of connections using a hexnut. As the size of hex nuts increase, the size of the wrench required for tightening this connection increases. Flanges are annular metal plates with through-bolted connections that connect the pipe to the housing. Flanges are rated at different pressures and are available as split flanges (typically for larger sizes).

NPT
National Pipe Thread connections are the most popular connections used in industry. NPT connections are tapered thread design and do not use an o-ring or gasket for sealing. An appropriate thread sealant may be used. This is not compatible with BSP connections.

SAE
The Society of Automated Engineers (SAE) has specified a number of connections that utilize a SAE-defined thread size and profile, and an o-ring for sealing. These connections are typically used for automotive and high-pressure applications.

Sanitary
Sanitary connections are commonly used in the pharmaceutical, dairy and food processing industries. Sanitary connections utilize thread-free, crevice-free flanges welded to the upstream and downstream pipe, with an o-ring or gasket to provide the seal. An external tri-clamp applies radial pressure to the two flanges. The wedge nature of the clamp transfers this radial force to an axial force on the o-ring to create a seal. Most sanitary connections are electropolished stainless steel.

Housing connections are selected by application and based upon requirements for the individual industry. The primary considerations in selecting a connection are:

• Flow rate
• System pressure
• Piping material and size
• Allowable housing pressure loss or pressure drop
• Housing material
• Industry governing laws or rules related to acceptable connections

For example, in the pharmaceutical industry, components that touch the product are generally made from stainless steel and the connections for housings used are generally sanitary connections. In this case, the industry governing rules takes precedence in selection of an appropriate housing connection.

In sizing a housing for a process gas filtration application, one must determine the system flow and pressure, then match an appropriate housing to the required flow and pressure, taking into account the standard piping for this process. A housing connection is selected to minimize the pressure drop loss from entering and exiting the housing. A good general rule of thumb is "you can bush a housing down but you cannot bush up." This means that it is better to select a port size larger than the piping size connecting to the housing, but not recommended to bush the piping size down to accommodate the housing. It is acceptable to throttle the inlet, but not to expand the outlet of a housing. There are exceptions, such as when the flow rate is so low that a smaller connection size will not negatively affect housing pressure drop.

Connection sizes larger than 2" generally become flanged connections. Threaded connections larger than 2" become expensive, heavy and impractical for most applications. Flanged and split flange connections are less costly and are rated in a range of pressure ratings.

Many filter manufacturers offer connections in keeping with industry standards. The users task is to select the connection that makes the most sense for them from the manufacturers offerings.