Choosing the wrong filter housing is one of the most common — and costly — mistakes in process gas and compressed air system design. An undersized housing creates excessive pressure drop; the wrong material corrodes within months; an incorrect port configuration means the filter simply cannot do its job. This guide walks through the three decisions that matter most: material, configuration, and sizing.
Step 1 — Material Selection: Matching the Housing to Your Process
The housing material must withstand the maximum operating pressure, the peak process temperature, and the chemical aggressiveness of the gas stream. For the vast majority of industrial applications — natural gas, nitrogen, instrument air, compressed air — 316L stainless steel is the correct starting point. It offers excellent corrosion resistance, is compatible with most seal materials, and is available across the full R+F process gas filter range.
Where 316L SS is insufficient, R+F FilterElements can supply housings in exotic alloys:
- Hastelloy C-276 — for wet chlorine, hydrochloric acid vapour, and aggressive reducing environments
- Monel 400 — for hydrofluoric acid service and seawater applications
- Inconel 625 — for high-temperature oxidising environments above 300 °C
- Titanium Grade 2 — for chloride-rich streams where even Hastelloy may pit
Seal material selection follows the same logic. NBR seals are rated to 100 °C and suit most hydrocarbon streams. FKM/Viton seals extend the temperature limit to 200 °C and resist aromatic hydrocarbons. EPDM seals are mandatory for oxygen service. PTFE seals handle the most aggressive chemistries up to 260 °C.
Step 2 — Configuration: Inlet-Only, 2-Port, or 3-Port?
Filter housing configuration determines what type of filtration is possible and how the housing integrates into your pipework. There are three fundamental configurations available across the R+F compressed air and process gas ranges:
Inlet-Only (Catchpot / Liquid Separator)
A single inlet port with no outlet — the housing acts as a dead-end vessel. Liquid droplets and bulk contamination fall to the bottom of the bowl and are drained periodically. Used upstream of sensitive instruments or as a pre-separator before a coalescing stage. No filter element is required, though a mesh screen can be fitted.
2-Port (Standard Particulate or Coalescing)
The most common configuration: one inlet, one outlet, with a filter element fitted inside. Suitable for particulate removal (using RF-P elements) or coalescing (using RF-C elements). The RF-H-150 process gas housing is a 2-port design rated to 100 bar, accepting standard 25178-series elements. For higher pressures, the RF-H-160 (250 bar) and RF-H-170 (400 bar) extend the range into analyser and high-pressure sampling applications.
3-Port (Coalescing with Bypass or Membrane Separator)
A third port — typically at the base — allows separated liquid to drain continuously under pressure, or connects to a bypass line for maintenance without process shutdown. Three-port housings are essential for coalescing applications where liquid loading is high, and for membrane separator installations such as the RF-GMS-170, which uses a PTFE hydrophobic membrane to provide an absolute liquid barrier.
Use our free Engineering Tool to get a filtration recommendation for your specific application in under 2 minutes.
Step 3 — Sizing: Balancing Cost, Pressure Drop, and Service Life
Once material and configuration are fixed, sizing determines the physical dimensions of the housing and element. The key inputs are: gas flow rate (Nm³/h or m³/h free air), operating pressure (bar g), inlet temperature (°C), contaminant type and loading, required filtration level (µm or ISO 8573-1 class), and line size (port connection).
The fundamental trade-off in sizing is between a small housing and a large housing:
| Factor | Small Housing | Large Housing |
|---|---|---|
| Initial cost | Lower | Higher |
| Pressure drop (clean) | Higher face velocity → higher ΔP | Lower face velocity → lower ΔP |
| Element service life | Shorter — element loads faster | Longer — more element area |
| Response to flow surges | Faster — smaller internal volume | Slower — larger internal volume |
| Maintenance frequency | More frequent element changes | Less frequent element changes |
For most process gas applications, R+F FilterElements recommends sizing the housing so that the face velocity across the element does not exceed 0.05 m/s at maximum flow. This keeps clean pressure drop below 20 mbar and provides adequate element life between scheduled maintenance intervals. Use the R+F Engineering Sizing Tool to calculate face velocity and pressure drop for any combination of housing and element.
Port Size and Connection Type
Port size must match the pipeline connection — undersized ports create a permanent restriction regardless of element size. The RF-H series is available with BSP, NPT, and flanged connections from ¼" to DN100. For instrumentation and analyser applications, the RF-H-110 to RF-H-170 instrumentation filter range uses compression fittings (Swagelok-compatible) in ⅛" to ½" sizes, keeping dead volume to a minimum in sample conditioning lines.
Contaminant Type Drives Element Selection
The housing size must also accommodate the correct element type for the contaminant present:
- Solid particulate only → RF-P particulate element (99.99% ≥ 0.3 µm)
- Liquid aerosols and oil mist → RF-C coalescing element (99.99% ≥ 0.1 µm)
- Hydrocarbon vapour / odour → RF-AC activated carbon adsorption element
- High-temperature particulate (up to 450 °C) → sintered metal element
- Sour gas (H₂S) → RF-C K-type element with H₂S-resistant binder
For point-of-use protection of instruments and analysers, the RF-DIL disposable inline filter provides a compact, low-cost solution that requires no housing at all — the element body is the housing, and the entire assembly is replaced at service intervals.
Need help selecting the right filter housing?
- The housing material must withstand the maximum operating pressure, the peak process temperature, and the chemical aggressiveness of the gas stream.
- Filter housing configuration determines what type of filtration is possible and how the housing integrates into your pipework.
- Solid particulate only
Putting It All Together: A Worked Example
Consider a natural gas metering station operating at 80 bar g, 40 °C, with a maximum flow of 500 Nm³/h. The gas contains entrained liquid hydrocarbons and fine particulate from the pipeline. The required outlet quality is ISO 8573-1 Class 1 for particulate (≤ 0.1 µm) and Class 1 for oil aerosol (≤ 0.01 mg/m³).
Working through the three steps: (1) Material — 316L SS housing with FKM seals for hydrocarbon compatibility at 80 bar. (2) Configuration — 2-stage: a 3-port coalescing housing upstream (RF-H-150 with RF-C element) to remove liquid aerosols, followed by a 2-port particulate housing (RF-H-150 with RF-P element) for final polishing. (3) Sizing — at 80 bar g, the actual volumetric flow is approximately 6.2 m³/h; a 25178-series element in the RF-H-150 housing gives a face velocity well within the 0.05 m/s limit, with a clean pressure drop below 15 mbar per stage.
This configuration meets the ISO 8573-1 Class 1/1 requirement with a compact, maintainable installation. For more complex applications — biogas, hydrogen, or oxygen — see the coalescing vs particulate element guide and the ISO 8573-1 quality class reference.
Try our Engineering Sizing Tool → or discuss your requirements with our team.
