Dust collector

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A highly speculative image of an array of dust collectors feeding atmospheric compressors on Mars. Sized for 50 m3/s of martian air, or about 1 kg/s.

A dust collector is a device used to separate dust from air in an atmospheric processing system. In some cases clean air is the desired result, in others, the dust is the object of interest.

Most dust collectors operate below atmospheric pressure with the fan on the clean side, after the dust separation system. In the thin martian atmosphere, the available pressure is very low, much lower than the standard operating pressure of dust collectors. So the fan might be required to be put on the dusty side, before the filters.

Dust collectors are filtration systems. For effective filtration of small particles they will often operate in cascade fashion, with a pre-filter and secondary filters.

Prior to the first stage the air can be routed through a torturous path with baffles. The incoming air hits the baffle and dust particles presumably fall down. This wikipedia article also states that heavy particles get flung to the outside wall and slide down to the bottom. It also mentions that smaller tube diameters give a higher speed for the air, causing greater separation. And that multiple stages of centrifugal air separation tubes cleans the air that much better.[1]

In the traditional first stage, cyclonic separation, a type of inertial separation, can remove a large fraction of the heavier particles.[2] To observe how a cyclone separator works, put some dirt into a glass of water and swirl it. The dirt collects in the center of the bottom of the glass. A cyclone separator puts high speed air into a vertical tube and spins the air around the edge of the tube. The particles fall down, clean air rises out.

HEPA grade filters can filter down to 5 microns, removing bacteria. However they have no effect on gases, only dust and particulate matter.[3] Filters can clog over time. A way to handle that is with paddles that knock the particles off of the filter. This company has a cyclone dust extractor with a huge paper filter for the exhaust and there is a hand crank that swirls a paddle around inside the filter to knock dust off of the filter. They claim that their 1 micron filter is viable for years. The motor on top is connected to an impeller inside the casing.[4]

Electrostatic precipitators[5] might be more effective than filters in removing the fine particulates in the martian atmosphere, and are part of most process proposals. Electrically charge the air particles and drive them to a plate of the opposite charge. Periodically clean the plate by vibrating it. This works best with a slow air flow so that the particles have time to be charged and attracted to the plates.

Dust collectors might be used in a number of cases:

  • Extraction of in situ resources from the martian atmosphere.
  • Cabin filtration in vehicles.
  • Airlock pump down systems.
  • Interior air filtration of a settlement.
  • Interior air filtration of settlement production facilities.

The background (average) dust loading of Mars is estimated at 1,8e-7 kg/m3[6]. A dust collector treating 1 kg/s of martian atmosphere, about 50 m3/s, would need to remove 283 kg of dust per year. This would be sufficient to produce the propellant for 40 Starship type vehicles. So for a single vehicle about 7 kg per year would need to be removed from about 1,1 m3/s (2400 cfm).

Dust collection average background
Volume flow rate m3/s 50
dust loading kg/m3 1.80E-07
dust capture kg/s 9.00E-06
per day s 86400
kg/day 7.78E-01
per year kg/year 283.82

These figures would go up during dust storms, except for a solar powered settlement that would probably need to cut back on propellant production during storms.

The MEDUSA instrument[7], part of ESA EXO Mars 2020 mission, is designed to measure in situ the dust conditions of the Martian atmosphere.

Electrostatic dust collectors

An alternative to cartridges and bag filters is the use of electrostatic separators, that do not require air pressure differences to operate[2].