COVID-19 English to Spanish Medical Medical: Instruments felts

Buenos días,

Espero que puedan ayudarme con una expresión al español: “felts” relacionada con el mantenimiento de un concentrador de oxígeno. Si pudieran también con el término "Inlet Pre-Filter". Muchas gracias:

“Periodic Maintenance for your unit
- Clean the air filter at least once a week.
- Change the felt of the Inlet Pre-Filter at least once half a year. Follow those steps: Open the Important Safeguard pull out the Inlet Pre-Filter, Pull out the old felts from the Inlet Pre-Filter (two sides). Then insert the new felts to the Inlet Pre-Filter each side, insert the Inlet Pre-Filter and close the Important Safeguard.”

GVL