20/05/2020
APPLICATION OF FILTERS INSIDE OT - ALL YOU NEED TO KNOW
THERE IS A DIFFERENCE BETWEEN HME AND HMEF
Also read about these in the Humidification UNITS under Physics.
HME is purely for Humidification. They can still trap large particles but not bacteria or viruses.
HMEF have both humidification and filtration properties and can filter out most bacteria and some of the viruses.
Image 1 is of HME and Image 2 is of HMEF
SO, HOW DO HMEF WORK?
Filter material generally consists of fibres formed into a non-woven wad or sheet. There are five main mechanisms by which the filter material removes particles from a flow of gas (Image 3):
a. Interception
Particles will tend to follow streamlines in a flow of gas. However, if a particle in the gas stream comes within one particle radius of the surface of a fibre, the particle will adhere to the fibre.
b. Inertial impaction
Particles have mass and are, therefore, not always able to follow a gas streamline around a fibre due to their inertia. The particles may, therefore, strike the fibre, even though the gas streamline is more than one particle radius from the fibre.
c. Gravitational settling
Large particles in slow-moving air do not follow gas streamlines, but settle due to gravity, and can, therefore, fall onto, and adhere to, a fibre.
d. Diffusion
Small particles do not remain on particular streamlines in the gas but undergo Brownian motion due to interactions with gas molecules. This effectively increases their cross- sectional area and so increases the probability of them striking a fibre.
e. Electrostatic attraction
Some filter material is electrostatically charged during manufacture to enhance its ability to capture particles. There are three mechanisms of capture: charged particles in the gas stream are attracted to oppo- sitely charged fibres; neutral particles are attracted to a charged fibre as the electric field on the fibre induces a dipole in the particles (positive and negative charges on opposite sides of the particles); and charged particles are attracted to neutral fibres by inducing image forces on the fibres.
DO HMEF PROTECT AGAINST CORONA VIRUS (COVID-19)
The size of COVID-19 varies between 60-140nm or 0.06-0.14 micron.
The relative efficiencies of the five mechanisms of filtration vary with the size of the particle (Image 4). In particular, particles of a certain size, typically in the range 0.05– 0.5 μm, pass through the filter more easily than others. This size is known as the most penetrating particle size. Particles of this size are too small to be directly intercepted by fibres and too large to undergo substantial Brownian motion.
So as can be seen from the image, HMEF will not provide protection completely from COVID-19, they will still trap some and reduce the load.
Footnote
IMAGE 3 shows mechanisms of filtration. (a) Interception; (b) inertial impaction; (c) gravitational settling; (d) diffusional impaction; (e) electrostatic attraction
Courtesy - Dr Shiv singh
