litre per square metre per hour
One litre of permeate through one square metre of membrane per hour — universally written LMH, and the unit every membrane datasheet, pilot report and design guide uses. Equal to 2.78 × 10⁻⁴ kg/(m²·s) for water.
Watch out: LMH is a VOLUME flux, and it converts to mass flux only by taking a litre as a kilogram — true for fresh water and progressively less true as the stream gets saltier or warmer. It is also always quoted alongside a temperature and a transmembrane pressure, because flux rises with both; a bare LMH figure with neither is not a specification. And normalised flux, in LMH per bar, is a different quantity again.
| 1 | 0.00027777778 |
It survives because it is the number an operator can picture: a membrane running at 50 LMH puts fifty litres an hour through each square metre of area, and the area is what you are buying. Expressed in SI it would be 0.0139 kg/(m²·s), which conveys nothing to anyone standing in front of a skid.
About the litre per square metre per hour
Membrane flux is the single number that sizes a plant. Divide the flow you need by the flux the membrane will sustain and you have the area, and area is what the capital cost is made of. Push the flux higher to save money and you foul the membrane faster, which is the whole engineering tension in the field expressed as one division.
The conversion to SI is for water, from a litre massing a kilogram and an hour holding 3600 seconds. That water assumption is doing real work: on a concentrate stream at 70,000 mg/L the density is around 1.05 kg/L, and the mass flux is 5% above what the volume flux suggests.
Two numbers never travel far from a flux figure and both belong with it. Temperature, because permeability rises roughly 3% per °C — the same membrane delivers substantially more in August than in February, and a plant designed on summer data is undersized. And transmembrane pressure, because flux is very nearly proportional to it, which is why normalised flux in LMH/bar is the number used to track fouling: a plant holding flux while its pressure climbs is not holding steady, it is losing membrane.
One litre per square metre per hour in every mass flux unit
| 0.000277778 | kilogram per square metre per second (kg/(m²·s)) | |
| 0.277778 | gram per square metre per second (g/(m²·s)) | |
| 1 | kilogram per square metre per hour (kg/(m²·h)) | |
| 1 | litre per square metre per hour (L/(m²·h))this unit | |
| 0.0000568934 | pound per square foot per second (lb/(ft²·s)) | |
| 0.204816 | pound per square foot per hour (lb/(ft²·h)) |