The relationship of gallons per day productivity to membrane area is not fixed for a particular size element. Membrane is manufactured in a range of productivity values that goes from low to very high. Each style of element has a preferred amount of active membrane area for ease of manufacturing. Whenever possible, we manufacture elements with the optimum size and length of materials because that gives us the most consistent production techniques.
Before membrane goes into production we run quality control tests that classify the membrane productivity and rejection characteristics. To fill an order we choose the roll of membrane that comes the closest to using the optimum material lengths. The amount of membrane needed to fill an order might effect the decision to use one roll or another. In other words, we could use a large amount of lower productivity membrane, a medium amount of medium productivity membrane, or a small amount of high productivity membrane to make the same GPD element. All lots of elements have variations in productivity, e.g. ± 15%. Standard products can have overlapping productivity ranges, so it is possible to classify an element either as the low end of one range or the high end of the next lower range. In most cases, we try to classify in the higher range.The typical 112 style element will have approximately 4.5 sq. ft. of active membrane, but the range of active areas can be from approximately 2.9 to 5.0 sq. ft.
FOR EXAMPLE: Given membrane at 5.5 gallons per sq. ft.-day productivity:
GPD = 4.5 sq. ft. area X 5.5 gfd = 24.7 gallons per day ~ 25 GPD
GPD = 3.0 sq. ft. area X 5.5 gfd = 16.5 gallons per day ~ 15 GPD
Given membrane at 11 gallons per sq. ft.-day productivity:
GPD = 3.0 sq. ft. area X 11 gfd = 33 gallons per day ~ 35 GPD
GPD = 4.5 sq. ft. area X 11 gfd = 49.5 gallons per day ~ 50 GPD
GPD = 4.5 sq. ft. area x 22 gfd = 100 gallons per day