SpectraPure
SpectraPure®Inc.
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CALCULATING EXPECTED GPD/LPD FROM THE MEMBRANE
Membranes produce the rated gallons per day (GPD) at 60 psi (4.1 bars) operating pressure, 77°F (25°C) operating temperature and 500
ppm total dissolved solids.
Membrane output gallons per day (GPD) depends on operating pressure, water temperature and the ppm TDS in the feed water.
Expected GPD = Rated GPD × PCF × TCF × OCF
where PCF is the pressure correction factor, TCF is the temperature correction factor and OCF is the osmotic correction factor.
1. Calculation of PCF:The output GPD from the membrane are directly proportional to the applied pressure. Note: The membrane
is rated to produce the rated GPD at 60 psi. For any pressure other than 60 psi the output GPD is multipled by the PCF (Pressure
correction factor).
PCF = Pressure available (in psi) ÷ 60
2. Calculation of TCF: The output GPD also decreases with decrease in temperature. This is because the water viscosity increases with
decrease in water temperature.
The GPD increases by approximately 3% for every °C rise in temperature. (Refer to the following table for TCF values).
Table 1: Temperature Correction Factor
(TCF
3. Calculation of OCF:The output GPD of the system decreases with an increase in ppm TDS of tap water. This is because the osmotic
presssure increases with increase in ppm TDS, and the increased osmotic pressure reduces the net driving force for pure water
passage through the membrane. The osmotic pressure is approximately 1 psi for every 100 ppm TDS.
Note
: Generally speaking, the effect of osmotic pressure can be neglected for most tap water supplies and the OCF can be assumed to
be 1.
Example: What is the expected GPD from a 50 GPD DWS system at 40 psi pressure and 60°F water temperature?
PCF = 40 ÷ 60 = 0.666
TCF = 0.754 (from Table 1)
OCF = 1
Expected GPD = 50 ´ 0.666 ´ 0.754 = 25.1 GPD
Important Note: This calculation makes the assumption that purified water from the system is going into an open tank or reservoir.
Actual System GPD will be lower for the DWS System because of the back pressure exerted by the pressurized storage tank. Addition of
the Permeate Pump Retrofit-Kit (PPRFK-DW, Part #: KT2300-015) eliminates the back pressure and operates your DWS System as if the
product water were going into a open tank.
°F (°C)
TCF
°F (°C)
TCF
°F (°C)
TCF
41.0 (5)
0.521 59.0 (15) 0.730 77.0 (25)
1.000
42.8 (6)
0.540 60.8 (16) 0.754 78.8 (26)
1.031
44.6 (7)
0.560 62.6 (17) 0.779 80.6 (27)
1.063
46.4 (8)
0.578 64.4 (18) 0.804 82.4 (28)
1.094
48.2 (9)
0.598 66.2 (19) 0.830 84.2 (29)
1.127
50.0 (10)
0.620 68.0 (20) 0.857 86.0 (30)
1.161
51.8 (11)
0.640 69.8 (21) 0.884 87.8 (31)
1.196
53.6 (12)
0.661 71.6 (22) 0.912 89.6 (32)
1.232
55.4 (13)
0.684 73.4 (23) 0.941 91.4 (33)
1.267
57.2 (14)
0.707 75.2 (24) 0.970 93.2 (34)
1.304
