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 Process Considerations  Feed Water Conductivity Equivalent (FCE) The concept of EDI feed water conductivity equivalent arose from the need for a simple field method of estimating the ionic load on a EDI device. Certainly the best way to determine the ionic load is to perform a complete water analysis and determine the concentration of all ionized and ionizable constituents, but in some cases this is not practical. What had frequently been substituted for a complete water analysis was a simple measurement of the conductivity of the EDI feed water. This could introduce considerable error, as a conductivity measurement does not detect the full amount of weakly ionized species such as carbon dioxide (CO2) and silica (SiO2). For example, 10 µS/cm water could contain 4 ppm of NaCl or 60 ppm CO2. Just as this would have a huge impact on the service cycle of a conventional demineralizer, it can have a major impact on sizing a EDI system. For this reason we have developed the concept of EDI feed conductivity equivalent, which attempts to take into account weak ions such as CO2 and silica. Calculating FCE Measure EDI feed water conductivity (µS/cm) Measure the ppm CO2 ppm as CO2 x 2.79 = µS/cm Measure the ppm SiO2 ppm SiO2 x 1.94 = µS/cm Add measured conductivity to CO2 & SiO2 µS/cm In the field, the CO2 concentration in the EDI feed water can be measured using Hach test kit Model CA-23 (#143601). The smallest increment for this test kit is 1.25 mg/l.  The EDI feed water conductivity can be measured with a hand-held conductivity meter such as the Myron L Model 4P or with the permeate conductivity meter on the RO system. Example FCE Calculation Measured conductivity = 6 µS/cm Measured CO2 = 5 ppm as CO2 Measured SiO2 = 0.5 ppm as SiO2 FCE = 6 + (5 x 2.79) + (0.5 x 1.94) = 20.9 µS/cm DC Volts and Amps Voltage (potential) causes current to flow Current causes transfer of salt, regeneration of resin Amount of current required proportional to product water flow, amount of salt being removed Use Faraday’s law to calculate current required Calculating Amps Required Faraday’s Law I = 1.31 (Q)(FCE)/(# cells)(eff) I = DC current, amps (per module) Q = product flow rate, liters/min/module FCE = feed conductivity equivalent, µS/cm eff = current efficiency, % (assume 10% current efficiency) Example DC Current Calculation Product flow = 50 lpm Feed = 5 µS/cm + 3.75 ppm CO2 FCE = 5 + (3.75 x 2.79) = 15.5 µS/cm I = 1.31(50 lpm)(15 mS/cm)/(24 cells)(10%) = 4.2 amps per module Setting DC Amperage Use constant current power supply Estimate amps required per module Set amperage to calculated value Power supply adjusts voltage to maintain current Important to record (trend) voltage & amperage Definition of EDI Recovery % Recovery (R) = (QP)(100)/(QP + QR) QP = Product (Dilute) flow rate QR = Reject (Concentrate) flow rate Calculating Reject Flow Reject = ((100 - R)/R) x product flow where R is the percent recovery Example: Product flow = 50 m3/h Recovery = 90% Reject = ((100-90)/90) x 50 = 5.6 m3/h