The TECTRON™ Cell is one part of the Membrane Electrode System. This Bulletin features a design specification guideline, which is followed by a discussion of each section of the guideline. Where appropriate, options to the specification will also be mentioned. The function of the Membrane Electrode System is to provide the opposing electrode, which drives the electrocoating (ED) paint process and to maintain the proper pH of the ED paint bath.
Approved Materials: Use only materials specifically approved by the vendor of the ED paint.
Design Specification: A basic UF system specification shall include the following major items: TruFlux UF Machine, Sepro or Koch UF brand 7640 type UF Elements, permeate storage tank, ter source.
Membrane Electrode Cells: Use the 4:1 Ratio or Average Electrode Current Density method to calculate the amount of Electrode area. Cell configuration and length determine the number and spacing of Cells.
Electroylte Holding Tank: 304 stainless steel with volume approximating the electrolyte in all the Cells.
Circulation Pump: Vertical CPVC pump design for about 2 lpm (0.5 gpm) per Cell at 1.5-2 bar (22-28 psi).
Controls: Conductivity controller operates DI water valve to dilute electrolyte when necessary.
Piping: PVC supply and return manifolds.
Mechanical Support: Rub rails, Cell support, and other related items.
Electrical Connections: Quick disconnects for cable leads, compression washers, and diodes for multi-zone systems.
DI Water: Meet necessary quality level and have adequate flow rate.
Membrane Electrode System Design Specification Guideline
An Membrane Electrode System shall include the following items: TECTRON Membrane Electrode Cells, holding tank, circulation pump, controls, plumbing, mechanical support and protection, electrical connections, and DI water source.
The function of the Membrane Electrode System is to serve as the opposing electrode and also maintains the proper pH of the ED paint bath. With cathodic ED paints the Electrode is an Electrode (i.e. the ware is the cathode) and the Membrane Shell removes anions (i.e. small negatively charged ions). For anodic ED paints the opposite applies in each case.
Direct current is supplied by a DC rectifier to each Cell. Current is then transmitted through the electrolyte solution, membrane, paint, and eventually the deposited paint film. The current then travels back to the rectifier through the conveyor, brushes, and cables.
A pump circulates an electrolytic fluid called “electrolyte” because it is comprised of ions and DI water. This fluid is responsible for transporting the ions removed from the paint bath to drain. It also cools off the face of the electrode because there is heat generated in this process. Note that there is no separate heat exchanger for the electrolyte.
A conductivity controller continuously monitors the electrolyte conductivity (units are Siemens/cm or Mho/cm). The controller instructs a DI water solenoid valve to open when the conductivity rises above a set point. The tank has a natural overflow opening, which allows excess electrolyte to leave the system and thus the electrolyte’s conductivity is diluted. When the controller senses a level below the setpoint, the valve closes.
First Class Quality & Approved Materials
All material and workmanship shall be first class. The following materials are generally approved for use with ED paint: PVC; 304 stainless steel (except for Electrodes, which are to be 316L stainless steel, or better); polypropylene; polyethylene; hypalon; viton; Teflon; neoprene; and EPDM. If there is any question, then the ED paint supplier should be consulted first.
The amount of Electrode area shall be calculated using the 4:1 Rule, which sates that the Electrode area shall equal one-quarter of the total painted surface area that passes one point in a two (2) minute period. See the example below:
Work Area Basis
= painted through-put rate x 2 minutes
= m2/minute x 2 minutes
Electrode Area Basis
= WAB 4
If the desired film thickness is less than 22 (0.9 mil) or more than 28 (1.1 mil), then an alternate method may be employed. First, the paint deposition factor (amp-minute/m2-micron, or amp-minute/SF-mil) must be known. TECTRON HD™ Cells have been operated for many years now up to levels of 50 amps/m2 (approximately 5 amps/SF) and the objective of this method is to estimate the actual current to be expected when the e-coat system performs work. The calculation is as follows:
= painted through-put rate x deposition factor x film thickness
= m2/minute x amp-minute/m2- x
For high speed, high painted through-put systems, typical Electrode current densities are set at about 35 amps/m2 (approximately 3.5 amps/SF) and slower, lower painted through-put systems approach the higher Electrode current density.
= estimated current 35 amps/m2
Some automotive firms have revised their specifications and now require the use of 2.5 minutes (not 2 minutes) when employing the 4:1 Rule.
Membrane Electrode Cells
The Membrane Electrode Cells shall be TECTRON Cells manufactured by UFS Corporation. The only metallic portion shall be the Electrode. All other components shall be made from entirely non-metallic, light-weight, non-conductive materials. The ion-exchange membrane shall selected based upon the type of ED paint and the expected duty cycle:
The Electrode shall selected in a similar manner:
The effective length of the side Cell shall be at least as tall as the height of the work package envelope. If possible, the effective length of the Cell should be equal to the height of the work package + submergence (distance from liquid level to top of work package envelope) + 50 mm (2”). The Cell can be made in any length up to 2.9 m (114.2 in) as an individual unit, with the standard lengths shown below:
Cells can also be ganged together to span up to about 6 m (236.3 in). In a conventional Membrane Electrode System the Cells are placed along the side walls of the ED tank. The number of Cells can then be easily established:
Number of Cells
= Electrode area area/Cell + 2 Cells
= m2 m2/Cell + 2 Cells (round up to an even number)
Newer ED systems as well as higher through-put systems are employing Electrode cells not only on the side walls of the ED tank, but also on the floor and above the roof of the auto body. This is being done for several reasons: reduce paint consumption, improve film build on roof and interior, and lower energy consumption.
Cell Layout Spacing - Monorail
The first Cell is placed at the end of the Pre-wet Zone, which is generally 10 to 20 seconds past the point where the ware is fully submerged (LPI). The last Cell is generally at the point where the ware breaks thought the liquid level (FPO).
The first 3 to 5 Entrance Cells should be at the minimum spacing. The spacing of the last two Cells should be at and exit can be at the 1-1/2 times the minimum. The balance in the Cells in the middle should be spaced accordingly.
Cell Layout Spacing - Monorail
Cells are generally placed along the two long sides of the ED tank. For Ed tanks with an aspect ratio closer to 1 (i.e. square tank as seen in the plan view) Cells can be placed on all four walls. Add to this section. In either situation, the Cells generally begin near the placement of the edge of the ware and extend to the other edge of the ware.
The holding tank shall be constructed from 304 stainless steel. All wetted seams shall be double-welded. A baffle shall be used to separate the pump from the returning electrolyte. A tank skimmer shall be used to remove floating debris. It shall have a removable lid for inspecting the inside of the tank. A strainer shall be fitted to the inlet of the tank (from the return manifold) above the usual liquid level. A stainless steel stud shall be welded to the tank for grounding purposes.
The pump shall be a seal-less type vertical CPVC style. The pump flow rate shall be calculated by using 8 l/m2 (2 gpm/10 SF Electrode area) and then adding 20% as a safety factor. The pump head capacity shall be at least 1.5-2 bar (22-28 psi), more if the pump is located more than 3 meters (10 feet) below the rim of the ED tank. There shall be a pump by-pass loop back to the holding tank with a throttling valve. The electric motor shall be 3 phase, 460 volt, TEFC style. The required flow rate for any horizontal Cell needs to be about twice, or 16l/m2 (2 gpm/10SF) in order to completely purge oxygen for the Cell.
The electrolyte circulation system shall be fitted with the following controls: 0-10,000 (or 0 to 1000 milli Siemens/cm) microSiemens/cm analog conductivity controller, plastic/stainless steel conductivity sensor, 0-2 bar (0-30 psi) guarded pressure gauge, roto-meter flow meter, check valve, main control valve (NO), 110 volt DI water solenoid valve, low tank level switch, and tank drain valve. The conductivity controller should be located near eye level about 1.5 m (5’) away from the holding tank.
Electrolyte Manifold Piping
All piping shall be PVC. Supply Manifold branch piping (i.e. on each side of the ED tank) shall be at least a PVC 50 mm (2”) Schedule 80 minimum and sized so that the average flow rate is no more than 0.25 – 0.5 meters/sec (3 –5 ft/sec). The size of the Supply Manifold main branch piping to the tee (i.e. where the branch piping begins) should be at least one size larger than the branch piping.
The Return Manifold branch piping shall be at least 75 mm (3”) PVC Schedule 40 minimum with PVC DWV type fittings. It shall be sloped downwards (i.e. towards the electrolyte holding tank) at a 21 mm per meter (¼ in per foot) slope and sized so the branch piping is never more than ¾ full. The size of the Return Manifold main branch piping to the tee (i.e. where the branch piping begins) should be at least one size larger than the branch piping. A 0-2 bar (0-30 psi) guarded pressure gauge shall be placed at the termination of each supply manifold leg. A siphon-breaker shall connect the supply and return manifold and there shall be at least a 50 mm (2”) vent located 200 mm (8”) above the top of the Cells.
The side Cell support strut channels shall be 41 mm square (1.625”) and made from steel. Cell support channels shall be supported at least every 1.5 m (5’). Two-piece clamps (use two clamps for each Side Cell) hall be used to attach Cells to the strut channels. Supply and return manifold shall be supported with the same type of strut channel every 1.5 m (5’). Metal two-piece clamps should be used to attach the manifolds to the strut channel. There shall be a FRP or PVC Schedule 80 (no more than 25 mm [1.5”]) OD rub rail located such that there is at least 250 mm (10”) gap from the ED tank wall to the rub rail. The Electrolyte holding tank shall be placed on a flat, level pad as close to the ED tank as possible.
The cable shall have a THHN, THWN-2, Oil and gasoline resistant, and MTW type inslutation. and be sized for at least 15 amps/m (5 amps/foot) of Cell length. All washers shall be made from stainless steel and be a compression type. There shall be a quick connect built into the cable lead for each Cell (does not apply to Hoist type ED tanks). Several Cell cable leads may be ganged together into a copper set screw lug. For systems with more than one voltage zone, diodes shall be used with the Cells in the lower voltage zone. The rating of the diode shall be twice the application voltage and 1.5 to 2 times the application amperage.
The supply of DI water shall be 60 to 80% of that of the circulating pump. There shall be a UV light source to minimize the existence of bacteria and fungus. There shall be a means to easily clean the UV bulb. DI water quality shall meet the requirements of the ED paint manufacturer. Carbon filter is also required to remove organic matter from the feed water.
DI water usage is a function of the following variables: coulombs consumed by the ED system, electrolyte conductivity setpoint; MEQ value of the replenishment ED paint; and the specific neutralizer used in the ED paint.
DISCUSSION OF MEMBRANE ELECTRODE SYSTEM COMPONENTS
Holding Tank –The function of the electrolyte holding tank is to act as a reservoir, in order to maintain a near steady-state conductivity level and also cool (from the ambient) the electrolyte fluid. The volume of the tank should approximate the total volume of the electrolyte in all the Cells. The smallest tank volume should be about 100 l (25 gal).
The baffle keeps foam away from the pump and skimmer removes floating debris. A nylon, or equal, strainer bag, maybe 40 mesh or less, is used to collect dead fungus. The bag should be located above the liquid level so a maintenance person can easily remove and clean it. The tank needs to be grounded to avoid potential electric shock injury.
Circulation Pump – A horizontal pump is not recommended because if there is ever a membrane cut, paint solids will enter the Membrane Electrode System and cause fast wear on the pump seal. A vertical CPVC pump, on the other hand does not use mechanical seals and is not affected by cut contaminated electrolyte solutions. The pump suction piping should not be smaller than the suction opening of the pump. It should include a foot valve (butterfly check valve) and inlet strainer. The electric motor should be a 3 phase, 460 volt, and TEFC style.
Generally the more electrolyte flow the better because this creates: greater turbulence inside the Cell (scrub oxygen off face of electrode) and more cooling of the Electrode, which lead to greater life. Note that for Low Profile Cells (i.e. those Cells with a Bulkhead Fitting) the pressure drop across the Cell should be less than ½ Bar (7 psi) to avoid damage to the membrane.