Electrolyte circulation

Type follows from how much flow you need, how accurate it has to be, and how leakage is contained. Centrifugal pumps push with a spinning impeller: high flow and low cost, but the pressure falls away as flow rises. Positive displacement pumps (gear, diaphragm) move the same volume every revolution, so flow tracks speed and holds up against pressure. Circulation therefore goes to centrifugal pumps, and make-up, where small doses have to be accurate, to positive displacement.

Micro gear

Displaces the volume between the teeth. For small flows that must track speed accurately

Brushless centrifugal

Pushes with a rotating impeller. High flow, low cost. The standard for 2 kW stack circulation

Vertical multistage centrifugal

Stacked impellers build pressure. For 30 kW scale, where flow and pressure are both needed

Magnetic drive gear

No shaft through the casing, so nothing leaks at a seal. For accurate small doses such as make-up

Brushless diaphragm

Reciprocates a membrane. Self-priming and high pressure, at the cost of pulsation

Manufacturer catalogues usually state that pressure as head. Head is the pressure expressed as the height of water the pump can lift, and on water roughly 10 m is 1 bar. So 32 m of head is about 3.1 bar and 16 m is about 1.6 bar. The table uses those converted figures so the column reads in bar throughout. Read it as the force available to overcome the resistance of the pipework and the parts in the loop. The stack alone drops about 0.11 bar at 4 L/min, and the filter, heat exchanger and pipework add to that. Our preliminary selection basis is therefore a variable-speed pump with at least 0.3 bar of available differential at 4 L/min. The maximum head of a centrifugal pump is its value at zero flow, so the real duty point is read off the flow–head curve. On a vertical multistage pump that curve shifts entirely with the number of stages and the drive frequency, so rather than print one figure we set the duty point from the manufacturer curve. Flow reads the same way. In a Wilo MVI part number the leading digits are the nominal flow in m³/h and the last two are the number of impellers, so MVI-202 is nominal 2 m³/h in two stages; the manufacturer manual sets out that key. The table carries that nominal figure only, and what the pump will actually deliver is settled at the duty point.

Circulation flow has to be trimmed while running and ramped gently at start-up, so variable speed is a condition rather than an option. Which method sets the speed also decides the control panel wiring, so it is fixed before the order goes in.

Front panel

Speed set on the pump's own display or dial. No external signal wiring

0–5 V

An analogue voltage from the controller sets the speed. The most common PLC interface

PWM

Pulse width sets the speed. The signal line tolerates electrical noise well

Potentiometer

A hand-turned resistor fixes the speed. For sites with no controller

RS485 (MODBUS)

Two-wire serial communication, with MODBUS as the protocol on top. Unlike an analogue voltage, which only sends a setpoint, it also reads the actual speed and alarms back

Inverter

Varies the frequency to a three-phase motor. Vertical multistage pumps take a separate inverter

Hot KOH degrades common materials quickly. The catch is that which material arrives depends on the order code rather than the model number. The Boxer 19K takes a nitrile, FKM or EPDM diaphragm, and only the EPDM build is usable. The Wilo MVI splits between a 304 and a 316L body, and the seal choice, EPDM or FKM, changes even the permitted liquid temperature: -15 to 120 °C with EPDM, -15 to 90 °C with FKM. Both sets of choices are printed in the manufacturers' own published data. Other models publish no material choice at all. For those we do not guess at the options: we confirm what the standard material is, ask the manufacturer whether it holds under our operating conditions, and order on that answer. Where the driver sits sets a temperature limit as well. Brushless pumps come with the controller built into the body or mounted separately, and the manufacturer data says the built-in build suits low-temperature media and environments while the external build suits high-temperature ones. That is why we order the external build for hot KOH. The same model number splits here too. Temperature divides at the same point. The liquid temperature column is the limit for the parts the electrolyte touches, but some manufacturers publish only an ambient figure. Rather than guess, we leave those as not published: the Lead Fluid CT3000F states ambient 0 to 40 °C and the Nanjing magnetic drive gear pump ambient -30 to 50 °C only. Our condition is 40 to 60 °C, so for both we confirm the liquid temperature with the manufacturer before ordering.

Generally survives

Fluoropolymers (PTFE, FEP), PEEK, PPS, EPDM, ceramic, 316L stainless

Does not survive

Nitrile rubber and ABS, which is what often arrives as the standard build

Depends on conditions

Polyamide (PA66) and similar, where temperature and concentration change the verdict. We confirm with the manufacturer against the operating conditions

Where the manufacturer stays silent

The two ZKSJ pumps publish only the housing material, PPS+30%GF, and say nothing about the shaft, impeller or seals. We do not guess at options that are not published; we put the operating conditions to the manufacturer and order once we have an answer

Three things are easy to leave out when the loop is built for the first time. The pump has to sit below the liquid level so it is primed at start-up. The tank needs level measurement so you can see the electrolyte falling. And without dry-run protection the pump keeps going after the electrolyte is gone. None of the three is in the pump price, and the loop does not stand without them.