Alkali-resistant pumps, tanks, tubing and flow measurement
The loop that carries KOH electrolyte through the stack and back to the tank. The same pump can be built with different rubbers and metals inside. What we do is not sell the pump, but specify the build that survives hot KOH and order it that way.
What this part does and what decides it
Our stacks circulate KOH electrolyte on the anode side only, and hydrogen alone leaves the cathode. What sets the circulation rate is heat and differential pressure, not water consumption. The electrolyte carries the heat out of the stack, so too little flow lets the temperature rise; too much raises the pressure drop across the stack, widening the difference between the anode and cathode sides, and KOH crosses the membrane to the cathode. Hence the 4 L/min standard. A pump trades how much it moves against how hard it pushes. The more the outlet is restricted, the higher the pressure and the lower the flow. So the catalogue's maximum flow is the figure with nothing in the way, and its maximum head the figure with the outlet closed and no flow at all — neither appears in real operation. So we select on the pressure a pump can actually deliver at 4 L/min, not on the catalogue figures. That value is only settled once the pipework design exists, so tell us your conditions and we work it out. The same product name can also ship in a material that does not survive hot KOH, so we send the operating conditions to the manufacturer, confirm the wetted material and the seal, and order that build. The wetted materials column in the table records that check.
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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 at 2 kW is centrifugal by default, while a test cell's very small flow, an operation that has to hold the flow accurately, and make-up where small doses have to be accurate, go to positive displacement. The two rows in the table below are both positive displacement. The centrifugal pump for standard 4 L/min circulation has its wetted material and seal set by the order code, so we specify that build against your conditions and quote it.
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. When we quote a centrifugal pump we apply that conversion so the pressure reads in bar throughout. 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, once the pipework design exists. The gear pumps in the table do not lose pressure with flow that way; their figure is a transfer limit instead.
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.
Hot KOH degrades common materials quickly. The catch is that which material arrives depends on the order code rather than the model number. Body material, and the seal or diaphragm material, are set by the order code, and whether the seal is EPDM or FKM changes even the permitted liquid temperature. The default build is not always the alkali-suitable one. 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 only. Our condition is 40 to 60 °C, so we confirm its liquid temperature with the manufacturer before ordering.
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.
The options and the models we handle
Start here and we adjust to your conditions. We can usually source what is not listed.
A micro gear pump with the electrolyte tank and flow measurement as one set. A centrifugal pump cannot hold a flow this small, so circulation uses a gear pump here too. The HXS-0 row in the circulation group below is the one.
Quote this option→A brushless centrifugal pump at 4 L/min on the anode side. Temperature control and level measurement come with it. The table lists only the gear pump that holds a low flow accurately; the centrifugal pump for 4 L/min circulation is specified and quoted against your conditions.
Quote this option→A vertical multistage centrifugal pump on three-phase power, with its own inverter. The control interface is wider, so we define the specification together. Tell us your conditions and we quote the build they call for.
Quote this option→Every model below is one we judge usable at our operating condition, 0.1 M KOH at 40–60 °C. Tell us your conditions and we pick from these and quote. Both rows are Lead Fluid dispensing gear pumps: the same CT3000F drive with a different pump head.
| Model | Type | Max flow | Pressure | Power | Speed control | Liquid temperature | Wetted materials | Origin | Quote this |
|---|---|---|---|---|---|---|---|---|---|
| Micro gear | 0.015 – 0.9 L/min | 14 barTransfer limit | AC 100–240 V | 0–5 V, RS485 (MODBUS)Front panel too | Not published | PEEK | China | Get a quote→ |
| Model | Type | Max flow | Pressure | Power | Speed control | Liquid temperature | Wetted materials | Origin | Quote this |
|---|---|---|---|---|---|---|---|---|---|
| Micro gear | 0.045 – 2.7 L/min | 8 barTransfer limit | AC 100–240 V | 0–5 V, RS485 (MODBUS)Front panel too | Not published | PEEK | China | Get a quote→ |
What to settle before a quotation
Only what has come up more than once in real enquiries.
We use a brushless centrifugal pump for electrolyte circulation by default, and a magnetic-drive gear pump for water make-up. Circulation needs flow; make-up needs a small, accurate feed, so the roles separate. Even on circulation, holding a flow accurately below the 4 L/min standard calls for a gear pump, and those two gear pumps are what the table lists. The centrifugal pump for 4 L/min is specified and quoted against your conditions. We do not list a diaphragm pump: the one we have handled runs around 800 mL per minute, short of the 4 L/min circulation standard. If you want a diaphragm pump on circulation we can source one that reaches the flow, but we have not applied one in our own systems and hold no operating data for it. Whichever type, the wetted parts see KOH, so material compatibility is the first condition, and a general purpose pump that is not alkali resistant cannot be used.
No. Electrolysis consumes water; the KOH stays in the loop. That is why the make-up pump feeds water, not electrolyte. KOH is replaced on condition and performance trend, not on consumption.
The same model number comes in different materials. The diaphragm or gasket material and the body material are set by the order code rather than the model number, and the default specification is not always the alkali-suitable one. The models in the table publish no material choice, which does not remove the check: we ask the manufacturer whether the standard material holds under our operating conditions, and order on that answer. Tell us your conditions and we place the order in the specification they call for.
An electrolyte tank with level measurement, a filter, a heat exchanger with temperature measurement, and a flow meter come as one set. The order they are plumbed in is in the flow diagram above. At HXS-0 scale we put that set together for you.
If the electrolyte drops or air breaks the suction, the pump is damaged. The gear pumps in the table do not carry dry-run protection in the manufacturer documentation, so external protection has to be provided. Some centrifugal pumps have it built in, and we confirm that at the quotation stage. Either way we recommend pairing it with tank level measurement.
The standard is 4 L/min and the allowable range is 1 to 10 L/min. Above that we have not operated and validated it ourselves, so we will not commit to performance or lifetime figures. Higher flow raises the stack differential and widens the gap between anode and cathode pressure, which drives KOH across the membrane to the cathode and complicates gas handling and downstream purification. The bottleneck is the internal flow field and manifold cross section, not the external pipework. Where a process needs high flow, we recommend separating a stack loop at 4 L/min from a process loop on a shared electrolyte tank.
Fill in only what you know. We ask about the rest.
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