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Quotes - Engineering - SalesLiquid dual circuit
Two independent liquid circuits on one common side
A dual-circuit single-phase brazed plate exchanger carries three liquids in one 316L block: a common fluid on one side and two separate circuits on the other, each with its own pair of ports and no connection to the other anywhere inside the pack. Six ports, one braze, no gaskets. The two circuits are either interleaved, common, A, common, B, repeating through the stack, or split, with circuit A in the first group of plates and circuit B in the second. Interleaving gives each circuit a share of the whole common flow; a split pack makes two exchangers in series on the common side.
The duties that call for it are all liquid and all single-phase. Two chilled-glycol loops from a duty chiller and a standby chiller cooling one process water stream, so that a changeover needs no valves on the process side. A cooling loop and a reheat loop acting on the same product water in one unit, one after the other. Heat recovery from a warm drain stream into one circuit while hot water in the second circuit trims the last few degrees.
Everything that applies to a liquid-to-liquid brazed plate applies here: 1–3 mm channels, strained fluids on all three sides, counterflow piping, 435–650 psig standard ratings, and 316L plates with copper braze unless one of the fluids says otherwise. What is new is the operating-point question, and that is where the sizing goes wrong.
Both liquid circuits and the common side run on 316L plates; copper braze for water and glycol loops, nickel when any of the three fluids is deionized water or a copper-sensitive liquid. The Material & Construction page compares the grades.
Size it for the case where both circuits run
A standby circuit sized for full duty and a duty circuit sized for full duty are not two full-duty units in one block. Tell us which combinations run together, the common-side flow in each case, and the leaving temperature each case has to hold.
The duty: three liquids, several operating cases
The common fluid is usually the one with the process requirement, product water, a tool-cooling loop, a tank recirculation, and it has one flow and one target temperature. The circuits are the utilities: chilled glycol at 20–30°F, tower water, hot water at 140–180°F, recovered heat at whatever the source gives. Each has its own flow, temperature pair and pressure drop allowance.
Write the cases down. Duty only, standby only, both, neither. Cooling only, reheat only, both in sequence. In a split pack the common side leaves circuit A at the temperature it enters circuit B, so the second stage works on a smaller temperature difference than a first glance suggests. In an interleaved pack an idle circuit leaves half the plates passive, and the active circuit does its whole duty on the other half, which is why the one-running and both-running cases size differently.
The stainless build and what to watch
Plates are 316L on all three fluids, so the chloride question gets asked three times: a glycol loop is clean, tower water is not, and the dirtiest of the three sets the strainer and the chloride check. Copper braze is standard in every channel, because the filler is one braze cycle for the whole block; a single copper-sensitive fluid moves the entire unit to nickel or stainless braze. There are six connections, so leave room for them; a crossed pair is the most common start-up fault.
Watch the idle circuit. Stagnant glycol is harmless, but stagnant tower water in a 316L channel for a season is the under-deposit corrosion case, so an idle water circuit is flushed or kept in slow circulation. Watch the pressures when the circuits are not alike: the plate between a 300 psig circuit and a 30 psig common side carries the full difference. And watch port orientation, since each circuit must be counterflow to the common side on its own.
What to send for a liquid dual-circuit unit
Enough to size every operating case, not only the largest one.
- Common fluid: what it is, its flow, inlet temperature, and the leaving temperature it must hold in each case
- Circuit A and circuit B: fluid and concentration, flow, inlet temperature, allowable pressure drop
- Which combinations run together, and which one is the sizing case
- Chloride, pH and filtration on any open or treated water among the three
- Design pressure on each of the three sides, and the code stamp if required
- Connection type and size for all six ports, and the mounting orientation
Call and talk it through with an engineer: 1-805-484-2992
Questions on two liquid circuits in one pack
Yes. The circuits share nothing but the plates that separate them from the common side, so one can be 30% propylene glycol and the other open tower water, and the pack is sized channel by channel for each. What they do share is the braze, one filler throughout, and the 316L plate, so the tower-water circuit brings its strainer and its chloride limit along for the whole block.
Thermally, very little: the idle channels sit near the common-side temperature and conduct a small amount of heat through the plate, which the sizing allows for. Mechanically, the idle fluid is stagnant, and that is the thing to manage. Glycol can sit; untreated water should not. Isolate it clean or leave a trickle of circulation, and expect the active circuit to meet its duty with no help from the idle half.
Split. Cooling and reheat act in sequence on the common fluid, so circuit A takes the first group of plates and circuit B the second, and the common side passes through both in order. An interleaved stack would have the two circuits fighting across every plate. For two standby utilities doing the same job, interleaved is the normal choice, since either circuit then has the whole common flow to work on.
Here all three fluids are liquid and the questions are hydraulic: which cases run, which fluid is dirtiest, which ports are which. The refrigerant version puts two boiling or condensing circuits on one water side, and its questions are distribution, oil return and freezing at part load. The plate stack looks alike from the outside; the design work and the risks are not alike.
Adding the two circuits up. A duty circuit sized for the full load and a standby circuit sized for the full load get specified, both at full flow, and the unit arrives sized for twice the heat the common side can ever carry, with double the pressure drop. State the standby case as a case, not as a second duty, and the pack comes out the right size for the way the plant will run it.
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