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Quotes - Engineering - SalesRefrigerant dual circuit
Two refrigerant circuits boiling against one water stream
A dual-circuit two-phase brazed plate exchanger is the evaporator, or the condenser, of a chiller with two compressors. Each compressor has its own refrigerant circuit with its own expansion valve, distribution insert and pair of ports, and the two circuits share one water or glycol side inside one 316L plate pack. Against two separate evaporators on the water side it saves a vessel, two water connections and a run of pipe, and it gives the chiller something more valuable: at part load, with one circuit running, the active circuit sees the whole water flow across the plates it touches, so its approach falls and its suction pressure rises.
The construction is an interleaved stack, water, circuit A, water, circuit B, and so on, so every water channel has one plate facing each circuit. Each refrigerant inlet port carries its own distribution device, because a two-phase feed split unevenly across half a stack behaves no better than across the whole of one. Refrigerant enters at the bottom and leaves at the top as superheated vapor; water runs top to bottom in counterflow to both circuits.
This page is the refrigerant case. Two liquid loops on one common side, standby glycol chillers, cooling plus reheat, are a different construction with different sizing questions, covered on the liquid dual-circuit page. Here the questions are staging, oil return and the water-side freeze line, and the pack is built to the 435–650 psig HFC rating with copper braze, or nickel braze for an ammonia pair.
Two-circuit evaporators use 316L plates and copper braze for HFC and HFO refrigerants; an ammonia pair takes nickel braze, and the water side's chloride still sets the plate limit. The Material & Construction page compares the grades.
One circuit running is the case that freezes plates
At low stage the active circuit boils against the full water flow, which is good for efficiency and bad for margin. Hold the design water flow, set the leaving-water low limit and a low-pressure cutout per circuit, and run glycol on anything that leaves below 40°F.
The duty: staged refrigerant on one water side
Each circuit has a capacity, an evaporating temperature and a superheat setting, and the pair may be equal or unequal; a 60/40 split is common on chillers that want a small first stage. The water side has one flow and one temperature pair at full load, and a different leaving temperature at every stage below it. Oil leaves each compressor with the refrigerant and has to come back with the vapor: a circuit unloaded to the bottom of its own range can fall below the velocity that lifts oil up the channels.
As a condenser the same pack takes hot gas from two compressors at the top ports and sends liquid out the bottom pair, water in counterflow; subcooling comes from the lowest plates and is set per circuit. Staging is gentler on the condensing side, but an idle circuit holding liquid refrigerant at water temperature is a migration path on restart, and the receiver and check valves have to allow for it.
The stainless build and what to watch
Plates are 316L; the water side decides whether that is enough. A treated closed chilled-water loop is easy. A tower-water condenser can reach 600–900 ppm chloride from a 150 ppm makeup, a review case that needs a strainer, a chloride check and a cleaning plan, because a brazed pack does not open. Copper braze suits HFC and HFO with POE oil; an ammonia pair takes nickel braze and stainless connections throughout. The pack has six ports: four refrigerant stubs sized to the suction and liquid lines of each circuit and two water connections, with the refrigerant inlets at the bottom on an evaporator.
Watch the freeze line. Water next to a plate boiling refrigerant at 30°F can drop below 32°F inside the channel while the bulk leaving temperature reads 42°F, so the guards are about flow, not only temperature: a flow switch, a low-pressure cutout on each circuit, a leaving-water low limit, and 25–35% glycol for anything leaving below 40°F. Watch distribution per circuit; a flooding circuit at low stage sends liquid to its compressor while its neighbor sits idle. And watch the piping drawing, because mirrored ports are easy to cross.
What to send for a two-circuit evaporator or condenser
The stage cases matter as much as the full-load case.
- Refrigerant, and the oil that travels with it
- Capacity, evaporating or condensing temperature, and superheat or subcooling for each circuit
- Water or glycol: concentration, flow, entering and leaving temperatures at full load and at each stage
- The minimum water flow the controls will allow, and the freeze guards already in the design
- Design pressure for the refrigerant side and the water side, and the code stamp if required
- Connection sizes for all six ports, and the mounting orientation
Call and talk it through with an engineer: 1-805-484-2992
Questions on two refrigerant circuits
Because the running circuit gets the whole water flow. With two single evaporators piped in parallel on the water, the idle one bypasses half the flow and the active one sees only half the water. In the interleaved dual-circuit pack the active circuit's plates face every water channel, so at 50% load it works on more water per ton than at full load, the approach shrinks, and the suction pressure comes up a few psi for free.
Unequal capacities, yes: a 60/40 or 70/30 split is built by giving the circuits different channel counts, and each has its own distribution insert sized for its share. Different refrigerants in one pack are possible but rarely worth it, since the pressure rating is set by the higher-pressure fluid and the water side sees two evaporating temperatures at once. Same refrigerant, same oil, two compressors is the normal order.
Per circuit it is the same problem as a single evaporator: oil comes back only if vapor velocity in the channels is high enough to carry it up and out of the top port. What changes is that each circuit must hold that velocity at the bottom of its own unloading range, independently of what the other is doing. A compressor that unloads to 25% on a circuit sized for 100% is where oil pools, and the fix is channel count, not plate size.
It works until the water side decides otherwise. Tower water at 600–900 ppm chloride is a review case for a brazed pack, accepted only cool, clean and treated, and a condenser runs its water warm, which eats into it. A strainer finer than the channel gap, a treatment program that keeps the plates clean and a chloride check on each season's water analysis keep a two-circuit condenser in service; on water past 1,000 ppm, or warm water that fails review, we quote a gasketed titanium plate instead.
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