Power plants are swapping open cooling for sealed, ultra‑pure water loops on turbine lube‑oil coolers — and the payoff is reliability, compliance, and years of steady heat transfer. The blueprint: a closed intermediate loop and a dedicated heat exchanger tied to the main cooling circuit.
Fouling in heat exchangers is a massive hidden tax on industry. One widely cited review pegs the cost in Germany alone at about €6.5 billion per year, implying global losses in the hundreds of billions (heatxglobal.com). Steam‑turbine operators have zero appetite for that kind of drag on bearing temperatures and oil life.
The countermeasure is straightforward engineering: isolate the turbine lube‑oil coolers behind a sealed, high‑quality water loop, and cool that loop via a dedicated exchanger with the plant’s main cooling water. Closed recirculating systems “provide better control” and can use make‑up water so clean that “scale deposits are not a problem” (watertechnologies.com) (watertechnologies.com).
It’s not just theoretical. Marine and power applications routinely run a demineralized freshwater secondary loop cooled by raw seawater in a primary exchanger specifically to “minimize corrosion [and] scaling” and “ensure smooth operation” (swep.jp).
Two‑loop architecture and isolation
The standard arrangement is: lube oil → lube‑cooler heat exchanger (HX) → lube return, and a separate closed water loop → main cooling water HX → pump → back to the lube‑cooler HX. Shell‑and‑tube units built to TEMA (Tubular Exchangers Manufacturers Association) standards are common for robustness (enggcyclopedia.com), and many heavy‑duty packages specify stainless or copper‑alloy tube bundles. Compact brazed‑plate HXs can work if pressure drops allow; marine designs place seawater on the primary side and freshwater on the closed side (swep.jp).
Strainers and differential‑pressure indicators on both exchangers are standard for fouling detection, a role well served by strainers sized for the loop’s debris load. The oil, the closed water, and the main cooling water remain fully separated, preventing cross‑contamination.
Figure: Cross‑section of a typical shell‑and‑tube lube‑oil cooler (closed‑loop water on one side, turbine oil on the other). High‑grade materials (copper/alloy tubes, stainless shell) and tight tolerances are used to maximize heat transfer while avoiding leakage or corrosion.
Ultra‑pure coolant specifications and chemistry
The closed‑loop coolant is essentially particle‑free deionized (DI) or distilled water with tightly controlled chemistry (ibm.com). Typical targets are laboratory‑like: ionic conductivity around 1 μS/cm or less (μS/cm is a conductivity unit), total hardness below 1–2 ppm as CaCO₃, dissolved oxygen as low as practicable, often <0.1 ppm via deaeration, and pH at roughly 8–9.
To achieve that, plants commonly rely on deionization packages, with a demineralizer supplying the base DI quality and a mixed‑bed polisher holding very low ions in service. Suspended solids are filtered to below 1 μm; in practice, a cartridge filter in a 316L housing fits the duty, and a sanitary housing such as an SS cartridge housing is aligned with the metallurgy cited for closed loops (304/316).
Biological growth is suppressed by trace biocide — often a non‑chlorine formula — and corrosion is controlled with inhibitors. Industry notes warn that closed loops are “particularly susceptible to corrosion…after being recently cleaned” because fresh oxygen enters, so keeping O₂ down and inhibitors up is crucial (chardonlabs.com). Operators typically meter these with an accurate dosing pump, drawing from a closed‑loop treatment suite such as closed loop chemicals, corrosion inhibitors (nitrite or molybdate for steel), and low‑toxicity biocides. Reviews advise caution with phosphate, which can precipitate as calcium phosphate if hardness is present; phosphate is seldom used unless feed water is ultrafiltrated (chardonlabs.com). Where pretreatment is needed ahead of DI, an ultrafiltration step helps ensure low solids.
Materials selection follows suit: copper/brass (with <30% Zn), stainless steel (304/316), and industrial EPDM rubber are common choices for wetted parts (ibm.com). Glycols are typically avoided unless freeze protection is needed; IBM cautions they “can adversely affect…cooling performance” and complicate treatment (ibm.com).
Pumped loop design and heat‑exchange duty
Loop sizing starts with the oil heat load. One published example cooled SAE‑30 turbine oil from 70°C to 60°C using 1 kg/s of oil flow (about 3.6 t/hr) and 8 kg/s of cooling water at 20°C, achieving a 10°C oil drop (researchgate.net). In larger turbines, oil flows can run on the order of 200–1000 kg/hr depending on bearing count and power, with exchangers handling a few hundred kilowatts of heat. Designers often target an oil‑side ΔT (temperature difference) of 5–10°C and a water‑side ΔT of 10–15°C.
The dedicated intermediate HX to the main cooling water is sized to that duty. Redundancy matters: an N+1 (one extra unit available) cooler lets maintenance proceed without interrupting oil cooling. Temperature control is commonly by thermostatic bypass or a motorized valve on the closed‑loop pump, keyed to oil‑inlet (or bearing) temperature. Layouts avoid flow chokepoints — even small throttle valves or bypasses can raise fouling risk sharply (stewart-switch.com) — and maintain velocities above ~0.3 m/s to prevent sedimentation.
Why closed loops stay clean
Closed circuits recirculate more than 90% of their water over a year, so only minimal makeup is required; that enables deionized or distilled water and tight control of chemistry (chardonlabs.com) (ibm.com). With high‑quality water, “scale deposits are not a problem” because there is little, if any, evaporation (watertechnologies.com).
By contrast, open circuits ingest dirt, salts and air; over time this “blackens the water” and creates sludge that “drastically” reduces flow and heat transfer (cqm-tech.com). Even in closed loops, trace oxygen or impurities can trigger corrosion — especially “oxygen pitting” in stagnant zones (boquinstrument.com) (chardonlabs.com) — which is why the DI standard and inhibitor regime are non‑negotiable.
Measured performance, oil health, and maintenance
Empirical plant observations are consistent: fouling in open systems can lift oil outlet temperatures by 5–10°C within months, while closed loops hold performance steady for years. A modern power plant study reported that an unrefrigerated lube cooler with moderate water fouling lost ~10% capacity in three months; with a closed loop, no capacity loss was seen over a year, maintaining a stable 55°C oil temperature (swep.jp) (cqm-tech.com).
Oil health follows. Turbine oil life in service ranges roughly 5–15 years depending on maintenance; “water, heat [and] contamination” are among the most important factors (machinerylubrication.com). A closed cooler keeps bearings at ~40–50°C with no water carry‑over, which slows oxidation and varnish formation; in practice, oil drain intervals extend to multi‑year cadence, and oil‑system monitors see minimal contaminant ingress from the water side.
Regulatory discharge and the intermediate HX
The dedicated exchanger to the main cooling loop simplifies compliance. Indonesian effluent rules (Permen LH 08/2009) cap free chlorine at 0.5 mg/L in cooling‑water discharge (text-id.123dok.com); closed circuits typically use no continuous chlorine dosing, so their rare blowdowns inherently comply. Local limits for ground‑level salt‑water cooling discharge often include ΔT ≤ 40°C and TSS/oil < 50 mg/L (text-id.123dok.com); because the closed loop never contacts raw contaminants, the main cooling discharge carries nearly none of the lube system’s burden.
Operations math and lifecycle gains
Because makeup water is typically less than 10% of closed‑loop volume annually, chemical usage and waste volumes are low (chardonlabs.com). That keeps dissolved solids and biocides near potable levels in any blowdown, while the main cooling system handles the heat duty.
The capital cost is modest — an extra pump and a plate or shell HX — with lifecycle payoffs in avoided cleaning, pump power, and unplanned downtime. Industry sources underscore that closed loops “make excellent use of high‑quality water” and virtually eliminate scale (watertechnologies.com). For turbine owners, higher net availability and reduced risk of lube failures translate to tangible savings often counted in the millions per year.
