Henk de Swardt
EASA Technical Support Specialist 

While air-to-air cooled machines (WPII or TEAAC) dominate the market, water-to-air cooling is typical for motors rated above 5,000 kW (6,700 hp) or where water is readily available, or with specific applications such as wind generators. Service centers encounter two configurations: external tube heat exchangers and integral water jacket heat exchangers.

Tube Heat Exchangers
The first type consists of separate, removable units featuring bundles of cooling tubes (often stainless steel, copper or copper-nickel). This design is common in high-power pump motors; an example of installation is illustrated in Figure 1.

An example of the removable cooler bundle is shown in Figure 2. By order of magnitude, up to 8.9 million m³ (2.35 billion gallons) of cooling water circulate through the heat exchanger between maintenance intervals. 

Tube material significantly affects thermal performance because thermal conductivity influences the heat transfer rate through the tube wall. Common materials include copper (thermal conductivity 391 W/m-K = 225.9 BTU/(hr.ft.°F), yellow brass (117 W/m-K = 67.6 BTU/(hr.ft.°F), copper-nickel alloys and stainless steel (14 W/m·K). Changing tube material to mitigate corrosion or inadequate cooling capacity may have unintended consequences. For example, replacing copper tubes with stainless steel reduces tube wall conductance by ~25x. In practice, this typically derates the heat exchanger’s overall thermal rating (kW capacity at design conditions) by 30%+, leading to higher internal air temperatures and an associated increased motor temperature rise.

Water Jackets
Integral cooling "pockets" surround the stator frame and are commonly used in mining, hazardous location motors and wind generators. (See Figure 4.) Unlike tube exchangers, the cooling water flows between the inner barrel and outer jacket, resulting in heat conduction that occurs directly through the stator frame. (CAUTION: During insulation processing (e.g., dip‑and‑bake or VPI), significant care is required to prevent resin ingress into the water‑jacket flow paths. Cured resin within the jacket degrades heat‑transfer performance and is extremely difficult to remove. All water passages must therefore be sealed air‑tight and vacuum‑tight prior to processing.)

In the drawing (see Figure 5), the water jacket outside enclosure is transparent to reveal the cooling water path as indicated.

Buildup of Mineral Deposits
Generally, distilled water is not available in industrial plants. Where demineralized water is used, the dissolved mineral content is not eliminated. Thus, the cooling water contains dissolved minerals, which precipitate as scale and coat the inside of the cooling tubes/water jacket. An example of an inlet pipe to a typical cooler showing severe buildup is shown in Figure 6. 

The "Why" – Thermal Implications
Industry practice that relies solely on pressure testing of coolers for leak detection is insufficient to ensure thermal reliability. Thermal analysis indicates that mineral buildup significantly impairs cooling, yet because the fouling is "out of sight" inside the tubes (not externally visible), it is often overlooked. 

Reduced heat transfer elevates winding temperatures. Per the Arrhenius relationship, every 18°F (10°C) increase in operating temperature halves insulation life. Therefore, restoring thermal efficiency – not just mechanical integrity – is critical to protecting design life. 

The Procedure: Cleaning and Testing
This method should be adapted for the specific cooler to be cleaned. Throughout the cleaning and testing process: use calibrated gauges; record pressure and ambient water temperatures; record leakage rate; record process steps and findings. 

a) Initial pressure test: Always perform a hydrostatic (water) pressure test before cleaning. Leaks are often symptomatic of wall weakness; finding them early prevents wasting hours on cleaning a scrap unit. 

Safety first: Never use only compressed air; hydrostatic (water) testing is mandatory to prevent violent rupture. (If the required pressure is higher than the available water pressure, fill the cooler with water and then use a regulator and compressed air to obtain the desired test pressure. To be clear, the heat exchanger is first filled with water.) 

Pressure limits: Test to the value listed on the rating plate. If unlabeled, consult the original equipment manufacturer (OEM) or agree on a test pressure with the customer (typically 1.3 to 1.5 times operating pressure). Do not overpressurize, as heat exchangers are not “pressure vessels.” 

Connect the water supply to the inlet of the cooler and secure the connection. Open the water and let the water run freely until all the internal air has been displaced. While keeping the water inside the cooler, close the incoming water supply and install a pressure gauge to the heat exchanger outlet, thereby blocking the outflow of water, while measuring the pressure. (Alternatively, if more convenient, the pressure gauge can be installed on inlet.) 

Usually, the pressure is held for one minute. If no leaks occur, the test issuccessful. 

b) Verify tube material: Before introducing chemicals, validate the tube or jacket material (stainless steel, copper, copper-nickel, brass or mild steel). Often a visual indication will be sufficient but alternatively use an X-ray fluorescence analyzer. This determines the suitable flushing fluid. (See Table 1.)

c) Descaling flush: Perform a controlled descaling process to remove mineral deposits inside the cooling tubes. If the cooling tubes are not accessible for tube brush cleaning, perform the descaling flush before mechanical cleaning. Where tube interiors are inaccessible for mechanical brushing, conduct the chemical descaling before mechanical cleaning; where accessible, a preliminary mechanical brushing may be used to remove heavy deposits. Begin cleaning with less aggressive agents and progress to more aggressive chemistries only if earlier stages are ineffective.

Start with a 30-minute low pressure municipal water supply to remove loose debris. Then, flush the cooler with hot (initially ~ 50 °C (120 °F) detergent water for one hour. Inspect tubes (with a borescope or by removing end plates, for instance). If buildup remains, repeat the detergent flush for a longer period. Detergent poses minimal risk to the to the tube material, so extended flushing – even for several days – is acceptable. 

With sufficient precautions in place, the hot water flush can be repeated several times and for extended periods of time, even with much higher water temperatures. (The water shouldn’t boil.) 

Additionally, free‑flow steam cleaning may be employed (non‑pressurized steam only), with steam discharged through the open outlet. After each cleaning cycle, inspect the cooler to assess remaining deposits. If deposits persist, proceed with a low-flow acidic descaling fluid compatible with the cooling-tube material.

Table 1: Common Acid Solutions for Descaling Coolers
Acidic Solution	Availability & Cost	Toxicity	Material Compatibility	Effectiveness
White Vinegar 5% acetic acid	Easy to obtain and low cost	Low	Safe for stainless steel, copper and copper-nickel in short exposures. May cause slight surface rust on mild steel if not rinsed, but minimal attack at ambient strength.	Moderately effective on light limescale. Will dissolve calcium carbonate deposits given sufficient contact time, though slower than stronger acids. Best for thin scale or regular preventive flushing (can require hours or overnight soak for thick scale).
Citric Acid lemon juice ~5–7% or citric acid powder solution ~5–10%	Easy to obtain and low cost	Low	Safe for stainless steel, copper, brass and mild steel; citric acid is often used to passivate steel and won’t aggressively attack base metal in short use.	Good effectiveness on typical mineral scale. Citric acid dissolves calcium, magnesium and iron oxide deposits at moderate rates. Often used for descaling stainless equipment to avoid corrosion. Suitable for regular maintenance flushes to prevent heavy buildup.
Sulfamic Acid typically used at ~5% in water, often with corrosion inhibitor	Common in industrial descaling products; still reasonably low cost and easy to source online	Low-to-moderate hazard. Will irritate or burn on contact.	Safe for stainless steel, copper, copper-nickel, mild steel, aluminum, cast iron, etc., especially with an inhibitor present. (Inhibited formulations protect base metal during cleaning.)	Very effective descaler for calcium and magnesium deposits. Efficiently dissolves carbonate scale and also removes light rust/oxide buildup. It's considered one of the safest effective acids for heat exchangers: it provides high cleaning power comparable to strong acids without violent reactions or excessive metal attack. Typically, a few hours of circulated cleaning will clear heavy scale.
Phosphoric Acid 10–20% solution	Available in some commercial descalers and rust removers. Relatively inexpensive.	Moderate hazard and can burn skin/eyes on contact. Toxic if ingested.	Not compatible with mild steel, since it tends to form an iron phosphate film on metal surfaces. Stainless: Safe. Copper/copper-nickel: Generally safe in short exposure; a mild concentration (~5%) is used to avoid excessive metal attack.	Highly effective at dissolving carbonate scale and rust. Often used for limescale removal and steel surface treatment.
Oxalic Acid ethane dioic acid, typically ~5–10% solution	Available as a crystalline powder (sold as wood bleach, rust remover, or deck cleaner). Inexpensive and readily available.	Moderate hazard and can burn skin/eyes on contact. Toxic if ingested.	Particularly good for dissolving iron oxide (rust) while being gentle on the underlying steel. Recommended to dissolve rust blockages in water jackets. Safe on stainless steel and mild steel. Not compatible with copper and copper-nickel, since it tends to form a calcium oxalate film on metal surfaces.	Moderately effective for rust removal but less effective in removing other mineral deposits.

Choose acids based on safety, material compatibility and effectiveness. Refer to Table 1 for commonly used acidic solutions (ranging from household‑strength to industrial formulations). Consult the applicable safety data sheets and conduct spot testing prior to full application. Use inhibited formulations where possible (particularly for sulfamic acid). Start with a less aggressive acid, flush, inspect, and if needed, proceed to a more aggressive acid. Always follow strict safety procedures and personal protective equipment (PPE) requirements. Heated acid is generally avoided because it increases corrosion risk and handling hazards. Circulate the acidic solution for several hours, checking deposit removal at approximately hourly intervals. Neutralize effluent to pH ~ 7 prior to disposal and comply with local wastewater regulations. 

d) Wire brush cleaning: (Not applicable to water jackets; when accessible, each tube should be wire brushed internally to remove loose debris dislodged during the descaling flush.) 

e) External cleaning: (Not applicable to water jackets; the outside of the tubes and surrounding enclosure must be washed and any bent heat exchanger plates repaired.) 

f) Post-cleaning water flush: After chemical and/or mechanical cleaning, flush the system thoroughly with clean water until neutral pH is restored.

g) Pressure vs. flow test: An often overlooked but essential verification is the pressure versus flow test. This test compares measured water flow rate and differential pressure against rating‑plate values. Increase the water flow rate progressively until either the rating‑plate flow rate or the corresponding rated pressure is reached. Restrictions or internal blockages within the heat exchanger will result in a reduced flow rate at the specified rating‑plate pressure. Bypass flow paths (applicable primarily to water‑jacket designs) will result in a lower than expected pressure drop at the rated flow rate. If either condition is identified, corrective action is required, as both conditions reduce effective heat transfer and will result in increased motor operating temperatures. An example of such a pressure vs.flow test on a 3060 kW wind generator is show in Figure 7. 

h) Final pressure test: Repeat the initial hydrostatic test procedure to verify the cooler remains leak‑free following cleaning and any mechanical rework. Use the same test pressure and hold time specified for the initial test and record the results. 

i) Drying: Dry the cooler using a temperature‑controlled oven. A typical drying cycle is approximately 150 °C (300 °F) for four hours, unless otherwise specified by the OEM or limited by materials, coatings or installed seals. Ensure all internal cavities are drained prior to heating to prevent pressure buildup or localized overheating. 

j) Gaskets and seals: Replace all gaskets and seals. Inspect sealing flanges for leaks and repair as necessary. 

Conclusion
Restoring heat‑transfer efficiency is essential for motor reliability and preservation of winding insulation life. Standard leak testing alone is insufficient to ensure operational thermal performance, and additional actions are required to restore cooling capability consistent with the original design condition. Accordingly, effective thermal restoration requires: 

• Hydrostatic safety verification 
• Material-compatible chemical descaling 
• Neutralization and thorough flushing 

Restoring heat‑transfer efficiency constitutes a critical reliability and sustainability activity, directly influencing the operational life of the winding insulation system. Implementation of the procedures described reduces the risk of overheating, extends winding service life and preserves mechanical and structural integrity. Selection of material‑compatible descaling agents – such as sulfamic or citric acid – based on exchanger metallurgy ensures effective deposit removal while minimizing the risk of corrosion or material degradation.

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Categories: Technical topics, AC motors, DC motors, Frame, Repair procedures & tips

Tags: Cleaning

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