Sizing Air Cooling for a 2 MW Dewatering Pump Motor

Four dewatering pumps use cooling water delivered down a mining shaft. The recorded supply condition is about 10 °C and 6 L/s, while the surrounding underground air is about 40 °C. Water leaving the motor-cooling circuits is discharged locally and must later be pumped to the surface. Converting the 2 MW motors to air cooling is technically possible only if a purpose-designed motor cooling arrangement and the mine ventilation system can reject the operating heat load continuously.

What are the field readings really saying?

Start with the operator-visible conditions: cold water enters, warm water leaves, and the mine already has hot air. The proposed change removes both the water flow and its heat-carrying capacity. It transfers that duty to the ventilation system; it does not eliminate the duty.

The motor rating alone does not define cooling duty. Obtain electrical input power, mechanical output or shaft load, efficiency at the operating point, motor speed, duty cycle, and measured coolant temperatures. Check: reconcile the four branch flows with the header flow before performing any equipment selection.

What heat load must the replacement system reject?

Motor cooling removes motor losses, not the full 2 MW shaft output. Copper, magnetic, mechanical, and ventilation losses become heat in or near the motor. The useful shaft power passes into the pump. Pump inefficiency adds heat elsewhere, mainly to the pumped fluid and surrounding installation, and belongs in the mine-wide heat balance rather than the motor-cooler calculation.

The evidence introduces a 90% efficiency and an 8-hour operating period as assumptions. Two interpretations of the 2 MW value must be retained:

For four equally loaded motors, the simultaneous motor-loss load is about 800 to 889 kW under those assumptions. Use the manufacturer’s efficiency at actual load when available; a nameplate efficiency at another operating point can distort both water and airflow calculations. Check: compare calculated loss with measured electrical input minus calculated or measured shaft output before sizing the replacement cooler.

What does the existing water circuit reveal?

Water provides a useful cross-check on the loss estimate. Using a water density near 1 kg/L and specific heat near 4.18 kJ/(kg·K), a 6 L/s stream carries approximately 25.1 kW for every 1 K rise:

Heat = mass flow × specific heat × temperature rise

The measured outlet temperature resolves this ambiguity quickly. A much smaller rise can indicate lower motor loading, more than 6 L/s per motor, bypass flow, or a measurement location that includes unmixed cold water. A larger rise points toward restricted flow, fouling, unequal branch balance, or higher loss.

Also measure the inlet temperature at the motors. A long uninsulated down-shaft pipe can gain heat before reaching them, so the stated 10 °C surface condition may not be the actual coolant temperature. Check: calculate heat pickup independently for every motor from its measured inlet temperature, outlet temperature, and branch flow; the result should track motor load.

How much air would each motor require?

For a first-pass sensible-heat calculation, use:

Air mass flow = motor heat loss / (air specific heat × allowed air rise)

With air specific heat approximated as 1.0 kJ/(kg·K) and an allowed 10 K rise, a 200 kW loss requires about 20 kg/s per motor. A 222 kW loss requires about 22.2 kg/s. Four simultaneously loaded motors require approximately 80 to 88.9 kg/s. With 40 °C inlet air, a 10 K design rise produces air near 50 °C at the motor outlet.

Convert mass flow to volumetric flow using the actual underground air density:

Volumetric flow = mass flow / actual air density

Use pressure, temperature, and humidity at the motor location rather than a standard-air density. Fan selection also requires motor-cooler pressure drop, duct losses, filter loading, elevation effects, and leakage.

An estimate of 6.7 kg/s follows from averaging 5.76 GJ across an entire 24-hour day: 66.7 kW average divided by a 10 K rise. That is not the required airflow while the motor operates. It works only if a thermal-storage system absorbs the difference during the 8-hour run and releases it during the remaining period. Check: confirm that the selected fan delivers the required mass flow at the total installed pressure drop, with the motor running at maximum expected loss.

Can the mine ventilation system carry the added heat?

Underground air is a managed utility. Every kilogram used for motor cooling must reach the motor, pass through the cooler, and leave without recirculating into the inlet or heating occupied areas beyond their limits. A local fan that moves hot air around the same chamber provides airflow at the motor but may provide little net heat removal.

Prepare a ventilation heat balance that includes the 800 to 889 kW simultaneous motor losses calculated above, plus fan power, pump losses released underground, electrical equipment, people, rock heat, and any other operating loads. Then map the complete supply and return paths. Account for other ventilation duties such as fresh-air delivery and dust removal.

Air at 40 °C also reduces the temperature margin available to windings, bearings, lubrication, cables, and nearby controls. Read the allowable coolant-inlet and component-temperature limits from the proposed motor and cooler documentation. Do not infer acceptability solely from a winding insulation designation or from the fact that air continues to move.

A successful airflow check therefore has two parts: adequate flow through the motor cooler and adequate mine exhaust capacity to remove the discharged heat. Check: run the ventilation model with all four pumps at the maximum coincident duty and verify inlet temperature without credit for recirculated outlet air.

Will the motor enclosure and drive arrangement still work?

Do not treat this project as a fan substitution. A water-cooled motor may depend on its water jacket or heat exchanger for enclosure temperature, internal circulation, bearing cooling, and certified operating limits. Select a motor or manufacturer-approved cooling conversion designed for the 2 MW duty, 40 °C inlet condition, contamination level, and installation pressure.

Changing from low-TDS cooling water to air does not by itself establish a Zone 0 or Zone 1 requirement. The plant electrical engineer must classify the atmosphere and verify the complete motor-and-fan assembly. Adding an external fan or opening a previously closed cooling path can change the certification basis.

For VFD service, a separately powered fan is often the workable configuration because its airflow does not fall with motor speed. It adds an auxiliary supply and a run-permissive that must trip or derate the motor after loss of cooling according to the approved design. Check: prove the fan-running feedback, airflow protection, motor thermal protection, and VFD permissive before applying process load.

Would recovering the water be the better conversion?

The existing water system may remain thermally superior while its disposal arrangement causes the operating cost. Evaluate recovery before replacing the motor cooling method. If the motor jacket and connected equipment can withstand the down-shaft static pressure, a supply and return column can offset much of the elevation head. A return booster then supplies the differential pressure needed for pipe friction, equipment pressure drops, level imbalance, and control margin.

Do not expose a motor jacket to full shaft pressure until its maximum allowable working pressure, transient rating, seals, hoses, valves, and cooler connections have been checked. If the jacket cannot accept that pressure, use a pressure-isolated heat exchanger and a low-pressure secondary motor loop.

The descending water also contains recoverable hydraulic energy. A turbine, hydraulic drive, or pressure-recovery device could reduce pressure while contributing power to the return duty. Size that alternative from measured head, flow, conversion efficiency, and operating range; the energy balance must include losses in both water columns and all heat exchangers.

Compare two workable configurations on total power and maintainability: purpose-designed air cooling transfers roughly 800 to 889 kW into ventilation at full coincidence, while a closed water circuit retains the water’s heat capacity and removes the local discharge. The closed circuit is preferable when pressure management and a surface heat-rejection path cost less than the additional ventilation and air-cooled motor package. Check: obtain one pressure-and-energy balance for the closed water option and one heat-and-pressure balance for the air option on the same operating schedule.

How is the complete installation commissioned?

1. Record each motor’s electrical input, load, speed, operating hours, cooling-water inlet and outlet temperatures, and branch flow under stable operation.
2. Calculate motor loss from the confirmed meaning of the 2 MW rating and cross-check it against water heat pickup.
3. Confirm the worst underground air inlet temperature, actual air density, allowable motor air rise, and required mass flow.
4. Select the motor cooling package and fan from the required flow at installed pressure drop, including dirty-filter or fouled-cooler conditions defined by the design.
5. Verify the mine ventilation balance with four motors operating together and with the proposed discharge-air routing.
6. Confirm hazardous-area suitability, VFD turndown cooling, auxiliary-fan power, thermal protection, and loss-of-cooling interlocks.
7. Run one motor through its maximum expected load and minimum VFD speed while trending inlet air, outlet air, winding indication, bearing indication, fan status, and electrical load.
8. Repeat at the maximum planned number of simultaneous pumps. Stop the test if any approved motor, bearing, cooler, cable, or mine-air limit is reached.

Check: accept the conversion only after temperatures stabilize below documented limits at maximum coincident duty and loss of cooling produces the intended alarm, derate, or trip.

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