Cooling a Dynamometer: Why a Cooling Tower Fails When Wet-Bulb Temperature Hits 40°C

A real customer conversation that reveals a critical principle in industrial cooling

Recently, a customer approached us with a cooling requirement for his dynamometer. The parameters seemed straightforward:

- Inlet water temperature: 40°C

- Outlet water temperature: 25°C

- Flow rate: 6000 L/h (6 m³/h)

He asked: “Will a cooling tower work?”

We asked about local climate conditions. His reply: “Wet‑bulb temperature is 40°C.”

At that moment, the cooling tower solution was ruled out completely. The only viable option is a chiller.

This article explains why – and covers the key principles of cooling towers, wet‑bulb temperature, and chillers for dynamometer cooling.

1. Why Does a Dynamometer Need Aggressive Cooling?

A dynamometer (or “dyno”) measures torque, speed, and power of engines, electric motors, or other rotating machinery.

Eddy current and hydraulic dynamometers are particularly demanding: they convert nearly all the absorbed mechanical power into heat.

- Example: During a full‑load test on a 100 kW motor, the dynamometer generates roughly 100 kW of heat.

- Without effective heat removal, the dynamometer overheats → measurement drift → component damage or failure.

Thus, a reliable cooling system is mandatory for dynamometer test cells. The two most common solutions are cooling towers and water‑cooled chillers.

2. Cooling Tower: Low Cost, but with a Fatal Limitation

A cooling tower removes heat by evaporative cooling. Warm water is sprayed over fill media while air flows through. A small portion of water evaporates, absorbing latent heat and lowering the remaining water temperature.

The “Achilles’ Heel” – Wet‑Bulb Temperature

Wet‑bulb temperature is a psychrometric property. In simple terms: it is the lowest temperature that can be achieved by evaporative cooling.

- If the ambient wet‑bulb temperature is 28°C, a cooling tower cannot produce water colder than 28°C (in reality, the practical limit is wet‑bulb + 3~5°C).

- If the wet‑bulb temperature is 40°C, the theoretical minimum outlet temperature is ≥43°C (approach of 3°C under ideal conditions).

The customer’s target outlet temperature is 25°C – a full 18°C below the wet‑bulb limit.

No cooling tower can achieve this, regardless of size or cost. The laws of evaporative cooling make it physically impossible.

Key takeaway: A cooling tower’s outlet temperature is always higher than the ambient wet‑bulb temperature.

3. Water‑Cooled Chiller: Climate‑Independent, Precise Cooling

A water‑cooled chiller uses a vapor‑compression refrigeration cycle (compressor, evaporator, condenser, expansion valve) – the same principle as an air conditioner or refrigerator.

- Ambient wet‑bulb temperature does not affect the chilled water outlet temperature.

Whether it’s 40°C or 50°C outside, the chiller can steadily deliver 7°C, 15°C, or 25°C water as set.

- The chiller’s condenser does require cooling (often via a separate cooling tower or dry cooler), but the evaporator produces chilled water at a controlled temperature, independent of outdoor conditions.

Applying to the customer’s requirement (40°C → 25°C, 6 m³/h)

- Required cooling capacity ≈ 104 kW

- A cooling tower cannot do it. A water‑cooled screw or scroll chiller handles this easily, maintaining ±1°C accuracy, essential for repeatable dynamometer testing.

4. Recommended Solutions for Dynamometer Cooling

Solution	Suitable for	Pros	Cons
Cooling tower only	Low wet‑bulb (≤28°C) and loose outlet temp requirement (e.g., only need <35°C)	Low initial cost, low operating power	Climate‑dependent; outlet temp rises in summer
Water‑cooled chiller only	High wet‑bulb, or precise temp control required (e.g., 25°C ±1°C)	Stable, precise, climate‑independent	Higher initial cost, higher electricity consumption
Cooling tower + chiller in series	Large dyno labs that need both energy savings and guaranteed summer performance	Tower pre‑cools water, chiller finishes to target – best overall efficiency	More complex system, larger footprint

For the customer’s case (wet‑bulb = 40°C, target = 25°C), the only feasible solution is a water‑cooled chiller (or an air‑cooled chiller if condenser water is unavailable, though high ambient temperatures reduce air‑cooled efficiency).

5. Why Not Just Buy a Cooling Tower?

The “tonnage” rating of a cooling tower is defined under standard conditions:

- Inlet 37°C, outlet 32°C (ΔT = 5°C), wet‑bulb 28°C.

The customer’s duty has a ΔT of 15°C – three times larger. Even under normal wet‑bulb, a standard 6 m³/h tower would be severely undersized; a 15–20 m³/h tower would be needed.

But when wet‑bulb = 40°C, no tower can reach 25°C outlet, regardless of size. The physics of evaporative cooling fails.

6. Conclusion

- Dynamometer cooling – especially for eddy current and hydraulic types – requires stable, high‑capacity heat rejection.

- Cooling towers work only in dry, cool climates with loose temperature requirements. Their performance is strictly bounded by wet‑bulb temperature.

- Water‑cooled chillers are not limited by wet‑bulb. They deliver precise outlet temperatures (e.g., 25°C) reliably, making them the preferred choice for precision dynamometer testing.

Remember: If your local wet‑bulb temperature is 40°C, expecting a cooling tower to deliver 25°C water is like expecting your home to stay at 25°C in winter without heating – it violates physical law.

When selecting cooling equipment for a dynamometer (or any industrial process), always provide these parameters:

Inlet temperature, outlet temperature, flow rate, (local summer design wet‑bulb temperature).

We help customers avoid “cheap but unusable” solutions. For reliable, precise dynamometer cooling – choose a water‑cooled chiller.

Written by the industrial cooling engineering team – specializing in precision temperature control for dynamometers, induction heating, lasers, injection molding machines, and more.