Stationary cooling systems with low noise emission

17 June, 2015

The sources of noise on coolers are varied. Almost all the main components such as heat exchanger, housing, fan, protective guard, motor support mount and drive (see Fig. 1) play their part in noise emission. On large coolers, the fan is the main culprit because many factors cause it to produce noise. Tonal noises occur for example as a result of moisture being driven out by the fan blades, whilst low frequency noises can be caused by turbulence and scoring on the blades.

It is, however, clear that the noise is affected not only by the fan alone, or its shape, but also by other factors such as the incident flow and the installation situation of the fan.

Airborne noise from the fan makes the greatest contribution

In order to better understand the relative contributions made by individual cooler components to overall noise emission, Hydac undertook a mass balance study of noise sources using an OK-EL11 cooler. Various measuring procedures were used to determine overall noise, structure-borne noise and partial noise. The flow noise was produced by an external air stream which was forced through the cooler.

The measurement results show that the largest proportion of airborne noise was produced by the fan. With a differential of 3.3 dB, it was more than twice as loud as that of the cooling element which produced the next largest airborne sound. The other portions were again much lower and the airborne noise of the motor fan had the third highest noise level overall. These measurements showed the need to optimise the fan noise level. It was therefore necessary initially to consider the theory in order to identify a starting point for concrete proposals.

The occurrence and intensity of the noise are clearly non-linearly related to the flow rate and at the same time are related to the effective back-pressure. This means that the fan reacts very sensitively to deviations from the optimum operating point. When selecting and adapting a fan for a cooler, it is therefore vital to ensure that the operating point of the fan in the cooler is as close as possible to the optimum fan operating point and also does not deviate far from this, as a result of enclosures used by the customer or contamination relating to operation.

The incident flow onto the fan is of great significance

it affects the efficiency and noise emission. If the flow is not uniform, or is otherwise impaired, the uneven loading of the fan will lead to higher noise emissions as well as losses in efficiency.

Particular importance is attached to the head clearance, in other words the gap between the outer edge of the fan and the static housing. The smaller this gap is, the smaller are the noise emissions which develop and the greater is the efficiency of the fan.

Based on these findings a series of design and production-engineering changes were carried out on the Hydac OK-EL8-14 standard cooler series which brought about substantial noise reduction. Particular importance was placed on optimising the airflow in the cooler. Amongst other things, an inlet nozzle was used to create a more even, and therefore improved incident flow onto the fan. Further design and production engineering modifications made it possible to reduce the head clearance by approximately 30 per cent which led to a sharp improvement in noise emission and airflow.

In addition, the design of the fan was also adapted to possible operating points in the particular system resulting in more efficient and quieter fan wheels. Now that these modifications had had such a marked effect on reducing the noise emissions, the focus turned to those emissions caused by structure-borne noise and the emission characteristics of the cooler were altered to the effect that less noise was emitted to the air.

Furthermore by designing a new motor support mount with an integrated protective guard, it was possible to further improve the noise characteristic.

Comparing old and new brings the results to light

It has been possible to achieve significant improvements in all cooler classes in the standard range from OK-EL8 to OK-EL14. For the same cooling capacity, the noise level was reduced by an average of 10 dB(A) at a distance of 1m – an important enough reason to re-launch the OK-EL series under a new name, to distinguish it from its predecessors. Since then, the new name has been AC-LN (Air Cooler – Low Noise). Significant improvements can be seen, particularly in the case of the size 11 coolers where the reduction is 13 dB(A). As an illustration of the scale of this improvement, it should be explained that a reduction of 3 dB(A) equates to reducing the noise power by half. On average, therefore, a cooler from the previous series is perceived as being as loud as ten comparable coolers from the AC-LN series – without affecting the cooling capacity.

The knowledge gained in developing the AC-LN range therefore provides a sound basis for further optimisation work on Hydac Cooling coolers.

www.hydac.co.uk