Fans solve noise for supermarket

Energy-efficient fans from ebm-papst have been used in a unique application to resolve a noise issue at a supermarket in Guildford.

The Chelmsford based manufacturer’s latest EC fans have been fitted by Enetech Products onto a condenser to replace the previous standard AC fans that were being speed controlled via a Triac speed controller, and which caused the fans to emit a very noticeable tonal noise.

Using a Triac speed controller is a simple way to regulate the speed of an AC fan. Reducing the voltage to the motor windings reduces the magnetic field within the motor leading to a reduced output torque and hence lower speed. Transistors are used to switch the supply voltage on and off at different points during the voltage cycle, which distorts the voltage and current wave forms. This distortion gives rise to a humming noise, at around 100Hz, which is a typical characteristic of Triac control.

Typically, this noise will reduce as you reduce the fan’s speed and therefore its volume flow output. The speed of a fan on a condenser is controlled to vary the volume flow across the coil depending on the heat load within the refrigeration system. Sometimes the fan speed is reduced at night to minimise the noise when the refrigeration load is lower.

In this instance, the condenser was located very close to a new block of apartments and the residents, irritated by the tonal noise, complained to the store’s management.

Noise measurements for the condenser unit were taken by BJA Refrigeration Consultant Engineers (BJA), before and after the EC fans had been retrofitted and resulted in an overall noise reduction of 3dB(A) compared to the AC fans previously fitted.

For the purposes of resolving the noise complaint, the noise levels at the nearby residential properties were measured, but BJA faced an unexpected difficulty as the noise emission from the unit fitted with the EC fans was so low that the noise level was not distinguishable from the background levels at the site boundary. Also, the noise monitoring showed that the noise levels just outside the windows of the closest apartment dropped to as low as 38 dB(A), which was found to be the background noise level, rather than noise from the condenser unit.

Although the EC fan is primarily used for energy saving applications,

ebm-papst has received several enquiries from the market for noise sensitive applications. This is particularly relevant to supermarket stores located in residential areas, where the use of EC fans can negate the additional requirement and cost of acoustic enclosures