Problems Using Oversized Centrifugal Fans

Problems Using Oversized Centrifugal Fans

Neel Rao Writes on Centrifugal Fan & Centrifugal blower technologies for Reitz India

The size of the industrial fan is often overlooked when selecting a fan system for a new or replacement industrial application. Generally,

industries use oversized fans to accommodate for uncertainties of future requirements. While some moderate oversizing is typically acceptable, choosing an excessively large equipment than the actual system requirement should be avoided. The size of the centrifugal fan should be optimal to the anticipated normal demand where it can operate at maximum efficiency for a given volume of air. Proper sizing of the fans involve determining all aspects of fan efficiency including airflow requirement and system effects at the fan inlet and discharge.

Key factors that effect fan sizing Airflow requirement The actual volume of the air required depends on the application need. If the required air volume and static pressure that the fan must overcome are known, then the fan can be selected with optimal size which can operate near to the point of maximum efficiency. However, if the application demands variable airflow volumes, then it is less likely to select a proper fan size because of the varying system conditions. In such cases,

volume control methods such as VFDs (Variable Frequency Drives) are used where the fan rotational speed will control the air volume and static pressure to match with the system demand.

System effects In applications with widely varying operating conditions, vanes and ducts are used to alter or control the airflow at the inlet and discharge. Centrifugal fan systems will have difference in performance than as rated due to the system effect developed by these air control devices. In order to compensate for the losses in the airflow due to system effect, often industrial applications are designed to use oversized fans. If a larger fan size is selected, the fan operation can become significantly inefficient. It causes the fan to have stall or surge in rotation during minimum volume demand, resulting in excess low frequency noise and waste of energy.

Necessity to avoid using over-sized centrifugal fans High energy consumption Not all industrial applications require larger air volumes for their industrial process. Having too much airflow can be counter productive for some industrial applications and may even drive up the energy consumption and cost. Since oversized fans come with large sized motors which consume more power than the moderate ones, it is better to use an optimal fan size that is suitable for current demands of the process.

Delivers poor performance To avoid under performing systems in industrial and commercial applications, fans are oversized for their service requirements. Under such conditions, fans do not operate at their best efficiency because they generate excess airflow and have system performance problems. High airflow can cause unstable fan conditions resulting in increased noise and system vibration ultimately leading to degraded fan performance.

High system vibration or noise Fans are typically quiet when operated at their peak efficiency. However, when oversized fans are used it results in inefficient operation which is often indicated by high system vibration or noise. The vibration & noise can be airborne or due to system disturbances at the inlet or outlet. When large fans are made to run at greater speed, the load on the fan system will be higher. This will cause imbalance in system design resulting vibration in duct supports, bearings, fan casing etc.

Extra maintenance Fan systems are mechanical equipment which require maintenance to ensure its proper working. Since oversized fans generate high pressure and deal with large volumes of air, they often have issues because of increased stress on system components. This can result in structural vibration caused by the components such as unbalance in the motor or rotor mounting, improper bearings, worn out or misaligned belts, gap or leakage in the duct work etc.

About REITZINDIA Reitz India has set a benchmark to deliver high quality fans for heavy duty industries which can work at precise operating conditions like high temperatures and pressures. Reitz offer idealistic Centrifugal Fan and Centrifugal blower design that can be used in all large scale process industries like cement, iron & steel, power plants, fertilizers/ chemicals and more.

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