WHICH CENTRIFUGAL FAN IS BEST FOR MY APPLICATION?

Specifying the best centrifugal blower for your application can be challenging if fan engineering is not your specialism. Given that there are three different kinds of centrifugal fans – forward-curved, backward-curved and radial – even choosing the right type can pose difficulties.

Opt for the wrong type or model of centrifugal fan and you risk compromising on airflow and pressure – and/or wasting highly expensive energy. So it always pays to get expert advice. These are just some of the factors that you’ll need to consider…

Drying is a key application for ACI fans and air knives – especially throughout industries such as:

Major manufacturers depend on high-performance air knives and the fans that power them to dry their products quickly, efficiently and cost-effectively – accelerating production, increasing output and enhancing production line reliability.

Those production lines can be anything up to 4-5 metres wide. Every application is unique; each has its own highly specific challenges. So every solution must be tailor-made, though they’re based on standard components to maintain cost-effectiveness and deliver a faster return on investment.

The fans that power the air knives for these applications can feature either forward-curved, backward-curved or radial impellers, depending on the requirements of the task.

Forward-curved centrifugal fans are more compact than radial fans – and they deliver higher pressures than backward-curved fans. This makes forward-curved fans ideal for drying applications, especially where long, winding ducting or other resistances are involved.

Backward-curved centrifugal fans are more energy-efficient than their forward-curved counterparts. They’re ideal for applications where large volumes of air need to be moved at lower pressures than those created by equivalent forward-curved and radial fans. This makes backward-curved fans cheaper to operate but it can rule them out of applications where higher pressures are required.

Radial fans are more suited to applications that require higher pressures and larger volumes of air.

Many factors affect the choice and performance of centrifugal fans in drying applications. They include the:

• dimensions of the object being dried
• surface temperature of the object
• composition of the water or coolant (water/oil mix) being blown away
• hardness of the water in the geographical location
• speed and width of the production line.

The finish on the surface of the object can also have a significant effect on how long it takes to dry. Even a slight roughness on the surface can make the object more likely to retain water, given how fast the production line is running and, therefore, how little time the object may spend under the air knife.

This application is very much an offshoot of drying because seldom is air alone used to cool an object: water does it much quicker but then the object must be dried. Applications such as this include the colling/drying of steel or rubber strips.

These cooling/drying applications require high volumes of low-pressure air. A compact forward-curved fan is often the unit of choice in this instance. However, where wider ducting is involved, there is the opportunity to specify a larger backward-curved fan that uses less energy.

Some objects – such as ultraviolet lights – are cooled by air because of the fragility of the components and the nature of the application. In this instance, forward-curved fans such as the VBW6 are used.

The VB, HFM and EV series fans are available in various designs, including single- and three-phase motors with ATEX/IECEx-rated versions for explosive atmospheres. Aside from industrial process cooling, VBW fans are good for the extraction of fumes (including degreasing), screen print drying, crop drying/conditioning, and gas burners.

Another option for industrial process cooling is a multi-stage fan, such as the MS11/080. Its modular construction enables each fan to have multiple impellers (up to 10 stages). This is extremely useful when you need to ramp up the pressure gently. It helps to reduce or prevent turbulence, especially in applications in which the discharge outlet must be either blanked off or its diameter significantly reduced at various stages of operation.

How quickly do you want to cool the object in question? This is essential to the specification of the fan. Furthermore, how hot is the object that needs to be cooled? And what is it made from? Aluminium cools faster than steel because of its high thermal conductivity. Aluminium dissipates heat up to 15 times faster than stainless steel – which is one of the reasons it is specified for electro-thermal applications such as car radiators and air conditioning units. (Aluminium also happens to be strong, light, abundant and easy to machine.)

Air curtains use fans to create an invisible but very real and reliable barrier between two environments. They are common in applications such as painting or shot blasting. They’re also vital to the operation of Class I and Class II microbiological safety cabinets, which use curtains of true laminar airflow to protect the operator and the biotech products being worked on.

Industry also uses air curtains to help prevent dust and other impurities from entering sensitive manufacturing areas. People and products can pass through the air curtains but dust cannot.

Applications such as these typically use centrifugal fans with either forward or backward curved impellers.

This short video – Popcorn: The Movie: (Popped In 60 Seconds) – demonstrates how air under pressure can be used to sort a finished product from debris.

Applications such as this use air to lift the desired lighter object from one place to another, while letting the heavier debris fall away. Or vice versa, air is used to blow light debris away from a heavier product.

The type of fan you’ll need will depend on how much pressure you need to create the required movement. If you need higher levels of pressure to achieve this, then a forward-curved or a larger radial fan is the best choice. If less pressure is required, consider a more energy-efficient backward-curved fan. These fans are particularly good for an air knife with a wide inlet (because of the larger volume of air that they move).

Paint booths require centrifugal fans that can extract compromised air from a space. That space may be the size of a garage door, and the air may need to be pulled through a fan inlet, perhaps only 300 mm to 400 mm in diameter. We can provide specialist advice on these fans to ensure they are the correct:

• width – which governs airflow
• diameter – which determines pressure.

Dust in mining, construction and other similar industries can contain carcinogens and silica. Workers exposed to dust on their clothing are 10 times more likely to be affected by fatal or debilitating respiratory diseases.

JetBlack safety cleaning booths protect industrial workers moving from dusty work areas to clean zones. These booths enable them to remove residue powder or contaminants from their workwear quickly and easily.

The booths feature forward-curved centrifugal fans from our HFM range. These fans employ a modular construction, enabling them to be manufactured more efficiently – resulting in cost savings for the customer.

ATEX/Ex fans are an important specialism for ACI. We can create hundreds of different air delivery system configurations for sectors including petrochemicals, oil, gas and pharmaceutical products. ATEX/Ex fans from ACI are capable of withstanding the harsh punishment meted out by the marine/offshore and other hazardous environments.

Our range of non-sparking ATEX/Ex fans includes units with forward-curved, backward-curved and radial impellers – offering a wide choice of pressure and airflow options.

At this point, the air being moved will stop moving in the desired direction and back up down the inlet (unlike in a positive displacement compressor which creates a ‘one-way door’ so the air can move in only one direction – it cannot travel back down the inlet).

Read our Frequently Asked Questions for more information. Find out more about the principles of fan engineering.

For more information on the fan applications, contact our Fan Engineers for specialist advice.