There are many fans to choose from when specifying a ventilation system.
Over time, the options have multiplied and now building engineers have hundreds of units to choose from,each offering something different and specific to certain applications or environments. The selection process has become a task in itself,aiming to find the right product for the given application that is also energy-efficient.
What follows is a brief breakdown of the fan selection process and what to look out for in both construction and performance.
A fan is a mechanical device that enables a movement of air,vapour and other gases in a given system.Fans are widely used for circulating air in rooms and buildings for cooling or heating people,motors, materials and products.
They are also used for exhausting dust and noxious fumes,forced draughts in steam boilers and in heating, ventilating and air- conditioning systems.
Fans may be generally classified as centrifugal,axial,or mixed flow. In general, an axial-flow fan is suitable for a larger flow rate with a relatively low pressure gain, and centrifugal and mixed flow fans for a comparatively lower flow rate and a higher pressure rise.
Sound, airflow, design and efficiency are highly important fan features, but the most topical issue, in accordance with the new environmental standards, is how they contribute to energy efficiency and whether they make any considerable impact on fuel savings in larger environments.With so many variations of fan types on the market, correct selection is a key ingredient in determining the efficiency.
There are many aspects that influence energy saving in fans,all of which should be addressed to fully understand their independent contributions.Aside from fan selection,which plays a key role,other factors include noise,motor-efficiency,speed,design,outercasing and maintenance.This article aims to briefly breakdown these elements.
Fan selection
Factors including cost optimisation, power rating and noise levels govern the selection of a fan. The fan selected should, ideally, allow for optimised energy usage.Various combinations are possible to meet any given duty or operating requirements for best performance, lesser noise, power and cost characteristics. Once the volume of airflow and the static pressure of the system are known, it is possible to specify a fan. To select a fan one must consider the following parameters: Total airflow required, total operating pressure, fan installation space, noise, reliability and operating environment.
One should also consider the speed. Most fans these days are capable of speed-control so the unit can be set at levels needed,rather than allowing for over-ventilation.It is considerations such as these that contribute heavily to energy use. By reducing the extraction rate of the fan, the heat loss from the ventilated space is also reduced.
Size is another key influence on correct fan selection. Space limitations in an application will provide certain specification guidelines. Additionally, fan size affects energy-efficiency considerably. A small fan normally is less energy-efficient than a larger unit of similar type. Selection reflects on building and ventilation design.
To summarise the selection process,it should be considered as a part of the whole ventilation system. Building design and air tightness, combined with an accurately selected and well designed fan driven ventilation system is the recipe for an effective and energy-saving solution.
While fan efficiency is an important consideration, it should not be used as the sole measure. Cost, availability, noise, maintenance, and projected life span are among the factors affecting long-term expenditure and serviceability. All of these factors, as well as effectiveness, must be evaluated.
Noise
Turbulence is a primary factor influencing the noise level of a fan. High sound levels are an indication of poor design and/or poor equipment selection. Many may agree that inline centrifugal fans are amongst the quietest on the market. The inline-fan arrangement allows for the placement of a fan to be installed in a space only slightly larger than the duct itself. This is where correct fan selection is important because some units are specifically designed for quiet operation while others are not.
Also, while recent advances in inline-impeller design have reduced noise levels, not all manufacturers provide the quieter impellers. As a matter of course, certified noise-performance data should be requested if the fan is being used in quiet areas such as classrooms and auditoriums.
Vibration also plays a part in sound levels and involves more than a flexible connection.Although duct linings and attenuators can help control sound,reducing HVAC noise at its source is the most effective solution.
Motor-efficiency
The motor can have a major influence on the fan’s overall ventilation energy-efficiency and operating costs. Some manufacturers offer fan models with high-efficiency(HE)motors. These may cost more initially, but offer a longer durability and less energy consumption than the lower efficiency units.
Speed
Speed greatly influences fan performance.The higher the fan speed, the more the turbulence increases and efficiency decreases. Fan speed should be controllable, allowing for energy to be used when needed and at the correct pressures. Understanding the ventilation requirement at hand is essential for setting the correct fan speed during installation.
Keeping the fan tip speed as low as possible may assist in quieter operation. Lower tip speeds generally mean lower sound levels but this is dependent on impeller design.
Design
Design and blade shape are important for determining the energy efficiency of a fan and no discussion of modern fan design would be complete without mentioning the motorised impeller.With the motor built into the fan wheel, the unit becomes as compact as possible.
In addition, some fans utilise blade designs that reduce the build up of dirt. The space between the blades and outer-casing is important for determining the flow pressure and efficiency of a fan. Fans with large gaps generally have low efficiencies due to the open airway that occurs during operation.
Outer casing
The housing of the fan influences general performance. As air enters and moves through an opening, turbulence is created and the effective cross-sectional area of the opening is reduced.
Reducing the effective area of the opening minimises the amount of air that can move through it. The shape of the opening, particularly the point at which the air enters, determines the degree of reduction that occurs.For industrial and larger commercial applications, building engineers look for units with corrosion-proof housing materials. Pressure galvanised steel or powder coated steel casings are ideal materials.
Fan maintenance
There is no point in purchasing an energy-efficient model if the unit is not well maintained. Poor maintenance can reduce fan-efficiency by 50 percent or more. Ventilation system components require regular maintenance.
Dust build-up on fan blades and shutters can drastically reduce fan performance. Dust on the safety grilles or on the shutters further restricts performance. Dirt on the motor can act as insulation,causing the motor to overheat. These higher operating temperatures may cause shortened motor life and premature burnout.
Belt adjustment is one of the primary,and often overlooked, maintenance problems with belt-driven fans. These must be regularly adjusted if full air movement is to be achieved,and efficiency maintained.Therefore, belts should be easy toadjust. When a new fan or a new belt has been installed, the belt should be checked for tension after a few weeks of operation to ensure correct tension is being maintained. Regular checks are advisable thereafter.
Summary
To sum up, there are several factors that play a part in the fan’s overall performance and energy consumption levels. Correct product selection,noise,motor-efficiency, speed, design, space limitations and maintenance. These elements are key considerations, but that does not eliminate the influence of the initial building design and the combination with other ventilation systems, if any. Installing incorrect products can be highly unfavourable to energy costs so adhering to a clear and concise guide on product options is the safer plan of action
