Fan options and key characteristics

Which are the different types of fans in the market and the main features for you to consider?

There are multiple fan types available in the market, but ceiling fans, in general are preferable compared to other fan types due to their air circulation effectiveness and low energy consumption, especially for serving multiple occupants within the same space. In addition, the installation and operation of ceiling fans are specific to the building design which is different from other portable fan options. This guide highlights a standalone section on ceiling fans, while other fan choices (i.e., other air movement devices) will be discussed in a separate section.

Ceiling fans

The US Department of Energy (DOE) defines various ceiling fan types in the "Uniform Test Method for Measuring the Energy Consumption of Ceiling Fans." Table T1 presents the criteria of ceiling fan blade thickness and tip speed adopted from DOE, and Table T2 summarizes the definition of several common ceiling fan types with supporting remarks.

Table T1. Ceiling Fan Blade and Tip Speed Criteria (adapted from DOE definitions).

Airflow Direction

Thickness (t) of edges of blades

Tip speed threshold

Table T2. Common ceiling fan types (adapted from DOE definitions)

Ceiling fan types

Fan diameter (D)

Remarks

In general, a larger diameter fan (i.e., high volume low speed (HVLS) fan) blade can move a larger volume of air than a smaller diameter fan blade. As fan diameter increases, the rotational speed is typically limited to prevent excessive noise from the fan blades, especially near the blade tip. Additionally, where fans can be mounted at blade heights below 3 m [10 ft], the rotational speed must be limited to meet safety criteria for the maximum speed of the blade tips. HVLS fans require higher ceilings (typically at least 3.3 m [11 ft]) and larger spaces free from obstructions to accommodate their increased diameter, and these fans are most often found in non-residential stings, like commercial and industrial applications. Some large-diameter ceiling fans include “winglets” or blade tip fences to maximize airflow and minimize noise, which is a less common problem in standard fans as the blade tip speed is already constrained for safety reasons.

Fan blades

Blade shape, number of blades, and blade pitch are important factors in increasing energy efficiency while maximizing airflow through the fan blades. Generally, there are two main types of fan blades: flat blades and airfoil-style blades. The curvature of the airfoil blades helps increase airflow through the ceiling fan, minimizing air turbulence at the trailing edge, which it is more efficient and quieter when compared to flat blades. However, airfoil blades will not operate as efficiently (i.e., lower airflow) in reverse direction. Increasing the number of fan blades and blade’s angle will increase the airflow of a ceiling fan. Nevertheless, the increased weight and drag can cause energy efficiency loss. Standard ceiling fans typically have 3 to 5 blades with blade’s angle to be 8 – 15°.

Motors and drives

Typically, ceiling fans have three main types of motor (AC induction; permanent magnet DC (PMDC); and brushless direct current (DC) motors) and two types of drives (direct drive; and gear-driven). Table T3 highlights the characteristics of these motors and drives.

Table T3. Comparison between ceiling fan’s motors and drives

Diameter and rotational speed

Assuming all other factors being equal, a larger diameter fan will produce greater airflow, average air speed and uniformity of air speed in space than a smaller diameter fan at the same rotational speed. In general, the total airflow and the air speed from any measurement point are linear with rotational speed. This relationship begins to break down at very low air speed, very low rotational speeds, or where the fan blade height is unusually far from the floor.

Rated airflow

The test methods for rating the airflow of these fans are regulated in the US under 10 CFR 430 Appendix U. For standard fans, the rating is determined by a modified Energy Star method, while for large diameter fans (above 2.1 m [7ft]), the rating is determined by the AMCA 230-15 test method.

Air speed and uniformity

Ceiling fans' air speed is defined as the average air speed that passes through the fan blades sweeping area. It is calculated by dividing the rated airflow of the fan by its diameter. Fan air speed is a useful metric because it is more directly representative of the air speed that will occur in the space. Typically, the maximum air speed in the space is within 1.3 – 1.5 times of the fan air speed and will occur below the fan blade tip. Fans with higher maximum fan rotational speed will produce higher possible air speed in the room regardless of fan diameter. If the room design target is based on maximum air speed instead of rated airflow, the designer can directly compare fans with different sizes.

The variation of the air speeds in a space is an important design consideration. Figure T2 demonstrates an example of air speeds distribution from a 1.5 m [5 ft] diameter ceiling fan. The airflow ‘jet’ from the fan immediately narrows to a slightly smaller diameter than the fan blades. The jet then impinges on the floor, creating a stagnation point, and then spreads radially outwards along the floor. Away from the fan diameter and above the floor spreading zone, there is a still air zone where air speeds are almost unaffected. The depth of air spreading zone along the floor and its average air speed is dependent to the fan diameter: smaller fan (shallower) and larger fan (deeper). In general, larger diameter fans have lower air speeds directly under the fan center (i.e., near the stagnation point), but the air speeds distribution is more uniform than the smaller diameter fans.

Fan operates in reverse direction

Most of the ceiling fans in the market can operate in reverse direction (i.e., blowing air upwards). One application is destratifying spaces in the heating season, where downwards flow may cause a draft (overcooling) on the occupants even when the fan is operating at a minimum rotational speed. The air circulation pattern generated by reversely operated ceiling fans is similar to the normal operation condition, but the air speed is much lower and much more uniformly distributed in space. The upward blowing airflow is depended on the fan type and blade geometry. For the same diameter fans, fan with flat blades would generate higher upward airflow than airfoil-style blades. The average air speed for upwards blowing is approximately 60 – 70 % of the downwards case for flat blades fans. Furniture and ceiling obstructions are likely contributing to the drop of airflow.

Fans speed control

Most standard fans typically have several fixed fan speed levels. Standard AC motor fans typically have three levels, whereas DC motor fans tend to have more. Large-diameter fans usually have a variable speed regardless of motor type. Having a high minimum speed can be problematic in some applications. It may generate too much of a cooling effect for the occupants when temperatures are mild or cool. A reasonable approximation is that the minimum fan air speed should be below 0.4 m/s [80 fpm] or a 1.7 °C [3 °F] cooling effect at the minimum allowed blade height, depending on the specifics of the application.

Power and efficacy

According to the US Department of Energy (DOE), ceiling fan efficacy is generally defined by the amount of airflow generated by the fan divided by the power consumption. Typical fan efficacy at the lowest and highest operating speeds are, respectively, 0.08 m³/s·W [165 CFM/W] and 0.04 m³/s·W [79 CFM/W] (MAEDbS database). It is noted that ceiling fans with the same efficacy could perform differently, whereas fans with lower-rated maximum airflows will have a better-rated efficacy even if they consume more power to provide the same airflow. It is because when a fan operates at a lower rotation speed, the reduction of power consumption is higher than airflow (i.e., fan power ∝ (rotational speed)3, while airflow ∝ rotational speed), resulting in higher fan efficacy metrics (airflow/power). Figure T8 shows three fans have the same efficacy 0.11 m³/s·W [234 CFM/W], but the fan represented by the leftmost curve has a smaller overall efficiency (i.e., lowest maximum airflow) than the rightmost curve (i.e., highest maximum airflow).

Meanwhile, the ceiling fan energy index (CFEI), a ratio of reference fan power input to actual fan power input, is a more reliable alternative in reflecting the above blind spot. It helps to make inefficient fans less likely to comply with slower speeds and to remove the unintentional barrier to compliance for high-performing high-utility fans by comparing fans to a standardized baseline. Taking the same example in Figure T3, CFEI at high fan speed for the leftmost and rightmost curve is, respectively, 0.63 (less efficient) and 1.72 (more efficient). See the section “Ceiling fans testing regulations” for more information.

Generally, the ability of a fan to operate efficiently at a lower speed improves as the diameter increases, but there is considerable variation in performance between models of fans with the same diameter. Fans with a lower turndown ratio (i.e., minimum fan speed divided by maximum fan speed) can be more flexible for different applications.

Other air movement devices

Places with lower floor-to-ceiling height or a ceiling with compacted fixtures could be limited for ceiling fan installation. Moreover, installing a ceiling fan may be challenging due to the presence of other equipment,fans like lights. These limitations can be mitigated by using other air movement devices. There are numerous kinds of fan available in the market, and this technical guide only illustrates six common types of air movement devices: desk fan, pedestal fan, tower fan, wall-mounted fan, bladeless ceiling fan, and air circulator. The selection criteria for these fan types are discussed as follows.

Typical functions and airflow patterns

The above-mentioned air movement devices basically have two major functions: (1) move air directly towards the human body and (2) room air circulation. Moving air towards the human body aims to cool the subject using convective heat loss, while air circulation aims to keep air in motion within the enclosed space to enhance air and temperature mixing. Nevertheless, most fan types can deliver both functions by adjusting the fan speed and operation distance between fans and occupants. Therefore, fan selection should be based on the intent, whether it is an individual adjustment for personal cooling (group 1) or producing a general air movement effect for multiple occupants’ usage simultaneously. Table T4 summarizes the typical function, application location, and approximate price range of different fan types for reference.

Table T4. Summary of other air movement device’s function, noise level and approximate price range.

Airflow patterns from different devices can be varied by fan size, blade types, and fan structure. There is no requirement or standard on typical air speed for the air movement devices described in Table T4. The air speed and airflow requirements are mainly dependent on the typical functions and locations of users.

For fans that intend for direct cooling towards the human body, customers tend to select a stronger fan that can produce more airflow and faster air speed. However, in some situations (e.g., operating the fan together with air-conditioning), the occupants do not require air movement that is too strong. Therefore, choosing a fan with a possible lower airflow turndown (minimum speed divided by maximum speed) capability could be the key to better comfort in terms of direct convective cooling. Meanwhile, if the function of a fan is used to circulate the air in a room, the fan selection approach should be either bigger in size (i.e., produce more airflow) or able to generate a high airflow jet with high speed to drive the in-room air. Figure T4 illustrates examples of airflow flow patterns for different fan types.

Noise levels

Noise from a fan can initiate dissatisfaction and unwillingness to its usage from occupants. The noise sources of a fan are mainly from the operating motor and the fast movements of the blades (i.e., turbulence ingestion). The sound level of domestic fans ranges from 30 to 70 dBA. Fans operating at higher speeds usually correspond to higher noise levels due to turbulence. Nevertheless, the noise cannot be characterized only by sound level. Its frequency is also an important aspect. Low-frequency noise has been identified to be annoying. It is not practical to conclude one fan type will always be quieter than another. Designers/users are encouraged to experience fan usage before purchasing.

Power and cooling fan efficiency

All the elevated air movement devices described in Table T4 consist of a motor and drive. The types of motor and drive used for these elevated air movement devices are similar to those that operate in a ceiling fan, which we have discussed in the former section. It is worth noting that a larger energy-saving potential can be achieved by using a DC motor instead of an AC motor, especially for small fans like desk fans or pedestal fans.

It is noted that the cooling effect depends on the effectiveness of convective cooling towards the subject’s body and is affected by air speed, airflow pattern, and fan operation distance. Eventually, the intent of fan usage (local cooling vs. air circulation) should have been considered when quantifying the fan's effectiveness.

Flexibility and other features

One major advantage of the above-listed elevated air movement devices over ceiling fans is the high flexibility of usage in terms of location, operation height and oscillating angle. Such flexibility enhances adjustment to occupant’s needs and comfort demands regarding elevated air speed under different circumstances.

In addition, some fans are also equipped with modern technologies that enable them with special features while being equally efficient. For example, fans equipped with filters for air purification, fans installed with UV-C light for air disinfection, and fans that emit water mist for air humidification and evaporative cooling.

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Last updated 1 year ago