HVAC Diffusers and their applications for airflow supply

Diffusers are designed for airflow patterns needed to distribute cold or hot air as intended.

There are many different types of HVAC diffusers available in the market. Each diffuser has its own unique characteristic and application. To figure out which diffusers to use and why, herein are 6 common types of HVAC diffusers: Directional Diffuser, Linear Slot Diffuser, Round Diffuser, Swirl Diffuser, Double Deflection Diffuser and Jet Diffuser. 

What are HVAC diffusers! HVAC diffusers are used to deliver cold air or hot air, depending on whether it is for cooling or heating. The main difference between diffusers and grilles is that diffusers are often associated with airflow control while grilles are mostly not. So, when we refer to something as a diffuser, it means that we are talking about supply airflow. Similarly, most people refer to grilles when they are discussing return airflow. Some of the common names given to airflow distribution materials are diffusers, grilles, vents, registers and louvres. 

Directional diffuser: most common 

Directional diffusers include 4-way diffusers, 3-way diffusers, 2-way diffusers and 1-way diffusers. Directional diffusers are considered standard HVAC supply air diffusers. The size of directional diffusers are mostly 2×2 ft (width x length) and they fit perfectly on a 2×2 ft ceiling. However, they do come in 1×1 ft and other sizes upon request. “Beware when you purchase directional diffusers because you need to specify whether the ceiling is measure in Imperial unit or SI unit.” The dimension of directional diffusers in the SI unit is 595x595mm. If you put an Imperial unit directional diffuser (603x603mm) on a SI unit ceiling, it will not fit. The same applies to other squareshaped ceiling diffusers. Standard directional diffusers are made of aluminium. Galvanized steel directional diffusers are likely to rust. Directional diffusers can be found almost everywhere, especially in office buildings and shopping malls. Particularly, the 2×2 ft 4-way diffuser is the most common type of HVAC diffuser. 

Way diffuser on a 2×2 ft ceiling 

Directional diffusers offer excellent airflow distribution. Each of them supplies about 300 cfm of airflow at low noise, barely audible. They are mostly coupled with a radial damper, a crown damper or an opposed-blade damper for airflow control. Furthermore, most of them come with a round-to-square adaptor (or plenum box) for a flexible duct connection. If you have a directional diffuser that has a radial damper, you can use a screwdriver to adjust the damper and thus, the airflow without having to open up the diffuser at all. 

Linear slot diffuser: most beautiful 

Linear slot diffusers include 4-slot linear diffusers, 3-slot linear diffusers, 2-slot linear diffusers and other numbers of slots depending on the manufacturer or upon request. The more common ones are 3-slot and 2-slot. Linear slot diffusers always have two lengths; 600mm and 1200mm. Other lengths such as 1500mm are possible depending on the manufacturer. As for the width, it’s depending on the number of slots. The more slot it has, the wider it gets. To give you an idea, a 2-slot and a 3-slot linear diffuser is about 100mm and 150mm in width respectively. 

Most linear slot diffusers are made of aluminium. The fins inside linear slot diffusers, which are used to control the airflow direction, are black in color. You can squeeze the fins to concentrate the airflow or widen the fins to spread out the airflow. I would suggest not to concentrate the airflow because often, it is uncomfortable for the people sitting below it. Linear slot diffusers look more premium than other types of supply air diffusers. Therefore, they are very common in hotels and luxury homes.The number of slots in linear diffusers corresponds to the amount of airflow. The higher the airflow requirement, the more slots it needs to have. Hence, commercial buildings often use 3-slot and above. 

“3-slot linear diffusers are more common in commercial buildings while 2-slot linear diffusers are more common in residential buildings.” Some luxury places even have curved linear slot diffusers. They are placed side-by-side to form a continuous length of the diffuser. In such cases, some linear slot diffusers are dummies and are not connected to any supply or return air duct. 

Leisure room linear slot diffusers 

Linear slot diffusers required a rectangularshaped plenum box with an opposed-blade damper for airflow control. The plenum box has a round duct collar for a flexible duct connection. It is very important that the duct collar on the plenum box of linear slot diffusers is put on the side of the plenum box rather than on the top. Linear slot diffusers have lower airflow capability than directional diffusers. Each of them supplies about 200-250 cfm of airflow only. If you allow too much airflow through a linear slot diffuser, it starts to generate noise. Besides, its vertical throw characteristic may cause uncomfortable if too much air is blasting onto people’s heads. 

However, when noise is not a concern, linear slot diffusers are good for high ceiling rooms. They are capable of throwing air from the ceiling to the ground at a height of around 9-12 meters most of the time. If you allow more air to pass, you can even throw up to 30 meters vertically. If you have ceilings with a height of 12 meters, you can control the supply airflow based on the diffuser datasheet to cap the air throw at around 9 meters so that you don’t blast cold air on people’s head but you allow the air to travel close enough to the ground and let it sink naturally. Thus, you don’t waste energy. 

Round diffuser: uniform air distribution 

Round diffusers are similar to directional diffusers but come in round shapes. They offer even better air distribution than directional diffusers. To many people, round diffusers are better looking than squareshaped directional diffusers. 

Naked round diffuser 

Round diffusers are available in aluminium, plastic or galvanized steel. Most round diffusers are made of plastic material and they are white in color. With that, they are cheaper than directional diffusers. The diameter of round diffusers is available from 150mm up to 600mm or more depending on the manufacturer. However, most round diffusers are 250mm and 300 mm in diameter since these sizes are appropriate for about 250-300 cfm of airflow. Furthermore, supply air flexible ducts are mostly 250mm and 300mm as well. Hence, they fit very well to round diffusers. Round diffusers can be seen in homes, office buildings, hotels and factories. They are often installed on plaster ceilings. Similarly, round diffusers are often coupled with a radial damper or a crown damper but not an opposed-blade damper. Many round diffusers have an openable front cover for easy damper airflow adjustment. They may not need an adaptor for the flexible duct connection since they have a round collar. 

Swirl diffuser: high air change rate 

Swirl diffusers have angled blades to “swirl” the supply air. They can be either round or square in shape. Directional and round diffusers don’t supply air vertically. Meanwhile, linear slot diffusers don’t supply air horizontally. On the other hand, swirl diffusers are the combination of both the directional and linear slot diffusers. They throw air horizontally and vertically, and the amount of airflow in each direction can be adjusted by setting their blades. 

FCU swirl diffusers 

Although swirl diffusers have vertical air throw, they are not capable of throwing the air as far as linear slot diffusers. Most of the time, they throw air maxed at about 4-5 meters only. The main reason to use swirl diffusers is to mix up the air faster and thus, cool faster and achieve a higher air change rate per hour. Swirl diffusers are also known as high induction diffusers. Most swirl diffusers are made of galvanized steel. Some swirl diffusers are the combination of two materials with the body made of galvanized steel and the blades made of aluminium. By default, swirl diffusers are white in color. Their blades have different arrangements for different airflow patterns. Swirl diffusers can be installed at places just like directional diffusers and round diffusers. However, swirl diffusers are less likely to be seen in lowend office buildings and hotels due to the higher cost. As swirl diffusers are available in both round and square shapes, they can be installed on 2×2 ft ceilings, plaster or even without the ceiling. 

Double deflection diffuser: large airflow 

Double deflection diffusers are like a mesh; many square-shaped holes. They can be viewed as cheaper linear slot diffusers but handle larger airflow. They are made of either aluminium or galvanized steel. By default, double deflection diffusers are white in color and square-shaped. Double deflection diffusers are mostly used in school halls, low-end theatres and auditoriums where there are high ceilings and aesthetics is not a major concern. 

Double deflection diffuser 

Double deflection diffusers are excellent vertical air throwers. They can handle a high airflow supply yet remain quiet. However, they are not able to throw air as far as linear slot diffusers since the air is not concentrated. Nevertheless, double deflection diffusers are still suitable for most high ceilings where the ceiling height is about 9-12 meters. These diffusers also can be installed on the side to throw air sideways instead of always from the top. Since double deflection diffusers are square-shaped, they need a round-to-square adaptor if connected to a flexible duct. Also, they can be coupled with a radial damper, a crown damper and an opposed-blade damper for airflow control. 

Jet diffuser: long distance throw

Jet diffusers are round-shaped supply air diffusers that look clean and smooth. Most of them only have one big opening for air to pass through unlike any other diffusers that have many openings. Jet diffusers are made of aluminium. They are like a ball in a round cover which allows them to supply air to almost any direction. Jet diffusers also have standardized sizes from 100mm up to 600mm or any other sizes upon request depending on the manufacturer. The outer appearance of jet diffusers is very clean and smooth. Thus, they are suitable for highend places such as airports, auditoriums, cinemas and museums. Nevertheless, they are mostly seen in airports, mounted on sidewalls along the hallway. 

Airport jet diffusers 

Generally, jet diffusers are an option for places where linear slot diffusers, double deflection diffusers and other types of diffusers are not possible. Because jet diffusers have a large opening, they deliver air at high velocity and yet, very quietly. Furthermore, jet diffusers have very little air resistance which is gentle on the requirement of the static pressure of air conditioners. Jet diffusers can throw air up to 20 meters. 

Conclusion 

There are some other types of supply air diffusers for certain applications which I have not mentioned here such as perforated diffusers or displacement air diffusers and linear bar grilles as supply air diffusers. However, the above 6 diffusers are the most common types of supply air diffusers used in air conditioning. Ventilation systems mostly use grilles with a volume control damper instead of diffusers. 

Diffusers are designed for certain airflow patterns which are needed to distribute cold/hot air evenly or as intended. Each type of diffuser has its own purpose and we should always use the suitable one according to the needs. 

Expertise shared by 

Yu Chang Zhen 

Aircondlounge.com