The process of ventilating a building involves the removal of stale or unpleasant air and its replacement with fresh air. This essential process, necessary in various settings, always follows the same basic principle: expelling unwanted air and drawing in clean air. The specific fan type and the rate of airflow, measured in cubic feet per minute (CFM), vary depending on the application. Other important factors include the resistance to airflow, known as static pressure (SP), and the fan's noise level, measured in decibels A (dBA).

Selecting an appropriate exhaust fan in Singapore for a specific function can be challenging, especially when unsure of the required model or air capacity. Because of this, it becomes necessary to engage in fan specification, a process that is not an exact science but can be approached confidently when the fan's intended use is clearly understood.

For proper fan selection, four key parameters must be determined: the model of the fan, the airflow rate in CFM, the system's static pressure, and the acceptable noise level in the environment.

1. Fan Model

Various models of exhaust fans in Singapore are available, each designed to move air effectively from one space to another. Due to the wide range of fan applications, manufacturers offer diverse models with advantages for specific uses. The challenge for users lies in identifying the model that best meets their requirements. Here are some key considerations:

Direct drive or belt drive?

Direct drive fans are cost-effective and require minimal maintenance for low-volume air movement (2,000 CFM or less) and low static pressure (0.50 inches or less). Some fans can be adjusted using a speed control to alter the airflow. On the other hand, Belt drive fans are more suitable for moving larger volumes of air (over 2,000 CFM) and for higher static pressures (above 0.50 inches). They feature adjustable pulleys that allow a 25% change in fan speed and airflow. Beltdriven fans are typically preferred for operating temperatures above 50º C (122º F).

Axial or centrifugal?

Axial fans, resembling propellers, are economical for moving large volumes of air (over 5,000 CFM) at low static pressures (0.50 inches or less). However, since the motors are usually placed in the airstream, they are best used where the air is relatively clean and not too hot, with maximum temperatures around 40º C (104º F). Centrifugal fans, in contrast, are more efficient at handling higher static pressures and are quieter than axial fans. Many centrifugal fan models are designed with motors outside the airstream, making them suitable for moving contaminated or high temperature air.

Roof, wall, or duct?

Fans are designed for three common mounting locations: roofs, walls, or ducts. The fundamental components of the fan remain the same regardless of the location; only the housing changes to facilitate installation. The desired airflow pattern and the building's physical layout determine the optimal location for a fan. Assessing the building's structure and visualising the desired air movement makes the best placement for the fan clear.

2. Cubic Feet per Minute (CFM) of Airflow

When selecting an exhaust fan in Singapore, it's essential first to identify the fan type and then calculate the volume of air that needs to be exchanged. Local building codes typically guide the air changes required for adequate ventilation. These guidelines generally suffice for most situations.

However, the necessary airflow may fall outside this suggested range in extreme conditions. To accurately determine the number of air changes needed, consider factors such as the geographic location and the usual intensity of use in the area.

Choose a lower number within the specified range in hotter climates or areas with heavier usage to facilitate faster air exchange. Conversely, a higher number within the range may be adequate in moderate climates or spaces with lighter use.

To calculate the volume of a room, multiply its length, width, and height. Then, apply the following formula to ascertain the cubic feet per minute (CFM) required for sufficient ventilation of the space:

Recommended Air Exchange Rates for Effective Ventilation

 

Area	Minutes to Change
Assembly Hall	3 to 10
Attic	2 to 4
Auditorium	3 to 10
Classroom	4 to 6
Corridors/Halls	6 to 20
Dormitories	5 to 8
Engine Room	1 to 3
Factory	2 to 7
Garage	1 to 10
Generator Room	2 to 5
Gymnasium	3 to 8
Kitchen	1 to 5
Laboratory	2 to 5
Machine Shop	3 to 6
Meeting Room	3 to 10
Office	1 to 8
Packing House	2 to 5
Residence	2 to 8
Restaurant	5 to 10
Toilet	5 to 7
Warehouse	3 to 10

 

3. Static Pressure

Accurately measuring static pressure is essential for correctly selecting an exhaust fan in Singapore.

Static pressure in fan systems is measured in inches of water gauge, where one pound per square inch is equivalent to 27.7 inches of static pressure (SP). Typically, the static pressure in these systems is less than 2 inches SP or 0.072 psi.

The accompanying illustration shows how a manometer measures static pressures in the ductwork. A differential in pressure between the duct and the atmosphere causes the water levels in the manometer to vary. This variation represents the static pressure, measured in inches of water gauge.

In the example of the exhaust fan, the air is pulled upward through the ductwork, creating a low pressure area at the top of the duct. This principle is like how liquids are drawn up through a straw. The static pressure that the fan needs to overcome depends on several factors, including the air velocity within the ductwork, the amount of bends in the duct, other elements that resist airflow, and the duct length. For well-designed systems with adequate makeup air, the following guidelines can be used to estimate static pressure.

 

Static Pressure Guidelines for Exhaust Fans in Singapore

• Non-Ducted Systems: 0.05 inches to 0.20 inches
• Ducted Systems: For every 100 feet of duct, the static pressure should be between 0.2 inches and 0.40 inches, provided the air velocity in the duct is between 1,000 feet per minute and 1,800 feet per minute.
• Fittings: Each fitting (elbows, registers, grills, dampers, etc.) adds 0.08 inches to the static pressure.
• Kitchen Hood Exhaust: The static pressure ranges from 0.625 to 1.50 inches

It's crucial to note that the static pressure requirements are greatly influenced by the volume of makeup air supplied to the space. Inadequate makeup air can lead to increased static pressure and a decrease in the volume of air being exhausted. It's crucial to ensure that for every cubic foot of air exhausted, an equivalent cubic foot of air is supplied.

 

4. Loudness Limit

When selecting an exhaust fan in Singapore, it's essential to consider the noise level it generates. In the fan industry, noise is commonly measured in sones. To put this into perspective, one sone is roughly as loud as a quiet refrigerator heard from a distance of 5 feet in a typical room. Sones are a linear measure of sound intensity, meaning a fan producing ten sones is perceived as twice as loud as one producing five sones.

For choosing a fan that suits your needs, refer to a sone chart to find a noise level acceptable for your application. Generally, it is advisable to select a fan whose sone rating falls within the recommended range for its intended use.

Recommended Limits for Room Loudness

Room Type	dBA
Private homes (rural and suburban)	32 to 48
Conference rooms	36 to 51
Hotel rooms, libraries, movie theatres, executive offices	38 to 54
Schools and classrooms, hospital wards, operating rooms	41 to 58
Court rooms, museums, apartments, private homes (urban)	44 to 60
Restaurants, lobbies, general open offices, banks	48 to 64
Corridors and halls, cocktail lounges, washrooms and toilets	51 to 67
Hotel kitchens and laundries, supermarkets	56 to 72
Light machinery, assembly lines	64 to 80
Machine shops	67 to 84
Heavy machinery	67 to 84

 

Expert Ventilation Solutions from JA Redden

If you're looking to ensure optimal ventilation in your space with the right exhaust fan, look no further than JA Redden, your trusted HVAC expert. Our team provides you with tailored solutions that meet your specific needs.

You’re welcome to register in our Download form, via the below link, to receive a free QuickFan selection software that is easy to use and loaded with various tools to assist in your design & calculation.

https://www.sodeca.com/en/software

 

At JA Redden, we understand the importance of proper air circulation, noise levels, and energy efficiency. Contact us today on +(65) 6451 6168 or email sales@jaredden.com.sg to find the perfect exhaust fan solution and experience the difference a professionally selected and installed system can make in your environment. Trust JA Redden for all your HVAC needs— where quality air meets quality service.

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