It is very important that ventilation systems comply with any Fire Regulations, Building Regulations, Codes of Practice, etc., relevant to the installation and the components being used.
As much information as possible must be obtained from the customer or other sources regarding the application of these regulations to the building and/or area to be ventilated before attempting to design a system.
A simple procedure for designing a ducted system is as follows:
| a)
| Calculate the Room Volume to be ventilated Width x Length x Height = m³ (cubic metres)
| | b)
| Calculate the Air Volume requirement by multiplying the Room Volume by the Air Change Rate per hour (see Table 1) = m³/h.
| | c)
| Decide on the best position for the intake and the extract outlets to the atmosphere and the best route for duct runs. The design should provide good air distribution in the room, whilst keeping the duct layout as simple as possible.
| | d)
| Determine each section of main and branch duct, the size and shape of each grille and duct bend.
|
There are several ways of approaching designing and sizing ducted systems. The simplest is the
velocity method, which involves selecting main and branch air velocities as in Table 1, used in conjunction with trial calculations.
| e)
| Sizing the duct(s). A calculation is necessary to establish a duct size, which will provide the Air velocity which equates most closely to the velocities required, see Table 1.
|
Substitute the Air volume for the room (m³/h) you have previously calculated, and the velocity
(from Table 1) in the equation below.
| Duct cross section area =
|
Volume (m³/h)
| = m²
| | Velocity (m/s) × 3600 (Factor)
|
Select the next size up duct dia. from (Table 2) and calculate the air velocity in the duct in the equation below:
| Velocity (m/s) =
|
Volume (m³/h)
| = m/s
| | Area (m²) × 3600
|
or
| Velocity (m/s) =
|
Volume (m³/s)
| = m/s
| | Area (m²)
|
TABLE 1 - Air Changes Per Hour (ACH) and duct/grille velocities:
|
|
|
TYPICAL VELOCITIES OF DUCTED SYSTEMS (m/s)
| | Location
| ACH
| Main Duct
| Branch Duct
| Supply Grilles
| Exhaust Grilles
|
| Assembly Halls | 4-8 | 5-8 | 4-6 | 3-5 | 2-3
|
| Bakeries | 20-30 | 8-11 | 6-8 | 5-8 | 3-4
|
| Banks | 4-8 | 5-8 | 4-6 | 3-5 | 2-3
|
| Bathrooms | 6-10 | 4-5 | 3-4 | 2-3 | 1.5-2
|
| Bedrooms | 2-4 | 4-5 | 3-4 | 2-3 | 1.5-2
|
| Billards Rooms* | 6-8 | 5-8 | 4-6 | 3-5 | 2-3
|
| Boiler Rooms | 15-30 | 8-15 | 6-10 | 5-10 | 4-10
|
| Cafes & Coffee Bars | 10-12 | 5-8 | 4-6 | 3-5 | 2-3
|
| Canteens | 8-12 | 5-8 | 4-6 | 3-5 | 2-3
|
| Cellars | 3-10 | 5-8 | 4-6 | 3-5 | 2-3
|
| Changing Rooms - main area | 6-10 | 4-5 | 4-6 | 3-5 | 2-3
|
| Changing Rooms - shower area | 15-20 | 5-8 | 4-6 | 3-5 | 2-3
|
| Churches | 1-3 | 4-5 | 3-4 | 2-3 | 1.5-2
|
| Cinemas & Theatres * | 10-15 | 5-8 | 4-6 | 3-5 | 2-3
|
| Club rooms | 10-12 | 5-8 | 4-6 | 3-5 | 2-3
|
| Compressor Rooms | 10-20 | 8-15 | 6-10 | 5-10 | 4-10
|
| Conference Rooms | 8-12 | 5-8 | 4-6 | 3-5 | 2-3
|
| Dance Halls | 8-12 | 5-8 | 4-6 | 3-5 | 2-3
|
| Dental Surgeries | 12-15 | 5-8 | 4-6 | 3-5 | 2-3
|
| Dye Works | 20-30 | 8-15 | 6-10 | 5-10 | 4-10
|
| Electroplating Shops | 10-12 | 8-15 | 6-10 | 5-10 | 4-10
|
| Engine Rooms | 15-30 | 8-15 | 6-10 | 5-10 | 4-10
|
| Entrance Halls & Corridors | 3-5 | 5-8 | 4-6 | 3-5 | 2-3
|
| Factories & Workshops | 8-10 | 8-15 | 6-10 | 5-10 | 4-10
|
| Foundries | 15-30 | 8-15 | 6-10 | 5-10 | 4-10
|
| Garages (Showrooms) | 6-8 | 5-8 | 4-6 | 3-5 | 2-3
|
| Glasshouses | 25-60 | 4-5 | 3-4 | 2-3 | 1.5-2
|
| Hairdressing Salons | 10-15 | 5-8 | 4-6 | 3-5 | 2-3
|
| Hospitals - Sterilising | 15-25 | 5-8 | 4-6 | 3-5 | 2-3
|
| Hospitals - Wards | 6-8 | 4-5 | 3-4 | 2-3 | 1.5-2
|
| Kitchens - Domestic | 15-20 | 4-5 | 3-4 | 2-3 | 1.5-2
|
| Kitchens - Commercial ** | 20-30 | 8-11 | 6-8 | 5-8 | 3-4
|
| Laboratories | 6-15 | 5-8 | 4-6 | 3-5 | 2-3
|
| Launderettes | 10-15 | 8-11 | 6-8 | 5-8 | 3-4
|
| Laundries | 10-30 | 8-11 | 6-8 | 5-8 | 3-4
|
| Lavatories | 6-15 | 5-8 | 4-6 | 3-5 | 2-3
|
| Lecture Theatres | 5-8 | 4-5 | 3-4 | 2-3 | 1.5-2
|
| Libraries | 3-5 | 4-5 | 3-4 | 2-3 | 1.5-2
|
| Living Rooms | 3-6 | 4-5 | 3-4 | 2-3 | 1.5-2
|
| Mushroom Houses | 6-10 | 5-8 | 4-6 | 3-5 | 2-3
|
| Offices | 6-10 | 5-8 | 4-6 | 3-5 | 2-3
|
| Paint Shops (not cellulose) | 10-20 | 8-11 | 6-8 | 5-8 | 3-4
|
| Photo & X-ray Darkrooms | 10-15 | 5-8 | 4-6 | 3-5 | 2-3
|
| Public House Bars | 10-15 | 5-8 | 4-6 | 3-5 | 2-3
|
| Recording Studios | 10-12 | 4-5 | 3-4 | 2-3 | 1.5-2
|
| Recording Control Rooms | 15-25 | 4-5 | 3-4 | 2-3 | 1.5-2
|
| Restaurants | 8-12 | 5-8 | 4-6 | 3-5 | 2-3
|
| Schoolrooms | 5-7 | 5-8 | 4-6 | 3-5 | 2-3
|
| Shops & Showrooms | 8-15 | 5-8 | 4-6 | 3-5 | 2-3
|
| Shower baths | 15-20 | 5-8 | 4-6 | 3-5 | 2-3
|
| Stores & Warehouses | 3-6 | 5-8 | 4-6 | 3-5 | 2-3
|
| Swimming baths | 10-15 | 8-11 | 6-8 | 5-8 | 3-4
|
| Toilets | 6-10 | 4-5 | 3-4 | 2-3 | 1.5-2
|
| Utility Rooms | 15-20 | 5-8 | 4-6 | 3-5 | 2-3
|
| Welding Shops | 15-30 | 8-15 | 6-10 | 5-10 | 4-10
| |
* Increase by 50% where heavy smoking occurs or if the room is underground
| |
** Some commercial kitchens may require higher ventilation rates, based on cooking equipment in use.
|
If the resultant velocity is too high then the duct diameter is too small, the system is likely to be noisy and it is unlikely that there is a fan to suit.
If the resultant velocity is lower than recommended, the system will be extremely quiet but the ducts oversized and the overall cost may rise unnecessarily.
The Cross Section area in m² for Circular Ducting is as follows:
| TABLE 2
| | Diameter
| =
| Area
| | 100mm | = | 0.00785m²
| | 125mm | = | 0.01227m²
| | 150mm | = | 0.01767m²
| | 200mm | = | 0.03142m²
| | 250mm | = | 0.04909m²
| | 315mm | = | 0.07794m²
| | 355mm | = | 0.09898m²
| | 400mm | = | 0.12566m²
|
| f)
| In order to proceed to the next stage you need the following information:
i. The preferred duct diameter (calculation e)
ii. The air velocity m/s (calculation e)
iii. A list of duct items including duct length and the number of bends. Select the duct diameter
and component resistance chart and list the resistance in Pa (Pascals) against each item
shown.
|
Add up all of the component resistances for the Total System Static Resistance.. Using the
manufacturers information, select a fan with the same duct diameter as the system, ensuring that the fan produces a slightly higher performance than required, as a speed controller can be used to reduce the fan's performance to the correct level.
| 
