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Ventilation design – Air Velocity Air velocity within a room or workplace should be between 0.15 and 0.50 m/s, depending on the amount of activity. Sedentary tasks such as desk work will fall into the range of 0.15 to 0.30 m/s, whilst more active assembly work, shop work and manufacturing, between 0.30 and0.50 m/s. These figures are designed to provide a feeling of freshness, to relieve stagnation without noise distraction from air movement equipment.Conveyance of air and discharge through ducting and outlet diffusers will produce some noise. This should not be distracting and must be maintained at an unobtrusive level. As the extent of occupancy activity and/or machinery and equipment noise increases, so may the ducted air velocity, as background noise will render sound from air movement unnoticeable. For design purposes, the greater the ducted air velocity, the smaller the duct size and the less space consuming the ducting. However, some regard must be made for acceptable ducted air noise levels and the following table provides some guidance:

Ventilation Design – Duct Sizing Chart

Duct size

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Estimation of duct size and fan rating can be achieved by simple calculations and application to design charts. The example below is a graphical representation of the quantity of air (m 3 /s), friction or pressure reduction (N/m 2 per m) or (Pa per m) and air velocity (m/s) in circular ductwork.

For mechanical supply and extract systems, the air volume flow rate or quantity of air can be calculated from the following formula:

Air velocity

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The ducted extract air system shown is a simple straight run, with duct A effectively 8 m long and duct B effectively 16 m long. Where additional bends, tees, offsets and other resistances to air flow occur, a nominal percentage increase should be added to the actual duct length. Some design manuals include ` k ' factors for these deviations and an example is shown on pages 7 (see below).

Disposition of extract grilles and room function will determine the quantity of air removed through each grille and associated duct. In this example the grilles are taken to be equally disposed, therefore each extracts 1„5 m 3 /s. Duct A therefore must have capacity for 3 m 3 /s and duct B, 1„5 m 3 /s.

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There are several methods which may be used to establish ventilation duct sizes, each having its own priority. The following shows three of the more popular, as applied to the design chart on page 3.

1. Equal velocity method - applied mainly to simple systems where the same air velocity is used throughout.

For example, selected velocity is 7 m/s (see page 3), therefore the design chart indicates:

2. Velocity reduction method - air velocity is selected for the main section of ductwork and reduced for each branch.

For example, selected air velocities for ducts A and B are 8 m/s and 5 m/s respectively:

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3. Equal friction/constant pressure drop - air velocity is selected for the main section of ductwork.

From this, the friction is determined and the same figure applied to all other sections.

For example, selected air velocity through duct A is 7 m/s:

Using the example on page 2 with the equal velocity method of duct sizing, the fan will be required to extract 3 m 3 of air per second at a pressure of:

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Plotting these figures graphically against fan manufacturer data will provide an indication of the most suitable fan for the situation:

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Air changes per hour can be obtained from appropriate legislative standards for the situation or the guidance given below:

Measures for control:● Health and Safety at Work, etc. Act.● The Factories Act.● Offices, Shops and Railway Premises Act.● Building Regulations, Approved Document F † Ventilation.● BS 5925: Code of practice for ventilation principles and designing for natural ventilation.

The statutes provide the Health and Safety Executive with authority to ensure buildings have suitably controlled internal environments. The Building Regulations and the British Standard provide measures for application.Requirements for an acceptable amount of fresh air supply in buildings will vary depending on the nature of occupation and activity. As a guide, between 10 l/s of outdoor air supply per person can be applied between the extremes of a non-smoking environment, to an extract air rate of 36 l/s per person in a room dedicated specifically for smokers.

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Converting this to m3/h (divide by 1000, multiply by 3600), equates to 130 m3/h per person.

Air changes per hour or ventilation rate is the preferred criteria for system design. This is calculated by dividing the quantity of air by the room volume and multiplying by the occupancy.

E.g. 50 m3/h, 100 m 3 office for five persons: 50/100 X 5 X = 2.5 a/c per h. (a/c = air change)

Guide to Ventilation Rates

E.g.Conversion to equivalent size square or rectangular ductwork is shown below.

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Some ventilation design manuals limit data presentation to circular profile ductwork only. It is often more convenient for manufacturers and installers if square or rectangular ductwork can be used. This is particularly apparent where a high aspect ratio profile will allow ducting to be accommodated in depth restricted spaces such as suspended ceilings and raised floors.

The numerical relationship between dimension a to b. Square = 1:1.Conversion of circular ductwork to square or rectangular (or vice versa) using the equal velocity of flow formula:

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See next page for a simplified graphical conversion

Most ducting is sized using the same pressure drop or pressure loss per metre length. Larger ducting in a ventilation system will require a higher velocity to maintain a pressure drop equivalent to the smaller distribution ducting that it serves. The higher velocity will generate some increase in air movement noise, but this is not usually a problem as larger ducting is generally remote from occupied areas.

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