Sizing a fan for a system

Determine required flow rate:

Flow = Velocity (m/sec) X Area (m²) = m³/sec

Flow = 0.4 X (0.3X0.5) = 0.06 m³/sec

For m³/hr x3600 = 216 m³/hr

Determine pressure

In this simple system there will be a filter which gives rise to two areas of restriction.

• Pressure required to pass the flow through the aperture
• Pressure required to overcome the filter resistance

With a velocity of 0.4m/sec across such a large area there should be very little pressure required to produce this flow. However for future reference the following formula can be used.

Pv = [ Flow/(Area X 47,496)]² Where Flow=m³/hr Area= m²

Pv = [ 216/(0.15 X 47,496)]² = 0.001mBar

The filter will contribute the largest restriction to flow and this information is normally provided by the filter manufacturer when given a desired flow rate.

Ducting losses

All ducting, bends louvres etc. will have an effect on the air passing through that system. This will nearly always take the form of a restriction to the flow and consequent pressure requirement.
At the earliest stage in the design process the ducting should be carefully evaluated and made as efficient as possible. This will reflect beneficially on the final design in the overall cost, the compactness of the fan and the running costs.

Points to consider:

• The ducting should be as large a cross section as practicable
• The total length should be kept as short as possible
• The number of bends kept to a minimum and should have as large a bend radius as possible
• Sudden expansion or contractions should be avoided. Smooth blended taper pieces should be used to make transitional sections.

If air is ducted outside in a total loss system consideration should be given as to how the air will be replenished.

Ducting manufacturers should be able to provide standard information as to the pressure losses in both straight sections and bends. At the back of the notes there is an old imperial table showing losses in various diameters of ducting per 100Ft run in InWg. The loss is proportional to the length. Hence if you were trying to pass the 216 m³/hr (130 cfm) down a 200 Ft run of Ø4” duct a pressure drop of around 2 InWg (5mBar) would occur.

Dwell time (Carbon filters)

For Carbon Filters to work effectively the air passing through the filter needs to be evenly distributed across the filter and be exposed to the carbon for a specified period of time. This is commonly known as the dwell time.

Parameters:

Filter Area (m²) = Flow X Dwell Time ÷ Filter Depth

Flow (m³/sec) = Filter Area X Filter Depth ÷ Time

Dwell Time (sec) = Filter Depth X Filter Area ÷ Flow

Filter Depth (m) = Flow X Time ÷ Filter Area

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