Calculating Fan Pressure Rise

If the fan pressure rise is not known it can be calculated approximately from Specific Fan Power (SFP) data using:

 

Delta P = 1000 * SFP * Fan total efficiency

 

Annex E of ISO 5801 shows that by rearranging the formula it can be derived that the SFP is a function of fan pressure divided by the efficiency of the fan system. Therefore the SFP will increase or decrease with a respective increase or decrease in the system pressure.

 

The SFP is a function of the volume flow of the fan and the electrical power input and is quoted for a particular flow rate;

 

SFP = Pe/ V

 

Where:

 

V is volume flow (l/s)

Peis electrical power input (W) to the fan system or complete air movement installation

 

[Reference FMA, UK, 2006]

 

Typical values for various system types are shown in the table below.

 

System Type

Specific Fan Power

(W/l-s)

Central mechanical ventilation including heating, cooling and heat recovery

2.5

Central mechanical ventilation including heating and cooling

2.0

All other systems

1.8

Local ventilation units within the local area, such as window/wall/roof units, serving one room/area

0.5

Local ventilation units remote from the local area, such as ceiling void or roof mounted units, serving one room/area

1.2

Fan coil units (rating weighted average)

0.8

 

Source ESTA.

 

Note: In reality the pressure rise across the fan has to meet the pressure drop across the index circuit of the actual system (i.e. the circuit with highest resistance to airflow). A more accurate estimate of fan pressure can be obtained by estimating the overall length of the ducting index circuit, assuming it’s sized on 1 Pa/m, adding approximately 20% for fittings and pressure drops for specific items such as diffusers, HEPA filters, etc.