Fan Performance Curves

FAN PERFORMANCE CURVES

Fan performance curves give us a broad idea of how a fan behaves during its entire operation. By definition “ Fan Performance Curves provide a graphical relationship between the pressure produced by the fan ( total or static ) for different amount of air it delivers ”. These curves are of great aid when one is trying to select a fan to suit their purpose as they also display the power the fan takes up during its operation at different air flows.

NOTE: Each fan curve corresponds to a fan operating at a fixed speed and deals with air of particular density.

The behaviour of the curve can be easily understood by the following video…….

We often plot the fan performance curves by taking static pressure on the Y-axis instead of total pressure because static pressure can be viewed as the total resistance the fan has to overcome to get the desired air flow.

One can ask how do we know that a fan acts like this? Well, these curves are plotted using the results from various experiments by utilising a duct system that offers different pressure drops or resistance to the flow.

So now that we an idea about what a fan curve is and what all quantities it can help us figure out……let us find out how to select a fan so that it meets our requirements.

SYSTEM CHARACTERISTIC CURVE

One of the most common application of fan curve is to select the most appropriate fan that serves the purpose and also minimizes the energy consumption thereby avoiding any wastage in the process.

For instance, lets say we have to select a fan for a duct system shown below………..

The following fittings and accessories used:

TEE
REDUCER
BRANCH
ELBOW
AIR FILTER
MANUAL DAMPER

Before proceeding we have to find out how much resistance does our duct offer or how much pressure does our fan has to apply so that it can provide the required CFM of air.

All of these provide resistance which the fan has to overcome and the pressure drop (ΔP) of these fittings have their own method of calculations.

NOTE: It is very important to keep in mind the fact that for any given duct system , we will get the required flow only when a particular static pressure is maintained inside the duct. For example, lets us suppose we get 100 CFM of air at 50 Pa from an air grill. Then, if the static pressure of the system changes due to some reasons ( dirty filter, changing of dampers positions ,etc.) , one should not expect to receive 100 CFM of air since the static pressure ( or amount of push ) that the air is experiencing after the observed change is reduced.

Once we figure out the static pressure drop (ΔP) we can then use the law which states that the resistance provided by the system is proportional to the square of the airflow rate.

Here Q is the airflow rate required to meet the room design conditions which is obtained using the heat load calculations.

Once we have calculated the static pressure drop and the required airflow rate at POINT 1, we can use the above mentioned law to find the static pressure at POINT 2 and eventually plot the system characteristic curve.