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# Total pressure2

## Total pressure

(OP)
Good day,
Can anybody explain me the relationship between static pressure, dynamic pressure and total pressure when selecting a fan for a ventilation system. I normally look at only the total static pressure on the duct and the other components such as filters etc, and select the fan can handle the static pressure. Is that correct?

### RE: Total pressure

What you say is not correct(i.e total static pressure), technically. You will calculate system resistance (for a fixed duct size and for a fixed flow) and this is the dynamic pressure. Then you add pressure drop across filters, cooling coil and other accessories. Pressure drop across these accessories is also a function of flowrate.

Now you select a fan that can develop static pressure which is equal to the dynamic pressure of your duct system.

This is a simple explanation. However, there may be some accessories which may offer resistance to the flow at a constant value and not proportional to the flowrate (for ex. HEPA Filters during their use).

(OP)

### RE: Total pressure

The fan will impart energy in two ways to the system, Kinetic and Potential.  These are proportional to the Velocity Pressure (VP) created by the momentum of the airflow and the Static Pressure (SP), which is consumed by eddy currents and turbulent flow through the system.

It's the operating characteristics or impedance that will determine the combination of flow and pressure that each fan will generate.  In order to obtain a certain flow (Q), the fan must impart enough energy to the air to account for the losses in SP and the VP throughout the connected ductwork, filtering and treatment accessories on both the fans inlet and outlet sides.

Since TP = SP + VP, the Fan Total Pressure (TPfan) is the differential pressure supplied by the fan where:

TPfan =     TPoutlet       -     TPinlet

or

TPfan = [SPfan + VPfan]out - [SPfan + VPfan]in

You'll find that most vendors rate their fans by the Fan Static Pressure (SPfan), which is an artifact of fan testing standards.  (arbitrarily defined as: SPfan = TPfan - VPatfanoutlet)

Vendors will offer Fan Tables where Fan Brake Power (Pbrake) and Rotation Rate (w) are found in the table by intersecting the desired Flow Rate (Q) and the SPfan.

Once Pbrake is known, use the Efficiency Rating of the fan (supplied by vendor) to calculate the Fan Air Power (Pair), Motor Power (Pmotor), and Consumed Power (Pconsumed):

Pbrake = Pair / [Fan Efficiency]

Pmotor = Pbrake / [Drive Efficiency]

Pconsumed = Pmotor / [Motor Efficiency]

These values will help to determine overall operation costs of the fan you choose.  I hope all this helps.  Good luck.

Braxton Lewis

Braxton V. Lewis
The Vaughan Associates
Engr. & Tech. Development
136 Eastgate Drive
Morgantown, WV  26508

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