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RE: [EnergyPlus_Support] Fan Pressure Rise for a PTAC

Pressure rise is the force needed to move the air mass for a certain distance. If you have dust on the filter, the pressure will rise and the flow rate may not reach the design maximum.

There are two fans in a air-conditioning unit for cooling, one for the evaporator, and for the condenser coil.  With a fan coil unit, the air speed and the flow volume are usually quoted by the manufacturer.
At the rated capacity, a COP (coefficient of Performance) is used to convert the cooling demand to the electrical supply.  The adjustment of capacity is is done by turning the compressor on and off.  That involves another motor.
The fans are not usually variable speed ones.  The fan coil unit may have three speed, and it is done by motor tap connection.
The manufacture provides the efficiency curves for COP, air speed, PLR of compressor, etc. 
Knowing the cooling demand, and assume a COP value, the electrical power requirement can be calculated.  You can calculated the fan power in detail, but is not usually done.  The efficiency of the fan is included in the COP.

Electrical supply power for the fan = the mechanical power to move the air mass + Electrical I2R loss + friction loss. 
Pressure rise is in the mechanical power section only, for calculation.
Hope this help. 
 Dr. Li  


To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: felipe@xxxxxxxxxx
Date: Fri, 24 Aug 2012 13:10:51 -0400
Subject: [EnergyPlus_Support] Fan Pressure Rise for a PTAC

Dear All,

I am wondering how to calculate the fan pressure rise for a PTAC. The max. flow rate is 0,16 m3/s = 339 CFM x 0,3 = 101 W.

Since I ve got the fan power (101 W) and the max flow rate (0,16 m3/m) ¿Does anybody knows how to calculate the fan pressure rise?

Kind Regards,

Felipe Durán Palma
Arquitecto UC, MPhil. U. Cambridge, LEED AP BD+C
Gerente de Proyectos
B-green Chile S.A.


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