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Conversion of Air Volume, Air Pressure, And Power for Fan Coil Units

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Fan coil unit is one of the core components in central air conditioning systems, mainly responsible for regulating indoor temperature and humidity to provide a comfortable indoor environment. Understanding the relationship between air volume, static pressure, power, and motor speed is crucial for the correct selection and installation of fan coil units.


Air volume refers to the total volume of air sent out or sucked in by a fan coil fan per minute. If calculated in cubic feet, the unit is CFM; If calculated in cubic meters, it is CMM, and the air volume unit commonly used in fan coil units is CFM. The air volume of the fan coil unit is related to factors such as the diameter of the fan impeller, the number of blades, and the inlet diameter.

Calculation formula: Q=π D ²/ 4 x v x 3600

Among them, Q is the fan air volume, unit: m ³/ H; D is the diameter of the impeller, in meters; V is the inlet wind speed, in m/s.


The static pressure value refers to the pressure value measured by the air pressure in the absence of airflow movement. It refers to the total pressure at the fan outlet minus the static pressure at the fan inlet, which is the ability of the airflow generated by the fan to drive the movement of objects. It directly affects the air flow rate and the wind energy efficiency of the fan coil unit.

The static pressure of a fan coil unit is related to factors such as fan speed, impeller diameter, number of blades, and inlet diameter

Calculation formula: Δ  P=K* ρ*  N ²  D ²/  10 ⁶

Among them, Δ  P is the wind pressure of the fan, in Pa; K is the fan coefficient, approximately between 0.008 and 0.065; ρ  Is the air density, in kg/m ³;  N is the fan speed, in RPM; D is the diameter of the importer, in meters.


Fan power refers to the work done by a fan per unit of time, usually expressed in kilowatts (kW). The power of the fan is related to the power required by the fan, so it is necessary to calculate the power of the fan in order to determine the required motor power.

Calculation formula: P=Q* ρ*Δ P/3600* η

Among them, P is the fan coil unit power, in kW; Q is the air volume, in meters ³/ H; ρ Is the air density, in kg/m ³;Δ P is the pressure difference, in Pa; η the total efficiency of the fan is generally between 0.75 and 0.85.


Fan speed refers to the speed at which the fan impeller rotates, usually expressed in units of revolutions per minute (RPM). The fan speed is related to factors such as impeller diameter, number of blades, and inlet diameter.

Calculation formula: n=60 × v/π D

Among them, n is the fan speed, in RPM; V is the inlet wind speed, in m/s; D is the diameter of the impeller, in meters.

When facing the following two scenarios, we need to apply the conversion of air volume, static pressure, and fan power.

1 When selecting fan coil units, given the air volume and pressure, it is necessary to estimate the installed power of the fan.

2 After the fan is running, estimate the operating air volume based on the known static pressure and fan power.

The relationship between the three elements:

N1=Q * P/ (3600 η 1* η 2 * 1000)

N=KN1;

N1- shaft power (kW);

N-actual power (kW);

Q - Air volume (m3/h);

P-static pressure (Pa);

η The efficiency of the fan can be taken as 0.719 to 0.8

η 2- Mechanical transmission efficiency. The mechanical transmission efficiency can be selected as following situations:

When motor direct drive the η 2 is 1 ; When coupling direct connection the η 2 is 0.98 ; When V-belt drive the η 2 is 0.95

K is the motor capacity reserve coefficient, selected according to the following table:

Axis power (kW)

Motor capacity reserve coefficient: K

<0.5

1.5

>0.5~1

1.4

>1~2

1.3

>2~5

1.2

>5

1.15

The performance parameters of fan coil units are not fixed and unchanging, and there is a certain inherent connection between them. When the fan coil unit is working in a central air conditioning system, these parameters are also affected by the network characteristics. Therefore, it is important to choose and use a fan coil unit well. Not only should one be familiar with the performance of the fan coil unit, but also understand the network characteristics and their relationships.


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