FAN LAWS: 

These are laws governing the performance of a fan and are used to convert the performance of  a fan from one set of variables (e.g. size, speed and air density) to another. Thus, if the performance of a fan is known for a given size or rpm or air density, its performance for another size, rpm or air density can be computed.

	Air Volume	Q2  = Q1x (N2/N1)1x(D2/D1)3
	Pressure	P2  = P1x (N2/N1)2x (D2/D1)2 x (ρ 2/ ρ 1)1
	Absorbed Power	W2 =  W1x (N2/N1)3 x (D2/D1)5 x (ρ 2/ ρ 1)
	Sound Power	Lw2 = Lw1 + 50 log (N2/N1)+ 50 log (D2/D1)+22 log (ρ 2/ ρ 1)
	Where	N = RPM
		D = Fan Size ( Impeller Diameter)
		ρ =  Air Density

 For application of fan laws to size , the fans must be “geometrically similar”. Fans are geometrically similar if-

•   The no. of blades and vanes  are the same.
  
  
•   All angular dimensions are same.
  
  
•   All linear dimensions change in proportion to the wheel diameter.

The efficiency of a fan does not change with rpm or air density. However, there is a minor  increase in efficiency for larger sizes due to what is known as “size effect”.

GET MAXIMUM MILEAGE OUT OF YOUR FAN:

Fan Outlets

• To minimize shock losses, avoid sharp bends at fan outlets and sudden expansion or contraction.
  
  
• By putting a straight length of duct or diffuser at fan outlet, some dynamic pressure can be converted to static pressure, a process  known as static regain.
  
  
• Bends at centrifugal fan outlets should be in the direction of wheel rotation. Placing a straight duct between the bend and fan outlet is even better.
  
  
• Air at the fan outlet is turbulent. By keeping system elements such as coils, filters and dampers at some distance from fan outlet permits the air flow to approach laminar which results in less pressure drop and lower noise.
  
  
• Flexible ducts or connections at fan outlets must be smooth and taut.
  
  
• Use as large a duct as possible at fan outlets. The dynamic pressure required for a certain amount of flow in a 6” round duct is more than 3 times that in a 8” round duct.

Fan Inlets

• Do not obstruct fan inlets.
  
  
• If a fan is mounted inside a box, the inlet should have the same axis as that of fan inlet to avoid swirl or spinning of air. Also the clearance at fan inlet should be atleast 0.5 x wheel diameter.
  
  
• Air approaching fan inlets should have smooth entry. Use of properly designed bell mouth inlet cones reduce pressure losses and noise. Inlet cones should be properly aligned with impeller inlet axis to minimize turbulence and noise.
  
  
• Two or more fans operating in parallel must have a separation of atleast one impeller diameter at fan inlet to avoid erratic or uneven inflow.

Fan Drives

• Estimation of proper system pressure drop is crucial especially in case of forward curved fans. Because of overloading power characteristics of these fans, motors should be oversized not only to take care of transmission losses (belt driven fans) but also to allow for actual system conditions being different from estimated.