Basic Fan ConceptsFan Laws, Fan Performance
The Fan Laws are basic tools in air handling. Three of the most common relationships are illustrated as follows.
Example: A fan handles 40,000 cfm at 2" static pressure. It runs at 760 rpm and draws 18 brake horsepower. The fans is speeded up to 800 rpm. What are the new cfm, sp and bhp?
1. Air flow rate varies directly with shaft speed.
cfm1 = rpm1 = 800 , cfm = 42,105 -------
cfm2-------
rpm2-----
7602. Pressure varies as the square of shaft speed.
P1 = rpm12 = 8002 , SP = 2.22" ----
P2---------
rpm2-------
7603. Horsepower varies as the cube of shaft speed.
bhp1 = rpm13 = 8003 , bhp = 21.0 -------
bhp2---------
rpm2-------
760
The relationships stated here apply when the air density remains constant and when there is no change in the fan or the system. The are based on Fan Laws 1a, 1b and 1c. For a complete presentation of the Fan Laws, see the ASHRAE Handbook, Equipment.
Each fan design has its characteristic set of performance curves. Those shown in the figure below are typical of a centrifugal fan with forward curved blades in the wheel, as commonly used in fan powered terminals. For a full discussion of the characteristic of the various types of fans, see the ASHRAE Handbook, Equipment.
The solid curves represent a fan running at constant speed, as it is throttled from free delivery to close-off.
The broken-line square curve represents the pressure drop though the complete air handling system in which the fan operates. Intersection (A) is the operating point of the fan.
The relationship between curves:
Fan Static Efficiency = cfm x PS ----------------
6356 x bhpSimilar curves for total pressure and total efficiency are also used, so that:
Fan Static Efficiency =
(Mechanical Eff.)cfm x PT ----------------
6356 x bhp
Full Load Currents: Three-Phase A-C Induction Type - Squirrel Cage and Wound Rotor Motors***
HP 200V 230V 460V 575V 1/2 2.3 2 1 .8 3/4 3.2 2.8 1.4 1.1 1 4.15 3.6 1.8 1.4 1 1/2 6 5.2 2.6 2.1 2 7.8 6.8 3.4 2.7 3 11 9.6 4.8 3.9 5 17.5 15.2 7.6 6.1 7 1/2 25 22 11 9 10 32 28 14 11 15 48 42 21 17 20 62 54 27 22 25 78 68 34 27 30 92 80 40 32 40 120 104 52 41 50 150 130 65 52 60 177 154 77 62 75 221 192 96 77 100 285 248 124 99 125 358 312 156 125 150 415 360 180 144 200 550 480 240 192 Over 200 HP
Approx. Amps/HP2.75 2.4 1.2 .96 Branch-circuit conductors supplying a single motor shall have an ampacity not lass than 125 percent of the motor full-load current rating.
Based on Table 430-150 of the National Electrical code, 1993.
***For conductor sizing only
Full Load Currents: Single-Phase Motors
HP 115V 200V 230V 1/6 4.4 2.5 2.2 1/4 5.8 3.3 2.9 1/3 7.2 4.1 3.6 1/2 9.8 5.6 4.9 3/4 13.8 7.9 6.9 1 16 9.2 8 1 1/2 20 11.5 10 2 24 13.8 12 3 34 19.6 17 5 56 32.2 28 7 1/2 80 46 40 10 100 57.5 50 Branch-circuit conductors supplying a single motor shall have an ampacity not lass than 125 percent of the motor full-load current rating.
Based on Table 430-150 of the National Electrical code, 1993.