Fan installation mistakes
There are two possible mistakes when fan impellers are installed on site:
• Incorrect rotation—due to the motor wired for running in the wrong direction
Way round of a double inlet impeller or to a wrong handed impeller sent in error.
This is common particularly for fans with the impeller mounted directly on the motor shaft extension. In this arrangement, with ducts fitted on inlet and discharge of fan, it is not easy to see any rotating part. Observation has to be made on the shaft as seen down the gap between the motor and the fan. This mistake can arise when the erector leaves the job before it is wired. Many people think that if a fan runs in the wrong direction it will “blow from where it should suck”, which is of course not true.
It is important to note that in some installations the reduced flow due to incorrect rotation is not obvious to the customer. Hence if the job is wrong and not checked he may not complain but in time will be dissatisfied with the work. Examples from experience will illustrate this.
In a sawdust collecting plant a backplated paddle fan handled 1.65 m3/s with incorrect rotation and actually worked in a poor manner. When corrected the flowrate was 2.41 m3/s. Other sawdust collecting plants have given similar results. A paddle bladed centrifugal fan was installed for handling exhaust from paint spraying booths with a textile bag filter on the discharge. It was put into operation, with another similar plant, with incorrect rotation. They worked this way for some time until a visit was made and the fault noted. The volumetric flowrate was 2.029 m3/s as compared with 3.303 m3/s when corrected, see Figure 5.8.
The only means of checking by the customer was the feel of the air entering the booths. It was designed for a face velocity of
0. 825 m/s but in the wrong fan rotation was about 0.5 m/s. As
0. 5 m/s is common for cut-price work, it is easy to see that a customer might never complain, although not satisfied.
Narrow cast iron centrifugal fans are liable to this mistake. A 225 mm fan on a small job handled 0.035 to 0.038 m3/s in the wrong rotation and 0.069 m3/s when corrected.
A cast iron fan with forward curved bladed impeller handled 81 % of specified flow with power about the same either way
One case is known of a cast iron fan which had been running in the wrong direction for seven years before it was noticed!
On forward curved multivane fans the wrong rotation is obvious as the flow is so much reduced and cannot fail to be noticed. The same applies to wide backward bladed fans, (see Figure
5.9).
Very narrow backward inclined bladed fans installed for blowing might not be noticed. In Figure 5.10, a 760 mm diameter type 30/25 fan which was designed of duty on 0.66 m3/s (140 cfm) against 7.47 kPa (30 ins. swg) handled about 0.52 m3/s (1100 cfm) at virtually the same power consumption. This is based on the system resistance following a square law relationship p oc Q2. The customer is interested in the flowrate handled and not in the pressure set up, this flow being judged by very rough observation in many cases.
With wide backward bladed fans a wrong handed impeller, with rotation correct, cannot fail to be noticed owing to the effect on power. For example, a double inlet backward curved bladed fan had its impeller inserted the wrong way by the erector. When the customer started up after the erector had left, he reported 5 times the normal power with the starter impossible to keep in. It will be seen that the effect on flow of the wrong hand is very slight, but the power characteristic is altered completely, because it has become, in effect, a forward curved impeller. See Figure 5.11.
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Backward curved fans: DIDW
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A = Normal B = Wrong hand runner C = Normal: incorrect rotation |
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Forward curved impeller |
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To (/) <D |
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> J5 A> Tr |
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1 Cl A> > S A> CC |
Multivane fans
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A = Normal B — Wrong hand runner C = Normal: incorrect rotation |
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Backward curved impeller |
Relative flowrate Figure 5.9 Forward curved multivane fans
Relative flowrate Figure 5.11 DIDW backward curved fans with installation errors
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I) Uniform Flow into fan on a duct system |
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Iii) Vena contracta at duct inlet reduces performance |
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I N»’ V)ldeal smooth entry to duct |
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N Vi) Bell mouth inlet produces full flow into fan |
Paddle bladed fans can normally be left out of this consideration as if put in the wrong way it means that the spider is in front of the blades instead of behind. This will reduce flow to some extent but not seriously.
With forward curved bladed fans a wrong handed impeller with the rotation correct should not fail to be noticed by its results. It might just pass, however, as flowrate in average cases could be down to around 63%, with less power absorbed.
Note: Fans of the backplated paddle type for wood refuse collection usually have greater clearance at the throat of the casing, and in the wrong rotation will handle relatively more air than normal paddle bladed fans. This is confirmed by experience.
Posted in Fans Ventilation A Practical Guide