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RyanC83
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Join Date: Jul 2011
Posts: 43

Ventilation Flow Rate in Lieu of Motor Power Rating (W)

09/05/2011 2:08 AM

Dear All

I am relatively new in this forum and I would like to clear an issue that I had some time ago. I know that this forum will be fruitful and will gain interesting recommendations and advises.

The issue is as follows:

The company I am working for bought two ventilation fans (box-type) of flow-rate 5800m3/hr each, these fans have a two-speed configuration and thus there are two-installed motors in each unit. Upon delivery of these units I have noticed that the power rating of each units are not the same having the following specifications:

FAN 1: RPM-->1400/900 POWER: 2.2/0.75KW CAPACITY: 5800m3/hr

FAN 2: RPM-->1400/900 POWER: 1.5/0.5KW CAPACITY 5800m3/hr

I find it difficult to understand the above since my understanding from fan laws is that

W2=W1(N2/N1)^3 , Q2=Q1(N2/N1) and P2=P1(N2/N1)^2 and n=(PxQ)/W

where W=Power Q=flowrate and P=pressure

n=fan efficiency

From the above formulas I cannot understand how two fans quoted as having the same flowrate and pressure can have variances in power rating. The only plausable reason maybe efficiency but nothing else I can think off.

Can you please help me out on this.

Thanking you in advance,

Ryan
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Codemaster
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#1

Re: Ventilation flow rate in lieu of motor power rating (W)

09/05/2011 7:28 AM

Your formulas relating pressure rise, flow and power to speed apply to a particular system, and strictly speaking then only if the resistance is all dynamic (which it's likely to be for a ventilation fan).

If the system resistance at a given flow is lower for Fan 2, there's no reason both sets of data are not correct. For Fan 2, assuming efficiency 50%, back-calculated pressure rise comes to ~ 50 mmwg, reasonable for this type of fan.

Codey

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pritam
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Engineering Fields - Mechanical Engineering - New Member India - Member - New Member

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#2

Re: Ventilation Flow Rate in Lieu of Motor Power Rating (W)

09/05/2011 11:29 AM

You have to clearly see whether both the fans are identical, check impeller outer diameter, eye diameter, number of blades, blade configuration (backward curved or forward), blade angle, blade width etc. There are several possibilities for higher power for FAN 1:

1. Higher pressure rise at discharge.

2. Blade angle near radial or forward.

3. Higher impeller outer diameter.

I would have gone for single motor with VFD instead of two motors.
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Codemaster
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#3

Re: Ventilation Flow Rate in Lieu of Motor Power Rating (W)

09/05/2011 1:26 PM

What pressure rise was specified for each of the fans?

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BigPete
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#4

Re: Ventilation Flow Rate in Lieu of Motor Power Rating (W)

09/06/2011 3:58 AM

I cannot see where you have quoted any values for pressure. The staic or dynamic pressure characteristics of a fan have vast effect on performance. It is obvious to me that the lower powered fan has a lower dynamic operating pressure than the other fan. We use a lot of blowers for odour control and gas filtration and when we order them have to specify the static plus dynamic pressure the fans operate under to ensure we get fans that will work. To put it simply, fans that have higher power for the same output usually suck more or can inflate against a load better.

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RyanC83
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#5
In reply to #4

Re: Ventilation Flow Rate in Lieu of Motor Power Rating (W)

09/06/2011 7:08 AM

Dear Pete

I stand to agree with you. The situation is as follows. Fan supplier has provided to the owner specs for two ventilation fans which upon customer's request had to be identical i.e. in both flow rate and in pressure drop values. Upon arrival I have noted that the size of the box fans are different and the power rating is also different. Now my understanding is that for the same flower rate the only plausable higher power consuption could be due to different static and dynamic rates. Howerver these were specified the same and thus there is no reason for the two fans having two different power ratings. As for the types and fan blade types these are the same and from the same manufucturer. Is there another parameter which I am missing which could dictate the power rating of the fan?

Kind Regards

Ryan
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Anonymous Poster #1
#6
In reply to #5

Re: Ventilation Flow Rate in Lieu of Motor Power Rating (W)

09/07/2011 3:55 PM

I would have to venture to say the difference you see in the power rating is a production revision, Rev. 0 -original design, Rev. 1-more efficient motor used. Are they both the same part number, production batch, production date and the same revision number? If, any one is different, then....
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Codemaster
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#7
In reply to #6

Re: Ventilation Flow Rate in Lieu of Motor Power Rating (W)

09/08/2011 6:59 AM

No, higher efficiency motor makes no difference, it just means less electric power is drawn. The output (shaft) power still has to meet the fan's requirement.

A higher fan efficiency could mean a lower motor rated power.

Codey

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