fan curves, volumetric air flow, temperature 1

[AO1958]

Hi there

hope somebody could help me in following topic.

Usually fan curves are provided @ 68F.

In this case, with the system pressure curve, I can get a certain volumetric air flow.

I am wondering if somebody could explain me what happens when ambient temperature changes.

In a first analysis, the fan curve should change.

question 1: how does this curve change ?

Question 2
I suppose that the fan is a volumetric equipment.
I would suppose that, in the same system, system volumetric air flow changes - since operating point changes - when ambient temperature changes because air density changes.
Nonetheless, I suppose that even mass flow changes.
My question is: what remains constant, then ?

A lot of confusion.

can anybody help me ?

Thanks in advance

 

[MTES93]

I will take a swing at this.

Fans are essentially constant volume devices. Mass flow DOES change. As air temperature increases, the density of the air decreases and vice-versa. So lets say a fan operating at 4,000 CFM at sea level and 68 degrees F will deliver 301 lbs/min of air. At 140 degrees F it will deliver 4,000 CFM and 265 lbs/min.

Question 2: Fan brake horse power decreases linearly w/ density changes as does the system losses (both in terms of duct loss and fitting losses (dynamic)).

Question 1: Yes, you are correct the fan curve will change. Primarily you need to speed the fan up to produce pressure X when the density is lower. In the old days if we needed 1.5 inches of pressure (actual) at 5,500 feet we would divide the 1.5 by the density ratio (site density/fan curve density or 0.075 lb/cf). The adjusted static pressure and our desired cfm would then be used to plot on the curve to determine RPM. We would then multiple the curve BHP by this same density ratio and determine actual BHP. Honestly w/ the advent of good computer selection programs I kind of forgot some of the old adjustment factors we used to do. Which ain't a good thing!

Try a good fan mfgrs program. A good one will adjust for density and altitude. Most are available free from local mfgr's representatives and most are available online.

Please see web link below..

http://www.greenheck.com/library/articles/12

 

[rmw]

A star for you 11241. Great explanation.

And, yes, isn't it terrible that computers and software have made us forget how to do it like we did back in the day with slide rules and pencil and paper.

rmw