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fan speed vs flow - indoor VRF units

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BIPVguy

Mechanical
Jul 23, 2007
37
Dear all,

I would kindly ask for an information please.
While reading indoor VRF cutsheet e.g. slim duct unit , I have noticed that external static pressure rises while air flow drops?! I was expecting to see increase in flow when fan speed is increased (and thus, static pressure). E.g. 46Pa corresponds to 450m3/h while 16Pa corresponds to 600m3/h. Does not make sense to me.
Please see the link below...

What seems to be the problem?
Cheers
 
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I take that you are looking at the 3.6 KW unit. This is simply saying that if you have a higher external pressure, you are going to get less air flow. Which makes sense.
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The fan capabilities are fixed, but the duct size and outside pressures are not. These outside influences are going to dictate how much air the fan can push through. For example, the smaller the duct size or the larger the outside air pressure, the less air flow you will get with your fan. This is the criteria they are stating here. So, for the same unit, you will get 450 m3/h @ 46 Pa and 600 m3/h @ 16 Pa. You are getting less flow at 46 Pa than 16 Pa external pressure is what it is saying. Hopefully this makes sense. This cut sheet is strictly talking about what the fan 'sees' as far as static pressure, which is dependent on velocity, duct size, and external pressure.

I hope this makes sense.
 
It's not that difficult to understand that flow decreases as pressure drop increases, is it?
 
why would you expect that?

learn the basics about fans, different types of fans have different system curves.
 
I don't mean any disrespect to the ones that have responded to this thread, but I'm really bothered by the responses. I thought the whole point of this was to impart knowledge, not to shun those that lack it. The two answers that I see provide no kind of insight whatsoever, only an offer of insult to the original poster. The OP comes with a question -- which, to the novice engineer could easily be misconstrued in the way that he has -- and the responses echo, essentially, to 'get a clue.' To me, this isn't the essence of what a site like this should be about. So what if what the OP doesn't understand something that makes perfect sense to you? Either try to answer his question in a helpful way, or don't say anything at all.
 
It would help if both you and the OP were to read forum Policies which begin as follows:


>>>>>These fora should not be used to bypass your own in-depth research on the issues that affect you, nor is it intended to be a substitute for appropriate professional assistance within your field or geographical region.
 
you can feel as an insult, or as an attempt to initiate your interest to grasp basic knowledge.

your question simply reflects lack of very basic knowledge, and the best you can do is to attain it before attempting anything else in hvac field. there is no shortcut for such things!

this is what i got when googling for "basic fan curves"


i think that i can reasonably assume that you are at least as good as myself in using google.

if you feel somewhat pushed, that's with best intetions...
 
tough crowd today....

BIPVguy: I think you mixed up ESP and the pressure drop of the unit.
the data of the VRF mso tlikley show the pressure of the (external) ductwork, which you need to design and know the pressure drop for that the design flowrate. So the more dP, the less flow.

I think what you were thinking you read is that is what flow you get out of the units with how much pressure,, like with an external fan. Then it would be the pressure drop of the unit, which indeed should increase with flow. Those units have an internal fan (like an AHU), so the data you see assumes what I described all above and the more pressure drop your ductwork has, the less flow.

For devices with their own fan, we typically don't see nor care about their itnernal pressuredrop (coil etc.). We only care about the external data... that specific unit manufacturer already takes into account his fan , and all the itnernal parts.
 
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