Equivalent area issue?

[golden eagle]

Hi ,

Question: can anyone know how to calculate the equivalent orifice area (%) of the top hang opening (as shown in the macroflow opening section)

I am trying to justify equivalent orifice area calculated by IES in macroflow opening section

the following is the calculation method for the macroflow opening (published by IES in the macroflow opening method document)

for all top hang openings

f=Cv*fa/100 , where fa is the opeanble area % and Cv is the opening co efficient , which is taken by the interpolated test resuls based on some window manufacture's performance figures , refer to macroflow calculation method

say L/H ratio between 0.5 <L/H<1.0 , from that table you get cv of 0.39 based on 20 degree opening and 90% opeanble area

so what I should be expecting to see in IES macroflow section for the top hand opening is

f=0.39x90/100=0.35, so 35% of gross

IES macroflow gives 56.61 if you put the same parameter ( 20 degree opening , 90% opeanble area , top hang)

As it can bee seen 2 figures are significantly different , IES calc does not seem to be matching this value

is that something wrong or missing in the formula above or does IES use another factor to arrive that value?

can please someone show me the calculation where they can get the same value,?

I have been questioned to find out equivalent area required for the windows , based on what is shown in IES, should I simply apply calculated value presented as an equivalent area % of the gross from the software , and then apply this % to my gross window opening ( structural ) , my concern is % of gross appears to me a bit of large value

[RossThompson87]

Hi,

This looks a bit complicated when written down.

Personally I don't use the MacroFlo tools for working out free areas of top hung windows.

To get the % opening area for IES I do:

(Window opening distance X window width) / (Window height x window width)

I hope that is relevant

Ross

[golden eagle]

Hi Ross ,

I understand the first part of equation where you get the projection area at horizontal level but why do you divide to window area, so say I have 2.1 x1.1window height and say window is allowed to open 100mm which is a typical restriction for safety, that gives you free area of 0.1*1.1/(2.1*1.1) =0.047 m2 , which is very small area
also does this include 2 triangle area created on both side of the window when open position?

[RossThompson87]

Hi

Sorry I have confused things slightly. Dividing by the window area will just give you the result in % terms (i.e. the fraction of the opening in your IES model that is free area).

For you example the free area in m2 would be:
0.1 x 1.1 = 0.11m2

But as a percentage of the whole 'IES opening':
(0.11 / (2.1*1.1)) * 100 = 4.76%

I have really struggled to find any official guidance on weather this is correct. CIBSE AM10 covers it and seems to suggest you can take into take into account the triangles on page 22:

With top hung vents, for example, the triangular opening each side of the open window is significant

But then on page 56 seems to suggest that is the area perpendicular to the air flow.

I tend to ignore the triangles to be on the safe side. Id be interested to see how other people approach it and if there is a guide I don't know about.

Ross

[PCully]

HI,

this is a fairly common error and is why IES researched and developed an opening types interface. The interface used to simply allow you to enter the equivalent orifice area but too many users were directly entering openable area or free area, neither of which is correct unfortunately and often results in nat vent being overestimated.

What we are providing an interface to calculate is equivalent orifice area and this is not same as openable or geometric free area.

The equivalent orifice area is the area of an ideal sharp edge orifice which would perform the same as the actual opening you are modelling. Click through help >> User Guides >> Thermal Reference category for the MacroFlo Calculation Methods guide and you will see that equations are provided to show Apache simulates all openings as sharp edge orifices so when we are defining windows, louvres etc we have to translate their performance characteristics into an equivalent sharp edge orifice. Don't take the lazy Google option to get this guide as you might have cached or find an old out of date version, go through the official http://www.iesve.com resource.

It takes into account the dimensions, the characteristics, typical pressure differences, if it is top hung, if there is a grille, if you have ducting etc which will all impact how it performs in reality compared to this ideal orifice.
To help with this we took listed manufacturer data and some empirical data from wind tunnel tests for typical types of openings and developed these into an interface for common types. This data is all published in the calculation methods guide as well.

should there be an option you don't like or if your type is not listed then you can calculate your own equivalent area (or manufacturer may have specifications on this) and you can choose the sharp edge orifice opening type and enter this value directly.

Sorry if this has turned into a bit of a lecture but it's been done wrong a lot in the past and we have tried to guide people on it but through misunderstanding why we're doing this people then question the interface provided. It's worth spending the time to get this right, compare your free area to the equivalent area for your top hung window and then consider this impact on your cooling loads as a %.

Phil

[golden eagle]

Phil


Thank you for your reply ,

I guess all user here understand equivalent orifice area is not the same as openable area and I assume we all configure each type opening as instructed in the IES Macroflow user guide. I suspect my original question is not well understood ?

I was trying to compare equivalent orifice area calculated by the software against the manual hand calculation,( which is based on the formula that IES has stated in the macroflow calculation method , this is also based on test data collected from the various manufacturer ) , I attempted to check this but I am not achieving the same value( as % of gross opening ) can you justify this by calculation ?, how the software ends up the equivalent free area based on the user macfroflow setting ? I am suspecting either the formula is wrong or the factors (cd) you declared for different type of openings in the guide are outdated or ies macroflow tool is not doing correctly or I am not doing correctly

I understand that the equivalent free area covers pressure differential and the characteristic of that opening , the software automatically calculates equivalent sharp edge orifice area (or equivalent free area as shown in the macroflow as % of the gross opening once the user has entered parameters ) , if the the equivalent free area information is not provided by the manufacturer, you can then configure this in the macroflow in the way as proposed by the architect and enter openable area( this is the % of gross after the window frame,) max angle of opening and L/H ratio of that opening , the software then calculate equivalent free area for you ,

I agree you do not enter free area information ( either provided by the manufacturer or you calculated by yourself) into the openable area % section , otherwise you undersized the opening

the free area information is not being used in IES at all, it is only the terminology used by the architect and others , just to present what % of the window area available as free area , however this is on the other hand triggers another question that me and Ross was discussing, should we need to include 2 triangle openings created on both side of that opening , I think we should not include it ?

another relevant point to raise is that I assume effective free area and the equivalent free are both the same thing ? some guideline requires this as effective free area ,which is the free area multiplied by the coefficient of discharge of that opening , so it appears to me giving the similar result
Regards

[RossThompson87]

Hi Phil,

I'm happy to keep rolling on with this discussion, but is there any chance of a webinar on this one?

Even just a couple of slides could do the trick. I expect 90% of the time people are just trying to model simple louvers, side hung, top hung windows etc. Probably mostly from opening distances as manufacturer's wont be involved until later.

Ross

[waleeed12]

I have been questioned to find out equivalent area required for the windows , based on what is shown in IES, should I simply apply calculated value presented as an equivalent area % of the gross from the software , and then apply this % to my gross window opening ( structural ) , my concern is % of gross appears to me a bit of large value

[maiquesk]

Hi golden eagle,

I am not english and may be I don't understand everything, but to me the problem come to your formula "f=Cv*fa/100". The Macroflow formula is "f=Cv/Cd*fa/100" where Cd is the discharge coefficient for a sharp edge orifice (0.62). When you take your result (35%) and you divide by 0.62, you have 56% (the result of Macroflow equivalent orifice area).

The equivalent orifice area give the free opening sharp edge orifice equivalent at your opening.

Regards

Kevin

[khimnavany]

Hi Kevin,

I'm using VE 2016, maybe this has been corrected in the latest version of IES but you are absolutely right on the coefficient:
I've set an opening as follows:
- Category: Top Hung
- Max angle open: 10degrees
- Proportions: L/H>2

According to the helpguide (http://www.iesve.com/downloads/help/The ... ethods.pdf) the coefficient should be
Cv = 0.1
When I set the openable area to 62%, I get the equivalent orifice area f = Cv x 100. And that is because of the discharge coefficient of 0.62.

So the formula really is %f = Cv/0.62 x (%fa)

I WISH IES support team could update the calculation sheet for macroflow!