Orifice and Regulator Openings

[Kingsley Hortin]

Hi Ventsim Team,

I have a couple of queries with respect to orifices/regulators, mainly just to make things clear in my mind. When I fix a flow through an airway requiring a restriction, the resultant returns an Equivalent Orifice area. For instance in the example below, to achieve 65 m³/s, an equivalent orifice of 3.6 m² is defined as being required.

Am I right in my understanding that this is area represents a round opening? If so, then the hydraulic diameter is 2.14 m. Yet, if I am using this to estimate the size of a rectangular opening (for example to represent a drop board regulator), then this area would seem to be underestimated. For a opening width of say, 3 metres, an area of 5.0 m² would be required to retain the 2.14 m hydraulic diameter. If the width of the opening is 4 metres, an area of 5.8 m² would be required to retain the 2.14 m hydraulic diameter. Am I looking at this the right way, or am I misunderstanding something?

Edited October 20, 2020 by Kingsley Hortin

[Martin Griffith]

Hi Kingsley, thanks for your question.

Firstly, the equation behind the orifice does not use an hydraulic diameter, it calculates the shock loss based only on the ratio between the orifice and and airway areas. There isn't any consideration of the shape or hydraulic diameter of the orifice. While the shape of the orifice will have an effect on the shock loss, we don't model it in Ventsim, since the area ratio will generally be the greater determining factor.

You've included a picture of the Regulator Resistance option; a note of caution about comparing the orifice and regulator resistance options. The orifice has its own equation which relies only on the orifice-airway area ratio.

We made the decision that Regulators only reference the Regulator area and not the airway area.

For example, if you want a fully-open regulator of 16 m2 area in a 5.0 x 5.0 m2 airway to give the same result as a 16m2 orifice, then you need to provide a Regulator curve that gives that resistance at 100% open.

This means that your curve for a 16 m2 area regulator is actually for a 16m2 area regulator in a 5.0 x 5.0m2 airway. The reasoning is that normally the shock or resistance curve of a regulator is not provided; usually it is obtained by the engineer taking measurements across the regulator at different openings and entering the curve in Ventsim themselves. If you do this then you can simply enter the total resistance measured, without having to work out yourself what proportion of the shock loss is due to the regulator-airway size ratio (which Ventsim would calculate) and what is due to the opening of the vents (what you would enter in the curve). This is different to the orifice option which is based only on the orifice-airway area ratio.

 I hope this helps. Let me know if you have more questions.

[Kingsley Hortin]

Hi Martin,

Thanks for your response regarding the calculation for orifice area. That's given me something to think about.

With regards to the picture of the Regulator Resistance, I accidently added that picture when I made the post. However, when I went back to edit my post, I found I couldn't delete the picture. With that said, thanks for the extra info regarding the Regulator Resistance option!