API RP 500 Naturally Ventilated Enclosed Areas

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Section 6.3.2.4.5 of API RP 500 (Nov. 1997, Reaffirmed Nov. 2002) provides equations to calculate 'minimum area for inlet and outlet openings in buildings to obtain a complete change of air each five minutes.' However, it recommends using these equations only for 'enclosed areas...of approximately 30 cubic meters (1,000 cubic feet) or less.'

Where can I find equations to calculate naturally ventilated inlet/outlet opening sizes for buildings larger than 1,000 cubic feet? Ultimately, the objective is to use Class I, Div II instrumentation and equipment inside the building.

Thanks for your advice ~ I appreciate it.

C
 

dicklaxt

Senior Member
I'd think an assessment must be made of all the parameters of the process at hand,the operating pressures of fluids & gases etc,,,,,,,,,then a look see at natural ventilation,historical data of prevaling winds directions and strengths etc .A qualified individual would have to make a call based on the above and establish a hazard radius and eventually an Area Classification document would be prepared applying limitations API RP 500.

I know this doesn't help a lot but it is quite involved sometimes with the presence of different media.

dick
 

rbalex

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Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
Theoretically, the equations work for larger enclosures; however for larger volumes there are two practical problems. One, as the enclosure gets larger the area (A) of the inlets/outlets approaches that of a stucture with only a canopy; two, the interior becomes more likely to develop "dead spaces" which may eventually become Division 1.

The Section actually points you to where you can find the equations for greater volumes: Chapter 22 of ASHRAE Handbook of Fundamentals.

Note: Even forced ventilation can develop "dead spaces". I recommend having someone with HVAC expertise with at least an understanding of hazardous locations to help evaluate.
 
ASHRAE

ASHRAE

rbalex,

I've got a copy of ASHRAE (1989 Fundamentals Handbook) and found the relevant equations on page 23.8 (equation 19, Flow Caused by Thermal Forces). I don't see any mention of the 1,000 cubic feet limitation, however, and I don't see separate equations for smaller and larger (volume) buildings.
 

rbalex

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Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
rbalex,

I've got a copy of ASHRAE (1989 Fundamentals Handbook) and found the relevant equations on page 23.8 (equation 19, Flow Caused by Thermal Forces). I don't see any mention of the 1,000 cubic feet limitation, however, and I don't see separate equations for smaller and larger (volume) buildings.
As I mentioned earlier, the formulas are general and theoretically apply to any volume; but classified locations are "special occupancies" and ASHRAE Fundamentals Handbook is not generally intended for classified locations.

I also mentioned the typical issues that arise as the enclosed space gets larger. Once you start attempting to use ventilation (natural or forced) outside the 1000 cubic foot limit, you need to know what you are doing and you won't get that from a text book. See Section 90.1 (C); it applies to Handbooks too. I routinely get my HVAC colleagues involved to identify dead spaces and assure proper airflow in complex enclosures and rooms. I've been doing it a long time and probably could identify most cases from experience, but I still get ventiation experts involved most of the time.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
A smoke test will enlighten you a lot about ventilation, especially inside an enclosed area of any size.

I did one once on a shooting range of a club I belong to. There were places where the smoke actually collected and got denser over time.
 
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