Roof Top Classified Air Conditioners 3

[timm333]

We have an explosive (class 1 Div 2) building that has to be air-conditioned. The air-conditioning units will be placed on the roof.

The problem is that the air conditioning unit will suck the explosive gas from inside the building, and so the roof top air-conditioning unit should also be classified.

I have been searching but am unable to find classified roof-top air conditioning unit. What should be done in this situation? Suggestions will be appreciated…

 

[timm333]

It is not pure ventilation; as a matter of fact only about 10% of air is ventilated in each cycle, and the remaining 90% of air "recirculates" and is thrown back into the hangar.

In other words, the roof top air-conditioner will suck the explosive gas from hangar, and will make it cool and then throw back into the hangar.

The explosive gas will be displaced in every cycle, and will keep moving throughout the hangar. So the whole hangar should be classified??

 

[davidbeach]

No, the potentially explosive mixture only exists below 18". That's because the potentially explosive stuff stagnates and might allow the bad stuff to concentrate. Mix it up with the whole volume of the hanger and you'll never get an explosive mixture. Ventilation only makes it better, not worse.

 

[PHovnanian]

Interesting anecdote relating to this subject:

Back in my days at Boeing, I was assigned to 'clean up' the design of a prototype automated tester for aircraft electrical power systems. Basically, it does a functional test of the electrical generating and distribution system on each airplane under construction. One component of this wheel mounted collection of equipment was an electrical load bank, consisting of 60 kW of resistive elements (exposed heating coils), contactors and a big fan. The people (not very bright) who built the prototype ATE mounted the load bank with its air intake at the bottom within 18" of the ground. And since there was the possibility that this unit could be used on the flight line, or in a hangar with fueled aircraft, this was a big problem.

My solution (after checking with our fire dept. and safety people) was to flip the load bank upside down. So it sucked in fresh air at the top (6' above ground) and blew it out at the bottom).

 

[timm333]

Does it mean that NEC 513.3 applies to both air-conditioned and non-air-conditioned hangars?

 

[rbalex]

Does it mean that NEC 513.3 applies to both air-conditioned and non-air-conditioned hangars?

Subsection 515.3(B) does. That's why I said to ignore the title. From a "grammar" standpoint, read the sentence without the parenthetic clause - and note the parenthetic clause would not change the hangar floor's status.

The entire area of the hangar...shall be classified as a Class I, Division 2 or Zone 2 location up to a level 450 mm (18 in.) above the floor.

Ordinarily, the area below 18" isn't volitile either - that's fundamentally what Division 2 means. See Section 500.5(B)(2); especially Subsections (1)&(2).

 

[timm333]

How about 513.3(C)(1).

Does this also apply to both air-conditioned and non-air-conditioned hangars?

 

[rbalex]

How about 513.3(C)(1).

Does this also apply to both air-conditioned and non-air-conditioned hangars?

Yes. Did you actually READ my 1 Sep 11 18:10 response? I told you ventilation CAN ONLY HELP. At worst, it makes no difference.

 

[timm333]

Yes but air-conditioning is "re-circulation" and not ventilation.

In the absence of air conditioning, the explosive gases will be present only up to 18" from floor level.

My whole point is that the air conditioning will "force" the explosive gases to move. And so the whole building, from floor to ceiling, will become explosive (instead of only up to 18")

 

[rbalex]

Under any condition from dead calm to wind tunnel, by definition, the Division 2 envelope in the hangar itself is 18” above the floor [513.3(B)] and 5’ from the aircraft power plants and fuel tanks [515.3(C)(1)]. If the air supply source is outside that envelope, you can ignore the effect on the HVAC system.

I don’t believe I’ll waste any more time on this. See Section NEC 90.3(C)

 

[rbalex]

Typo, that is 90.1(C)