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…

 

[davidbeach]

What should be done is a single pass system where you supply cold fresh air and vent the displaced hot air.

 

[waross]

Look at split systems. Possibly a combination of split for cooling and heating and the single pass system that David suggests for make-up air. In the split system only the evaporator is inside the explosive area. This has a fan, but finding or assembling a air handling unit with an explosion proof fractional motor for the fan may be easier than finding an entire A/C unit.
A chiller circulating cold water through the air handling units may work also.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[timm333]

These are good options but are costly. I think cheaper option is to build a custom explosion proof air conditioner.

My main question now is whether we really need explosion proof air conditioner or not. The code says that only 5 feet around the equipment should be classified. Do we really need the explosion proof air conditioning units (on the top of roof), or is it overly conservative? Please let me know comments.

 

[rbalex]

Just curious; which Code are you using to define the classified location envelope?

 

[timm333]

NEC 513

 

[timm333]

Actually NEC 513.3(C)

 

[rbalex]

So we're talking about an aircraft hangar?

 

[timm333]

Yes

 

[rbalex]

Your air supply will be within 18" of the floor?

 

[timm333]

If there is no air-conditioning, then the area shall be classified up to 18" from floor. But I think the air-conditioning shall force the air to circulate, and so the whole building shall be classified (not only 18").

Is it correct?

 

[rbalex]

Read Section 513.3(B) and ignore its title for the moment. It simply says:

The entire area of the hangar, including any adjacent and communicating areas not suitably cut off from 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.

Section 513.3(D)is somewhat complementary.

Adjacent areas in which flammable liquids or vapors are not likely to be released, such as stock rooms, electrical control rooms, and other similar locations, shall be unclassified where adequately ventilated and where effectively cut off from the hangar itself by walls or partitions.

From 513.3(B), the hangar area is always Division 2 up to 18" above the floor ventilated or not.

In any case, ventilation of the overall hangar can only help. The Division 2 envelope is only 18" above the floor and 5' from the aircraft power plants and fuel tanks [513.3(C)]. As long as the air supply source is outside the envelope ('you gotta draw the line somewhere') you can ignore the area classification for the HVAC.

 

[timm333]

Thanks rbalex, but confusion still prevails. This is true that the air supply source is outside the envelope, but it will re-circulate and suck the explosive air from within the envelope. In other words, air supply source (air conditioner) will contain explosive gases even though it is outside the envelope.

I am not saying that you are wrong; just I am looking for more clear explanation.

 

[ScottyUK]

You must have a signifcant dilution factor by the time the return air reaches the AHU. Anyway, fuel vapour is relatively dense so it will tend to hug the floor.


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If we learn from our mistakes I'm getting a great education!

 

[timm333]

Yes but how does it prove that the air conditiong unit does not need to be classified?

 

[ScottyUK]

Well, over here once the concentration is below the LEL then it doesn't require certified equipment. Part of the engineerijng process is to establish how far the hazardous zone extends before it ceases to be a hazard. This would look at the nature of the release point, the material released, the location and it's effect on concentrating the hazard, and so on. The hazard zone doesn't extend indefinitely, so an engineering decision has to be made to determine how far away is far enough based on your country's codes and regulations, and on standards like IEC 60079.


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If we learn from our mistakes I'm getting a great education!

 

[davidbeach]

If the NEC defines the Div 2 space as being up to 18" above the floor, you could draw in your return air from a louver with its lower lip at 18" above the floor with no need for a classification of the HVAC equipment. You don't have a Div 2 building, you have a building that has a Div 2 space. Stay out of the Div 2 space with unclassified equipment and you're good to go.

 

[rbalex]

I gave Scotty and Dave stars for their excellent interpretations of my 'you gotta draw the line somewhere' comment

 

[timm333]

I truly hate to ask same thing again and again but it looks that I have not made it clear what my concern is.

My concern is that the classification suggested in NEC is only for non-air-conditioned hangars. But in our case we have an air-conditioned hangar. So the entire hangar from floor to ceiling should be classified (instead of only 18" above the floor). Is it correct?

 

[davidbeach]

No. Under some conditions ventilation can be used to declassify a space. I'm not aware of any condition where ventilation increase the extent of classification. (But I've not worked with the NEC on a daily basis for nearly seven years.)