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Roof Top Classified Air Conditioners
3

Roof Top Classified Air Conditioners

Roof Top Classified Air Conditioners

(OP)
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...

RE: Roof Top Classified Air Conditioners

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

RE: Roof Top Classified Air Conditioners

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

RE: Roof Top Classified Air Conditioners

(OP)
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.

RE: Roof Top Classified Air Conditioners

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

RE: Roof Top Classified Air Conditioners

(OP)
NEC 513

RE: Roof Top Classified Air Conditioners

(OP)
Actually NEC 513.3(C)

RE: Roof Top Classified Air Conditioners

So we're talking about an aircraft hangar?

RE: Roof Top Classified Air Conditioners

(OP)
Yes

RE: Roof Top Classified Air Conditioners

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

RE: Roof Top Classified Air Conditioners

(OP)
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?

RE: Roof Top Classified Air Conditioners

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

Quote:

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.

Quote:

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.   

RE: Roof Top Classified Air Conditioners

(OP)
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.

RE: Roof Top Classified Air Conditioners

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.
  

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

RE: Roof Top Classified Air Conditioners

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

RE: Roof Top Classified Air Conditioners

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.
  

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

RE: Roof Top Classified Air Conditioners

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.  

RE: Roof Top Classified Air Conditioners

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

RE: Roof Top Classified Air Conditioners

(OP)
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?

RE: Roof Top Classified Air Conditioners

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.)

RE: Roof Top Classified Air Conditioners

(OP)
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??

RE: Roof Top Classified Air Conditioners

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.  

RE: Roof Top Classified Air Conditioners

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).   

RE: Roof Top Classified Air Conditioners

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

RE: Roof Top Classified Air Conditioners

Quote:

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.

Quote:

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).
 

RE: Roof Top Classified Air Conditioners

(OP)
How about 513.3(C)(1).

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

RE: Roof Top Classified Air Conditioners

Quote:

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.

RE: Roof Top Classified Air Conditioners

(OP)
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")

RE: Roof Top Classified Air Conditioners

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)
 

RE: Roof Top Classified Air Conditioners

Typo, that is 90.1(C)

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