Chiller plant in return air path

[msnyder95]

I have an existing building that has the chiller plant in the return air path. They are looking at replacing the chillers and I am concerned about whether this is allowed today.

Local code references ASHRAE 15 Mechanical Room Safety, but I am reading this two different ways.

Section 7.3.2 defines calculations for when refrigerant equipment is located in the air distribution system, but 8.11.7 says no machinery room shall have an open air path to the occupied space. (I am referencing ASHRAE 15 - 2007).

I think I understand the difference, but am looking for second opinions or anyone with experience regarding this issue.

 

[MechEngNCPE]

Whether it is specifically mentioned in the code or not - i'm going to say its a bad idea, and would avoid at all cost.

 

[DRWeig]

I can't cite chapter and verse from here in the Korean coffee shop, but I believe I've read a prohibition of that kind of return arrangement in the IMC (International Mechanical Code).

Wait for others to respond before taking my word -- or look in the IMC to see if I'm remembering right...

Good on ya,

Goober Dave

 

[msnyder95]

MechEngNCPE,

I agree, but this is an existing commercial office building with three 450 ton chillers up top. I don't have a lot of options for building a new mechanical room.

 

[MechEngNCPE]

Any way to duct it?

 

[msnyder95]

It's a built up system, so it would take some massive ductwork and I have limited space.

I can find a way to do it if we have to, but I need to justify to the customer that it is required. They aren't going to spend that kind of extra money when it's been operating this way for 30 years unless they have to.

 

[MechEngNCPE]

Sorry I don't know the code off the top of my head, but ASHRAE 15 does come to mind... maybe shutdown via leak detectors is a loophole, don't know. Someone on here will though.

 

[KiwiMace]

You are prohibited from ducting through a chilled plantroom under an IMC or fire code provision (sorry also out of office) It can be done with permission of your AHJ if you fire rate the duct, which is likely your best option.

 

[willard3]

It is prohibited both by ASHRAE and IMC to have refrigerant gas/machinery in occupied areas.

Given that refrigerants will suffocate/kill people, this is just good Engineering practice and never mind the codes.

Tell the Owner he's going to need a new mechanical room.

 

[MechEngNCPE]

Well after 30 years, I'd tell the owner the ahu's are at the end of their "anticipated life cycle anyway" and that might make him feel a little better about shelling out the money.

 

[ChasBean1]

Agree with all, it IS a bad idea in principle. However, the whole chiller refrigerant alarming and purge systems are built from past issues with confined spaces, such as sealed navy vessel auxiliary machinery rooms, and lives lost as a result. If the chiller room represents a relatively small portion of the AHU return, and is for a building and not an enclosed vessel such as a lower level ship engine room, a possible work-around may be an automated AHU shutdown (circuited in with the AHU smoke relays for example) for any refrigerant purge alarm/event.

 

[317069]

Sorry, but I couldn't understand this exactly
"I have an existing building that has the chiller plant in the return air path"

then you said "this is an existing commercial office building with three 450 ton chillers up top"

 

[msnyder95]

@317069
This is an existing building with built up AHU's in the penthouse. The chillers are also up there and the mechanical room was never isolated. The return air comes up a central chase. Outside air comes in through the exterior walls. They mix right in the penthouse where the chillers sit and then go through the coils and back to the fans.

 

[mauricestoker]

You might want to look at ASHRAE 62.1 as well.

 

[317069]

in this case I think you better consult the city first.
- does this job (chillers replacing) need a permit from the city or it could be considered as a maintenance job like when a contractor replace chiiler's drivers for example.
- even it is old design but still strange to use the penthouse as a mixing box.
- I guess the penthouse includs boilers too,
- I don't see a massive duct required in this case unless the owner doesn't want to pay like usual
- check with the city first to protect yourself and public, not the owner's pocket.

 

[cry22]

Some known facts:

Air distribution chapter in IMC, says that Rooms cannot be used as RA plenums.
Also RA plenum is prohibited where Gas-fired equipment are present such boiler rooms.
Gas valves are prohibited from being installed in RA plenums.

I'd say that refrigerant is just as equally dangerous, and should be avoided the same way as gas valves in RA paths.

We have seen several chiller plants sharing space with boilers, the trick around it was to shut down the main gas valve upon detection of refrigerant in the room.
One could implement similar preventive measures to shut down AHU upon sensing refrigerant as suggested above.

You have asked the question, and several engineers tend to agree on the answer - stay out of it if you can or implement safety measures and alarms.