5 Chillers and Pressure relief

[tbedford]

This is a new problem for me.

I am commissioning a hospital with 5 Trane series RTHD chillers. Unfortunately, the machine room is located in the basement and in the center of the building footprint.
The chiller pressure relief valves (3 per chiller) are piped and sized correctly into a common header.
The 12" black iron header runs horizontally for 20 meters then turns vertical for 4 stories.

My current copy of B52, the "go-to" book only provides pipe sizing to 6" It does not provide acceptable configurations for common headers.
Is there reference material for me to check out the header?

Thank you,

Tom

 

[LittleInch]

No idea what B52 is, but this sounds like a simple pipe sizing and pressure drop issue to me for which there are many on line calcualtors and charts.

The rise up 4 stories is not good for a relief system due to the static back pressure involved. It would be interesting to know if the relief valve design has taken this into account.

What are the pressures, flows, volumes we're talking about here?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[tbedford]

LittleInch,

B52 is the Canadian refrigeration Installation Code. I have also read ASHRAE 15 and 34 which cover mechanical room design.

Each chiller contains 525 pounds of R134a, class A1 refrigerant.

Evaporator has a 1" relief valve with a 200 psig rating at a flow of 48 lbs./min.
The condenser has a 1" relief valve with a 200 psig rating at flow of 48 lbs./min.
The compressor has a 1" relief valve with a 200 psig rating at a flow of 78.8 lbs./min.
Depending on the cycle, the refrigerant could be equally distributed throughout the piping.

I will look on-line for a sizing calculator.

Thank you

 

[LittleInch]

Interesting.

I didn't realize this was for the refridgerant in which case forget most of what I said as I assume the stuff will turn to gas as soon as it's vented. Gas flow is then what you need to calculate.
You'll need to figure out the density but also look at the temperature of this stuff. You might still have a significant back pressure depending on what actual volumetric rate that translates to. If it's coming out and vaporizing then you're down to minus 25C or more?? "Black Iron" doesn't sound like it's a low temperature material to me...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[tbedford]

Thank you LittleInch,

The refrigerant will evaporate immediately at approx. -14F(-26) and then it is all gas flow after that. It won't even accumulate for more than a couple of minutes.
It also begs the question should the pipe be insulated; although an un-insulated pipe with condensation in the corridor above my head may be a good sign of something amiss in the chiller room
A very odd installation to be sure.
I will make sure design assumptions and drawings are recorded.

Regards,

 

[LittleInch]

At that temp make sure your material is strong enough and that the contraction of the pipe won't break it.

I don't know what "black iron" is - it seems to cover a wide variety of iron materials so just check - you don't want this stuff pouring out of a broken pipe.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[tbedford]

thanks BI

 

[dbill74]

R-134a is heavier than air with a molecular weight of 102.03 g/mol. If I understand the construction, the relief valves would dump excess R-134a into a horizontal 12" dia. header. At one end of the header the pipe turns and goes up 40+ feet.
What is driving the R-134a up the vertical pipe?
Maybe when its venting pressure in the chillers, but what happens when the relief valve closes?
Any R-134a is going to settle in the horizontal header.

Relieve enough and you eventually get a 20m x 12" dia. cylinder of R-134a. I've not done the math, but seems like a lot of refrigerant to be just sitting there.

 

[tbedford]

Thanks dbill74,

The refrigerant sitting in the pipe after a release is what got me questioning the design and configuration.

 

[SAK9]

For the basement chiller plant rooms I have worked on, I have never installed a pipe work from the relief valve to the external. A supply & extract ventilation system is a must for basement plant rooms and the relief valve is usually piped into the extract duct. Gas alarms should be installed if you have several large chillers and can be linked to the extract system.