Basic question - why are chilled water systems pressurised?

[MechEng92]

It's very basic and I expect to be shamed for asking, but I want to know simply why are chilled water systems pressurised to around 10-15bar? Is it to prevent vapour bubbles being released in certain areas of the system as pressure changes? Why choose 10-15bar?

Thanks

 

[LittleInch]

Depends on the chilled water system.

If the building is 80m high then at the base it will need to be about 10 bar.

No examples or details here and any general statement tends to be just what you've seen/ epxperienced.

It seems high, but without context it is not easy to say why.

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[MechEng92]

Thanks LittleInch, and is that pressure just whatever it happens to be based on the height difference between a chiller and cooling unit? It's not necessarily a target pressure?

 

[LittleInch]

There's more to it than just that, but that's a key consideration to avoid pressure< say 1 bar at the highest point of the system taking into account pressure losses through the AHU's or whatever, losses in the pipes, height difference, pressure rating of the chiller or AHUs.

In general you want to design to as lower as pressure as you can as this reduces cost of components and reduces leaks and drips from passing seals or valves.

You also tend to get jumps in pressure rating and hence cost at certain points - depends which design codes you're working to, but many units will be rated up to 10 bar or 16 bar so maybe people see that as a target and not a maximum pressure?

You've not given anyone much to go here so it's all vague and general.

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

 

[MechEng92]

Sorry it's not really based on a real life situation just more for my own benefit. Basic example I can think of is CRAH units in a data centre connected to a chiller, I know the system is pressurised but curious as to why and to what value for what reason.

 

[LittleInch]

Probably differential height is the key consideration.

You might find that the vendor just works on that sort of pressure so that they know the same design and components work in 95% of their locations without needing to change anything. Commonality / repeat design is a lot cheaper than re-designing every one slightly different.

So in many places you might only need 2 or 3 bar, but 10 works for everywhere just about so it's easier and cheaper to design it for 10.

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

 

[MechEng92]

Brilliant thank you

 

[HVAC-Novice]

Consider pressure at what elevation. Highest point (de-aerators) needs to be above atmospheric pressure or the de-aerators become aearators.

 

[IRstuff]

It seems to me that you're driving past the forest to see a tree, to paraphrase.

A "chilled water system" is intended to provide a means to remove heat from something. It must do so in the face of pressure head, fouling, etc.; there was a posting from someone concerning their glycol cooling system that finally got some maintenance, and the resulting conclusion was that there was gunky sludge being pumped around their cooling system. Likewise, if your chiller system doesn't get routine maintenance, it'll need excess capacity to overcome whatever indignations the cooling system has endured between maintenance cycles while not resulting in so much pressure that the pipes burst every time water hammer is induced from closing any valve, and flow rates that are so high that cavitation and pipe erosion occurs.

That's the ideal world, and in the real world, some design point works "well enough" and provides sufficient overcapacity to mitigate poor maintenance, etc., that customers will often do, or not do, while blaming the designer/installation for the results of poor maintenance.

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[MEP.Learner]

the pipe pressure that u see is result of pump head (depends on the pipe hydraulic loss u need to overccome) and + static head

With example above, 80m height building gives about 8 bar static pressure. + the operating pump pressure and some safety factor, u will get about about 10+ bar at the lowest point of the CHW pipework (assuming pump at the lowest point too).
Assuming if the question you are asking is about what's the pipe and fittings pressure rating to be allowed for.