System Working Pressure for Closed Loop Chilled Water System

[Salah Zantout]

Dear All,

This is my first post in this forum and after I have been following it for sometime now. I have really found it to be informative and helpful in so many ways so I genuinely thank each and everyone of you for your contribution.

I would kindly like to ask your help for the following:

I have a closed loop chilled water system. Machine Room including chiller package, circulating pumps and expansion tank will be all installed at the ground level. The highest level for the system is at 12meters height. Fluid here is PG 33% with supply/return temp -6degC/-2degC. Expansion tank with pressurization unit to be installed on pump suction line as close as possible to pump inlet to get all additive positive pressure from discharge to suction of pump. I have assumed 50kpa additive pressure over the highest point in the system for proper venting.

The lowest MAWP for the components at the floor level is the pump and PHE which are at 10bars (both are at floor level). Pump delivery is 41meters.

Now the thing is that I want to figure out:

1. What the Max System Working Pressure is to put it in expansion tank sizing formula. Cold fill pressure is static 12m + 0.5 bar for positive venting.

Acceptance factor= (max system working pressure absolute - cold fill pressure absolute)/ (max system work pressure absolute)

2. Safety relief valve setting for the system which I suppose needs to be at the tank location different
than the one installed for other for components.
3. How to choose the system working pressure.

Please let me know if the above is clear or requires further elaboration.

Thank you all in advance.

Ps. I have attached the engineering handbook extrol which I found really helpful.

 

[Salah Zantout]

FYI

 

[LittleInch]

Hello Salah,

First point is to be careful what you use. The excerpt you attached seems to be based on a hot water system where you start at a certain temperature 4C/ 40F and the water heats up and expands into the expansion chamber.

Your situation is the other way around, so your first fill / system off temperature represents the maximum expansion volume and then when the system starts up and cools down the expansion tank needs to maintain your min pressure at the inlet to the pump by flowing liquid into the system and therefore needs to start say half full and at a higher pressure.

So the static design pressure you need to work out is what that is at say 35C on a warm day when the system isn't running.

The system working pressure at the pump inlet would appear to be about 1.7bar. Discharge in running mode is then 5.8 bar.

So settle out would probably be about 3.5 bar? at -4C. This is one of the downsides of placing the pump etc at the lowest point.

Now what pressure this rises to when the loop rises in temperature during a shutdown is what you need to calculate. I would first work out what that volume is and then start thinking about your expansion tank size to cope with the different volumes and pressure and also how to fill it correctly.

Safety relief I would set at some suitable level to protect the lowest rated part of the system. This should be sufficiently separate from the normal operating pressure not to go off in operation.

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

 

[Salah Zantout]

Hi LittleInch

Thank you so much for the heads up.

To be precise, my cold fill conditions I have used are at -6C. When it comes to Max conditions I have used +35C. and that is to get the densities in Ashrae formula. There is as well another formula which you would find in the attached below from Grundfos, which is also mentioned in amtrol for chilled water systems.

So basically for Ashrae formula we will need the densities, while for amtrol or grundfos we will need the net expansion factor of glycol.

I am completely clueless about the system working pressure and how to decide it. Is it normally a value in-between suction and discharge pressures while the pump is running as you mentioned. I mean could it be anywhere between those 2 values? and what is a good trade off or a standard practice?

In regards total system volume, it is 33,000 liters

"Safety relief I would set at some suitable level to protect the lowest rated part of the system. This should be sufficiently separate from the normal operating pressure not to go off in operation."
So basically if the PHE MAWP is 10 bars, I would set the SRV for 10bar? and what is thes best location for it?

 

[LittleInch]

System pressures are dependant on where you take them.

Thinking about this a bit more I was not correct about the settle out pressure - that ignores the impact of the expansion tank which is your moveable and changeable element.

At no flow and at -6C, your pressure at the pump inlet will be >1.7 bar.

When you establish flow it will change but what to depends on how big your expansion tank is and what the flexibility of the rest of the system is as the pump starts up. 30 cubic metres and a 41 m differential head are quite decent sizes.

But the running case should result in your expansion being at about 10% full as the only pressure rise is upwards as the water heats up when you stop flowing.
How much you fill the tank up is up to you, but if it's no more than 50% then the pressure rise in the tank will be limited to maybe about 3 bar?

but in running mode your pump discharge will be abut 4 bar higher initially and then fall as the frictional losses mount up.

You locate the PRV where you want, but take account of the hydrostatic head between its position and the point of highest pressure

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

 

[HVAC-Novice]

Study this book

At 12m below the highest point the air pre-charge pressure should be 22.6 psi or 156 kPa. that assumes 5 psi pressure at the high point.
Your relief valve pressure at that pressure depends on the height difference to the expansion tank. But should be larger. No need to account for pump head IF the relief valve is close to the suction side of pump.

with a 50 psi relief valve, the tank should be 1479 liters. You also could go to Wessels and use their online calculator. My values are based on Siegenthaler and more conservative. Also based on 30% PG since the program I wrote only works in 10% increments up to 60%.

No guarantee, I'm not designing for you. Do you own math and review above resources.

 

[Salah Zantout]

Hello Gents,

Apologize for late reply as I had some issues. Thank you all for your valuable inputs.

I am not sure if I can connect all the info.

I would agree with @LittleInch in regards to the cold fill pressure which is 1.7 bar in addition to the 5.8 bar at the discharge when the pump is running.

and thanks to HVAC-Novice.. but what height difference if you can be accurate please? Is it height difference between PRV and pressure tank? Assuming it is next to the pressure tank (installed on the same suction line connecting to the pump) How much larger should it be.

Taking into consideration now that the tank is to be installed at the suction side of the pump... I would like to know:

1. how to select or decide the normal working pressure of the system (as a complete. What decides it?

2. What PRV setting I must request for the pressure tank

Really appreciated for your feedback gents.

 

[HVAC-Novice]

Properly installed, it is all on the suction side. So the operating pressure of the pump doesn't factor in. All pressures are static pressures and only depend on density of the fluid and vertical height.

Height differences depend on your physical layout. Most mechanical rooms have that all on relatively same height. but it can be different.

 

[LittleInch]

The question is not the right one you need to ask....

"Normal" working pressure will vary around the system and depend on all sorts of things.

The worst case / design pressure is if the return line is turned of for blocked just before the connection tot he expansion tank and the pump is running.

Then you have your 1.7 bar plus the dead head pressure of the pump which will be higher than your 41m, so say maybe 50m. So the pressure at that point is about 7 bar. Hence why I think they used 10 bar for the pump pressure.

PRV for the pressure tank depends on the max working pressure of the expansion tank. It should be set to that pressure.

Your tank will find it difficult to ever see 10 bar though. So long as you expansion tank is big enough to cater for the expansion of the liquid contents and still be < 50-60% full then you're probably looking at MAWP of the expansion tank of maybe 5 bar? If there's no difference in cost then get a 10 bar unit to match the pump etc.

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