Closed loop chilled water system

[KZ001]

Hi,

Can someone help me with the following questions?

1.Is the minimum system pressure equal to the system static head (elevation)?
2.Does the system expansion tank precharge pressure have to equal the system minimum pressure?
3.Is it better and more conservative if the expansion tank precharge pressure is set higher than the system min. pressure and how much higher?
4.When calculating the total pressure losses for closed loop system do I need to add the system static head to the system TDH? Doesn’t the static head for a closed loop system cancel out?


Thank you!

 

[DRWeig]

Are you designing this system? How high is the top of the CHW piping? You need to learn to work these questions out.

1. Consider trying to fill the system with water. What pressure do you need to get the water to the top? What pressure do you need to insure that the automatic air relief valve at the top of the pipe actually lets air out? What pressure do you need to insure that a leak at the top of the piping won't suck in air?
2. Consider how much of the expansion tank you want to be air, and how much water, and what the system pressure is.
3. Same as number 2.
4. Consider the elevation of the water in the supply pipe and the elevation in the return pipe. Are they the same?

Best to you,

Goober Dave

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[Drazen]

1. no. minimum pressure equals expansion vessel presetting.
2. it's unavoidable, physically.
3. pre-charge pressure should generally be little higher than static head by margin which prevents air sucking in. how much? it depends on several factors, for low-temp system it's goes from 0 to 0,5 bar (different sources give different recommendations)
4. make considerations as per drweig's explanation and find solution yourself!

 

[KZ001]

DRWeig and Drazen,

Thank you for your help!

Drazen,

Regarding question 1 response, you said that the minimum pressure is not equal to the static head. How do you determine the system minimum pressure?
I agree with DRWeig answer that elevation and air relief valve minimum closing pressure should be considered to define the system minimum pressure.
Regarding question 4 response, I would agree with DRWeig response if we discuss an open system. I think the static head is equal to zero in a closed loop system completely filled with fluid. Please correct me if I am wrong.

Thank you!

 

[sprinkler1000]

Regarding your question 4, your system pressure is not zero if you have a vertical piping.

System pressure is pump discharge pressure plus piping height, at the bottom of the pipe. As the system is closed loop, you do not take into account the height of the pipe in the pressure required for the pump, as it cancels out (I mean you only have friction losses and pressure required in the elements, coils, etc.).

So, when the pump is not running, the static pressure at the bottom of pipe is the pipe height.

 

[Drazen]

you determine it by point 3, as explained. you have to decide on pressure margin over static head, apply it to expansion vessel pre-charge setting and that will make your minimal system pressure.

if you would be much more specific what are you trying to accomplish, some more tips could possible be available.

 

[317069]

ASHRAE has good articles about hydronic system design and fluid mechanic.
Do you want to select a pump or a pipe?

 

[DRWeig]

Click here: Expansion Tank Application Gude from VEMCO

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[MTES93]

What Drazen is referring to is what pressure do you want at the top of your system. You don't want it to be zero static (at highest element) if you charge pressure exactly equals highest element height. Hence you 'flood' or charge the fill pressure a bit. As he mentions it is dependent on system but 3 to 5 psi is typical. So if you have 60 feet from the X-tank location (connection point) to highest element a good fill pressure would be (assuming water is fluid) 60/2.31 + 5 = 30.9 psi or so... That is one possible solution. If this was my system I would look at a 30 psi fill... or locate the tank/pumps up top :~)

In this case, as Drazen mentions, the x-tank pressure is the minimum fill pressure and is slightly more than what you need to fully lift the water to the highest element. If you stuck a pressure gauge on the element it should read around 5.0 psid over atmospheric.

I believe the rest of the posts answered your other questions.

 

[ChasBean1]

1. At the highest point in the closed loop, you want to maintain some minimum pressure. This is typically about 15 pounds per square inch, gauge (psig). The minimum system pressure is therefore 15 psig.

2. The expansion tank pre-charge is almost always factory set at 12 psig. SO, what do you adjust it to? If it is located at the highest system point, no field adjustment needed. Keep the 12 psig factory value. If the expansion tank is in a basement of a 10 story building with 15’ slab-to-slab, the pre-charge would be:

12 psig (factory, delivered) + building elevation pressure

Building elevation pressure at 150’ is 65 psig. The conversion is 2.31’ per psig.

So if this expansion tank was installed in the basement, the pre-charge should be adjusted to 12 psig + 65 psig = 77 psig.

3. No.

4. No, do not add static head to TDH. Yes, it cancels out for the purpose for which you inquire. Do not try to figure restriction losses into this equation. Keep 15 psig at the top of the loop with all pumps off; the operating system will perform adequately.

 

[KZ001]

Everyone, thank you very much for your help!