working Pressure calculation? 1

[tcihvacff1]

Hi all! I am woking on a project where we are using 12 Nos of Water cooled chillers(each 415 Tr) for cooling 5 buildings(each G+5 Floors). And we have 12Nos of Secondary & Primary pumps each....
My question is that how to calculate the working pressure if Secondary pump has 18m head & Primary pump has 25m?
any kind of help is appriciated...

 

[Drazen]

you need to draw combined pump-system diagram, where pump curve is constructed by series connection of both pumps (pressure adding at the same flow).

of course, the question is basic, meaning that you presumably did not take adequate theoretical courses so my answer will mean little to you, so the only right answer would be the one frequently posted in many threads - you need help of hvac professional, you cannot become one before you pass adequate pre-education plus at least minimum number of years within field.

 

[KiwiMace]

Hmmm.. 12 small chillers? That is an unusual way of doing it.

The working pressure is the highest pressure in the system, which is usually at the physically lowest point of the circuit, at the discharge of one of the pumps.

 

[317069]

in primary secondary system you have two separate circuits, each of them has it owen working pressure.
in you case you have two working pressures, the primary at the design operation has 18m which is about 1.8 bar, the secondary is about 2.5 bar
in order to how to calculat pressure you have to read books about fluid mechanic or ASHRAE for example.

 

[KiwiMace]

That is not correct. You have the static head of the system plus the dynamic pressure of the pump. The static head is the same for the primary/secondary, but the working pressure will vary based on the pump head.

It is also unusual for the secondary head to be lower than the primary.

 

[317069]

primary is a separate circuit than secondary,they do not share any hydrloic parameter
when you size primary you do not involve secndary system and also when you size secondary you do not involve primary system.
working pressure as I understand is the operation point at pump curve

 

[KiwiMace]

Working pressure includes the static head. You select pipe wall thickness, valves and fittings based on working pressure. Therefore the calculation provides the highest pressure experienced in the system.

 

[317069]

you are right

 

[KiwiMace]

Maybe so, but we scared off the OP!

 

[tcihvacff1]

Thanks KiwiMace!

 

[tcihvacff1]

And thanks all for your valuable suggestions....

 

[ChasBean1]

Yes to above. To clarify:

Add:
- The minimum pressure you want at the highest point in the system in the highest building
- The pressure from the head of the highest elevation to the lowest
- The maximum pump head

So if you want 15 psig (or 103 KPa) at the highest point in the system which is pretty typical, and your highest system elevation is 80 feet or 24 m (which is also pretty typical of a water system in a 5 story building):

15 psig (or 11 m, or 103 KPa) PLUS
35 psig (or 24 m, or 241 KPa) PLUS
36 psig (or 25 m, or 241 KPa)

is the total system operating head.

 

[tcihvacff1]

Thanks All!
The thing is that we are getting a pressure of 7 bar at the highest point but our PMC is asking 10 bar pressure & for that they are giving the reference of ASHRAE 111....
So please can anyone help me from where i can get the procedure of testing pressure in chilled water lines or from where i can get this version of ASHRAE..
Thanks in advance!

 

[KiwiMace]

Not sure where ASHRAE 111 (testing and balancing standard) comes into this, but pumps, pipe flanges, valves and accessories are normally supplied in 10 bar, then 16 bar, then 25 bar standard pressure ratings. If you have 7 bar at the top of your system, you will certainly be higher at the bottom of the system. You pick the next highest rating level, you will probably be 10 bar.

Standard pipe wall thicknesses in pipe sizes up to DN300-350 usually far exceed the 10-16bar pressure ratings, so pipe itself does not present a problem in normal hydronic applications.

 

[tcihvacff1]

Thanks KiwiMace!