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Elevation Head in a Closed-loop Chilled Water System
4

Elevation Head in a Closed-loop Chilled Water System

Elevation Head in a Closed-loop Chilled Water System

(OP)
Hi,

I am just a beginner in HVAC & there are lots of things that are for me, quite confusing. One of those are Elevation Head in a Closed-loop Chilled Water System.

To calculate for the TDH of a Closed-loop Chilled Water piping system, one should consider the Velocity head + Friction Head (multiplied by 2 because of supply & return pipes). Now my question is, how can I raise the fluid at a certain vertical distance if I will not consider the Elevation Head? The system is new so the discharge line is empty.

Thank you so much!

RE: Elevation Head in a Closed-loop Chilled Water System

2
It's always much better to include a sketch so everyone can work out what it is you're thinking / looking at.

In general for systems that include a significant elevation difference, you are looking at two different pressures / pumps.

One to fill the system, usually from the bottom, venting at the top, but sometimes via your expansion tank or top entry. Either way you need sufficient pressure somewhere to get the water to the top of the system from ground level. This however is a rare occurrence so you either use a small high pressure pump or some other external source of pressure ( e.g. the water mains).

The main circulation pump does not include elevation within the calculation of Total Developed Head (TDH) - try using the full words the first time you use an abbreviation, TDH sometimes is used for Total Discharge Head... - as the elevations balances out for a closed filled system. Often this TDH is much smaller than the head required to fill the system, but otherwise the pump would be oversized for its main duty.

There are many posts similar to this if you search correctly.

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

RE: Elevation Head in a Closed-loop Chilled Water System

(OP)
Hi LittleInch,

Thanks for your immediate response.

I actually did a search related to this topic before I posted mine. Unfortunately I wasn't able to see one answering question on how to raise the liquid at a certain vertical distance from the ground level without considering the Elevation Head, considering the system is new. Threads related to this are questions like how to solve for Total Dynamic Head alone, or how to size a pump, etc. If any, none has answered already. Or maybe, I just missed it.


Thanks a lot Sir.

RE: Elevation Head in a Closed-loop Chilled Water System

This would likely be a means and methods problem for the installing contractor(s) (not something they teach in school, unless it is the school of experience).

You do not have to necessarily use your pump to raise the liquid in the beginning. I would make sure there is a hose bibb from the domestic water system near the top and bottom of the system. If there is not a hose bibb at the top, then there can be a threaded nipple with valve at the top to allow connection of a hose from the domestic water system (probably a mop sink)

These are usually (should) be located in every mechanical room or every janitors closet or toilet room for make up water and other uses. Also make sure there are vent points (small nipples of pipe with a valve or similar) to allow air to escape while filling.

Once the system is filled, you do not have to worry about elevation head.

RE: Elevation Head in a Closed-loop Chilled Water System

First, I found a great reference book endorsed by ASHRAE called "HVAC Equations, Data, and Rules of Thumb" check it out.

Second, you stated this is a closed loop. The pump should not start until the system is full of water. Once full the amount of lift required is negated by the drop. The pump does not need to add a lift factor to the process, only friction is a factor.

In an open system you need to worry about lift because the only force behind the pump is atmosphere. This is why the pump will need to be lower than the supply water, to build up pressure. In the open loop system, friction AND lift (Z-axis, vertical dimension) will need to be added to the pump head to determine TDH.

Third, if you are using the pump to build up pressure to push the water to the top of the system, you may oversize the pump so that after initial fill, the pump will not run efficiently and will probably burn the motor out faster. ALWAYS fill the system prior to turning the system pump on. Too many bad things can go wrong making everyone angry.

RE: Elevation Head in a Closed-loop Chilled Water System

You should find an experienced person in your office to help with this.

RE: Elevation Head in a Closed-loop Chilled Water System

Pay attention to very high elevations though.
If the water column is say over 350 feet high (30-story plus building), then watch for your valves and fittings pressure rating at the bottom of the riser, as they may see a pressure larger than their rating. valves rated at 125 PSI at the lower level will not stand the 150 PSI+ pressure from the water column. Make sure you specify 300 PSI rating on valves and accessories at the lower level.
Then again, when you get to that kind of water column, you may be having an intermediate floor with storage tank.

RE: Elevation Head in a Closed-loop Chilled Water System

The question itself is not clear, and it is ambiguous. The question stated that it is a closed-loop Chilled Water piping system, so there is no elevation head in the pump head calculation. Next, the question asked how to raise the fluid at a certain vertical distance. Is this referring to the vertical pipe or distance from the chilled water expansion tank at top to the chilled water distribution pipe network? Next, the question stated that the system is new (mentioned twice) so the discharge line is empty. Where is this referring to in a closed loop chilled water piping system? So, is this really a closed loop application?

RE: Elevation Head in a Closed-loop Chilled Water System

It might help to also clarify whether the domestic water pump will have sufficient head for filling at high point.

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