Chilled water buffer vessel 1

[dulenz]

I would very much appreciate if somebody could help me with few questions.

Chiller manufacturer recommends certain amount of water in the chilled water system per kw of installed capacity. Should I look at total kw of the chiller or total kw of the chiller divided by the number of compressors?


When I have constant primary and variable flow secondary chilled water system should I just look at the water content in the primary circuit or secondary circuit as well?


If I have multiple chillers should I look at kw of the total plant or kw of the smallest chiller in the plant divided by the number of compressors in that chiller.



Thanks

 

[trashcanman]

Ask the chiller manufacturer to explain what he means.

 

[cry22]

He means if I understand correctly: MINIMUM WATER VOLUME in the system.
The data is published in chiller catalogues. Typically 3GPm per ton of water volume in the system.

If you had your car with only one gallon of water in your radiator, your car will overheat, you could not possibly cool your engine if you had only a sip of water in the system.

We run into some problems when the distance between a chiller and an air handler is too short, you would need either a buffer tank to bring the TOTAL volume to that accepeted by manufacturer. One way we got around it one time was to increase the pipe size to the next size. Price difference is not that much and yet gives the necessary system volume.

 

[dulenz]

Sorry, maybe I did not explain properly what I wanted English is a foreign language to me

For example: if I have 100t chiller with 2 compressors. and Manufacturer recommends that minimum water content in the chilled water system should be 3gpm per ton so the questions are:

1. How much water in total do I need 3*100 =300gpm or 3*100/2=150gpm where 2 is the number of compressors in the chiller

2. should I have that water quantity in primary circuit only or in primary + secondary

3. If I have two chillers of 100 ton each with two compressors each how much water do I need

2*100*3=600gpm or 2*100*3/4=150gpm

I did ask chiller salesman this questions and he did not know the answer apart from the fact that we need a minimum volume of water in the system

 

[trashcanman]

"I did ask chiller salesman this questions and he did not know the answer apart from the fact that we need a minimum volume of water in the system"

If the Chiller Manufacturer doesn't know, how are you supposed to know? I would change manufacturer to find one that did know his equipment.

 

[DRWeig]

Dulenz --

I've never seen such a thing as minimum static water volume for a chiller, and in one of your posts you used the unit GPM. This is a flow rate, not a static volume. 3 GPM per ton is a good minimum to maintain.

Has nothing to do with volume external to the chiller. In your car example, a one-gallon radiator would work just fine if the flow rate were fast enough to capture the engine heat and the radiator were effective enough to reject ie.

Primary circuit only. If chiller is 100 tons, do 300 GPM. It's about heat transfer and freeze protection. Doesn't matter if only one compressor is running out of two.

Good on ya,

Goober Dave

 

[GMcD]

There are two issues here:

One: Minimum chilled water flow rates (function of flow and system delta T) so the chiller refrigerant compressors/heat exchangers are operating in their design band, and...

Two: Minimum chilled water system volume which relates to the "cycle time" of the water going through the CHW/HVAC system so you don't short cycle the chiller compressor(s), especially at low loads.

We are having this problem a lot on small chilled water systems where a high delta T is used to minimize pumps, pipe sizes and flow rates to reduce energy and if the circulation time of that "slug" of chilled water going through the HVAC system is less than the anti-short cycling time of the chiller compressor control, then you get trip-outs, and in some cases at low cooling load conditions, the chiller can feeze itself with low CHW return temps with a low system CHW volume.

Hence a buffer tank to increase the CHW system water volume to get a longer circulation time of that "slug" of water going around the HVAC system.

 

[DRWeig]

GMcD,

Thanks -- my bad, I neglected the part about the OP's chillers being reciprocating type.

Good on ya,

Goober Dave

 

[dulenz]

If the primary circuit should contain at least minimum required chilled water volume (3.5-4.5 l per kW of refrigeration or whatever the manufacturer recommends), does that mean that most of the primary-secondary chilled water systems have buffer vessel because primary circuits are usually short (supply and return headers, evaporator, pumps and fittings)and they do not contain a lot of water.

For example, if you have two 1200 kW chillers you would then need 10.8 m3 of water in primary circuit. That is a lot of water. If all pipes are 300 mm schedule 40 you would need 150 m of pipes to have that kind of water volume. I am yet to see a primary circuit with 150 m of pipes!

ok. there will be some water in evaporator and fittings as well, but again it looks like every primary-secondary system with short primary loop would need a buffer vessel.


You could oversize the primary circuit pipes and headers to have a bit more water but there is a limit to that.

 

[GMcD]

Dulenz: It depends on the chiller. 1200 kw chillers are usually centrifugal compressors, and are much easier to control capacity-wise at low loads/low CHW flows compared to small unitary refrigerant compressors like scrolls, recips and screws. So you wouldn't normally need any buffer tanks with a large centrifugal chiller.

You need to define the types of chillers and type of pumping systems and terminal cooling systems before more meaningful discussion can happen here. Consult the Trane Chiller Plant design manual (or your favorite Go-To chiller manufacturers's desihgn manuals). The manual I have here is a Trane Applications Engineering Manual, publication SYS-APM001-EN (July 2000) and it's full of good design information for multiple chillers and pumping schemes.