Do I need a chilled water buffer vessel

[Consultant]

I am installing a new chiller in a building which has three other chiller, total capacity 3000kW. The chillers feed a ten storey building and the terminal units are two port control. Overall volume of the system is about 10 metre cube.

Because the system is a two port system, the chiller manufacturer tells me that I need a buffer vessel. he tells me that the water content in the secondary side cannot be included in the water content calculation. The water in the primary circuit is insufficent to stop the chillers trying to start too many times.

is this true? Is the buffer vessel the correct solution? Seems to be a waste of kit and space.

[frank]

Hi Consultant and welcome to the forum.

On a chiller application, if you have a steady load then you do not need a buffer tank but where you have differing loads and demands, as with your 2 port controls then the return water temperurature will peak and trough i.e. it will not be steady and should have a buffer tank fitted.

As your 2 port valves open/shut or modulate in response to the local demand so the return temperature will go up and down. The point of a buffer tank is to smooth out these widely varying temperatures so allowing the chiller to have less starts and stops.

If you had all your indoor fan coils or whatever all calling for cooling then the return tempurature would steadily be handled by the chiller which normally controls on return temp, but when say, 50% of your fan coils have a satisfied thermostat after a short while (perhaps because they are in the shaded part of the building) without a buffer vessel the chiller will see a rapid drop in return temperature and will cycle off. With a buffer vessel the rapid drop in temperature will be absorbed in the buffer vessel allowing the chiller to continue running.

Have a read of this item I found on the net http://www.pfi.com/files/articles/No...apacitance.pdf

[Peter_1]

1 st, what is a 2 port system?
If chiller is capacity controlled so that actual capacity can be adapted to the cooling load, then mostly you don't need it.
It was more true in the past where most chillers were unable to adapt capacity.
Carrier handles a thumb of rule ,x liters of water/ kw capacity but I can't find it right now.

[Brian_UK]

1 st, what is a 2 port system?

The water control valves operate as an ON/OFF valve only and are fitted into one pipe. They do not divert or bypass an water when closed or closing etc.

Hope that helps Peter.

[Consultant]

Thanks for the helpful responses. Further question for the experts......

The system is a slow response system, so surges are unlikely.

Does anyone know the ratio of distribution system volume to chiller cooling capacity. I recall 13 litres per kW?

I believe that the volume in the pipework is sufficent to arrest fluctuations at low demand periods - mid season cooling of the building.

2 port systems are when the terminal units on the chilled water circuit have control valves which varies the flow dependant on demand. 3 port systems maintain constant flow by diverting the surplus.

Consultant

[frank]

Does anyone know the ratio of distribution system volume to chiller cooling capacity. I recall 13 litres per kW?

I'm not sure I follow your question or why you need to determine the distribution volume.The volume flow through the chiller should be determined by the formula Q = m c dt or by transposition m = Q/(c dt) where
m = mass flow of fluid (kg/s)
Q = duty of chiller (kw)
c = specific heat capacity of fluid(4.19kj/kg for water)
dt= difference between entering and leaving temperatures of the fluid

The water in the primary circuit is insufficent to stop the chillers trying to start too many times.

By adding a buffer vessel you are increasing the size of the primary circuit and diluting any tempurature fluctuations

[Consultant]

Frank

Chiller manufacturers require a distribution volume to ensure the chiller dont exceed the 6 starts per hour.

[Peter_1]

What type of chiller are you installing?
If it has a capacity control, then you won't need one.
Or a frequency controlled compressor perhaps?

[Consultant]

The chiller has four step control, which implies that 25% capacity will be the smallest load. Say the chiller is 750kW, then this equates to 187.5kW of cooling. If the system demands only 100kW - will the chiller try to stop and start too often?

I am told that the buffer in the system avoids this happening. Can someone tell me how to calculate that?

It must be a function of max starts per hour, system volume and cooling demand.

[Peter_1]

100 kW needed and 185 kW available is +/- 0.5hr/hr running, so OK if you ask me.
That's indeed the purpose of a buffer.
You can also increase DT so that it runs longer and has longer standstill periods.

[frank]

You can also increase DT so that it runs longer and has longer standstill periods.

With only 25% capacity running I would expect less than 2K dt which will increase running time and delay the compressor switch off.

[caseyjones]

Hi Guys, i am currently designing an 8000 kW centralised chilled water system with 5 individual chillers( ammonia screw chillers ) all connected to a large low loss header with seperate primary pumps and a big mother of a main system pump. my initial thoughts where (because part of the load is steady process load and because of the number of chillers and because the header holds 4000 litres) i would not need a buffer vessel. However i am wracked with self doubt now. What do you dudes think ?

[Peter_1]

Nice intro.

[Peter_1]

Only 1 main pump for 800 kW?
What if this one fails?
You also have the contenance of the whole system, not only the header.
Is your smallest step is 1600 kW, then you certainly don't need one.

[Peter_1]

Frank

Chiller manufacturers require a distribution volume to ensure the chiller dont exceed the 6 starts per hour.

Not correct if more compressors are installed like most chillers nowadays.

[1torr]

Hi,we work on a minimum of 4 litres/kw for air conditioning and a minimum of 7 litres/kw for process cooling.

[Frudge it]

After Having read about this problem posed , there is another peice of flow equipment in the CHW circut , that is designed to handle precisely the expected circut performance characteristics.This item is the " Mains CHW Bypass Control Valve".There are a number of theorems that determine precisely how it is design configured But essentially it must be installed in 2 port valve systems to achieve (1) Maintaining minimum CHW flow across the Operating Chiller Vesels & (2) When flow demand drops off to under the flow capacity of CHW Pumps - Bypassing to maintain minimum flow to the CHW Pump. Buffer Vessels are not required if this valve is sized / controlled correctly after careful & exacting analysis of the Building & Chiller Circuts Flow demands

[Frudge it]

[Old Frudgies Never Die - they just run out of gas]After Having read about this problem posed , there is another peice of flow equipment in the CHW circut , that is designed to handle precisely the expected circut performance characteristics.This item is the " Mains CHW Bypass Control Valve".There are a number of theorems that determine precisely how it is design configured But essentially it must be installed in 2 port valve systems to achieve (1) Maintaining minimum CHW flow across the Operating Chiller Vesels & (2) When flow demand drops off to under the flow capacity of CHW Pumps - Bypassing to maintain minimum flow to the CHW Pump. Buffer Vessels are not required if this valve is sized / controlled correctly after careful & exacting analysis of the Building & Chiller Circuts Flow demands[/quote]

[Deepesh Gupta]

Can anybody explain the process cycle with Buffer tank in the system. If pictorial, it helps a lot.

Regards,

[Brian_UK]

^^
Chilled Water Buffer Tanks (CBT) are designed for chilled water systems with insufficient water volume capacity in relation to chiller capacity. Relatively low water volume systems require additional buffer capacity for the system to eliminate problems such as excessive chiller cycling, poor temperature control, and erratic system operation. The properly sized CBT adds the necessary volume to buffer the system volume.

http://www.cemline.com/products/steeltank/cwbmenu.asp

http://www.cemline.com/products/Isom..._secondary.pdf

[frank]

Nice BUMP - nearly 3 years

[charlie n]

1torr has it right. AC systems need a minimum of 3.5l/kw. his 4l/kw is a good safe design.

[mr_madee83]

Hi...

Just wondering what is the function of Primary Circuit and Secondary Circuit in Chilled Water System? On what type of system to apply them?

Hope somebody can help me with this?

[Goober]

Just to Hijack a thread....Buffer Tank, Expansion Tank...same thing? Or a White horse of a different colour?

[Sridhar1312]

Expansion tank is to take care of expansion when the chiller is not operated. Buffer tank is just before the pump forthe chilled water return to the Chiller is always constant.
For buildings with multistorey the chilled water can be with primary and secondary pumping system then the chilled water buffer tank is not essential as all the terminal units may not be in operation to full load.If you have modulating two way valve then the pressure difference can increase r decresae the speed of the secondary pump and the exces flow is tane back to the primaru suction which may be lower when the load is less and chiller will unload.
Ifthe system has Centrifugal chiller there may be issue of surging when it operates around 20 - 25 % although and more than three frequent start stop is not recommended due to the centrifugal machine charterstics. Due to the frequent start stop within an hour the centrifugal impellers have broken in quite a few cases.

[Sridhar1312]

Primary circuit is between the Primary pump to chiller and suctin of secondary while econdary pump is to feed all air handling units / fan coil units and returns to the suction of primary with a by pass arrangement between primary suction and seconday suction. Primary pumps are constant flow pumps with constant flw across the chiller where as the Secondary pumps are with Variable frequency drive and two way motorized valves to modulate flow across terminal equipments a per load. Secondary pump senses pressure drop from intial to the further most unit and varies the speed of the pump. By this good energy savings are feasible.

[Sridhar1312]

For this sort of system where process load may not be continous it is better to have two pumps one if primary pump from hotwell ofthe tank to chiller nad back to cold well. Then secondary pump between the cold well and process returning warm/hot well. The mixing tank can be sized with 2/3 for cold well and 1/3 for hot well so that here is over flow of cold to hot which will modulate the chiller. This will reduce the risk of chiller freeze compared to only primary pump. Both pumps it is always rccommended to have standby and all pumps can be connected to common header so th any chiller can be operated with any pump.

[Lowrider]

@ Consultant:
If you have a system with 2way valves you are most likely making a decoupled primary/secundairy system with variable speed pumps on it.
A buffer vessel, placed in the bypass will be a great help in keeping the system more stable.

A system as shown in the picture attached is a proven and reliable system. The pumps on the left side are the ones with the variable speed.

[Lowrider]

just noticed the original post is 5 years old

[omairfarooq]

Performance sheet@48C.pdfBuffer-Tank_BTH_401-085.pdfBuffer-Tank_BTL_401-086.pdfDear All,

I have two air cooled chiller with built-in hydro-kit (chilled water pump). Infact there are two of the pumps on each chiller (one working - one stand by). Now the client wants us to provide a buffer tank along with the pump. The pump flow is 11.1 l/s with a head of 280 meters. The water volume of the condenser for the chiller is 51 galons with a design water flow rate of 188.4 gpm.

Can anyone advise me on how we can size a buffer tank for the system. Do we need independent buffer tanks for the standby pumps also?

Attached is the selection data sheet for the chiller and the data sheet for the buffer tanks. Can anyone advise which buffer tank to use and why?

Thanks in advance.

[Peter_1]

Important question: how many steps does your chiller have and what is your smallest capacity step you need?
Most modern chillers doesn't need a buffer any longer. A buffer was only needed with chillers with big compressors in it and unable to reduce their capacity to a sufficient low level.

[omairfarooq]

Thanks for the quick response... The scroll chiller has 5 compressors... 5 steps with a minimum part load of 14%.

Do i still need a buffer tank? What if the contractor insists... Generally how do we size this thing?

[Peter_1]

You don't need a buffer in your case.

According to York: V=N x 60 x Z / 4.18dt

V= total system water content (Litres)
N= chillers first capacity step (kW)
Z= minimum allowable running time (min 5 mins)
dt= temp difference at minimum partload condition (say 2 deg C.).

This allows the chiller to operate smoothly at low load capacities and should provide sufficient thermal storage to give at least 5 mins operation after the machine has stopped running. This ensures the equipment doesn't keep stopping and starting at low load conditions.....by now, you probably already know all this!!

Or rule of thumb 3 litres/kW