Basic Chilled Water System Weir Tank

[johnhodgson]

Hi - I am a newbie to this forum and seek the help of much wiser members (that I think includes everybody). I run a maintenance department and our factory has a number of small internal process chillers of various sizes. I am installing a couple of external chillers of 50kW each with the intention of getting rid of the internal ones. I have been advised to use a buffer/weir tank and due to limited space have opted for a 1000 litre unit. My question is about the connections to the weir tank - basically where to take the water out and where to return it too. Also the best point for connection of the two chillers, flow and return. I could really do with a simple schematic also showing the weir and principal of operation including thermal flow. Any help is greatly appreciated.

[Brian_UK]

Hi John and welcome to the forum.

There has been a previous discussion on this which might prove useful to you. There are a couple of layouts included.

http://www.refrigeration-engineer.co...ad.php?t=19429

[johnhodgson]

Hi Brian - thanks for your response - I have visited the discussion you mention but the help I am after is probably a bit more fundamental. Basically I am trying to understand the thermal dynamics of the weir tank as I need to work out the best place to make connections from two chillers to the tank as well as the process flow-pump and return lines. Six connections in total. The tank supplier has asked where I want these connection points.
I have attached a doc file showing a basic schematic (I hope it makes sense).
Regards John

[Brian_UK]

The purpose of the buffer tank is to provide sufficient water for the system to operate, the temperature within the tank will be basically the same on both sides of the weir.

The chillers need to be connected in parallel with the common flow pipe connected to the buffer tank on one side and the return from the other side going back to the chillers.

The process flow is taken from the chiller common flow and returned to the chiller common return.

The layout shown in the previous link shows this.
http://www.refrigeration-engineer.co...4&d=1245786971

[sedgy]

hi john, interesting, but you say you want to put the chillers outside, if they are air cooled, you are waisting a lot of heat , in the north of the uk the heat would be wisley used inside even if you used a swing flap door to divert the heat, indoors for winter or outside in summer hope this complements your thoughts on the situation

[mad fridgie]

I would go for a split buffer.
You need to fit a deviding wall in the middle of the tank (finishing close to the top but not at the top).
Side "A" warm return water from process, supply to chillers.
Side "B" cold return from chillers
supply to the process.
When there is an inbalance in flow, water will flow over the deviding wall.
A thousand litre tank is not very big for this application. I would recommend bigger, if you can get 2 *1000l in then install balancing pipe between

[Lowrider]

there is a simple calculation to see what the minimum buffer capacity should be!

Minimum size = (smallest capacity(%/100)*Maximum Capacity chiller * Minimum on time) / (specific heat of medium * control dead band)

[multisync]

Why buy a chiller with an integral pump/buffer set if you are installing a larger tank?

With the flow and return mixing it up in the tanks I suspect putting them at diagonals top bottom and angled away would suffice

What chillers are you using?

[johnhodgson]

Hi All, thanks for your input, I'll try and answer some questions.
1) Chillers are Daikin EUWAB20KAZW - 46.6kW cooling each (one is for backup), I could only get the model with internal pump/buffer at short notice.
2) External siting is due to lack of space, plus the heat they produce in summer makes for a very warm factory!
3) The buffer tank on order is a split type with a weir in the middle (1000L due to limited space).
We are flexo printers operating 24/7, I intend getting rid of a number of smaller internal chillers. I intended connecting each chiller to the buffer/weir tank separately with a 2 pumps feeding the processes (one for backup).
I just can't quite work out the best points to connect to the weir tank.
Regards - John

[johnhodgson]

Hi people, thanks for all your input. At the risk giggles from the more knowledgeable I have uploaded another sketch showing my proposed connections. I have inverted the weir plate so water can flow UNDER rather than over the plate; this is so if the water level in the tank falls it should not affect the operation. Your opinions are valued.
To be honest I am beginning to think that I may be making too much of a meal of this!

[mad fridgie]

Hi people, thanks for all your input. At the risk giggles from the more knowledgeable I have uploaded another sketch showing my proposed connections. I have inverted the weir plate so water can flow UNDER rather than over the plate; this is so if the water level in the tank falls it should not affect the operation. Your opinions are valued.
To be honest I am beginning to think that I may be making too much of a meal of this!

Nice Drawing!
I personally do not like the inverted weir (you can potentially loose some of the benifits of this system)
I would draw water (suction of the pump) on all from the bottom of tank. (Process and to the chillers) Even if the level fails the plant will run. If you are really concerned about loosing water then install 2 Non Return Valves at the bottom of the weir plate (one in each direction, size to meet max pump flow)
Where you introduce the returns, there are 2 arguments "1" introduce both to the top, this aids in mixing "2" Introduce at the bottom (almost a tee) this gives biggest possible temp difference. Without really knowing your process in depth, I would choose option "1" very safe and reliable
Mad

[Lowrider]

It's better to connect both chillers to a common pipe and then go into the buffer, the same goes for the return pipe! Trust me, I've done this many times before and it works much better!

[Lowrider]

The layout Brian_UK used (http://www.refrigeration-engineer.co...4&d=1245786971) was a drawing I made. This is the most efficient way to use multiple chillers. The two pumps can either be used as each others back up, or if the system calls for it, a small and larger one.

If you connect the system up the way the changes are both sides will short cycle, meaning the chillers will draw most of their water back to themself, as will the proces. In the way I draw it, this can't happen!

[mad fridgie]

The layout Brian_UK used (http://www.refrigeration-engineer.co...4&d=1245786971) was a drawing I made. This is the most efficient way to use multiple chillers. The two pumps can either be used as each others back up, or if the system calls for it, a small and larger one.

If you connect the system up the way the changes are both sides will short cycle, meaning the chillers will draw most of their water back to themself, as will the proces. In the way I draw it, this can't happen!

In this case, it is not a multiple chiller application
(duty /stand by) The units come complete with their own internal water pump. So would be difficult, even though not impossible to use each pump for the others application.
Without knowing the actual application and process control methods, it makes it some what difficult to determine the best process. In my experience in the printing industry (limited) then tend use a fixed flow through the process heat exchangers and simply control the water temperature (in many cases refrigeration on and off) They tend to have a wide temperature variance within the process (not a lot of need for PID controllers driving some sort of modulating valve)

[Lowrider]

The pumps I was talking about are the VSD pumps, the ones used to transport water to the proces!

Although your idee might work, if one wants a reliable and cost effective system, this is the way to go! The other one just consumes more energy than one should want! Futhermore, should the system call for it (start up or more load during high production ) the second chiller can be used to satisfy the load!

In this case, it is not a multiple chiller application
(duty /stand by) The units come complete with their own internal water pump. So would be difficult, even though not impossible to use each pump for the others application.
Without knowing the actual application and process control methods, it makes it some what difficult to determine the best process. In my experience in the printing industry (limited) then tend use a fixed flow through the process heat exchangers and simply control the water temperature (in many cases refrigeration on and off) They tend to have a wide temperature variance within the process (not a lot of need for PID controllers driving some sort of modulating valve)

[mad fridgie]

The pumps I was talking about are the VSD pumps, the ones used to transport water to the proces!

Although your idee might work, if one wants a reliable and cost effective system, this is the way to go! The other one just consumes more energy than one should want! Futhermore, should the system call for it (start up or more load during high production ) the second chiller can be used to satisfy the load!

Sorry for the mis-understanding!
I always focus on the process, the refrigeration is secondary. The two port valve system with the VSD pumps is great for controlling refrigeration effect example like coolstores or air cond.
many industrial processes need cooling of surface area, so fixed flow, (for close control 3 port valve set up in its many guyzzes)

[johnhodgson]

Hi Folks - many thanks for all your input, I have things sorted now.
As the buffer was referred to as a 'weir-tank' I expected there to be a 'thermal' flow over the weir, I overlooked the fact the weir might be there simply to allow water-flow from one side to the other in instances of imbalance; i.e. process pumps stop while chillers are running causing warm side to empty and cold side to fill. Talk about overlooking the obvious!:
Best regards to all
(now where do I start my thread for maintaining 2-bar pressure in the process flow?)

[Brian_UK]

(now where do I start my thread for maintaining 2-bar pressure in the process flow?)

Try Smedegaard Pumps, no connection.