High Loop Volume Chiller System

[sirilkt]

I am using an 800 TR chiller system with the load distribution as shown in the attached schematic.

So essentially we have an underground storage tank in which we are supplying chilled water and are also putting the return water from AHU into the same underground tank.

But in AHU we are not able to maintain the temperature because of this.

The underground storage tank capacity is 2,00,000 litres.

Loop Volume / Ton is coming out to be 260 Gallons/ Ton.

Can you please suggest on what can be done better so that we can get proper temperature in AHU.

For calculating the loop volume , I have added the entire 2,00,000 litres of underground storage tank for each chiller. Is this correct way of calculating

 

[dbill74]

First thing I have is do NOT mix your units (litres/gallons), choose one and go with it. It causes confusion and results in errors in calculations.

Second, you have not adequately defined the problem before starting to look for a solution.

But in AHU we are not able to maintain the temperature because of this.

This tells me nothing other than you are seeing temperatures in the system you are not expecting. I still do not know where in the system this is happening or if the temperature is high or low.

What is the temperature of the storage tank? What is the temperature of the water you are pulling out of the storage tank (should be same, if different this can help you find the problem). What kind of units are the load, WSHP, CHW AHU?

My first thought would be to ditch the storage tank, bypass it or at least reduce its size.

 

[LittleInch]

"Loop volume"??

I can't work out anything really from the (lack of) data on your schematic and your post.

The storage tank should just be acting as a cold buffer store so that your chillers are not sized for the max load which doesn't happen continuously so that e.g. your chillers operate continuously and are more efficient when it's cooler outside when load is therefore reduced.

However without seeing the relative size of the flows, temperatures, chiller load, or AHU load we can't see anything.

Inly be seeing what the temperature of the central store is over the day could you work out if the store isn't big enough or the total load is greater than total chiller capacity.

Maybe the temperature in the store rises too high, maybe your AHU isn't big enough, maybe your chiller isn't big enough - how are we supposed to tell from the data provided??

What is 2,00,000? Two million, two hundred thousand (200m^3)?? Only by knowing if the load flow is 200m^3/hr or 20 m3/hr can we see what we're dealing with here

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

 

[sirilkt]

Thanks Inly and Dbill for your replies.

The schematic of the chiller system is as follows. The load side of AHU is 1710 TR, where as the chiller capacity is 1610 TR. The flow rates on the chiller loop is 8200 LPM where as on the process loop it is 8700 LPM.

Generally we size the AHU capacity to be 30% more than the outdoor.

Also here for 1600 TR Installed capacity we are using 1600 TR Cooling Tower. Would there be any effect on the chiller delivered capacity because of that.

 

[LittleInch]

To me this is fundamentally wrong. The central tank will exist at some intermediate temperature which is maybe not high enough for the chiller to work effectively and hence may not be doing your 1600TR, but the AHU is getting water at a higher temperature than it would like.

What's wrong with a standard primary loop running through the chillers and the AHUs taking flow from the cold supply and then dumping it BA into the return piping?

Chillers work better, ahu gets colder water.




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

 

[dbill74]

With 1710 TR of heat effectively dumped into the storage tank and only 1600 TR being removed by the chillers, it should be expected that the temperature in the storage tank will rise and thus the CHW temp going to the AHU's will also increase. This raises the question of what is the temperature setpoint for the storage tank? If it is the same as the maximum expected by the AHU's then, yea you're gonna have problems.

If the storage tank is suppose to act as a "cold storage" system, then it sounds like you might have some control issues. Maybe its bad set points, maybe the storage tank is getting extra heat from the ground in which it is installed. At 2,000,000 liters, that storage tank is massive, perhaps you are getting banding or regions of hot/cold within the tank itself. If the connection points are too close together you could be getting short cycling of the water within the storage tank.

Right now, there are simply too many unknowns, too many variables to consider in determining the reason you have unacceptably high temps going to the AHUs.

 

[LittleInch]

I just thought about the physical issues as well. If the flow and return lines are relatively close together, then the flow may just not be circulating properly and even the rather odd arrangement of a single water temperature might not be working.

A true physical drawing would be good and also to find out what exactly 2,00,000 litres is. - sirilk has written this on the OP and on the attached diagram which makes me wonder exactly how big this storage actually is.


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

 

[dbill74]

I had that same thought about short cycling LittleInch.

For kicks, I googled the volume of water in an Olympic swimming pool, the result is 2,500,000 L.

OP also stated it was in ground which raises question of potential heat gain/loss to earth.