Chilled Water Buffer Tank - Primary/Secondary - Design Help

[erugg24]

We have been tasked with replacing an existing evaporator/condenser combination unit at a local resort & spa. The condenser is on the roof and the evaporator is in the basement. Rather than install a refrigerant monitoring system in the basement to bring the new system up to code, we are proposing installing an air-cooled chiller on the roof. The existing system provides cooling for a number of chilled water coils for comfort cooling in common areas.

I've included single line drawings of the existing system and the proposed new system. I'm looking for some advice to make sure we do not end up with a system that does not work efficiently... or at all.

Some key points we considered when selecting the new system:
-Added a buffer tank to prevent short cycling of the 70-ton Daikin Air-Cooled Chiller. Rough calculations from internet engineering sites lead me to believe we need to add a 300-gallon buffer tank.
-We opted to go with a primary/secondary system to avoid replacing the existing CHW pumps.
-The existing chilled water pumps are sized for the existing loop. The hope is that keeping those pumps on the secondary loop will continue to provide the correct flow & FTH to the secondary loop.
-We sized the primary pump to account for the new chiller design flow and additional FTH to pump the primary CHW to the air-cooled chiller on the roof.
-In theory, the new CHW pump, chiller, and buffer tank will create CHW and the secondary loop will continue to pull from the CHW buffer tank as if it were the original evaporator.

Am I on the right track? Do we need the buffer tank?
Should I keep the primary piping arrangement and replace the existing CHW pumps to accommodate the risers to the roof and ditch the buffer tank?
Any help & advice is greatly appreciated.

 

[Antifed]

Hi,

Please forgive my peppering you with questions.

Just for clarification, the existing set up is a compressor chiller, correct?

As far as the buffer tank goes, and if you need it, what is your system volume and how much buffer does your Daikin require?

Is system currently variable volume or constant volume? All 2 way valves, 3-way valves, or a combination thereof?

How old is the existing system? How old are the existing pumps?

Will you need chilled water year-round? Will you need glycol (given the current configuration, is it safe to assume that you currently don't have any in the system)?

R/

Antifed

 

[LittleInch]

Well if you don't have a buffer tank now, why do you need one in the future?

Is the chiller now short cycling?

Is the new one not a modulating type?

what does your cooling load like ion terms of variance over time.

You may need the buffer tank, you may not - a lot depends on your system design about which we know very little.

If your chiller is operating at a low percent, even a 300 gal tank won't make a big impact as you will still need to start and stop.

 

[erugg24]

Great questions. Your questions alone have made me re-think our approach. The chiller is 40 years old and has not had issues with short cycling. We are probably over thinking it because we are moving the chiller to the roof and trying to avoid replacing the existing CHW pumps.

Our new plan is to replace the existing CHW pumps to accommodate the additional FTH and match the design flow of the chiller. No buffer tank. We will simply add the new air cooled chiller to the loop. We will include VFD's for the new CHW pumps and increase the FTH to 80 (rather than 60). The chiller is approximately 50' higher in elevation than the pumps will be.

Thank you for the insight.

 

[LittleInch]

FTH? I can't find that in a set of abbreviations.

Elevation is not an issue, it's the extra length of pipes, but to be frank if you make those new pipes to the roof say double or more the diameter of your current pipes going to the evap then your current pumps will probably be ok. But you will need a new pressurisation tank on the roof to maintain the highest point at prob 5- 10 psi min.

Also check all your existing units for pressure rating as your pressure will increase by at least 22 psi due to the extra static head.

 

[erugg24]

FTH - Feet of Head (for the pump)
I was taught that you needed to increase the FTH to match the height of the piping as it went up in elevation.

 

[LittleInch]

FTH - Feet of Head (for the pump)
I was taught that you needed to increase the FTH to match the height of the piping as it went up in elevation.

Not in a closed system like I assume you have. You need to pressurise it and have an expansion tank like I guess you have now, but it needs to have more pressure so that at the high point you are still under positive pressure at all times. Hence why the pressure at the base increases.

But for a closed loop system filled with water and held under a positive pressure at the top, the pump head is purely frictional. So yes your friction head will increase, but if yu make the extension in bigger pipes than you have at the moment., that extra friction will be quite small.

If you were pumping up to an atmospheric tank or a tap then yes, you would need to increase head.

 

[BronYrAur]

FTH - Feet of Head (for the pump)
I was taught that you needed to increase the FTH to match the height of the piping as it went up in elevation.

Will the maximum water lever (in the new chiller) be significantly higher than the level now (presumably in the highest coil)? If so, the static fill pressure of the system will increase. You need to make sure it won't be excessive.