Different Sized Chillers In Parallel

[SRBKDW]

I'm reviewing an existing site that has two chillers in parallel. One chiller is sized at 120 tons nominal and one chiller is sized at 40 tons nominal. Both chillers operate off of a single pump. I'm trying to use the existing equipment more efficiently but it seems that if I use only the 40 ton chiller for winter operations I won't be able to get the delta T across this chiller necessary to allow the air handling units to operate properly due to the constant flow of the pump. All of the air handling units currently operate off of 3-way valves. Any suggestions or observations are appreciated. Thanks.

 

[remp]

Interesting situation...unusual...
Why don't you put a variable speed drive (frequency invertor) and programme it so that when the large chiller is working you get speed #1 (design water flow)and when the small chiller is working you get seepd #2 (design water flow #2). If the pump is sized to cope with 2 chillers working together you could have speed#3.

Dont install a second pump to operate both pumps in parallel with each other, this will not work with 3 way valve in all air handling units, unless you change the valves to 2 way only.

Good Luck

 

[Cooky]

Try this.....

Each chiller should have its own primary pump and motorized shut-off valve. When a chiller is not required on a multi-chiller installation, water should not flow through that chiller. This is achieved by switching off the pump and operating the motorized valve (but remember to allow the pump to run-on for about 10 mins).

You could re-use the existing chilled water pump, to operate as the secondary chilled water pump.

The chillers, pumps and motorized valves should be controlled by a single BMS system.

Have you contacted the chiller manufacturer?, they could also offer you some ideas.

I hope this helps.

Cooky.

 

[quark]

Cooky's idea is the best with respect to optimum power consumption within the limited data you provided.

Your clarification about the following points will help in analyzing the system better.

1. Do you have primary secondary pump setup?

2. Do you have only chilled water temperature control with the AHUs?

3. Do you have problems related to humidity?

In my view variable speed pumping with only one pump is not a better solution. Suppose you design your pump for a total load of 160 ton and if you want to use it for 40 ton chiller, you should reduce the speed by 75% for a fixed temperature difference across the chiller. This will drastically reduce the head developed by the pump ( to about 6% of the original head) and you will not be able to pump water to the AHUs. If you maintain head then you will have excess flow and this will defeat the purpose of the variable speed pumping.

You can have a flow control valve installed on the chiller and bypass the excess fluid (or you can run the pump at 25% flow rate as this is well above safe minimum flow rate). But some of your AHUs may starve.

Why can't you run both the chillers and rise the chilled water temperature control set point?

Regards,

 

[lilliput1]

You can operate just the 40 ton chiller & the common pump to both chiller. You may have to lower the discharge temperature off the 40 ton chiller to match AHU coil performance required to meet the load.
Assuming 10°F desig TD of both chiller, 44°F leaving & 54°F entering:
Gpm through 40 ton chiller = 96
Gpm through 120 ton chiller = 288
Total Gpm through AHU(s) w/ (3) way valves = 384 GPM
Now if the load is 40 tons & the LWT off the 40 ton chiller is set at 44°F, 40°F then 38°F respectively, the CHW temperature entering the 40 ton chiller & averaged AHU coil leaving temperature would be 54°, 50°F and 48°F respectivel. The corresponding mixed net CHW supply temperature to the AHU coils will respectively be 51.5°F, 47.5°F or 45.5°F respectively. So using the AHU coil performance & actual coil entering air conditions for the 40 tons load, select the appropriate LWT off the 40 ton chiller. Use this to set up a reset schedule of chiler LWT.
This avoids having to convert the valves to (2) way & adding dedicated pumps or reconfiguring to primary/secondary pumping.