Chilled Water - Variable Speed vs Conventional Pumps

[SeyBrew]

Hello everyone,

I have been looking everywhere for an informed take on this but am finding wildly different information so I was wondering if anyone on here has a view on this that they are willing to share.

We are in the early design stages of a chilled water system but are vacillating between whether it is a good idea to use variable speed pumps rather than conventional pumps.

Is there any significant advantage to using variable speed pumps that justifies the increased installation cost?

What is the usual payback period for something like this? My gut feeling is that this will be quite long....and that 'it depends'.

Don't like the loss of three way valves the variable speed pumps entails. Is there any positives to this anyone can think of other than an offset against the cost?

Any other thoughts on this would be welcome.

[majorzero]

Hi,

In my opinion, fixed speed pumps are better than inverter ones. But if you want to use an inverter, you MUST install a "water volume limiting mechanism" on your circuit. Otherwise, you can not guarantee a certain water volume for your each system.

[SteveCass]

In disscusions I have had regarding this topic, it was pointed out to me that if you reduce the chilled water flow rate then you decrease the chiller cooling duty and the unit has to run on longer to keep up to the load. ie it costs more to run the chiller, potentially offsetting the pump now costing less to run. Have'nt seen any numbers for this, but it seems to make sense?

Steve

[Tesla]

Hi SeyBrew
In a control perspective from what I have seen in the field it is viable for larger systems to use variable flow. The biggest I have seen used variable flow for primary loop and variable flow for the secondary loop with a decouple pipe. I think the primary loop pumped 10 or 20% volume more than the secondary loop. There were 7 x 4.4mW centrif chillers. The secondary flow was matched to the load from CRAC units. With load matching large energy savings can be made as part of the electricity charge is based on the highest 1/2 kWh each month. There is also less wear on components from soft starting and reduced speeds and mechanical loads.
In my mind three way valves are a waste of energy, the pump wastes energy and paid for chilled water is bypassed. With energy management control running costs can also be reduced with - optimal start/stop, load forecast from weather and historical data, precool and load shedding for high time tariff electricity charges.

[Tradewinds]

Hi SeyBrew,
I think a lot of Air Conditioning Inspectors in the UK are now recommending VSD's on pumps to reduce energy costs. It all comes down to the way they are controlled. Primary circuit pumps can be controlled to maintain flow thru the chiller whilst secondary circuit pumps can vary flow according to load demands. They also reduce the flow during night time setback if they cant be switched off.