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Constant Primary / Distributed Variable Secondary Chilled Water System

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Buildtech2

Mechanical
Mar 6, 2012
158
I would like to propose sustainable chilled water system as recommended by ASHRAE for my ongoing project. It is called “Constant Primary / Variable Distributed Secondary” system which is similar to district cooling system. The sequence of operation may be slightly similar to the “Constant primary / Variable Secondary” but still control sequence to be slightly modified to suite this system. I would appreciate if anybody can help me in preparing the controls sequence of operation suitable for this system design. I have also attached sequence of operation for “Constant primary / Variable secondary system which needs to be slightly modified for consideration in proposed system “Constant primary / distributed variable secondary”

Please see attached ASHRAE literature for “Constant Primary / Variable Distributed Secondary System”
 
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Do you want to install flow meters in each loop? Your sequence seems to depend a lot on flowrates. I would be caustious as they are not always accurate and can fail or read wrong. I've seen dirt stuck to them, unless you use expensive ultrasound meters.

you should talk to the controls contractor if this is all programmable.

I would highly recommend to draw in visio what you want to do. Sketch the pipes, shillers, valves, sensors etc. Add controlpoints and the sequence on the same sheet. teh controls contractors dothat for their submittals and I use visio to do that so I basically specify it in a format they use. Most designers have a schematic on plans (if at all), a separate point list, and a separate sequence. i find having it all on one sheet helps understanding and finding missing parts.

Here some resources to read on.

I'm not quirte sure what the difference between variable secondary and variable distributed secondary is.

It wasn't clear where you have the dP sensor, it saÿs at "top of riser". the dP sensor shoudl be 3/4 of the main run, or at the largest coil if a coil sticks out as significant. You also should add static pressure reset based on valve positions.

 
I do not recommend to using decoupler flow meter to control the chillers. At low flow conditions, the flow meter is not accurate. I prefer using the total load by putting a flow meter in the supply and temperature sensors in supply and return and calculate BTU load or stage and load chillers from temperature.

This system will have individual DP sensor for each distribution pump set that can be located 3/4 of the main run of that pump.
 
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