Primary/Secondary chilled water system pumping - variable primary - variable secondary

[remp]

Primary/Secondary chilled water system pumping

The traditional primary/secondary pumping system is variable secondary pumps and fixed speed primary pumps with a decoupling line between
Can you have variable speed primary pumps whereby the chilled water flow rate through the evaporator varies from max-min L/s. If I have a control valve and flow meter on the chiller CHW line will this work in the overall scheme?

Did anybody ever try this?

R

 

[lukaiENG]

Variable Primary is a thing. You will want to have a minimum flow bypass in the loop somewhere to ensure that you can pass minimum flow through your chiller. The minimum flow will be dictated by the chiller manufacturer and will me ~40-50% of the chiller nominal design flow. Note, the "chiller nominal design flow" may not match your project specific design flow depending on your capacity requirement and water side dT.
I suggest doing a little more reading on the topic before diving right in.

 

[moideen]

for more info, trane has published a newsletter for variable primary system.

 

[LittleInch]

This is for heating, but I imagine its the same thing.

The difference is the buffer tank for use when loads are low and to avoid cycling the chillers.

http://jmpcoblog.com/hvac-blog/cond...iable-primary/variable-secondary-flow-systems

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

 

[LowDeltaTSolver]

Yes
I designed a Variable Primary Flow (VPF)/Variable Secondary Flow (VSF) flow system for an existing University Campus Chilled water system.

The design approach was published in ASHRAE Journal October 2014 Technical Feature "Simplified Chiller Sequencing for a Primary/Secondary Variable Chilled Water System."

The primary variable flow was controlled by only using four temperature sensors.


See attached article.

 

[moideen]

LowDeltaTSolver: Thank you, I am also very interested to retrofit the chilled water system. On printing your attached document and would discuss more. But, have you any experience for converting the constant primary-variable secondary to variable primary pumping by removing the secondary pumping and decoupler line? Thank you

 

[LowDeltaTSolver]

Variable primary pumping requires a very sophisticated and expensive controls and highly trained chiller plant operators.

Sequencing chillers on and off is very complicated with a variable primary only system

The variable primary - variable secondary flow system (VPF-VSF)system in the article will result in similar energy usage to the primary only system without expensive sensors, expensive flow elements, expensive control valve, complicated algorithms, special training for plant operators, etc

Conversion to a variable primary only system would be more costly.

See attached ASHRAE Journal Article "Primary Only vs Primary secondary variable flow systems" by Steven Taylor.

Keep in mind that Steven Taylors article does not address a VPF-VSF system.

The ASHRAE Journal October 2014 Technical Feature "Simplified Chiller Sequencing for a Primary/Secondary Variable Chilled Water System." is the only example I know of that has been published for a VPF-VSF.

The beauty of the VPF-VSF system is the simplicity of design and control when compared to a variable primary only system.

 

[quark]

Been doing variable primary for 3 years now and not as bad as it used to be. Trane readily agrees with variable primary as long as you provide 40% minimum rated flow.