Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


What if there is no Decoupler line in CHW system?

What if there is no Decoupler line in CHW system?

What if there is no Decoupler line in CHW system?

Can anyone brief on facts, if we do not have decoupler line in any chilled water system..

RE: What if there is no Decoupler line in CHW system?

Say what?

RE: What if there is no Decoupler line in CHW system?

In which sense decoupler line helps in chilled water system of any building...

RE: What if there is no Decoupler line in CHW system?

No de-coupler, no primary secondary system, only a primary system with pumped circulation.

It means your flow through the main pumps / chillers will vary depending on the usage of the secondary system.


Pretty basic question.... A diagram would help a lot.

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

RE: What if there is no Decoupler line in CHW system?

Thank you @littleinch for your valuable response. I am going through it.

RE: What if there is no Decoupler line in CHW system?

Decoupler line or bypass line is of crucial importance in primary-secondary arrangement of chilled water systems.

In primary-secondary chilled water systems, the flow rate in the primary loop is maintained constant and the pumps (primary pumps) are NOT driven through VFDs. Assume that flow rates in this loop are reduced based on some parameter, possibilities are there for the pumps to NOT satisfy the min flow requirement of chillers. This will cause the chiller to shut down and therefore the primary loop is always equipped with constant speed pumps giving constant flow rate at all times.

Then, we have the Secondary loop or the DISTRIBUTION LOOP. This distributes chilled water to various loads per the demand. Obviously, these pumps are driven through VFDs to vary the flow based on the demand using liquid differential pressure transmitter at critical circuit(s).

The point to be noted here is both primary and secondary pumps are arranged in series. But, primary pumps are of constant flow rates and secondary pumps are of variable flow rates. Anyone now might get a question how is this even possible? They are arranged in series, where will the difference in flow rate goes? The answer is through DECOUPLER LINE which bypasses the secondary loop in the system.

The doupler line is a bypass line that is connected between the suction points of primary pumps and secondary pumps. Please note that chiller outlet point and secondary suction point are one and the same.

Consider a situation,where the system is designed for 100 GPM flow. Assume the system has a demand 50% of design. Then, the primary pump will have 100 GPM flow trough the chiller. The secondary pumps operating at lower speeds will now suck only 50 GPM for distributing to all AHUs and thereby meeting the demand. Another 50 GPM will now flow into the decoupler line at the junction in secondary suction point. Now at the other decoupler junction (primary suction point), we have two in-flows. 50 GPM from AHUs and 50 GPM from decoupler line. Thus making it to 100 GPM and completing the cycle.

So, without decoupler line, it is impossible to establish the primary-secondary arrangement. By default, it is understood that the secondary loop is of variable flow.

Hope you get it.

RE: What if there is no Decoupler line in CHW system?


Very good explanation of how a decoupled primary/secondary piping system arrange works.

However, you limited the system operation to a traditional constant primary flow (CPF) and a variable secondary flow (VSF) system.

You can also have a variable primary flow (VPF) and a variable secondary flow (VSF) decoupled system.

In the VPF/VSF system, the primary flow can be modulated to equal the secondary flow.

See October 2014 ASHRAE Journal Technical Feature "Simplified Chiller Sequencing for a Primary/Secondary Chilled Water System."

This article discusses in detail how a CPF/VSF system was successfully converted to a VPF/VSF system.

The VPF/VSF arrangement solved many issue associated with a traditional CPF/VSF system.

RE: What if there is no Decoupler line in CHW system?

Thanks for the information @LowDeltaTSolver.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close