Using Modular Chillers in Variable Flow Applications

December 1, 2014

Modular chillers offer a unique set of advantages to conventional machines - which we'll cover in a future post.  The focus here will be on incorporating a modular chiller in a variable flow system.  We will touch on the basics here.  More information can be found in the links below, or by contacting NEAP directly.  

 

Similar to a conventional single chiller Variable-Primary Flow system, you will still have VFDs on the chilled water pumps  and a minimum flow bypass.  However, because a modular chiller is more similar to multiple chillers piped in parallel, the way we control those components is slightly different.  

 

When we select a Multistack chiller as 'variable flow' we add motorized valves in each module.  As the load on the chiller decreases and compressors cycle off, these valves modulate to ensure no water is bypassing through off modules.  Because we are ultimatley changing both the number of modules that the chilled water is running through, and the minimum flow capability of the chiller as we stage, we will need to measure the differential pressure accross the chiller - and use that to help control our system.  

 

Chiller DP and system DP can then be used to control the chilled water pump speed or the system bypass (both control scenarios are discussed in the bulletin & webinar in more detail).  

 

Some important items to keep in mind:

  • System bypass valve needs to be fast-acting (<60 seconds)

  • Little to no delay with Building Management System (between bypass & pumps)

  • Minimum 2 minute loop time at all load conditions (this is ideal for all chiller applications)

    • Locating bypass near the chiller makes this very difficult

  • Proper proof of flow is very important (use thermal dispersion or DP switch in lieu of paddle type)

  • Accurate sizing of the system bypass is very important 

 

Variable Flow Bulletin

 

Variable Flow Webinar 

 

Contact NEAP with any additional questions.  

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