Improving Accuracy with Ground Loop Design Flow
Proper geothermal heat exchanger design is not a simple matter.
The following piece was written by Doug Carruthers of Geo-Connections, Inc.
Systems, especially big commercial systems, have a lot of moving parts and the chassis onto which they are all strapped is flow.
Flow through the coil, flow through the ground loop, and air flow all come into play… but until LoopLink PRO, the most important flow to loop design (flow through the ground loop) has only been given a passing nod. LoopLink PRO is the first and only ground loop design software to account for flow throughout the entire project. Tracking ground loop design flow unlocks some powerful capabilities and vastly improves the accuracy of the overall design process.
Three Flow Analysis Modes
Each zone group in a PRO project must have a flow analysis mode selected. Your options are:
Peak Block (Primary-Secondary)
In this mode, LoopLink PRO will calculate the flow based on peak block loads with the interior piping flow decoupled from the ground loop design flow. In this mode, you specify a desired amount of flow per ton of load and PRO automatically calculates the flow to the field based on the peak loads of the zone group.
Primary-Secondary operation allows you to account for “load sharing”. So if, you have a space that needs to be heated while another space in the same zone group needs to be cooled, the flow needed in the ground loop is reduced.
Peak Block (Unitary)
Peak Block (Unitary) is just like Peak Block (Primary-Secondary) except, the ground loop flow is now tied to the interior piping flow. In this method, the full flow for all running heat pumps (regardless of operating mode) is pushed through the interior system and the ground loop.
In each zone and hot water system you will either attach equipment or specify a fixed amount of flow. PRO will loop through the zone group and figure out how much total flow is connected to the ground loop and adjust accordingly based on the design day part load factor (PLF).
More Than Simple Accounting
The flow analysis mode isn’t really that exciting. It is just a strategy for how PRO should keep the books when it comes to flow. The exciting part about tracking flow is in the real time calculation of the fluid properties, flow regime and subsequent film resistance in your designs.
In other software, designers specify the flow regime directly as Turbulent, Transitional or Laminar. LoopLink PRO doesn’t need to ask because it already knows the answer and automatically updates all of its calculations for film resistance based on a direct calculation of fluid properties.
So, when you change the number of bores in your loopfield, or change your anti-freeze concentrations, or adjust your operating temperatures, the new lengths already account for any changes to the circulating fluid, its properties and the flow regime.
The importance of the film resistance calculation in PRO is hard to overstate. It gets rid of a lot of the secondary calculations that designers have been running by hand or through custom spreadsheets. In doing so, it improves the accuracy and speed of system design. Plus, accounting for flow gave us the opportunity to build an automated headering system… but, that’s a topic for another day!
Enroll in Commercial Geothermal Heat Pump System Design with Dr. Charles Remund, author of the International Ground Source Heat Pump Association (IGSHPA) Design Manual, and learn how to design a ground heat exchanger for a commercial geothermal system. Participants are quizzed on key concepts and then use LoopLink Pro software to work through design and load calculation assignments.
About the author, Doug Carruthers
Doug Carruthers is primary software developer for Geo-Connections, Inc. he is responsible for the design and development of LoopLink RLC, LoopLink PRO and LoopLink GSE. He holds a Bachelor of Science degree in Mechanical Engineering from South Dakota State University as well as a Bachelor of Arts in Fine Art from Washington State University.