Summary Panel Results

A designer needs to think about three primary goals when engineering a piping system. These are:

1) The contractor/installer must be able to purge/remove air from the piping system

2) Each circuit in the system must be able to transfer energy between the circulating fluid and the ground

3) The circulation pumps must be sized efficiently to not waste energy and create "parasitic" cooling loads.

The GLD Piping System Builder and Computational Fluid Dynamics modules help to ensure that the design achieves all three goals.

 

GOAL 1: Purging

Purge pump requirements are displayed at the bottom of the Overview Tabbed Panel. If the purging flow rate and/or purging pressure drop are too high, installers will not be able to purge the supply-return runout pairs and circuits. Please be sure that your design has a purging flow rate and purging pressure drop that can be handled by available purge pump equipment. If you have questions, contact your local installation resources.

GLD performs all purge calculations with water (systems are always purged with water). Furthermore if a design has balancing valves included, note that purging calculations are performed assuming that the balancing valves are all fully open.

In general, it is extremely difficult to purge a system that requires a flow rate greater than 150gpm and/or a head loss greater than 100 ft. hd. If your system has purge requirements such as these, it may be worthwhile redesigning the system in the Piping System Builder (larger diameter supply-return runouts, fewer circuits per supply return runout, etc. will help). The system below, for example, will be impossible to purge.

 

GOAL 2: Energy Transfer

To efficiently transfer heat energy between the circulating fluid and the ground, Reynolds numbers must be above a certain minimum value. Typically, designers aim for turbulent flow Reynolds numbers during peak load conditions. If Reynolds numbers are too low, system performance may suffer severely. Conversely, if Reynolds numbers are too high, circulating pumps may be oversized, resulting in much wasted operational energy and excessive operational costs.

The Reynolds number range for a system is reported in the Operational Performance section on the Overview tab. Range is defined as the maximum and minimum Reynolds numbers across the circuits in an entire system.

In addition to looking for Reynolds numbers that provide turbulent flow, a designer should also be interested in the spread of the Reynolds number range. In general, the tighter the range the better. For example, a range of say 3255 - 3511 is more optimal than a range of 2855 - 4211. The system below has appropriate Reynolds numbers and a beautiful Reynolds number range.

The system below has a wide Reynolds number range that should be tightened up, if possible.

While a number of design decisions can lead to a wide Reynolds number range, two of the most common factors include external imbalance and internal imbalance.

External Imbalance

External imbalance occurs when a system has multiple modules with different supply-return runout lengths. The image below, from the Piping System Builder, shows six modules (each with 10 circuits) with a wide range in supply-return runout lengths. Such a system suffers from signficant external imbalance.

The image below shows the Reynolds number range for the above externally imbalanced system.

A common solution for resolving external imbalance is to use balancing valves. The Piping System Builder has an "add balancing valves" button on the Manifolds tab. When this box is checked and a system is built in the Piping Module, the designer will have the option of pushing the "BV" (balancing valve) button, which automatically determines the optimal balancing valve values. With balancing valves, the Reynolds number range will tighten up. This can be seen in the image below, which has a tight Reynolds number range but still suffers from high absolute Reynolds number values (which could be lowered by adjusting down the system flow rate).

A less common, but more elegant, solution is to manually adjust the supply-return runout pipe diameters in the Piping Module to improve flow balance across GHX modules.

Internal Imbalance

Internal imbalance occurs when there are too many circuits on a single supply return runout. Internal imbalance can lead to significant performance variation across circuits in a single GHX Module. This is because flow rates are not equal across circuits in a module. The greater the number of circuits per module, the greater the internal imbalance. Please note that traditional design methods and materials, including simplistic software tools and spreadsheet tools, indicate that flow rates are equal across circuits in a module. This is wrong and can lead to system performance problems.

As can be seen in the image below, taken from the Piping System Builder, the module has 20 circuits on a 2 in supply return runout. Such a system has significant internal imbalance.

The image below shows the wide Reynolds number range for the internally imbalanced system.

Common solutions to internal imbalance include reducing the number of circuits on each supply-return runout pair and/or increasing the supply-return pipe diameter.

 

GOAL 3: Circulation Pump Sizing

An undersized circulation pump will not provide the Reynolds numbers necessary to achieve Goal 2. An oversized circulation pump will waste a lot of energy. In general, as of early 2016, many systems use significantly oversized circulation pumps.

The circulation pump power for a system is reported in the Power Performance section on the Overview tab. If the design has an associated Peak Load entered for a design, the Piping Module will also report a Pumping Power Score at the top of the Overview tab. The lower the score, the better. And of course, the more stars, the better as well.

As can be seen in the image below, this 1200 kBtu/hr (100 ton peak system) with 60 400 ft deep boreholes, requires only a 1.03 hP circulation pump, earning a Pumping Power Score of 1.0 (the lower the better) and five stars.

If you have more detailed questions about piping design, we encourage you to experiment with different designs in the Piping System Builder and see how the different design decisions impact System results. Since the design process is fast and easy with these tools, they can act as a superb learning resource. If you are interested in expert training on these tools and the design process, please contact us.

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