IS YOUR ENGINEER DUMBER THAN A BAG OF ROCKS?

Is Your Engineer Dumber Than a Bag of Rocks?
By John D. Manning, PE

I recognize that this is a pretty harsh statement; after all, rocks deserve more respect. All kidding aside, there are some very serious issues that we as an industry need to discuss. My intent is to simply capture the attention of all geothermal system designers such that collectively we can deliver the best possible system design to our clients. I wish I had a nickel for every time a well driller, loop installer or a mechanical contractor called me up complaining about a geothermal loop or system design, sharing their frustration on a project where it is obvious that the design engineer did not know how to apply the geothermal technology. Their frustration becomes my frustration as it becomes clear that the best interests of the client, the geothermal industry and our community at large are not being served. 

There are many aspects of a design that could be identified and discussed that reflect a fundamental lack of understanding and failure to execute a fiduciary responsibility on the part of the design engineer. Many are worthy of discussion and may form the basis for future articles in this “Bag of Rocks” series, however this initial discussion will focus on a particular pet peeve of mine – the use of balance valves on a geothermal loop field design.

To begin this discussion, it is essential to understand that the reason our clients buy a geothermal system is not because the word “geothermal” gives them a warm fuzzy feeling, it is simply for the operating efficiency that is the implied and expected result of a geothermal system. Consequently, every aspect of a design that undermines the final delivered efficiency either through extra costs that add no value or extra costs that have a negative effect on performance, or negative value. I would characterize balance valves on a loop field as negative value.

From a technical perspective it is easy to understand why an engineer may feel that a balance valve would add value, precise control of the flow rate is a admirable quest, however, with a little analysis it can be clearly shown that there is a significant performance penalty as well as a significant first cost penalty, a double whammy that the client will pay for the life of the system.

Commercial geothermal loop fields are most commonly designed with multiple vertical loops that are piped in parallel with a buried reverse return manifold for each group of loops. The number of groups and the number of loops per group is an engineering decision that reflects an understanding of cost/performance/reliability trade-offs. For example, keeping the size of supply and return piping to each group at 2” or less has some fundamental benefits for a variety of reasons, however when the overall size of a project gets bigger there will be a benefit to keeping the number of groups down and may result in stepping up the pipe size to 3” or larger. In other words, the approach in pipe sizing and number of groups has to be determined on a job by job basis, but there are some key parameters that are essential to a good design. First, maintaining good turbulent flow in the vertical loop to ensure good heat transfer under peak load conditions (this is often cited as having a Reynolds Number greater than 2500). Second, size the horizontal piping such that it does not dictate the natural balance of the system (I have referred to this as the 70% Rule - simply having a minimum of 70% of the loop field pressure drop occur in the vertical loop, thus the horizontal piping pressure drop should not exceed 30% of the total).

There may be times when there are significant differences in the length of horizontal piping required to reach all the different groups in a loop field. In these circumstances evaluate the group that is the furthest away and size the horizontal piping to comply with the 70% rule. Subsequently, evaluate the group that has the shortest amount of horizontal piping and determine the pressure drop with the same diameter horizontal pipe as the furthest group. A by-product of the 70% rule is that we know that a worst case difference between the furthest group and the closest group can be no greater than 30% and is often much less. If indeed the difference is 30% (our worst case scenario) the size of the horizontal pipe can be used to “tune” the natural balance. This is simply done by decreasing the diameter of the horizontal pipe serving the closest groups, and through the process of selective sizing and mixing different lengths of different diameters a perfect natural balance can be achieved. This technique can be applied to achieve a perfect balance even when the various groups do not have the same number of loops. The obvious benefit to this approach is perfect balance achieved while reducing the cost of the piping. This approach is desirable more from a cost reduction perspective and not the need to have perfect flow.

Before we continue with a discussion of the valves, let’s explore what a worst case situation actually does to the performance of a loop field. By the laws of fluid mechanics a loop field will only have one pressure drop under full flow conditions, therefore we know that each group will also have that same pressure drop, and the flow rate through each group is that specific flow rate that corresponds to the overall loop field pressure drop. Specifically, the groups further away will have a lower flow rate and the closer groups will have a higher flow rate. Bear with me while I use some simple algebra to illustrate. Under the worst case scenario when we adhere to the 70% rule the furthest group will have a pressure drop of X + Y; X being the pressure drop of the horizontal piping and Y being the pressure drop of the vertical loop. We also, by design have set X = 30% of (X + Y). Under this worst case scenario, the closest group will have a pressure drop of Y at design flow rates. The next step is to determine what these design flow pressure drop differences will do to the final actual flow rates when pressure drops all become equal.

The total design flow rate for the system is dictated by the required heat pump flow rates adjusted by reasonable diversity factors and/or an acceptable flow rate required under full load, which may very well be 2 ¼ to 2 ½ gpm/ton. Determining the design system flow rate is worthy of a whole discussion in itself, suffice it to say that maximizing value & performance need to be the guiding principles, as opposed to adding the flow rates of all the heat pumps at 3 gpm/ton and not considering the impact of load diversity and the benefits of operating at 2 ¼ or 2 ½ gpm per ton. Regardless of the means to arrive at the system design flow rate QSYS-DES there will be a resulting loop flow as determined by dividing by the number of loops in the system QLOOP-DES.

Since the loops are in parallel, which are in series with the group’s horizontal piping we can focus on a single loop to determine actual flow rate, using the pressure drop equations that simply state that a pressure drop is directly related to the square of the flow rate, or conversely the flow rate is directly related to the square root of the pressure drop.

We need to assume that regardless of how the loop field balances we will still need to have the same total system design flow rate, consequently the loops in the middle of the loop field will be pretty close to our QLOOP-DES and the loops further away will be at a lower flow rate and the closest loops will be at a higher flow rate. Referring to the pressure drop discussion above we can assume these middle loops will operate at QLOOP-DES and would have a pressure drop of:

PD = .5X + Y 

The following table steps through the appropriate calculations to determine the actual flow rate between the closest loop and the furthest loop:

Worst Case Flow Balance Without Balance Valves 

This table clearly indicates that even under the worst case scenario the flow rate deviation between the loops will be about +/- 10% from the desired design flow rate.

From a heat transfer perspective, as long as our flow is turbulent the predominant resistance to heat transfer is the dirt/rock and the slight difference in forced convection heat transfer coefficients associated with different flow rates is literally trivial. This fact combined with the fact that the same temperature water will flow into each group and thus the different flow rates will only result in a slightly different temperature change and thus the average temperature in each loop may be different by fractions of a degree. These fractions of a degree will be somewhat offsetting with the loops at a higher flow rate performing 1-2% better while the loops at a lower flow rate may be 1-2% worse. The net effect in the overall performance of the loop field is immeasurable, and there are so many other variables between loops that any measured difference is probably due to the slight difference in hydrogeology, or specific positioning of the loop within the bore hole or the variation in actual batches of thermally enhanced grout, etc.

Regarding balance valves, as a side note, every job I have visited that incorporated balance valves in the loop field manifold design, the valves were all in a wide open position….not sure if everyone got the memo that these valves can only affect balance when they are actually used. Have you priced a 3” balance valve lately? Generally balance valves will have a 2 to 4 psi (4.6 to 9.2 Feet of Head) pressure drop in a wide open position and with a general accuracy of 5% it should be clear to the reader that the presence of a balance valve will do nothing more than add to the overall pressure drop for the life of the system possibly forcing the designer to select a bigger pump. By the way, this extra pumping energy will eventually turn into heat raising loop temperatures forcing the heat pumps to work harder in the air conditioning mode and essentially be warming the loop with the equivalent of electric resistance heat in the winter time. Our goal is to have loop field pressure drops in the 20 Feet of head range (+10/-5), so it is possible that balance valves could add 25 to 50% more head pressure and that will translate into a significant amount of energy over the life of the system. Just think, your client got to pay extra for this feature.

So, to all those experienced geothermal installers and designers who cringe every time they see balance valves on a geothermal loop field manifold I share your pain. You know that the rock or soil you drill through will surrender its heat without regard to the precise flow rate, unfortunately as engineers we often fall victim to the delusion of precision. Geothermal loop fields have maximum value when we can achieve required flow and heat transfer without deluding ourselves and without burdening the system with cost and performance penalties. To borrow a line from Dr. Kavanaugh "Keep it Simple Stupid".  My goal is not to offend those engineers who have chosen to incorporate balance valves in their designs but to merely open up their thinking to the possibility that such valves are not needed. I will certainly be receptive to any arguments that could justify their use as well as any other comments regarding this subject.

Please send to jmanning@earthsensitive.com or enter below.

Be well & think geo !!!