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Saturday, October 19, 2013
- All loops are ¾”
- Nominal sizing is 150’/ton
- Desired flow is 3 gpm/ton
- Based on above, the nominal tonnage is 6 and the desired flow rate is 18 gpm
- Furthermore, the 200’ bore depth loops should have 4 gpm and the 100’ bore depth should have a flow of 2 gpm….. which, depending on antifreeze could open the dreaded low Reynolds # problem (<2500 span="">2500>
- Assuming pure water (unlikely) Reynolds # = 4,758
- 18% Propylene Glycol Reynolds # = 2,503
- 23% Propylene Glycol Reynolds # = 1,200
- 200’ bore = 3.4 gpm and the 100’ bore = 4.42 gpm at a pressure drop = 12.4 Feet
- This is 15.1% below design on the 200’ bores and 120.8% above design on the 100’ bore
- ((4/2.8)^2) x 2.34 x 1.192 = 5.69 Feet of Head for B&G Circuit Setter in a wide open position
Friday, August 9, 2013
By John D. Manning, PE
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:
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|
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.
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Be well & think geo !!!
Saturday, August 8, 2009
In the broad spectrum of commercial loop fields that have been designed and installed there is great diversity in what I am calling the grouping strategy. Specifically, this grouping strategy is the assembling of multiple vertical bores into a group that is connected to a reverse/return manifold and a single supply and return pipe which is then connected to a valved manifold located in either the mechanical room or a vault (the whole concept of using a vault is a subject for a separate Geo “Grey” Paper). The range of grouping that has been utilized in designs can be grouped into one of three categories:
- One Group – This approach uses a single supply and return pipe that is connected to all the vertical bores on the project either in a single reverse return strategy or even multiple reverse return manifolds that are then gathered into one major reverse return gathering manifold.
- Multiple Groups – In this approach a number of vertical bores are fed with a reverse return manifold which is connected to supply and return pipe that is then connected to a valved manifold in a mechanical room or a vault. The entire loop field is then comprised of these multiple smaller groups, all connected to the same pair of valved manifolds (one supply and one return).
- Home Runs – This concept is simply having each vertical bore be connected to a supply and return pipe that is connected to a valved manifold in a mechanical room or vault.
It is easy to visualize how each of these solutions will have a different impact in a variety of areas in the final product. Specifically, these areas would include the following:
- Material Cost – Polyethylene Pipe, Valves, Wall Penetration Seals and Antifreeze
- Installation Labor – Pipe Handling, Fusion Joints, Wall Penetrations, Manifold Assembly
- Pumping Energy – Pressure Drop
- Flow Balance – Flow Balance will impact Loop Performance or require Balance Valves
- Loop Field Reliability – Number and type of fusions and mechanical joints
- Ease of Flushing and Purging – Required Flush and Purging Apparatus
For the sake of presenting a quantitative discussion, as well as qualitative, I have elected to create a typical project scenario:
- Small – (20) 500’ vertical bores with 1 ¼” loops – Each loop will have a design flow of 10 gpm (System Flow = 200 gpm) and the loop field is located 400’ from the mechanical room. System will use 18% Propylene Glycol antifreeze.
This paper will be expanded in the future to quantify in a similar fashion the following scenarios:
- Medium – (80) 400’ vertical bores with 1 ¼” loops – Each loop will have a design flow of 8 gpm (System Flow = 640 gpm) and the loop field is 60’ from the mechanical room. System will use 10% Propylene Glycol.
- Large – (300) 200’ vertical bores with ¾” loops – Each loop will have a design flow of 3.5 gpm (System Flow = 1,050 gpm) and the field is 100’ from the mechanical room. System will use water only.
This nominal 65 Ton Loop Field is located 400’ from the mechanical room and the various grouping strategies would result in the following design details:
The table above illustrates that the selected pipe size has resulted in each design having generally a similar total pressure drop (between 31.7 and 34.6 Ft). This pressure drop is reasonable for a loop field and has the vertical loop creating the larger portion of the total pressure drop, which will enhance natural flow distribution (eliminating the need for any circuit setter valves on the manifolds). Generally speaking, pressure drop translates to operating costs as well as initial cost associated with a “stronger” pump.
Generally, the following table illustrates material cost differences for both the polyethylene pipe, valves, Metraseals and the antifreeze:
The assessment of impact on installation labor is difficult to make quantitatively, generally speaking the number of fusion joints and the amount of pipe to handle might be helpful to quantify labor, but they have severe weakness, due to the speed of fusion and overall productivity can be dealt with by “tooling up” appropriately, and it is a given that Socket Fusion Tools suitable for 2” & smaller can be purchased for significantly less then butt fusion equipment capable of fusing 6” PE Pipe. Additionally, core drilling is certainly labor intensive, but a linear relationship between number of holes to labor content is not valid, the size of the hole will also determine labor content.
Now, let’s discuss the flushing and purging requirements which can be quantified. The standard in the industry associated with the flushing and purging process is the minimum accepted velocity of 2 feet/sec. Consequently, it is simple to calculate the required flow to achieve this velocity in every section of the loop field to perform acceptable flushing and purging (the process that cleans debris and air out of the system). The following table illustrates the flow and corresponding pressure drop and the Pump Horsepower to achieve this flow and head condition:
To accomplish the flushing and purging of both the Home Run and Multiple Group Designs a standard residential Flush Cart, which has either a 1 ½ or 2 HP Pump will be acceptable. The One Group Design will require a larger Purging Pump, although 5 HP is certainly not an outrageous size and can be acquired and handled without a great deal of difficulty.
The final area of discussion is the overall loop field reliability. There is a general “fear” associated with having the source of all heating and cooling buried underground with no serviceability. This fear is often expressed as the “What if….” questions. “What if a loop fails?” “ What if a fusion joint fails?” “What if there is an earthquake?” “What if a ‘wild backhoe’ eats one of the pipes?” “What if the heat transfer were to stop?”. Granted, some of these questions may be more absurd than others, but frankly, the biggest threat to loop fields is the ‘wild backhoe’.
The second Achille’s Heal is the quality of the fusion joints. It is imperative that quality fusion joints be made, and simply put if the technician is not properly schooled in the “art” of polyethylene fusion then 1 joint in the system is too many and the long term loop field reliability is at risk. And conversely, if the technician is properly qualified to do this work then the failure rate is so incredibly small that the loop field reliability is unchanged whether there is 10 fusion joints or 1,000 fusion joints.
Another method of evaluating system reliability is by understanding what the consequences are associated with a “loop failure”. The entire loop field is essentially only required under full load, which occurs by ASHRAE definition 1% of the time. When a part of the loop field is not functioning the temperature difference required between the fluid in the pipe and the earth will adjust proportionately resulting in a different temperature entering the heat pumps then what was designed for the system. Each system and geographical location as well as any imbalance between heating and cooling will influence the degree of risk as to whether or not the actual operation of the heat pumps are at risk. Specifically, in the Northeast on a relatively small system where there is good balance between heating and cooling, the peak summer design temperatures may be 85 F, while the minimum winter temperature may be 35 F. Where the average earth temperature is 52o F this would result in a peak load temperature difference of 17 degrees in heating and a 33 degree difference in cooling. If we were to lose 25% of the loop field then the 17 degree difference would increase to 22.7 degrees resulting in a entering water temperature to the heat pump of 29.3 F under peak heating conditions. Correspondingly, in the cooling mode a 25% loss of loop field will result in a 44 degree difference under peak cooling load conditions or 96 F entering water temperature at the heat pumps. Both 29.3 F and 96 F are within the operational range of geothermal heat pumps and would result in approximately a 4-5% decrease in seasonal efficiency.
It is precisely this loss of loop field performance that makes the home run strategy seem attractive. It indeed makes a loop field more robust and reduces the system performance impact in the event that a loop or fusion joint or a wild backhoe “takes out” a loop. The Home Run approach reduces the percentage loss to a absolute minimum, the Multiple Group approach reduces the risk to a manageable level while not losing sight of the first cost challenge. Finally, the One Group approach certainly exposes the client to the total failure scenario while offering no significant cost benefits.
To summarize, the Multiple Group approach to loop field design offers well managed first costs while maintaining a very robust quality.
Obviously, geothermal loop field design is similar to any other technical design challenge, there is a fundamental art associated with balancing all the deign objectives while developing a strategy the reflects maximum value for the client. Opinions are great, they form the basis of constructive discussion, I have laid out my opinion and look forward to hearing back from those who may have a different opinion.
John D. Manning, PE
Earth Sensitive Solutions, LLC