Chapter 2 - IS YOUR ENGINEER DUMBER THAN A BAG OF ROCKS?

In response to the comment below, I submit the following:

There are several methods that could be employed when faced with such a situation – namely (4) loops at 200’ and (1) loop at 100’.

To better define the situation we will have to make some assumptions:

• 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="">
• Assuming pure water (unlikely) Reynolds # = 4,758
• 18% Propylene Glycol Reynolds # = 2,503
• 23% Propylene Glycol Reynolds # = 1,200

Simple Approach – Just let it naturally balance

Maintaining the total flow at 18 gpm, the question boils down to what flow rates in each loop will result in an equal pressure drop across all loops.  Basing our analysis on the 18% Propylene Glycol antifreeze will allow us to use specific numbers, but will not affect the final balance, but simply the final pressure drop and potential laminar flow issues.

As a reference, the pressure drop through ¾” DR11 HDPE pipe w/ 18% Propylene Glycol at 4 gpm is 4.3 Ft of Head per 100’ of pipe and at 2 gpm is 1.27 Ft of Head per 100’.  The desired flow of 4 gpm in the 200’ bore would result in a pressure drop of 17.2 Feet (4.3 x 4), and the 100’ bore would equal 2.54 Feet (2 x 1.27).  Being that this violates the laws of nature (pressure drops MUST equal each other), the flow will decrease in the deeper loops and increase in the shallow loop until pressure drops become equal. 

Using the parabolic relationship of pressured drop versus flow rate and a quick spreadsheet tool to perform many iterations of flow variation until the following flow distribution was arrived at:

• 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

Now, if there is anyone out there that can demonstrate that this flow imbalance will have a negative impact on overall heat transfer I would welcome such proof.  So, I would suggest that even under this radical example, that we would certainly like to avoid, the resulting flow imbalance will not be a significant detriment to performance, and I would further suggest that we will eliminate the potential laminar flow issue.

Put a Balance Valve on each circuit

Let’s see what happens if we were to put a balance valve on each circuit, an approach that many engineers would immediately turn to to achieve that perfect flow scenario.  Using the commonly available B&G CB model (3/4”), which has a Cv of 2.8 in a wide open position an extra pressure drop of 5.7 Feet of Head would be imposed on all the 200’ bore circuits (the Cv of 2.8 can be used by dividing the desired flow rate of 4 into 2.8 and then squaring the result and then multiply by 2.34 (Ft of Head per psi conversion) and finally multiply by 1.192 for the 18% Propylene Glycol pressure drop factor):

• ((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

The balance valve would be used to impose a larger pressure drop on the 100’ bore to reduce flow to the design of 2 gpm but would not increase the total pressure drop of the system (it would simply raise the 100’ bore/loop pressure drop to become equal with the 200’ bore/loop pressure drop).

So, this approach would create the desired flow balance and would have a pressure drop of 22.9 Feet of Head.  This approach would also cost nearly $300 in material costs, or perhaps nearly $500 to the customer.

Use one balance valve on the 100’ bore/loop

This approach would allow the 200’ bore/loops to free flow, and achieve the 4 gpm with no additional pressure drop penalty and certainly reduce the cost to perhaps $100. 

However, I would still suggest that the $100 extra cost and the extra pumping energy (17.2 feet versus the natural balance pressure drop of 12.4 feet) cannot be justified with any significant or measurable performance improvement.

Add 200’ of ¾” pipe to the 100’bore/loop

Certainly this technique of adding more pipe in series with the 200’ feet of pipe in the 100’ bore has been used by many contractors.  Perhaps if this pipe is run up and down existing trenches to gain a small amount of extra heat transfer capacity as opposed to just coiled up in one spot in the trench there could be a reasonable argument to spend another $50 on pipe.

Regarding this method, I would simply say that this “gut feel” contractor approach is far better than the highly engineered circuit setter on each loop.

I hope this helps and so I ask again “Is your engineer dumber than a bag of rocks?”  I would also suggest that geothermal’s bad reputation of “costing too much” is often driven by insufficient understanding of the basic principles and importance of establishing the value of throwing extraneous hardware or design strategies at a fundamentally simple technology.