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GEOTHERMAL CLOSED LOOP DESIGN
Closed Loop Basics
Closed Loop Earth Coupled Heat Pump systems are
commonly installed in one of three configurations:
horizontal, vertical, and pond loop. Each configuration
provides the benefit of using the moderate temperatures
of the earth as a heat source/heat sink. Piping
configurations can be either series or parallel.
Series piping configurations typically use 1-1/4", 1-1/2"
or 2" pipe. Parallel piping configurations typically use 3/4"
or 1" pipe for loops and 1-1/4", 1-1/2" or 2" pipe for
headers and service lines. Parallel configurations require
headers to be either "closed-coupled" short headers or
reverse return design.
Select the installation configuration which provides the
most cost effective method of installation after considering
all application constraints.
Loop design takes into account two basic factors. The
first is accurately engineered system to function properly
with low pumping requirement and adequate heat transfer
to handle the load of the structure. The second is to design
a loop with the lowest installed cost while still
maintaining a high level of quality. These factors have
been taken into account in all of the loop designs
presented in this manual.
In general terms all loop lengths have been sized by loop
sizing software so that every loop has approximately the
same operating costs. In other words, at the end of the
year the homeowner would have paid approximately the
same amount of money for heating, cooling, and hot
water no matter which loop type was installed. This
leaves the installed cost of the loop as the main factor for
determining the system payback. Therefore this analysis
says "install the most economical system possible given
the installation requirements".
Pipe Fusion Methods
Two basic types of pipe joining methods are available for
earth coupled applications. Polyethylene pipe can be
socket fused or butt fused. In both processes the pipe is
actually melted together to form a joint that is even
stronger than the original pipe. Although when either
procedure is performed properly the joint will be stronger
than the pipe wall, we prefers socket fusion in the fusion
of 2" pipe or less because of the following:
•
Allowable tolerance of mating the pipe is much
greater in socket fusion. According to general fusion
guidelines, a 3/4" SDR11 butt fusion joint alignment
can be off no more than 10% of the wall thickness
(0.01 in.). A hundredth of an inch accuracy while
fusing in a difficult position can be almost
impossible to attain in the field.
•
The actual socket fusion joint is 3 to 4 times the
cross sectional area of its butt fusion counterpart in
sizes under 2" and therefore tends to be more
forgiving of operator skill.
•
Joints are frequently required in difficult trench
connections and the smaller socket fusion iron is
more mobile and operators will have less of a
tendency to cut corners during the fusion procedure
such as can happen during the facing and alignment
procedure of butt fusion.
In general socket fusion loses these advantages in fusion
joints larger than 2" and of course socket fittings become
very expensive and time consuming in these larger sizes
as well. Therefore butt fusion is general used in sizes
larger than 2". In either joining method proper technique
is essential for long lasting joints. All supplied pipe and
fittings are IGSHPA approved. All fusion joints should
be performed by certified fusion technicians. Table 6
illustrates the proper fusion times for the Geothermal PE
Pipe.
Table 6 - Fusion Times for Polyethylene Pipe
Pipe Size
Socket Fusion
Time (sec.)-
3/4" IPS
8-10
1" IPS
10-14
1-1/4" IPS
12-15
1-1/2" IPS
15-18
2" IPS
18-22
Holding time of 60 sec.
Cure time of 20 min.
Always use a timing device.
Parallel vs. Series Configurations
Initially, loops were all designed using series style flow
due to the lack of fusion fittings needed in parallel
systems. This resulted in large diameter pipe (>1 1/4")
being used to reduce pumping requirements due to the
increased pressure drop of the pipe. Since the fusion
fittings have become available parallel flow using (3/4"
IPS) for loops 2 ton and above has become the standard
for a number of reasons.
•
Cost of Pipe - the larger diameter (>1 1/4") pipe is
twice the cost of the smaller (3/4" IPS) pipe however
the heat transfer capability due to the reduced
surface area of the smaller pipe is only decreased by
approximately 10-20%. In loop designs using the
smaller pipe the pipe length is simply lengthened to
compensate for the small heat transfer reduction,
however, it still results in around 50% savings in
pipe costs over the larger pipe in series. In some
areas 1 1/4" vertical bores can be more cost
effective, where drilling costs are high.
•
Pumping power - parallel systems generally can
have much lower pressure drop and thus smaller
pumps due to the multiple flow paths of smaller
pipes in parallel.
12
Butt Fusion
Time (sec.)
Bead (in.)
8
1/16
12
1/16
15
1/16-1/8
15
1/16-1/8
18
1/8
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