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The header shown in Figure 11B is a standard header
design through 5 tons for polyethylene pipe using 1-1/4"
supply and return runouts. Notice the reduction of pipe
from 2" IPS supply/return through circuits 12 to 8 and
then the line is reduced to 1-1/4" IPS pipe for circuits 7
to 4 and then finally the header line is reduced to 3/4"
IPS to supply circuits 3, 2, and 1. This allows minimum
pressure drop while still maintaining 2 fps velocity
throughout the header under normal flow conditions (3
gpm/ton), thus the header as shown is self-flushing under
normal flow conditions. This leaves the circuits
themselves (3/4" IPS) as the only section of the loop not
attaining 2 fps flush velocity under normal flow
conditions (3 gpm/ton & normally 3 gpm/circuit). 3/4"
IPS requires 3.8 gpm to attain 2 fps velocity therefore to
calculate flushing requirements for any PE loop using the
header styles shown simply multiply the number of
circuits by the flushing flow rate of each circuit (3.8 gpm
for 2 fps velocity). For instance on a 5 circuit loop the
flush flow rate is 5 circuits x 3.8 gpm/circuit = 19 gpm.
Headers that utilize large diameter pipe feeding the last
circuits should not be used. In PE1 1/4" IPS pipe requires
9.5 gpm to attain 2 fps and since increasing the flow
through the last circuit would also require increasing the
flow through the other circuits at an equal rate as well,
we can estimate the flush flow requirements by multiplying
the number of circuits by 9.5 gpm (in 1-1/4" IPS) or for
instance a 5 circuit loop in PE would require 5 circuits x
9.5 gpm/circuits = 47.5 gpm to attain flush flow rate.
This is clearly is an impossible flow to achieve with a
pump of any size.
Header Layout - Generally header layouts are more cost
effective with short headers. This requires centrally
Figure 12 - Typical 'Laydown' Header
2 foot wide trench
Supply
Line
Circuit 4
Circuit 4
Circuit 3
locating the header to all circuits and then bringing the
circuits to the header. One of the easiest implementations
is to angle all trenches into a common pit similar to a
starburst. This layout can utilize the laydown or 'L'
header and achieves reverse return flow by simply laying
the headers down in a mirror image and thus no extra
piping or labor. Figure 12 details a "laydown header".
Inside Piping - Polyethylene pipe provides an excellent
no leak piping material inside the building. Inside piping
fittings and elbows should be limited to prevent excessive
pressure drop. Hose kits employing 1" rubber hose
should be limited in length to 10-15 feet per run to
reduce pressure drop problems. In general 2 feet of head
pressure drop is allowed for all earth loop fittings which
would include 10-12 elbows for inside piping to the flow
controller. This allows a generous amount of maneuvering
to the flow controller with the inside piping. 3/8 to 1/2"
closed cell insulation should be used on all inside piping
where loop temperatures below 50 F are anticipated. All
barbed connections should be double clamped.
Flow Controller Selection - The pressure drop of the
entire ground loop should be estimated for the selection
of the flow controller. In general if basic loop design
rules are followed units of 3 tons or less will require only
1 circulating pump (7602-300). Units from 3.5 to 6 tons
will require a two pump system (7602-301). As a caution,
loop pressure drop calculation should be performed for
accurate flow estimation in any system including unit,
hose kit, inside piping, supply/return headers, circuit
piping, and fittings. Use Table 7A, B and C for pressure
drop calculations using methanol and various piping
materials. Tables showing other antifreezes are available
from Technical Support.
Circuit 3
Circuit 2
Circuit 2
14
Return
Line
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