System information

Carrier Geothermal Heat Pump Systems
23
Residential Products Technical Guide
Figure 4a: Typical Header Through 15 Tons
In smaller loops of two tons [7 kW] or less, the reasons for using
parallel loops as listed above may be less obvious. In these cases,
series loops can have some additional advantages:
• No header - fi ttings tend to be more expensive and require
extra labor and skill to install.
• Simple design - no confusing piping arrangement for easier
installation by less experienced installers.
Parallel Loop Design
Loop Confi guration - Determining the style of loop primarily
depends on lot (yard) size and excavation costs. For instance, a
horizontal 1 pipe loop will have signifi cantly (400%) more trench
than a horizontal 6 pipe loop. However, the 6 pipe will have about
75% more feet of pipe. Therefore, if trenching costs are higher than
the extra pipe costs, the 6 pipe loop is the best choice. Remember
that labor is also a factor in loop costs. The 6 pipe loop could also be
chosen because of the small available space. Generally a contractor
will know after a few installations which confi guration is the most
cost effective for a given area. This information can be applied to
later installations for a more overall cost effective installation for the
particular area. Depth of the loop in horizontal systems generally
does not exceed 5 feet [1.5 meters] because of trench safety
issues and the sheer amount of soil required to move. In vertical
systems economic depth due to escalating drilling costs in rock can
sometimes require what is referred to as a parallel-series loop. That
is, a circuit will loop down and up through two or more consecutive
bores (series) to total the required circuit length. Moisture content
and soil types also effect the earth loop heat exchanger design.
Damp or saturated soil types will result in shorter loop circuits than
dry soil or sand.
Loop Circuiting - Loops should be designed with a compromise
between pressure drop and turbulent fl ow (Reynold’s Number) in
the heat exchange pipe for heat transfer. Therefore the following
rules should be observed when designing a loop:
• 3 gpm per ton [3.23 l/m per kW] fl ow rate (2.25 gpm per ton
[2.41 l/m per kW] minimum). In larger systems 2.5 to 2.7 gpm
per ton [2.41 to 2.90 l/m per kW] is adequate in most cases.
Selecting pumps to attain exactly 3 gpm per ton [3.23 l/m
per kW] is generally not cost effective from an operating cost
standpoint.
• One circuit per nominal equipment ton [3.5 kW] with
3/4” IPS and 1” IPS circuit per ton [3.5 kW]. This rule can be
deviated by one circuit or so for different loop confi gurations.
Header Design
Headers for parallel loops should be designed with two factors in
mind, the fi rst is pressure drop, and the second is ability to purge
all of the air from the system (“fl ushability”). The header shown in
Figure 4A is a standard header design through 15 tons [52.8 kW]
for polyethylene pipe with 2” supply and return runouts. The header
shown in Figure 4B is a standard header design through 5 tons [17.6
kW] for polyethylene pipe using 1-1/4” supply and return runouts.
Notice the reduction of pipe from 2” IPS supply/return circuits 15 to
8 to 1-1/4” IPS pipe for circuits 7 to 4 to 3/4” IPS to supply circuits
3, 2, and 1. This allows minimum pressure drop while still maintaining
2 fps [0.6 m/s] velocity throughout the header under normal fl ow
conditions (3 gpm/ton [3.23 l/m per kW]), thus the header as shown
is self-fl ushing under normal fl ow conditions. This leaves the circuits
themselves (3/4” IPS) as the only section of the loop not attaining 2
fps [0.6 m/s] fl ush velocity under normal fl ow conditions (3 gpm per
ton [3.23 l/m per kW], normally 3 gpm [11.4 l/m] per circuit). Pipe
diameter 3/4” IPS requires 3.8 gpm [14.4 l/m] to attain 2 fps [0.6 m/s]
velocity. Therefore, to calculate fl ushing requirements for any PE loop
using the header styles shown, simply multiply the number of circuits
by the fl ushing fl ow rate of each circuit (3.8 gpm for 2 fps velocity
[14.4 l/m for 0.6 m/s]). For instance, on a 5 circuit loop, the fl ush fl ow
rate is 5 circuits x 3.8 gpm/circuit = 19 gpm [5 circuits x 14.4 l/m per
circuit = 72 l/m or 1.2 l/s].
NOTICE: Whenever designing an earth loop heat exchanger, always
assume the worst case, soil and moisture conditions at the job site in
the fi nal design. In other words, if part of the loop fi eld is saturated
clay, and the remainder is damp clay, assume damp clay for design
criteria.
Closed Loop Design/Installation Guidelines