Enertech VS Series - Water Flow Calculations and Selection

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Enertech Global IOM, VS/VT Models, Rev A
Water Flow Calculations and Selection
Proper ow rate is crucial for reliable operation of
geothermal heat pumps. The performance data
shows three ow rates for each entering water
temperature (EWT column). The general “rule of
thumb” when selecting ow rates is the following:
Top ow rate: Open loop systems (1.5 to 2.0 gpm
per ton)
Middle ow rate: Minimum closed loop system ow
rate (2.25 to 2.50 gpm/ton)
Bottom ow rate: Nominal (optimum) closed loop
system ow rate (3.0 gpm/ton)
Although the industry standard is adequate in most
areas of North America, it is important to consider
the application type before applying this “rule
of thumb.” Antifreeze is generally required for all
closed loop (geothermal) applications. Extreme
Southern U.S. locations are the only exception.
Open loop (well water) systems cannot use
antifreeze, and must have enough ow rate in order
to avoid freezing conditions at the Leaving Source
Water Temperature (LWT) connection.
Calculations must be made for all systems without
antifreeze to determine if the top ow rate is
adequate to prevent LWT at or near freezing
conditions. The following steps should be taken in
making this calculation:
Determine minimum EWT based upon your
geographical area.
Go to the performance data table for the heat
pump model selected and look up the the Heat of
Extraction (HE) at the “rule of thumb” water ow rate
(GPM) and at the design Entering Air Temperature
(EAT).
Calculate the temperature difference (TD) based
upon the HE and GPM of the model.
TD = HE / (GPM x 500).
Calculate the LWT.
LWT = EWT - TD.
If the LWT is below 35-38°F, there is potential
for freezing conditions if the ow rate or water
temperature is less than ideal conditions, and the
ow rate must be increased.
Example 1:
EWT = 50°F.
Model GXT048, high capacity. Flow rate = 6 GPM.
Air Flow = 1500 CFM. HE = 36,700 Btuh.
TD = 36,700 / (6 x 500) = 12.2°F
LWT = 50 - 12.2 = 37.8°F
Since the water ow is leaving at approximately
38°F, the ow rate is acceptable.
Example 2:
EWT = 40°F.
Model GXT048, high capacity. Flow rate = 6 GPM.
Air Flow = 1500 CFM. HE = 36,700 Btuh.
TD = 36,700 / (6 x 500) = 12.2°F
LWT = 40 - 12.2 = 27.8°F
Water ow rate must be increased to avoid freezing.
Performance Data Notes
1. Capacity data is based upon 15% (by volume) methanol antifreeze solution.
2. Desuperheater capacity is based upon 0.2 GPM Flow per nominal ton at 70°F entering hot water temperature.
3. Interpolation between above categories is permissible; extrapolation is not.
4. See Flow Rate Selection above for proper application.
Section 4: Unit Data Information