(GP/GT) for Additional Water Supply in the Lower Rio Grande

temperature and the heat gain curve. Proceed vertically to the
intersection with the heat input curve. From the intersecticn
proceed horizontally to the y-axis to read the heat innut
requirement in Btu/hr per square foot of sheltered pond surface.
The following example (see 60° graph) illustrates the use of the
graphs. Assume that a grower wishes to cover 5,000 ft' of pond and
maintain 60 0 p in the refuge area. The location is such that 20° can
be safely used for the design outside temperature. Based on these
factors, the heating requirement for the refuge would amount to 72
Btu/hr per square foot of fond surface under the cover. The total
requirement for 5, 000 ft would be 5, 000 ft' x 72 Btu/hr' ft Z =
360, 000 Btu/hr. As a result, the heater selected for this
application should be capable of a mini:num of 360, 000 Btu/hr
output.
I must stress that the values used to develop these graphs are
calculated heat losses. I have no direct experience with this type
of cover to use as verification of the calculations. As a result,
I have used a conservative approach to develop the numbers.
There are two considerations with regard to the use of this type
of thermal refuge which warrant emphasis.
1. When installing the cover, it is most important to keep it
above the water. Once the cover is permitted to rest on the
surface of the water, its effectiveness is severely
compromised. You may wish to consider using "floats" of some
sort (styrofoam, tire tubes, etc.) to prevent the cover from
falling onto the pond surface.
2. It is important to anticipate the need for the thermal refuge
and begin adding heat as far in advance of need as possible.
The heat loss values which appear in the graphs aSSlL"lle that
the water under the shelter is already at the required
temperature. Heating input necessary to bring the water up
from a lower temperature can be significant. Using the
example pond, and assuming an average depth of 4 ft, a total
of 150,000 gallons would be contained under the cover (5,000
ft'). To heat this water from 50° to 60° would require a total
of 12,500,000 Btu or 35 hours of continuous operation at full
heater capacity. If it will be necessary to bring the refuge
temperature up to the desired value (from a lower
temperature), this heaing load should be the basis for heater
sizing rather than the steady state approach outlined above.
B-4