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Cost of evaporative heat removal.


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At present operations, afternoon water temperatures have reached 84 degrees in the 1200 gallons of grow out tanks under greenhouse with 60% shade cloth. A 1/2 HP rotary blower moves 60 CFM with much circulation and some passive cooling. I also have the equivalent of 1000' tube and shell geothermal cooling from 78 degree Aquifier water which amounts to 3/4 HP of refrigeration cooling. Due to 30" depth of the eight 150G Rubbermaid tanks, I expect water temperatures to continue to increase. Last summer each 150G tank had a 20" box fan to maintain 82.0 degrees. I will duplicate this today with #8 tank. In so doing, I will set up a comparison of electrical cost comparing refrigerate cooling, evaporative cooling and geothermal cooling.

I consider geothermal to be a back-up method. Due to the warm temperature of Middle Trinity at 78 degrees, the temperature difference of the water drives the heat transfer. At a system temperature of 82 degrees, the cooling water heat gain was 1.4 degrees at 5GPM.. When I pay to pump that water it cost me $0.21 per hour compared to $.05 per hour for refrigeration heat removal cost. Yes, I could slow down the cooling water flow rate to reduce pump cost and increase temperature difference in cooling water. This also reduces total heat removed by cooling water unless the tank water temperature increases, not desirable.

I started the test at 0940 hrs with #8 tank isolated from system circulation. Air bubbles will maintain circulation. Tank water temperature was 77.3 degrees, when 20" box fan was turned on. Fan is 6" from water surface blowing at an angle of 90 degrees.

To get accurate results, this will have to be done when the sun is not contributing heat into tank. I will do these test again when the sun goes down. Perhaps a Jaccuzzi pool party.

La bonne temps roulee,

Patrick

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It is noon and system water temperature is 81.5 degrees. It is noon with an ambient air temperature in the greenhouse of 96 degrees and a humidity of 35%. Too hot to stay. I have brought 5 GPM of geothermal cooling on line. It is going to be hot today. I think inside with the AC is my place this afternoon.

Let it be cool.

Patrick

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I have added a high flow degassing column to the equation. I installed Sweetwater hi efficiency pump with 2" piping in and out. At this small head, I am pumping 50 GPM. Each system consist of four 150G Rubbermaid tanks. Each system receives air uplift for circulation and each system has 500' of 1/2" tube and shell heat exchanger using 2.5 GPM of Aquifier well water at a temperature of 78 degrees. System #2 is 83.8 degrees at 1800 hrs. System #1 is 2 degrees warmer at 85.8 degrees. High flow degassing columns promote evaporative cooling.

When the sun goes down, I will quantify BTU removed and power consumed.

Patrick

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In 20 minutes 100 gallons of water increased 2 degrees. That is more than 5 tons of AC at 50K BTU per Hr. I was dumb until I considered air temperature of 90 degrees. Summer operations require air temperature at 80.0 are lower. This was most important to find out.

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When you figure out how to keep the air temperature under 80 in Austin summers you will be a very rich man. smile.png.

The equipment that I was using was a downflow degassing column. It will be used at nighttime to remove carbon dioxide and replenish oxygen. I had planned on using it 24/7 until this heat increase of water resulted in cooler air. I bet the 90 degree air was reduced to below 80 degrees. Will I get rich with that? A conventional swap cooler will reduce humid air by 15 degrees and dry air by 20 degrees. The simple 20"box fan at 6" above water surface cooled 150 gallons from 82.9 degrees at 9:36 PM to 80.3 at 11:23 PM. That is 17K BTU per Hr or 1.5 HP of refrigeration. A 1.5HP load is 1KW per Hr compared to 85W of fan.

I proved what I needed to know.

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Between 7PM Sunday and 7AM Monday four 20" box fans cooled 1000 gallons of salt water from 82.3 degrees to 77.8 degrees.

An energy calculation was done with one tank and one fan. From 9:30PM to 11:30 PM 150 gallons of water was cooled 2.7 degrees.

Btu equals thermal mass times delta temperature or 1275 lbs times 2.7 degrees equals 3440 btu in 2 hours. A ton of AC equals 1200 BTU per hour. Considering 1720 BTU per hour as cooled, a 85W fan cooled the equivalent of 1000W of refrigeration.

Il fait chaud.

La bonne temps roulee,

Patrick

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Between 7PM Sunday and 7AM Monday four 20" box fans cooled 1000 gallons of salt water from 82.3 degrees to 77.8 degrees.

An energy calculation was done with one tank and one fan. From 9:30PM to 11:30 PM 150 gallons of water was cooled 2.7 degrees.

Btu equals thermal mass times delta temperature or 1275 lbs times 2.7 degrees equals 3440 btu in 2 hours. A ton of AC equals 1200 BTU per hour. Considering 1720 BTU per hour as cooled, a 85W fan cooled the equivalent of 1000W of refrigeration.

Il fait chaud.

La bonne temps roulee,

Patrick

Your last line says one ton is 1200 BTU, 1 ton of AC is 12,000 BTU 1,000 WAtt HPS/Mh lights will produce 4,000 BTU per hour including radiant heat, excluding ballast heat.

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Chris,

Considering my ten fold error in thermodynamics, I must correct the efficiency of 20" box fan. It is only 30% more efficient then AC with electricity and at about 1% of the capital cost.

Patrick

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At 3PM, daytime temperatures are soaring. Outside PAR values read 2000. Greenhouse ambient air temperature is 102 degrees. System water temperature reads 78.9 degrees. I have initiated 5GPM geothermal cooling water flow. Today, I will irrigate much.

La bonne temps roulee,

Patrick

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At 3:30PM, the greenhouse air temperature is 108 degrees. System water temperature is 79.0 degrees. With low humidity there is much evaporative cooling.

The normal heating/cooling for summer operations outdoors will be to run evaporative cooling fans 24/7. During the day heating/humidity cycle, the morning has the lowest air temperature with the highest humidity. As day time temperatures heat up with increasing air temperature and decreasing humidity, evaporative cooling is the most effective because the air is the hungriest for water vapor. Geothermal water to water is driven by temperature difference between ground water and system water temperature. Ground water comes in at 78.0 degrees. The higher the system water temperature the greater the BTU transfer to the cooling water.

I consider 85.0 to be my high temperature threshold for continual operation. At 82.0 degrees, I will control hi temperature with geothermal cooling. The water used to remove this heat goes into vegetable and ornamental gardens. I also have a 32" high dam that is 40' long. This will be designated Bear Pond or perhaps Bare Pond.

Go figure,

Patrick

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