PreChill System: The Science

The PreChill System helps cool the air around your air conditioner to make the heat exchange process more efficient. Pricing is based upon the size of your air conditioner. Please use the contact button for a price quote, or call us at 716-245-4SHI or 855-SHI-COOL

Evaporative cooling is nature's way of cooling. Water absorbs heat when it evaporates. The amount of heat required to evaporate 1 gallon of water is more than 8000 BTUs. How much cooling that can be realized from evaporating water is dependent on how much water can be evaporated in a given time. The more water evaporated, the more the cooling effect.

Physical Principals

The physical principal behind evaporative cooling is that water needs more energy to be in a gas state then it does in a liquid state. As water evaporates from a liquid to a gas, it absorbs energy (heat) from its surrounding environment. The result is that as water evaporates, the temperature of the water decrease, the temperature of the air absorbing the water decreases, and the temperature of anything the water is touching decreases. The amount of heat absorbed in the evaporative process is 1000 BTU’s per pound of water evaporated. The more water that evaporates, the more cooling will happen.

Practical Application

There are various evaporative cooling systems, the most common of which is a misting or fogging system. This is a system where water is injected directly into the air, in fine droplets that easily evaporate, and cool the surrounding air. More advanced than misting, are evaporative coolers. Evaporative coolers use fans to move air across wet pads and are often used to cool large spaces where humidity buildup isn’t a problem.

How Much Cooling can be Realized with Evaporation?

Since cooling is dependent on how much water is evaporated, the only limit to how much cooling can be done by evaporation is how much water can be held by the air. This is dependent on the temperature of the air and the Relative Humidity of the air. The Relative Humidity is a measure of how much water has already been absorbed by the air. Air at a given temperature can only go to 100% Relative Humidity.

Bucket analogy: If air is like a bucket, the Relative Humidity is the measurement of how full of water the bucket is.

When the air has very little water, or a low Relative Humidity, evaporation is fast and easy. When the air is nearly saturated, evaporation is slow and difficult. When the air is warmer, it will also hold more water.

Bucket analogy: If air is like a bucket; then the bucket grows with temperature.

The hotter and dryer the climate, the more water can be absorbed and the greater the cooling effect of evaporation.

In the warmer and dryer climates found in the Southwest, temperature drops of 25 to 35 degrees can easily be achieved. How to best utilize this reduction in air temperature?

Allowing very humid air directly into our homes is usually not the best solution because high humidity can leave the air feel stuffy and muggy and sometimes 25 to 35 degrees of cooling still leaves the temperature too high for comfort. However, a study done by the condenser can be lowered by 25 degrees, the efficiency can be improved 35%.

The chart shows the total amount of cooling that can be achieved through water evaporation at various humidity levels. At a relative humidity of 25%, typical in the hot dry areas of the U.S. during the Summer, the average daily cooling that evaporative cooling can provide is approx. 22 degrees F., with a maximum cooling of 25 degrees F. Lower humidity levels yield a greater cooling effect.

What efficiency increase can be expected from cooling the AC condenser 25 degree F?

How many BTUs per hour does this increased efficiency represent?

One ton of AC equals 12,000 BTUs per hour, therefore a 30% rise in efficiency equals an additional 3600 BTUs of cooling with the same energy expended. A 35% increase in efficiency equals 4200 BTUs/hour.

How much water is required to provide the additional cooling for a 30% to 35% efficiency increase?

Each gallon of water that is evaporated provides 8300 BTUs of cooling. Therefore the amount of water that must be evaporated to provide the additional cooling is 0.43 gallons to 0.5 gallons per ton of AC per hour. An evaporative water system typically allows for 80% evaporation which means that for optimum cooling efficiency, a misting system must spray 0.54 to 0.62 gallons per hour per ton of AC. This amount of water mist should yield an increase in efficiency of 30% to 35%. The PreChill System nozzles emit 0.35 to 0.50 gallons per hour each at 40 to 60 psi, therefore a 5 ton AC unit requires 6 nozzles.

 

References:


Application of Water Mist Pre-cooling on the Air-cooled chillers-2009

Colorado Springs Utilities—Evaporative Pre-Cooling of Large Air-Cooled HVAC Equipment

Excerpt_from_a_1996_Texas_A

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