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Comparison of Heat Rejection Methods

Solution

Overview
Cooling systems utilize one of three primary methods for heat rejection in the cooling
process: air cooling, water cooling, or adiabatic cooling. These methods are commonly
used to service any number of applications including dehumidifying air, fluid cooling,
and industrial applications.

Fundamental Methods
Air Cooled: A process by which air passes over a coil or channel containing fluid. Heat is transferred from the coil directly to the
air.
Water Cooled: This process utilizes a spray system to pass water over coils or fill media to reject heat to the atmosphere through
evaporation. The spray water itself or the fluid contained in the coil can then be used by a cooling system.
Adiabatic: This method is a two stage process that uses a combination of air and water to reject heat. Below a set temperature,
the process will run as dry operation. This is similar to an air cooled method where process fluid is run through a coil or micro
channel with air flowing over it. When a peak temperature is reached, the air is pre-cooled by pulling it through a pad moistened
with a small amount of water. This brings the air close to the ambient wet-bulb temperature, allowing for greater heat rejection
when it is blown over the coil. Unlike a water cooled method, the water does not flow directly over the coil.

The Value of Efficiency

 With higher performance and lower operating costs, water cooled methods offer the best long-term investment for any cooling
system. Systems that utilize a water cooled heat rejection method are approximately 35% more efficient than their air cooled
counterparts. Systems with adiabatic heat rejection operate with efficiencies mid-way between air and water cooled methods.
While not attaining the same cost savings as water cooled technology, they do offer a better investment over air cooled
technology, especially in locations where water availability is limited or where there is a large gap between the dry-bulb and wetbulb.
The savings from energy efficient cooling methods will continue to grow as time moves forward. The global demand for energy
is projected to increase nearly 50% by the year 2035.1
 For cooling systems this means the total cost of ownership will become
more dependent upon their efficiency. Installation costs, even for systems that are initially more expensive, are quickly offset by
highly efficient methods like water cooled technology, saving money in the long run.

In addition to direct energy costs, cooling system selection can be affected by many local and national entities that are
becoming increasingly focused on green programs, energy savings, and environmental responsibility. Standards such as ASHRAE
90.1, regulations such as California Title 24, and certifications such as LEED® are different facets of this movement, all of
which recognize the growing need for responsible energy consumption. Methods of heat rejection that meet these needs are
more likely to be compliant with codes, approved for job sites, and even qualify for incentives.
From project specification to daily operation, a cooling system’s efficiency will affect all aspects of a project. To get the best
value for system owners, the method of heat rejection should be carefully analyzed for the life span of the cooling system.

Installation Considerations

There are many factors that can influence the selection of a cooling system. Typically the size of the system, the required design
conditions, the operating sound level, along with the aforementioned efficiency, and price of the system all play a major role
during the decision making processes. Table 1 compares the three heat rejection methods against these criteria:

Table 1| Heat Transfer Comparision

 

Comparison Summary

Traditional systems using air cooled heat rejection will meet design specifications when absolutely no water is available;
however, these systems are quickly becoming out dated and require the most space, largest operating budget, and maintain
the highest levels of sound during operation. Adiabatic based cooling systems are a technological improvement to air cooled
methods, providing a middle ground on efficiency, sound, total cost, and ability to operate with minimal water. Systems
with water cooled heat rejection offer the best performance, smallest foot print, largest capacity, and most ideal operating
considerations. For a customized recommendation for the best heat rejection solution to meet your cooling needs contact your
local BAC Representative.

 
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Article details
Article ID: 10
Category: Articles
Date added: 2013-03-22 04:22:14
Views: 139
Rating (Votes): Article rated 3.1/5.0 (25)

 
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