Many types of air conditioners cool the air through direct contact with refrigerant lines. However, this system configuration can be difficult to install in large buildings, especially high-rise constructions. When water is used as a heat exchange fluid between indoor air and refrigerant, heat can be removed effectively even if conditioned spaces are far from the cooling equipment.
Without using chilled water, there would only be two possible configurations for space cooling:
Installing long refrigerant lines between cooling equipment and conditioned spaces.
Circulating air through the cooling units, and then distributing air with long duct systems.
Long refrigerant lines are more exposed to physical damage and leaks, and the loss of refrigerant can be significant if the installation is large. On the other hand, air ducts are very bulky and consume more fan power as their length increases.
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Advantages of Space Cooling with Chilled Water
Water can carry heat removed from indoor spaces more effectively than air, and hydronic pipes are much more compact than air ducts. These drastic differences in system configuration are due to the physical properties of each substance:
The specific heat of water is 4.187 kJ/kg-K, while the specific heat of air is 1 kJ/kg-K. In other words, if equivalent masses of air and water experience the same temperature rise, water absorbs over four times as much heat.
Water is also much more dense than air, having a density of 1,000 kg/m3, while air has a density of 1.225 kg/m3. Water not only carries more heat per kilogram, but it also uses less space than air for a given mass.
The higher specific heat and density of water represent design advantages in chilled water systems. When chilled water is used, indoor heat can be removed with a smaller fluid mass, and hydronic piping is more compact than air ducts.
Chilled water is also more convenient when vertical distances are involved. Water can be pumped vertically without problem, but blowing cool air to upper floors is impractical and inefficient.
For a specified cooling load, a chilled water system normally provides a higher efficiency than an air conditioning system with only air ducts. This can be a significant advantage in commercial buildings, where the extra efficiency yields thousands of dollars in monthly savings.
How Chilled Water Systems Contribute to Indoor Air Quality
When a building uses only air ducts, indoor areas are interconnected. With this configuration, air pollutants and unpleasant odors can spread easily to other parts of the building. On the other hand, a chilled water system allows the use of a separate air handling system for each area, isolating air pollutants and odors. If one area has an air quality problem, areas with separate duct systems are not affected.
Chilled water systems also produce less noise than air ducts, which improves comfort for occupants. This is a significant advantage in office buildings, where constant noise and vibration from an air duct system can cause distraction and lower productivity.
Boosting the Performance of Chilled Water Systems
Although chilled water systems are efficient, their energy consumption is still significant. However, there are several possible upgrades that make chilled water systems even more efficient:
NEMA Premium Efficiency motors are highly recommended for chilled water pumps and air handlers. These motors have the highest efficiency rating in the USA.
For applications where the load is less than one horsepower, electronically commutated motors (ECM) are recommended. The NEMA Premium Efficiency rating does not cover fractional horsepower motors
High-efficiency motors can be complemented with variable frequency drives (VFD), which reduce motor speed during part-load conditions.
Consider that ECMs already have built-in speed control, and therefore a VFD is not needed.
Chilled water systems can normally be integrated with building automation platforms, which optimize their operation based on the current cooling load. Even when a chilled water system uses efficient equipment, top performance can only be achieved with the correct set points for operation.
Mechanical installations are bulky, and poor design decisions can cause a significant increase in the system cost. For example, oversized equipment and inefficient duct layouts both drive up costs without bringing benefits. Professional MEP engineers can optimize the cost of chilled water systems, while integrating energy efficiency measures such as motor speed control.
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