For a specified cooling load, a chilled water system normally provides 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.
Benefits of Chilled Water Air Conditioning
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.
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 denser 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.