Most buildings have a choice between two water systems: Direct Expansion (DX) and Chilled Water (CHW) systems. DX systems are common in residential and commercial facilities but many businesses are not aware of the benefits associated with Chilled Water systems and choose to forgo the benefits provided by a CHW system.
How does a CHW System Work?
Chilled Water systems pump cool water while a Direct Expansion system uses cooled air. However, a CHW system distributes cooled refrigerant throughout the entire building to ensure a consistent thermal envelope, unlike a DX system. The refrigerant in a CHW system is continuously transformed from a liquid, to a vapor, and back again to cool down the water passing through an evaporator.
Unlike the centralized cooling unit in a Direct Expansion system, a Chilled Water system distributes the cooling fluid between the chillers and the load terminals throughout the building to maintain consistent temperatures. Cool air moves between occupied spaces by terminal devices throughout the building or through coils in air handling units. Automatic valves at these terminal devices ensure air temperature control and the heat absorbed by the water is transferred outside through a cooling tower.
The Benefits of a CHW System
Based on installation costs, a Direct Expansion system may seem like the easier choice. However, as mentioned, a DX system uses refrigerant to cool the air closest to the central unit. The further from this central unit the water moves the less effective it becomes at cooling the building. Direct Expansion systems will struggle to handle larger buildings and vertical spaces, making a Chilled Water system more effective over a longer period.
Additionally water is more effective in absorbing heat and takes up less space than air for a given mass. If equivalent masses of air and water absorbed the same amount of heat, water would absorb four times as much. This enables water to remove internal heat faster with a smaller fluid mass due to hydronic piping being more compact than air ducts. This reduces the amount of refrigerant needed to cool the system reducing long-term costs.
Chilled Water Systems have significantly longer lifespans than Direct Expansion systems due in part to the usage of water but primarily because the components involved are located in safe, dry locations and not on the exterior of the building, thereby limiting possible damage from the elements. Alone this extends the lifespan of the system but combined with the reduced stress on components and lack of moving parts a CHW system can outlast a DX system quite easily.
Ultimately, a Chilled Water system not only reduces energy consumption and costs for the building but it ensures a consistent temperature throughout any size of building, thereby protecting the people inside. The components are less likely to deteriorate as quickly as a Direct Expansion system and reduce noise during operation which is incredibly valuable for buildings like hospitals and laboratories. While it might cost more up front, a Chilled Water System provides incredible benefits and will always be an asset.