Chilled water systems work much the same way as direct expansion systems work except they used chilled water in the coil rather than refrigerant (Technically speaking, water can be classified as a refrigerant). Chilled Water systems can be rather complex and many chilled water systems are found in commercial and industrial applications. There are some chilled water systems used in residential applications. However, chilled water systems in residential HVAC systems are extremely rare. A typical chiller uses the process of refrigeration to chill water in a chiller barrel. This water is pumped through chilled water piping throughout the building where it will pass through a coil. Air is passed over this coil and the heat exchange process takes place. The heat in the air is absorbed into the coils and then into the water. The water is pumped back to the chiller to have the heat removed and make the trip back through the building and the coils all over again.
How a Chilled Water System Works
The chiller basically removes heat from the water so that it may used as a refrigerant to remove heat from the building. The chilled water circulates through a chilled water loop and through coils located in air handlers and other HVAC equipment designed to exchange heat such as computer room air conditioners or other specialized equipment. The chilled water absorbs the heat from the building and returns to the chiller where the chiller removes the heat from the water using the refrigeration process. Some chilled water loop arrangements are very complex while others are simple. Control of the chilled water from pressure. to velocity, to volume is up to the control system controlling the pumps and valve actuators in the system. Chillers range in size from smaller than 5 tons all the
way up to several hundred tons and can be found in residential applications, commercial buildings, and industrial process applications.
Chillers and Cooling Towers
Many chillers have cooling towers where the heat removed in the chiller barrel is transferred to another barrel. It is the condenser barrel where the refrigerant is condensed and sent back to the evaporator barrel to remove the heat. The process is in reverse in the condenser barrel. The water absorbs heat from the refrigerant and allows it to condense.
The water is then transferred to a cooling tower where the heat in this water is removed to the atmosphere. Once the heat is removed from the water it is pumped back to the chiller barrel to absorb more heat from the refrigerant. Some chillers do not have a condenser barrel to remove the heat. The refrigerant is pumped into a condenser coil where a fan blows across the coil and removes the heat. These chillers are cheaper to purchase upfront but not as efficient to run as those with the condenser barrels. The upfront costs are less but the energy costs are more over the long run. One bonus to an air cooled chiller is that it does not require a cooling tower and therefore the maintenance costs associated with maintaining a cooling tower.
Chilled Water and Direct Expansion
Chilled water systems provide comfort to mainly commercial buildings and are typically cheaper to operate than Direct Expansion (DX)systems. While many buildings have DX systems in the way of large roof top units, the cost of installation is generally cheap for the DX systems because all that is required to install them is duct work and electrical service to the unit. With chilled water systems, chilled water piping must be installed throughout the building and this can be far more expensive to install over the plain old DX roof top units which supply conditioned air to a VAV system that has electric reheat in them. Of course, adding state of the art controls like direct digital controls can improve any system and take energy management to a whole new level of control and savings. So whether your building utilizes chilled water or DX systems it will save energy dollars by adding a state of the control system like DDC or direct digital controls to make it work as efficiently as possible. In the picture to the left, a chilled water pipe had to be relocated. The gate valve that shut the water off to that section of the loop was broken and could not be closed. To prevent water from flooding all over the floor and in the space where the relocation of the chilled water piping was necessary, a piping freeze machine was used to stop the flow of water so that the chilled water pipe could be cut and re-soldered to accommodate its new location. Pipe freezing also allows the new piping to be soldered into place because it prevents water flow inside the pipe. Copper pipe cannot be soldered if it has water inside it. For years plumbers and HVAC technicians used bread or a mechanical stop to keep water out of the pipe so it can be soldered. The bread, rolled into a ball and inserted into the pipe, would prevent the water from forming around the solder joint allowing it to soldered. Later, when full flow is restored, the bread breaks up and is caught in a strainer where is can be removed from the system. With the freeze machine, once the water thaws, there is no bread to contaminate the system or plug the strainers.
High Performance HVAC: Your Resource for Chilled Water Systems Information