Chilled water systems are mainly used in large commercial HVAC air conditioning systems and industrial cooling applications although there are some residential and light commercial HVAC chilled water systems in use. Chilled water systems are popular because they use water as a refrigerant. Water is far less expensive than refrigerant. This makes them cost effective especially in commercial HVAC air conditioning applications. Instead of running refrigerant lines over a large area of the building water pipes are ran throughout the building and to evaporator coils in air handlers for HVAC air conditioning systems. The chilled water is pumped through these pipes from a chiller where the evaporator coil absorbs heat and returns it to the chiller to reject the heat.
Set Points and Evaporator
When the water returns to the chiller it is typically 10 degrees Fahrenheit warmer than when it left the chiller. Chiller set points are usually 45 degrees Fahrenheit unless a complex control system and algorithm is used so the return water temperature is typically 55 degrees Fahrenheit unless modern computer controls are used for set point control of the chiller and the chilled water system. Once the 55 degree Fahrenheit water returns to the chiller it enters the evaporator where the heat is absorbed into the refrigerant inside the chiller evaporator barrel. The refrigerant is then pumped into the compressor where it is compressed and sent to the condenser.
Some HVAC chiller condensers have air cooled condensers while others have water cooled condenser barrels and use a water tower to remove the heat from the water which is ran through the chiller water cooled condenser barrel. Chillers which are water cooled can cost more on initial purchase and installation because a water tower must be installed and piping ran to the water tower to distribute the water which cools the condenser. Air cooled chiller condensers do not need a water tower and generally require less maintenance and upkeep than a water cooled chiller system with a water tower.
There are two main types of chillers; the compression chiller and the absorption chiller. These are the most common types of chillers in use today. Compression chillers, depending on the size and load use different types of compressors for the compression process. Semi-hermetic, screw, scroll, and centrifugal compressors are the different types of compressors used in chillers.
- The compression chiller uses a compressor in the refrigeration process and requires all the major components of the refrigeration process.
- Metering Device
- The absorption chiller does not use a compressor for the refrigeration process. These systems use either a chemical process or natural gas to raise the temperature of the refrigerant and then use another chemical process to absorb the heat from the refrigerant.
The compression chiller can use multi-compressor staging and cylinder unloading to reach higher levels of efficiency. Under low load applications a multi-compressor chiller may only run one compressor to meet demand while a single compressor chiller that has unloading capabilities may unload on cylinder to meet low demand and load up that additional cylinder in higher demand situations. This makes them more efficient than having the chiller run full blast all the time.
Chillers that are not air source chillers need a cooling tower to remove the heat from the condenser water pumped through the condenser barrel. The cooling tower for a chilled water system uses evaporative cooling to remove the heat from the condenser water. The water is channeled to the cooling tower where it is sprayed into the top of the cooling tower. The water falls down panels that have micro channels and small holes. A large fan pulls air through the panels and as the air passes over the water heat is released into the air. The remaining water that does not evaporate falls into a sump and is then pump back to the condenser barrel to remove more heat. Since some of the water in the cooling tower evaporates there is a need for make up water to keep the condenser system full of water. If the system is set up correctly a water side economizer can be set up using the cooling tower and a heat exchanger that will provide chilled water without using a chiller. The water is pumped to the cooling tower outside where the water releases heat and is returned to the heat exchanger where the cold water absorbs heat from the chilled water return. Coming out of the heat heat exchanger is a chilled water supply pipe that is delivered to the load through the chilled water supply. For this to work the outside air temperature has to be cold enough to properly cool the water plus the proper mechanical and control system needs to be set up to make this function properly. If set up properly this type of system can save a lot of energy and give the chillers a rest.