Chilled Water System Basics
Air Handling Units cool the air by one of two methods. One way is to blow air over a coil that has cold refrigerant in it, and the other is to blow air over a coil that has cold water in it. The first way is called a Direct Expansion (DX) system, and the second is called a Chilled Water system. The DX system is usually used for smaller applications like a small store, apartment buildings, or houses, while a Chilled Water System is designed for larger buildings like office buildings, museums, schools, even groups of buildings that can run off the same system. It is generally considered to be more efficient to pump water than it is to pump refrigerant in large buildings and that is the main determining factor in deciding which system to design with.
DX systems are simpler than Chilled Water systems and are actually at the core of Chilled Water systems. Understanding the DX loop concepts will make learning about Chilled Water Systems, and there is a tutorial on this website about DX systems if you have any questions.
Here are the general concepts:
In a basic Chilled Water System there are two to three loops that are involved. There is a Chilled Water Loop, a DX Refrigerant Loop, and sometimes a Cooling Tower Loop. There is at least one Air Handling Unit (usually many), water pumps, a chiller, and either an air cooled condenser or a water tower. Since the system is usually made for larger buildings it makes sense that the Air Handling Units (AHUs) are capable of blowing thousands of Cubic Feet Per Minute (CFM) of air, the pumps are usually capable of pumping tens if not hundreds of Gallons Per Minute (GPM) of water, and the Chillers and Condensers are capable of creating hundreds of Tons of cooling.
Chillers are one of the essential pieces of equipment in a Chilled Water system. They all work on a DX loop system where the evaporation coil cools water that passes over it. Pumps push water into the Chiller where it is cooled to about 45 degrees F and then is piped out to the air handling units to cool the air that blows into the building. When the water absorbs the heat from the air streams in the Air Handling Units it warms up, and then the water returns to the Chiller at about 60 degrees F in order to be cooled again. This loop is called the Chilled Water Loop.
Inside the Chiller is a DX Refrigerant Loop. The general pieces of equipment that make up the Chiller are the DX valve, the evaporation chamber, a compressor, and a method of condensing the refrigerant. Sometimes the Chiller will use an air cooled condenser, much like that of a regular DX unit, and sometimes the Chiller will employ a method of condensing the refrigerant with water. If the Chiller uses an air cooled condenser then usually the condenser is not far from the chiller, and usually they are mounted on the roof of the building together as one piece of equipment.
A Water Cooled Chiller rejects heat from the condensing chamber by pumping dedicated water into the condensing chamber where the water absorbs heat from the super heated refrigerant, then is piped out to a Water Cooling Tower where the hot water is exposed to the atmosphere, releasing the absorbed heat through the process of evaporation. Water usually reaches the Water Cooling Tower at a temperature of 200 degrees F, and any water that does not evaporate off collects at the bottom of the tower and has cooled to a temperature of around 150 degrees F. The cooler water is pumped back to the Chiller to absorb more heat. This is called the Water Tower Loop.
That is basically it. As a review, there is a Chilled Water loop where water gets chilled and cools the air through coils in an air handler. There is a DX loop in the Chiller which uses refrigerant to chill the water, and there is a Water Tower Loop or Air Condensing Unit that condenses the super heated refrigerant. Chilled water systems are more complicated, but they do allow for much larger HVAC system designs.