Chilled Water System in BMS

Chilled water is used to cool the building's air and equipment. A chilled water system consists of chillers and pumps. Analog temperature sensors measure the cooling water supply and return lines. Chillers are switched on and off to cool the chilled water supply.

A chiller is a refrigeration unit designed to produce chilled (cold) water for space cooling purposes. The cooled water is then circulated to one or more cooling coils located in air handling units, fan-coils or induction units. Cooling water distribution is not limited by the 100 foot separation limit that applies to DX systems, thus cooling systems based on chilled water are used in larger buildings. Capacity control in chilled water systems is usually achieved by modulation of water flow through the coil; Thus, multiple coils can be fed from a single chiller without compromising control of any individual unit. Chillers can work on vapor compression principle or absorption principle. Vapor compression chillers can use reciprocating, centrifugal, screw, or rotary compressor configurations. Reciprocating chillers are commonly used for capacities below 200 tons; Centrifugal chillers are generally used to provide high capacity; Rotary and screw chillers are used less frequently, but are not rare. Heat rejection from the chiller can be through air-cooled condensers or cooling towers. A vapor compression chiller can be combined with an air-cooled condenser to provide a packaged chiller, which will be installed outside the building envelope. Vapor compression chillers can also be designed to be installed separately from the condensing unit; Normally such a chiller will be installed in an enclosed central plant space. Absorption chillers are designed to be installed separately from the condensing unit.

Chilled Water Cooling System is two types

• Air cooled chiller
• Water-cooled chiller.

Air Cooled Chiller

Air-cooled chillers are almost always outside the building and remove heat from the chilled water directly into the surrounding air.

Air-cooled chillers expel heat from the condenser coil. As the warm refrigerant passes through the condenser coil, outside air flows over the condenser coil and removes heat from the refrigerant.

The refrigerant then passes through the expansion valve, where it cools rapidly, and through the evaporator, where it cools the cooled water. This process is repeated continuously.

Water Cooled Chiller

Water-cooled chillers are almost always located inside the building. They work similarly to air cooled chillers. The difference is that they remove heat from the cooling water for a second, a separate water line called the condenser water line.

Condenser water flows through the chiller and absorbs heat. The condenser water is then returned to the cooling tower. A cooling tower is almost always located outside the building and removes heat to the atmosphere by evaporating some of the condenser water.

As some of the condenser water evaporates, heat is removed from the condenser water and the cooled condenser water flows back to the chiller. This process is repeated continuously.

Chilled Water System Problems

• insufficient flow
• Dirty coils
• Condensate leakage
• High cooling water temperature
• Winter freeze-up

1. Insufficient Flow

Insufficient water flow to individual air handlers is the most common problem in chilled water systems. In a mall, for example, piping to each tenant's air handler creates a parallel path between the supply and return mains. Water will therefore follow the path of least resistance; The greater the resistance to flow in each path, the less water will flow through that path. The total resistance to flow in any path is a function of run length, piping diameter, coil resistance, and valve and fitting resistance. Balancing valves are installed on each branch to add resistance to the flow to guarantee that each branch receives the water it was designed to handle.

2. Dirty Coils

The most common problem is a dirty chilled water coil. Air handlers have a useful life of up to 30 years. Over time, chilled water coils become clogged with airborne dirt that renders them inefficient. Unlike mechanical refrigeration units, which freeze-up and leak when indoor coils are plugged, chilled water coils become less and less efficient until they can provide enough capacity to cool the space.

3. Condensate Leakage

Like most air conditioning systems, chilled water systems leave the air below the dew point to remove moisture from the air. Moisture condenses on the surface of the cooling coil and falls down the fins until it collects in a drain pan that sits below the cooling coil. It is drained from the condensate pan by gravity into an indirect waste connection. If the pan outlet or drain line is clogged with debris, the condensate will overflow the drain pan. All air handlers are installed with a secondary drain pan to catch leaks. This secondary pan should be equipped with a water detector that shuts off the unit or with its own separate drainage piping.

4. High Cooling Water Temperature

Chilled water is delivered to the piping system at a temperature generally between 42°F and 46°F. This is the water temperature that must be present at the supply connection on the air handler. In some buildings, the cooling water temperature changes with the change in outside temperature. This is called a reset. In an effort to save energy, a building can install a control system that resets the cooling water temperature upward as the outside temperature drops. The theory is that as the outside temperature drops, the need for cooling decreases, and the need for cooling can be met with either higher temperature water or less water. They can provide 42°F water at 95° outside temperature and 50° water at 65° outside temperature. Unfortunately, a 65°F day load can be as high as a 95°F day load in a retail store. 

5. Winter Freeze-up

One of the most devastating problems that can occur with chilled water air handlers is freezing of the chilled water coils. When the water in the finned coil freezes, it expands. As it expands, because water is trapped in the tubes of the coil, it expands the tubes. Tubes, limited in their ability to expand and then split. The tubes actually look like blisters, which break open. Usually, when the coil freezes, the splits are so numerous that the coil cannot be repaired and must be replaced.