For large buildings a central plant system is generally used. In such a plant, the conditioned air is kept at a central source and is distributed using a network of ducts.
In larger and especially multi-storey buildings, the length of pipes and ducts becomes unmanageable. In such situations, we can consider using a chilled water distribution system. These systems are especially useful where control in individual rooms is required. These are commonly used in commercial and industrial buildings.
In this section we will discuss the operation of such a system.
The Basic Principle
The basic principle is that chilled water from a source is sent through the pipe or duct work. This chilled water takes heat away from the spaces and return to the source with an increased temperature. It is like a loop starting with ‘cold’ or chilled water and returning with ‘warm’ water.
What are the components of this system? The water gets chilled through Chillers, moves through pipes called chiller water supply system and enters into the a heat transfer device called a Heat Exchanger. Heat is transferred to the chilled water as it goes through the heat exchanger. Water is then returned to the chiller for re-cooling.
Chilled water systems are also known as Hydronic systems. We will discuss the components below.
A chiller removes heat from a liquid. In its simple form, it consists of an evaporator, a condenser, a compressor and a device to control the flow. A refrigerant flows through evaporator and condenser with the help of a compressor resulting in chilled water at temperatures between 4°C and 8°C. Chillers are of different types based upon the type of compressor used in them. These normally are packaged to lower cost and for ease of installation.
Absorption chillers use a heat source to produce chilled water. The cooling effect occurs when refrigerant evaporates thereby removing heat. These use less electricity and have low noise.
Absorption chillers are generally classified as ‘direct-fired’ or ‘indirect-fired’. In direct fired units, the heat source can be gas or some other fuel that is burned in the unit. Indirect-fired units use steam that brings in heat from a separate source, such as a boiler or heat recovered from combined heat and power (CHP).
Air-cooled chillers utilise air to cool heat rejection coils. Compared to water cooled chillers, air-cooled chillers are easier to maintain, require more space, and do not require a dedicated water supply. They are, however, generally less energy efficient than water-cooled units. Water-cooled chillers Water-cooled chillers are used where a high cooling demand exists, such as large commercial and industrial buildings. Centrifugal pumps.It distributes the chilled water through the system. Centrifugal pumps can be of different types. Their basic principle of working is that they increase pressure of the liquid entering them through a rotating component called an impeller.
In a heat exchanger the heat energy is transferred from one fluid to the other. This can be achieved by mixing the hot and cold fluids together although the heat transfer can still take place even if they are not ‘mixed’. Common types are flat plate, shell-and-tube and cross-flow.
Other important components include: Strainers which trap particles from fluid flow; valves which regulate flow and help to isolate the network for maintenance; and pipes.
The publications listed contain a more detailed explanation of chilled water distribution systems. You are advised to read these before attempting the tasks.
Some useful websites and video resources are listed with self explanatory titles which will help you understand the concepts.