The heart of any hydronic HVAC system is the central plant equipment. On most projects, the primary heating and cooling devices are situated in one location and consist of boilers, chillers, piping, pumps, delivery components (also called terminal devices), and accessories.
The boiler is the system component that produces heat in the form of hot water or steam for distribution throughout the system. Boilers may use a variety of fuel sources, including oil, natural gas, electricity, and geothermal energy. Three categories of boilers are commonly used in commercial heating/cooling applications: fire-tube boilers, water-tube boilers, and cast-iron boilers. In a fire-tube boiler, gases heated by combustion pass through tubes immersed in a water-filled chamber where heat is transferred to produce steam. Fire-tube boilers are most appropriate for large buildings that require steam for process or heating loads. In a water-tube boiler, water passes through tubes that absorb heat directly from combustion gasses outside the tubes. This flexible type boiler is used mostly for hydronic heating systems in medium-sized buildings. Cast-iron boilers, long the standard in many hydronic heating systems, offer installation flexibility. These can be shipped “packaged” as a fully assembled unit, or “knocked down” for applications where an existing building opening is too small to accommodate a factory-assembled unit. The many pieces of the boiler are then assembled on site into a complete, functioning boiler. The combustion efficiency of most boiler types traditionally has been limited to around 80 percent. However, newer, high-efficiency (modulating-condensing) boilers can achieve up to 95 percent efficiency by condensing the flue gases to recover additional energy.
For cooling, the central plant equipment that provides air conditioning to a large building is usually a water- or air-cooled chiller. Both types use a compressor—reciprocating, centrifugal, or rotary—to create the refrigeration effect. The difference between the two types of chillers lies in the way in which they reject heat. A water-cooled chiller has its condensing system connected to a cooling tower for heat rejection. An air- cooled chiller uses fans to pull ambient air through the condensing coil to eliminate heat. Cooling towers are used to remove or reject heat from water-cooled chillers. The two most common types for HVAC applications are the induced-draft cooling tower, which uses the cross-flow process, and the forced-draft cooling tower, which uses counterflow. In a cross-flow, induced-draft cooling tower, an upward-facing fan pulls air in from the sides of the tower and rejects it out the top. Water flows from the top of the tower down and across the air in cross-flow fashion, thereby cooling it. Counterflow towers have a forced-draft fan that blows air in at the bottom and dis-charges it up and out the top. Water is sprayed down on the medium, causing the water to be cooled.
Boilers and chillers are connected to heating and cooling devices throughout the building with piping systems. There are several commonly used distribution piping arrangements. Hot and chilled water is circulated from the central plant to the building using any of a broad variety of types of pumps ranging from wet-rotor circulators and in-line pumps to vertical and horizontal split-case pumps, multistage pumps, and base-mounted/close-coupled pumps.
There are numerous types of pumps commonly used in hydronic HVAC systems, each with its own characteristics and each type best suited for particular applications. Base-mounted pumps, for example, have been used for larger buildings; whereas inline have been used for smaller buildings. However, larger vertical inline pumps are now being used for larger buildings. They have the advantage of a smaller footprint and simpler maintenance when changing seals. Wet-rotor circulators serve individual loads such as heating and cooling terminal devices. Horizontal split-case pumps are usually installed in large buildings to provide hot- or chilled-water service.
