Heat exchangers are commonly used in centralized heating systems and water-source heat pump systems.
Heat exchangers are used in HVAC systems where it is necessary to transfer heat from one fluid stream to another without mixing the two fluid streams. Heat exchangers can be used to transfer heat from steam to water, water to water, water to brine, or brine to water. A common application of a steam-to-water heat exchanger is in the transfer of heat from a district or campus steam heating system to a heating hot water system within a building. Another application of a heat exchanger—where separation of the two fluid streams is of primary concern—is in the transfer of heat from a closed heat pump water system to an open cooling tower system. Heat exchangers can also be used
to isolate fluid systems in tall buildings, thereby preventing overpressurization of these systems due to static head
There are two types of heat exchangers commonly used in HVAC systems: U-tube shell and tube heat exchangers and plate and frame heat exchangers.
U-tube Shell and Tube Heat Exchangers
U-tube shell and tube heat exchangers consist of a copper U-tube bundle mounted within a steel cylindrical shell. The cold fluid stream normally circulates through the tubes of the tube bundle, and the hot fluid stream normally circulates through the shell (around the tube bundle). Heat is transferred from the hot fluid to the cold fluid through the walls of the tubes. Shell and tube heat exchangers are commonly used to transfer heat from steam to water or brine. However, they can also be used to transfer heat from water to water, water to brine, or brine to water. Baffles are installed in the shell to direct water flow across the tubes if the heat exchanger is used to transfer heat from water to water, water to brine, or brine to water. A minimum of two passes of the fluid through the tube bundle are required for U-tube shell and tube heat exchangers. For most HVAC applications, shell and tube heat exchangers are between 3 and 6 ft long and 6 and 12 in. in diameter, although larger heat exchangers are available. below Figure illustrates the piping connections to a steam-to-hot water shell and tube heat exchanger.
Plate and Frame Heat Exchangers
Plate and frame heat exchangers consist of two main components:
Plates: Multiple plates, normally constructed of stainless steel, form the surface through which heat is transferred from the hot fluid to the cold fluid. Channels formed between the stainless heat exchanger plates provide the narrow passages through which the two fluid streams circulate. The hot fluid circulates through the channels on one side of each plate while the cold fluid circulates through the channels on the other side of each plate. The heat exchanger plates separate the
two fluid streams and conduct heat from the hot fluid to the cold fluid. Elastomer gaskets are used to seal the joints between the plates. Plate and frame heat exchangers are used strictly to transfer heat from water to water, water to brine, or brine to water; they cannot be used to transfer heat from steam to water. A steel shroud surrounds the plate pack to protect the plates from damage.
Frame: The frame is normally constructed of steel and holds the plates together. The frame consists of a fixed steel front plate (called the fixed head) and a movable steel rear plate (called the movable head). The stainless steel heat exchanger plates are sandwiched between the two heads with compression bolts. There may also be a top carrying bar and a bottom guide bar; these bars support the plates. The carrying bar and guide bar are connected between the fixed head and a rear vertical
support frame. Heat exchanger plates can be added after the initial installation if additional capacity is required as long as the carrying bar, guide bar, and compression bolts are long enough to accommodate the additional plates and the system pump has sufficient capacity for the increased flow rates and pressure losses.
Plate and frame heat exchangers are commonly 5 to 7 ft high, 5 to 8 ft long, and 3 to 5 ft wide, although larger heat exchangers are available.