The term heat exchanger is referred to equipment that transfers heat from one medium to the other. With steam boilers, combustion gas passes its heat to water to achieve evaporation. It is known as a fired heat exchanger. The term is usually applied to shell and tube heat exchangers or plate heat exchangers for heating a process with a primary fluid like steam. Shell and tube heat exchanger is used for heating water for space heating by using water or steam. It is generally known as a non-storage calorifier, consisting of a hot water storage vessel with an internal primary heating coil.
Types of Heat Exchangers:
Plate heat exchanger:
A plate heat exchanger includes a series of thin corrugated metal plates, wherein several channels are formed in between. The primary and secondary fluids flow through alternative channels as the heat transfer occurs from the primary fluid steam to the secondary process fluid across the plate.
The rigidity of plates increases with a corrugated pattern of ridges that creates turbulent flows in the channel. It enhances heat transfer efficiency, resulting in a compact design of plate heat exchangers as compared to shell and tube heat exchangers.
The plate heat exchanger allows both condensing and sub-cooling of condensate within a unit. The condensate, if drained to an atmospheric receiver can be monitored by reducing the condensate temperature. It results in reducing the amount of flash steam lost to the atmosphere through the receiver vent.
Earlier, plate heat exchangers were majorly used in the food processing industry and water heating purposes. However, with the advancement in designs, plate heat exchangers are preferred for steam heating applications.
Gasketed Plate Heat Exchangers:
Gasketed plate heat exchanger consists of plates clamped together in a frame that is sealed with a thin gasket around the edge. The plate pack is compressed between the frame plate and the pressure plate with tightening bolts. Gaskets offer resistance to thermal fatigue and sudden pressure variations in plate packs and are best suited as a steam heater for immediate hot water supply.
The operating temperature rate of the gaskets restricts the steam pressure.
Brazed Plate Heat Exchanger:
As the name suggests, the plates are brazed together using nickel or copper in a vacuum furnace in the brazed plate heat exchanger. It offers more resistance to high pressures and temperatures at a low cost compared to gasketed plate heat exchangers. However, it cannot be dismantled and requires chemical cleaning.
Although the brazed plate heat exchangers are sturdy in design, it usually faces thermal fatigue. For preventing thermal stress, sudden or frequent changes in temperature and load must be avoided. It is widely used for applications in which the temperature variations are slow.
Welded Plate Heat Exchanger:
Welded plate heat exchanger holds the plate pack together with welded seams between the plates with a laser welding technique. It offers more resistance to pressure pulsation and thermal cycling as compared to the brazed plate heat exchanger. The high temperature and pressure operating limits enable the heat exchangers to have higher specifications that are ideally suited for applications of heavy process industries. It is widely used for high pressure and temperature performance or to heat viscous components like oil or hydrocarbons.
Shell and Tube Heat Exchangers:
The shell and tube heat exchanger are the most commonly found indirect heat exchanger in manufacturing operations. A shell and tube heat exchanger includes a cylindrical shell containing a bundle of tubes. The tube ends are fitted in the tube sheets that separate the primary and secondary fluids.
While using condensing steam as the heating medium, the heat exchanger is mostly horizontal, with condensation occurring inside the tubes. The process of sub-cooling is also performed for recovering excess heat from the condensate in the heat exchanger. In cases requiring a high degree of sub-cooling, it is advisable to switch to a separate condensate cooler.
Selection of Heat Exchangers:
Selecting a heat exchanger system depends on the design pressure of the exchanger, the amount of time required in operation, and the initial and final temperature of the fluid to be heated. It is advisable to maintain heat exchanger steam pressure at a minimum that enables adequate heat transfer to minimize the condensate temperature before the steam trap.
Calculating Steam Consumption for Heat Exchangers:
Shell and tube heat exchangers and plate heat exchangers are primary examples of flow-type applications. The heat exchangers are generally sized on the full running load. The startup load is ignored in rare occurrences. Shell and tube heat exchangers are ideal for preventing heat loss and mishaps or injuries to the operating staff. Plate heat exchangers are compact in design, requiring less surface area.
The steam consumption of heat exchangers are calculated with the following formula:
Q = m cp ΔT
Where,
Q = Quantity of Energy
m = Mass of the substance
cp = Specific heat capacity
ΔT = Temperature rise of the substance
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