A heat exchanger transfers heat from a liquid or gas to another cold fluid without direct contact. In a boiler heat exchanger, heat transfers from the hot gasses of combustion to water cycling through the exchangers internal piping system. This heats water up and cools gas down. It is essential for a boiler to properly function since hot water is the basis for a boiler.
Heat Exchanger Models
Shell and Tube Heat Exchangers
The shell and tube heat exchangers put two working fluids, such as combustion gas and water, together through tubes within the outer shell. The pathways are made out of conductive metals including stainless steel and aluminum alloys. These exchangers are compact, easily maintained, and very effective.
Plate and Frame Heat Exchangers
The plate and frame heat exchangers, also known as gasketed plate heat exchangers, transfer heat from one liquid or gas to another liquid. They offer maximum efficiency, are highly versatile, compact and easily accessible.
Heat exchangers are classified based off of their heat flow arrangement. In parallel-flow, or single pass, heat exchangers, the two liquids enter on the same side and exit on the opposite side. In counter-flow heat exchangers, the liquids flow in opposite directions. Finally, in a cross-flow heat exchanger, the fluids travel perpendicular to one another through the exchanger
Heat Exchanger Maintenance
Heat exchangers are a critical aspect of every boiler system and therefore must be cared for on a regular basis. Over the course of their operation, heat exchangers are exposed to tremendous amounts of heat and pressure, which can lead them to become faulty or crack. Proper cleaning and inspection is critical in making sure that your heat exchanger is functioning correctly and efficiently.
Service and Repair
- Common repairs include tube bundle replacements, plate replacements, weld repairs on larger heat exchangers
- Gasket replacement and general maintenance
- Seal the far end of each tube with a plug and insert nozzle into the near end to produce a vacuum in the tube
- When compressed air reaches a satisfactory level, trigger is released
- A steady gauge means good tube
- Falling vacuum means faulty tube
Cleaning and Inspection
- Open for inspection, depending upon size and use, anywhere from annually to every 5 years
- Cleaning the internal tube bundle is important for maximizing heat transfer efficiency
- Inspect for hot spots and holes in the tubes – important two fluids do not touch to avoid oil leaks and unclean water in domestic lines
- Process varies based on kind of heat exchanger and size – some large heat exchangers require sandblasting and recoating for some of the toughest scale buildups
Specialized Metals and Custom Styles
- Titanium, copper, or steel bundles
- Tube thickness is customizable
Properly Sizing New Heat Exchangers
- Consider total boiler system demand
- Engineering factor for future capacity (add stuff down the line that needs it)
- Take into account wear and tear – degraded capability over time
- Spacing consideration – sizes and placement of inlet outlet
Smart Controls and Heat Exchanger Monitoring
Controls can be as simple or complex as desired. Controls on larger heat exchangers can easily tie into a PLC system. You typically will want to monitor flow rates and temperatures for inlets and outlets as a great starting point to monitor the performance of your heat exchanger.
The logarithmic mean temperature difference (LMTD) is used to determine the temperature driving force for heat transfer in heat exchangers. The LMTD is a logarithmic average of the temperature difference between hot and cold feeds at each end of the heat exchanger.
Important variables for calculating heat exchanger efficiency:
- Heat exchanger inlet cold water temperature
- Hot water temperature exiting the heat exchanger
- Water volumetric flow rate through the heat exchanger
- Steam temperature at the heat exchanger steam inlet
- Hot water temperature further downstream from the heat exchanger
These measurements are essential to determine the amount of heat transferred from the steam to the water. The volumetric flow rate is converted to a mass flow rate by multiplying it by the density of water.
Common Brands We Work On
Frequently Asked Questions
- What are heat exchangers used for?
- Heat exchangers are used to transfer heat from a liquid or gas to another liquid without direct contact. They are used in boilers to transfer combustion heat to water.
- What is an SI unit of heat?
- The International System of Units (SI) heat unit is the joule (J). In many applied fields, however, the British Thermal Unit (BTU) is used more often for heat.
- What are the types of heat exchangers?
- The two main boiler heat exchangers are shell-and-tube type and plate and frame heat exchangers.