HISTORY
One of humankind’s most significant achievements has been the ability to generate heat and guide its movement for useful purposes. This ability is essential to modern life, especially when it’s used to establish and maintain thermal comfort in habitable spaces. Heat exchangers of all types are used for this purpose.
For centuries, heat was generated from fire, and methods were developed to guide that heat into occupied areas of buildings. An often-cited example of early heat exchange for improving human comfort was the Roman hypocaust. Wood-fueled fires would be maintained outside of occupied spaces. The hot combustion gases from these fires were channeled through spaces under stone floors supported on stone columns, as seen in Figure 1-1, as well as through hollow stone walls.
Because the combustion gases were significantly warmer than the building surfaces, heat would be absorbed into the stone. It would then transfer through the stone by conduction and be released into occupied spaces by thermal radiation and convection. This heat exchange process took place with minimal mixing of the combustion gases and the air in occupied spaces. As such, the hypocaust functioned as a heat exchanger. Today, modern heat exchangers allow heat transfer with zero mixing of the liquid or gas supplying the heat with the liquid or gas absorbing that heat.
Wood stoves are another example of centuries-old heat exchangers that allow heat from combustion gases to be transferred to occupied spaces with little, if any, release of those combustion gases into those spaces.
Some wood and coal burning stoves also included heat exchangers for heating domestic water. Figure 1-3 illustrates the concepts used in these early systems, which were available long before electrically powered circulators. Flow between the “water jacket” of the stove and the “range boiler” tank was created by thermosiphoning due to the differences in density between hot and cold water.
The evolution of heat exchangers was also critical to development of internal combustion engines. Some of the earliest engines were cooled solely by surrounding air. As engine design improved and horsepower increased, it became impractical to rely solely on surrounding air to keep the engine temperature under control. Engineers of that era turned to the superior thermal properties of water as a means of conveying heat from inside engine blocks to a location where it could be dissipated to surrounding air. Figure 1-4 shows an example of a Ford Model T radiator.
This radiator could be fundamentally described as a water-to-air heat exchanger. For its time — the early 1900s — it represented state-of-the-art-technology. Water from the upper portion of the engine block flowed into the upper portion of the radiator and divided up into multiple closed channels made of copper or brass. Air passed between these channels as a result of the car moving, as well as flow created by a simple fan connected to the engine’s crankshaft by a leather belt. The higher thermal conductivity of the copper and brass channels provided minimal thermal resistance between water and the outer surfaces of the radiator. After giving up heat, the coolest water settled into a reservoir at the base of the radiator and flowed back to the lower portion of the engine. No water pump was used. All flow was driven by the changes in buoyancy of the water between the top of the engine and lower portion of the radiator. Unlike modern vehicle radiators, this radiator was not pressurized. Model T drivers learned to carry extra water with them to replace the water lost through evaporation, and in some cases, boiling inside the radiator.
All fuel-burning boilers used for heating buildings have a combustion chamber combined with a heat exchanger. Figure 1-5a shows an example of a cast iron section that is used to build the heat exchanger of a cast iron boiler. Figure 1-5b shows how this heat exchanger, which is also called a boiler “block,” is made by joining several cast iron sections together.
Hot gases pass upward from the combustion chamber and across the “pins” on the cast iron sections. The pins increase the heat transfer surface area of the section. Heat from the hot gases passes through the cast iron walls of each section and is absorbed by the water inside.
Heat exchanger technology continued to progress through the 20th century. Hundreds of heat exchanger designs were developed for use in boilers, radiators, chillers, fan-coils and convectors. Wrought iron pipe and copper tubing were embedded into concrete slabs to create “radiant panel heat exchangers,” as seen in Figure 1-7. These panels transfer heat from heated water into occupied spaces using thermal radiation and convection.
Today, heat exchangers are precisely engineered for use in all types of stationary energy-processing equipment, as well as virtually all land-based vehicles, marine vessels, aircraft and spacecraft. These devices range from huge, multi-ton cooling towers used to dissipate heat from high-rise buildings (Figure 1-8), to tiny liquid cooling systems for microprocessors (Figure 1-9).
Every hydronic heating and cooling system involves the movement of thermal energy between water, or a water-based fluid, and one or more surrounding materials. This heat exchange begins at a heat source and ends at one or more heat emitters. With proper design, the rate of heat exchange at all locations within the system allows for safe operation and unsurpassed comfort.
Within hydronic heating and cooling systems, there are many situations where it’s necessary to move heat from one fluid to another without letting those fluids contact each other. Examples include:
- Transferring heat from the “system” water in a boiler to domestic water.
- Transferring heat from system water to an antifreeze solution circulated through tubing for melting snow on pavements.
- Transferring heat from an antifreeze solution circulating through piping buried in the earth to the refrigerant within a heat pump.
- Transferring heat from a district heating system to a building located several miles away.
- Transferring heat from water circulating through a cooling system to the refrigerant within a chiller.
This issue of idronics focuses on heat exchangers used within hydronic heating and cooling systems, as well as in some plumbing applications. The discussion ranges from fundamental heat transfer concepts to performance evaluation and novel applications within modern hydronic systems.