Heat-Exchanger Technologies

Summary

A heat exchanger transfers thermal energy from one flowing medium to another. A car radiator transfers thermal energy from the engine-cooling water to the atmosphere. A nuclear power plant contains very large heat exchangers called steam generators, which transfer thermal energy out of the water that circulates through the reactor core and makes steam that drives the turbines. In some heat exchangers, the fluids mix together, while in others, the fluids are separated by a solid surface such as a tube wall.

Definition and Basic Principles

There are three modes of heat transfer. Conduction is the method of heat transfer within a solid. In closed heat exchangers, conduction is how thermal energy moves through the solid boundary that separates the two fluids. Convection is the method for transferring heat between a fluid and a solid surface. In a heat exchanger, heat moves out of the hotter fluid into the solid boundary by convection. That is also the way it moves from the solid boundary into the cooler fluid. The final mode of heat transfer is radiation. This is how the energy from the Sun is transmitted through space to Earth.

The simplest type of closed heat exchanger is composed of a small tube running inside a larger one. One fluid flows through the inner tube, while the other fluid flows in the annular space between the inner and outer tubes. In most applications, a double-pipe heat exchanger would be very long and narrow. It is usually more appropriate to make the outer tube large in diameter and have many small tubes inside it. Such a device is called a shell and tube heat exchanger. When one of the fluids is a gas, fins are often added to heat-exchanger tubes on the gas side, which is usually on the outside.

Plate heat exchangers consist of many thin sheets of metal. One fluid flows across one side of each sheet, and the other fluid flows across the other side.

Background and History

Boilers were probably the first important heat exchangers. One of the first documented boilers was invented by Hero of Alexandria in the first century. This device included a crude steam turbine, but Hero's engine was little more than a toy. The first truly useful boiler may have been invented by the Marquess of Worcester in about 1663. His boiler provided steam to drive a water pump. Further developments were made by British engineer Thomas Savery and British blacksmith Thomas Newcomen, though many people mistakenly believe that James Watt invented the steam engine. Watt invented the condenser, another kind of heat exchanger. Combining the condenser with existing engines made them much more efficient. Until the late nineteenth century, boilers and condensers dominated the heat-exchanger scene.

The invention of the diesel engine in 1897 by Bavarian engineer Rudolf Diesel gave rise to the need for other heat exchangers: lubricating oil coolers, radiators, and fuel oil heaters.

During the twentieth century, heat exchangers grew rapidly in number, size, and variety. Plate heat exchangers were invented. The huge steam generators used in nuclear plants were produced. Highly specialized heat exchangers were developed for use in spacecraft.

How It Works

Thermal Calculations. There are two kinds of thermal calculations–design calculations and rating calculations. In design calculations, engineers know what rate of heat transfer is needed in a particular application. The dimensions of the heat exchanger that will satisfy the need must be determined. In rating calculations, an existing heat exchanger is to be used in a new situation. The rate of heat transfer that it will provide in this situation must be determined.

In both design and rating analyses, engineers must deal with the following resistances to heat transfer: the convection resistance between the hot fluid and the solid boundary, the conduction resistance of the solid boundary itself, and the convection resistance between the solid boundary and the cold fluid. The conduction resistance is easy to calculate. It depends only on the thickness of the boundary and the thermal conductivity of the boundary material. Calculation of the convection resistances is much more complicated. They depend on the velocities of the fluid flows and on the properties of the fluids such as viscosity, thermal conductivity, heat capacity, and density. The geometries of the flow passages are also a factor.

Because the convection resistances are so complicated, they are usually determined by empirical methods. This means that research engineers have conducted many experiments, graphed the results, and found equations that represent the lines on their graphs. Other engineers use these graphs or equations to predict the convection resistances in their heat exchangers.

In liquids, the convection resistance is usually low, but in gases, it is usually high. In order to compensate for high convection resistance, fins are often installed on the gas side of heat-exchanger tubes. This can increase the amount of heat transfer area on the gas side by a factor of ten without significantly increasing the overall size of the heat exchanger.

Hydraulic Calculations. Fluid friction and turbulence within a heat exchanger cause the exit pressure of each fluid to be lower than its entrance pressure. It is desirable to minimize this pressure drop. If a fluid is made to flow by a pump, increased pressure drop will require more pumping power. As with convection resistance, pressure drop depends on many factors and is difficult to predict accurately. Empirical methods are again used. Generally, design changes that reduce the convection resistance will increase the pressure drop, so engineers must reach a compromise between these issues.

Strength Calculations. The pressure of the fluid flowing inside the tubes is often significantly different from the pressure of the fluid flowing around the outside. Engineers must ensure that the tubes are strong enough to withstand this pressure difference so the tubes do not burst. Similarly, the pressure of the fluid in the shell (outside the tubes) is often significantly different from the atmospheric pressure outside the shell. The shell must be strong enough to withstand this.

Fouling. In many applications, one or both fluids may cause corrosion of heat-exchanger tubes, and they may deposit unwanted material on the tube surfaces. River water may deposit mud. Seawater may deposit barnacles and other biological contamination. The general term for all these things is fouling. The tubes may have a layer of fouling on the inside surface and another one on the outside. In addition to the two convection resistances and the conduction resistance, there may be two fouling resistances. When heat exchangers are designed, a reasonable allowance must be made for these fouling resistances.

Applications and Products

Heat exchangers come in an amazing variety of shapes and sizes. They are used with fluids ranging from liquid metals to water and air. A home with hot-water heat has a heat exchanger in each room. They are called radiators, but they rely on convection, not radiation. A room air conditioner has two heat exchangers in it. One transfers heat from room air to the refrigerant, and the other transfers heat from the refrigerant to outside air. Cars with water-cooled engines have a heat exchanger to get rid of engine heat. It is called a radiator, but again it relies on convection.

Boilers. Boilers come in two basic types: fire tube and water tube. In both cases, the heat transfer is between the hot gases produced by combustion of fuel and water that is turning to steam. As the name suggests, a fire-tube boiler has very hot gas, not actually fire, inside the tubes. These tubes are submerged in water, which absorbs heat and turns into steam. In a water-tube boiler, water goes inside the tubes and hot gases pass around them. Water-tube boilers often include superheaters. These heat exchangers allow the steam to flow through tubes that are exposed to the hot combustion gases. As a result the final temperature of the steam may reach 1,000 degrees Fahrenheit or higher. An important and dangerous kind of fouling in boilers is called scale. Scale forms when minerals in the water come out of solution and form a layer of fouling on the hot tube surface. Scale is dangerous because it causes the tube metal behind it to get hotter. In high-performance boilers, this can cause a tube to overheat and burst.

Condensers. Many electric plants have generators driven by steam turbines. As steam leaves a turbine it is transformed back into liquid water in a condenser. This increases the efficiency of the system, and it recovers the mineral-free water for reuse. A typical condenser has thousands of tubes with cooling water flowing inside them. Steam flows around the outsides, transfers heat, and turns back into liquid water. The cooling water may come from a river or ocean. When a source of a large amount of water is not available, cooling water may be recirculated through a cooling tower. Hot cooling water leaving the condenser is sprayed into a stream of atmospheric air. Some of it evaporates, which lowers the temperature of the remaining water. This remaining water can be reused as cooling water in the condenser. The water that evaporates must be replaced from some source, but the quantity of new water needed is much less than when cooling water is used only once. When river water or seawater is used for cooling, there may be significant fouling on the insides of the tubes. Because the steam leaving a turbine contains very small droplets of water moving at high speed, erosion on the outsides of the tubes is a problem. Eventually a hole may develop, and cooling water, which contains dissolved minerals, can leak into the condensing steam.

Steam Generators. In a pressurized-water nuclear power plant, there is a huge heat exchanger called a steam generator. The primary loop contains water under high pressure that circulates through the reactor core. This water, which does not boil, then moves to the steam generator, where it flows inside a large number of tubes. Secondary water at lower pressure surrounds these tubes. As the secondary water absorbs heat it turns into steam. This steam is used to drive the turbines. Steam generators are among the largest heat exchangers in existence.

Deaerators. Because the condensers in steam systems operate with internal pressures below one atmosphere, air may leak in. Some of this air dissolves in the water that forms as steam condenses. If this air remained in the water as it reached the boiler, rapid rusting of boiler surfaces would result. To prevent this, an open heat exchanger, called a deaerator, is installed. Water is sprayed into the deaerator as a fine mist, and steam is also admitted. As the steam and water mix, the water droplets are heated to their boiling point but not actually boiled. The solubility of air in water goes to zero as the water temperature approaches the boiling point, so nearly all air is forced out of solution in the deaerator. Once the air is in gaseous form, it is removed from the system.

Feedwater Heaters. Leaving the deaerator, the water in a steam plant is on its way to the boiler. The system can be made more efficient by preheating the water along the way. This is done in a feedwater heater. Steam is extracted from the steam turbines to serve as the heat source in feedwater heaters. Feedwater flows inside the tubes and steam flows around the tubes. Feedwater heaters are often multi-pass heat exchangers. This means that the feedwater passes back and forth through the heat exchanger several times. This makes the heat exchanger shorter and fatter, which is a more convenient shape.

Intercoolers. Many diesel engines have turbochargers that pressurize the air being fed to the cylinders. As air is compressed, its temperature rises. It is desirable to lower the temperature of the air before it enters the cylinders, because that means a greater mass of air can occupy the same space. More air in the cylinder means more fuel can be burned, and more power can be produced. An intercooler is a closed heat exchanger between the turbocharger and the engine cylinders. In this device, air passes around the tubes, and cooling water passes inside them. There are usually fins on the outsides of the tubes to provide increased heat transfer area.

Industrial air compressors also have intercoolers. These compressors are two-stage machines. That means air is compressed part way in one cylinder and the rest of the way in another. As with turbochargers, the first compression raises the air temperature. An intercooler is often installed between the cylinders. Compressed air flows through the intercooler tubes. Either atmospheric air or cooling water flows around the outside. Cooling the air before the second compression reduces the power required there.

Careers and Course Work

Heat exchangers are usually designed by mechanical engineers who hold bachelor or master of science degrees in this field. Students of mechanical engineering take courses in advanced mathematics, mechanics of materials, thermodynamics, fluid mechanics, and heat transfer. An MS degree provides advanced understanding of the physical phenomena involved in heat exchangers. Research into the theory of heat transfer is normally carried out by mechanical engineers with doctoral degrees. They conduct research in laboratories, at universities, private research companies, or large corporations that build heat exchangers. As mentioned earlier, convection heat transfer calculations rely on equations that are derived from extensive experiments. Much research work continues to be devoted to improving the accuracy of these equations.

Construction of heat exchangers is executed by companies large and small. The work is carried out by skilled craftsmen using precise machine tools and other equipment. Machinists, welders, sheet-metal, and other highly trained workers are involved. Students who pursue such careers may begin with vocational-technical training at the high school level. They become apprentices to one of these trades. During apprenticeship, the workers receive formal training in classrooms and on-the-job training. As their skills develop they become journeymen and then master mechanics.

Workers who operate, maintain, and repair heat exchangers have a variety of backgrounds. Some have engineering or engineering technology degrees. Others have vocational-technical and on-the-job training. At nuclear power plants, the Nuclear Regulatory Commission requires a program of extensive testing of the vital heat exchangers. This is carried out by engineers with BS or MS degrees, assisted by skilled craftsmen.

Social Context and Future Prospects

Although heat exchangers are not glamorous, they are an essential part of people's lives. Every home has several, as does every car and truck. Without heat exchangers, people would still be heating their homes with fireplaces, and engines of all types and sizes would not be possible. Heat exchangers are essential in all manner of industries. In particular, they play a key role in the generation of electricity.

The design of heat exchangers is based on empirical methods rather than basic principles. While empirical methods are reasonably effective, design from basic principles would be preferred. In the early twenty-first century, extensive research projects are under way with the goal of solving the very complicated equations that represent the basic principles of heat transfer. These projects make use of very powerful computers. As the cost of computers continues to drop and their power continues to increase, heat exchangers may come to be designed from basic principles.

Bibliography

Agrawal, Kamal Kumar, Rohit Misra, Ghanshyam Das Agrawal, Mayank Bhardwaj, and Doraj Kamal Jamuwa. "The State of Art on the Applications, Technology Integration, and Latest Research Trends of Earth-Air-Heat Exchanger System." Geothermics, vol. 82, 2019, pp. 34-50, DOI: 10.1016/j.geothermics.2019.05.011. Accessed 6 June 2022.

Babcock and Wilcox Company. Steam: Its Generation and Use. Reprint. Whitefish: Kessinger, 2010.

Blank, David A., Arthur E. Bock, and David J. Richardson. Introduction to Naval Engineering. 2nd ed. Annapolis: Naval Inst., 2005.

McGeorge, H. D. Marine Auxiliary Machinery. 7th ed. Burlington: Butterworth, 1995.

Thurston, Robert Henry. A History of the Growth of the Steam-Engine. Ithaca: Cornell UP, 1939.