Put questions to Text 2C and answer them.

22. Write a brief summary of Text 2C using the following key word combinations:

Process of transferring heat; thermal radiation; driving potential; raised to the fourth power; considerable challenges; electromagnetic spectrum; ideal thermal radiator; Stefan-Boltzmann law of thermal radiation; physical mechanism of radiation; monochromatic radiation; spectral distribution.

 

Unit 3.

Heat Exchangers

  New words and word combinations to be memorized:

 

shell-and-tube (heat) exchanger	кожухо-трубный теплообменник (с комбинацией параллельного и перекрёстного токов)
double-pipe exchanger	двухтрубный теплообменник
stacked-plate structure	уложенная рядами пластинчатая структура
plate-fin exchanger	пластинчатый ребристый теплообменник
plate-and-frame exchanger	пластинчатый каркасный теплообменник
conspicuous adj	видный, заметный; обращающий на себя внимание
boiler feed-water heater	водонагреватель котла
gas-fired hot water heater	газовый водонагреватель
condenser coil	охлаждающий змеевик
energy payback	окупаемость энергетических затрат
counter flow	противоток, обратное течение
cross flow	перекрёстный ток; поперечный ток
extruded	прессованный
the log (logarithmic)  mean temperature difference (LMTD)	логарифмическая средняя разность температур
fin n	тех. ребро, ребристый выступ
contact heat exchanger	смесительный теплообменник
core n	сердцевина; внутренность; ядро; тех. стержень
immiscible adj	не поддающийся смешению, несмешивающийся
annular adj	кольцеобразный, кольцевой
inlet header	впускной коллектор
doughnut baffle	кольцеобразная отражательная перегородка
orifice n	отверстие, сопло, насадка
tube bundle	пучок труб

 

Match the words in A with their definitions in B

A	B
1. baffle	a) the act of paying sth back
2. condenser	b) a container in which water is heated to provide hot water and heating in a building
3. core	c ) any flat or level surface
4. shell	d) a machine used for making air or water warmer
5. heater	e) the central part of an object
6. plane	f) any structure that forms a hard outer frame
7. payback	g) a device that cools gas in order to change it into a liquid
8. boiler	h ) (technical) a screen used to control or prevent the flow of sound, light or liquid

Translate the following attributive constructions.

Tubular structure, water - cooled steam condenser, boiler feed - water heater, combustion air regenerator, gas - fired hot water heater, pumping power requirements, heat transfer enhancement devices, heat storage device.

Make up sentences in writing with the following words. Use them both as verbs and nouns.

Transfer, increase, conduct, compress, progress, contract, transport.

 

4. You are going to read Text 3A. Before reading try to answer the following questions. Share your answers with your fellow students. What do you know about heat exchangers? Do you think it is profitable to optimize designs of heat exchangersin terms of an economic return on the investment but also in terms of the energy payback of a system and why?

Text 3A

Heat Exchangers

Heat exchangers are generally devices or systems in which heat is transferred from one flowing fluid to another. The fluids may be liquids or gases, and in some heat exchangers more than two fluids might flow. These devices may have a tubular structure, of which the double - pipe and shell - and - tube exchangers are perhaps the most prevalent, or a stacked - plate structure, which includes the plate - fin and plate - and - frame exchangers, among some other configurations. Perhaps the most conspicuous and historically the oldest applications can be found in a power plant. The steam generator or boiler, water - cooled steam condenser, boiler feed - water heater, and combustion air regenerator, as well as several other types of equipment are all heat exchangers. In most homes, common heat exchangers are the gas - fired hot water heater, and the evaporatorand condenser coils of a central air - conditioning unit. All automobiles have a radiator and oil cooler, along with a few other heat exchangers.

When a heat exchanger is placed into a thermal transfer system, a temperature drop is required to transfer the heat. The magnitude of this temperature drop can be decreased by utilizing a larger heat exchanger, but this will increase the cost of the heat exchanger. Economic considerations are important in engineering design, and in a complete engineering design of heat exchange equipment, not only the thermal performance characteristics but also the pumping power requirements and the economics of the system are important. The role has taken an increasing importance recently as engineers have become energy conscious and want to optimize designs not only in terms of a thermal analysis and economic return on the investment but also in terms ofthe energy payback of a system. Thus economics, as well as such considerations as the availability and amount of energy and raw materials necessary to accomplish a given task, should be considered.

 A heat exchanger is a device in which heat is transferred between a warmer and a colder substance, usually fluids. Common types of heat exchanger flows include parallel flow, counter flow, and cross flow. In parallel flow, both fluids move in the same direction while transferring heat; in counter flow, the fluids move in opposite directions; and in cross flow, the fluids move at right angles to each other. Common constructions for heat exchangers include shell and tube, double pipe, extruded finned pipe, spiral fin pipe, u-tube, and stacked plate.

When engineers calculate the theoretical heat transfer in a heat exchanger, they must contend with the fact that the driving temperature difference between the two fluids varies with position. To account for this in simple systems, the log mean temperature difference (LMTD) is often used as an "average" temperature. In more complex systems, direct knowledge of the LMTD is not available, and the number of transfer units (NTU) method can be used instead.

There are three basic types of heat exchangers:recuperators, regenerators and direct contact heat exchangers.

In arecuperator the hot and cold fluids are separated by a wall and heat is transferred by a combination of convection to and from the wall and conduction through the wall. The wall can include extended surfaces, such as fins, or other heat transfer enhancement devices.

In a regenerator the hot and cold fluids alternately occupy the same space in the exchanger core. The exchanger core or “matrix” serves as a heat storage device that is periodically heated by the warmer of the two fluids and then transfers heat to the colder fluid. In a fixed matrixconfiguration, the hot and cold fluids pass alternately through a stationary exchanger, and for continuous operation two or more matrices are necessary. Another approach is the rotary regeneratorin which a circular matrix rotates and alternately exposes a portion of its surface to the hot and then to the cold fluid.

In a direct contact heat exchanger the hot and cold fluids contact each other directly. An example of such a device is a cooling tower in which a spray of water falling from the top of the tower is directly contacted and cooled by a stream of air flowing upward. Other direct contact systems use immiscible liquids or solid-to-gas exchange. The Direct contact heat exchanger is used to transfer heat between the molten salt and air. The direct contact approach is still in the research and development stage.

The simplest arrangement of this type of heat exchanger consists of a tube within a tube. Such an arrangement can be operated either in counterflow or in parallel flow, with either the hot or the cold fluid passing through the annular space and the other fluid passing through the inside of the inner pipe.

A more common type of heat exchanger that is widely used in the chemical and process industry is the shell-and-tube arrangement. In this type of heat exchanger one fluid flows inside the tubes while the other fluid is forced through the shell and over the outside of the tubes. The fluid is forced to flow over the tubes rather than along the tubes because a higher heat transfer coefficient can be achieved in cross-flow than in flow parallel to the tubes. To achieve cross-flow on the shell side, baffles are placed inside the shell. These baffles ensure that the flow passes across the tubes in each section, flowing downward in the first, upward in the second, and so on. Depending on the header arrangements at the two ends of the heat exchanger, one or more tube passes can be achieved. For a two-tube-pass arrangement, the inlet header is split so that the fluid flowing into the tubes passes through half of the tubes in one direction, then turns around and returns through the other half of the tubes to where it started. Three- and four-tube passes can be achieved by rearrangement of the header space. There are three types of baffles used in shell-and-tube heat exchangers: orifice baffle; disk-and-doughnut baffle and segmental baffle. A variety of baffles have been used in industry, but the most common kind is the disk-and-doughnut baffle.

The shell-and-tube heat exchanger has fixed tube sheetsat each end, and the tubes are welded or expanded into the sheets. This type ofconstruction has the lowest initial cost but can be used only for small temperaturedifferences between the hot and the cold fluids because no provision is made to prevent thermal stresses due to the differential expansion between the tubes and the shell. Another disadvantage is that the tube bundle cannot be removed for cleaning.

These drawbacks can be overcome by modification of the basic design. In the improved arrangement one tube sheet is fixed but the other is bolted to a floating-head cover that permits the tube bundle to move relative to the shell. The floating tube sheet is clamped between the floating headand a flange so that it is possible to remove the tube bundle for cleaning. This heat exchanger has one shell pass and two tube passes.

In the design and selection of a shell - and - tube heat exchanger, the power requirement and the initial cost of the unit must be considered.

In gas heating or cooling it is often convenient to use a cross-flow heat exchanger. The cross-flow heat exchanger is widely used in the heating, ventilating, and air-conditioning industry. In such a heat exchanger, one of the fluids passes through the tubes while the gaseous fluid is forced across the tube bundle. The flow of the exterior fluid may be forced by natural convection.

In this type of exchanger the gas flowing across the tube is considered to be mixed, whereas the fluid in the tube is considered to be unmixed. The exterior gas flow is mixed because it can move about freely between the tubes as it exchanges heat, whereas the fluid within the tubes is confined and cannot mix with any other stream during the heat exchange process. The mixed flow implies that all of the fluid in any given plane normal to the flow has the same temperature. The unmixed flow implies that although temperature differences within the fluid may exist in at least one direction normal to the flow, no heat transfer results from this gradient.

In the design of heat exchangers it is important to specify whether the fluids are mixed or unmixed, and which of the fluids is mixed. It is also important to balance the temperature drop by obtaining approximately equal heat transfer coefficients on the exterior and interior of the tubes. If this is not done, one of the thermal resistances may be unduly large and cause an unnecessarily high overall temperature drop for a given rate of heat transfer, which in turn demands larger equipment and results in poor economics.

 

5. Answer the questions:

1. What kind of device is a heat exchanger? 2. Are economic considerations important in a complete engineering design of heat exchange equipment and why? 3. Where can the oldest applications of heat exchangers be found? 4. What common heat exchangers are used in most homes? 5. Why has the role of heat exchangers increased recently? 6. What basic types of heat exchangers are mentioned in the text? 7. What more common type of heat exchanger is widely used in the chemical and process industry? 8. How many types of baffles are used in shell-and-tube heat exchangers? 9. Where is it convenient to use a cross-flow heat exchanger?