6. Discuss the following topics using the Conditional Mood as well as the introductory and connective words,

1. Kinds of centrifugues. 2. Working principles and applications. 3. Their advantages and disadvantages.

Use the words of argument to develop and prove your idea.

I move that, because, according to, considering, taking into consideration that, provided, suppose, assuming the fact that, based on, on the ground that, there is no doubt that, it is quite evident that, support the statement with, illustrate the fact (phenomenon, theory), challenge, argue that, object, agree, exactly so, argue the statement, disagree, in view of these advantages, In contrast to this., It is essential / particularly valuable / desirable / is necessary , it goes without saying.

Unit 12

1. Learn the notions and their definitions.

Conduction the transfer of heat by molecular action.

Convection turbulent transfer, the transfer of heat by a mixing process.

Radiation the transfer of heat by emission and absorption of energy without physical contact.

Heat-transfer cofficient a method of calculating film coefficient.

Steam The invisible vapor into which water is changed when it is heated to the boiling point.

Boiler A vessel in which water is boiled to produce steam.

Condense To return to a liquid state, as when steam reverts to water. The process is condensation and a device that causes the process to take place is a condenser.

Vacuum A space devoid of matter. Vacuums are partial, not com­plete.

Valve A device that opens or closes to control the flow of liquids or gases.

Evaporation the operation of concentrating a solution by boiling away the solvent.

Text 1

1. Read, translate and retell the text.

Industrial Heat-Exchange Equipment Heat-Exchangers. Principles of Unit Operations

Two mechanisms of heat transfer have already been established:

Molecular ─ the transfer of heat by molecular action; this is referred to as conduction. Turbulent ─ the transfer of heat by a mixing process; this is usually referred to as convection. It has been established that these two mechanisms can occur simultaneously or individually.

A third mechanism of heat transfer which is common but which has not

yet been discussed is radiation, which is the transfer of heat by emission and absorption of energy without physical contact. Unlike conduction or convection, which depend upon physical contact for thermal-energy transfer, radiation depends upon electro­magnetic waves as a means for transferring thermal energy from a hot source to a low-temperature sink. Radiation may occur simultaneously with, or independent of, the other two mechanisms of transfer.

The one characteristic common to most heat exchangers is the transfer

of heat from a hot phase to a cold phase with the two phases being

separated by a solid boundary.

Heat-Transfer Cofficients. A method of calculating film coefficients was proposed by Wilson for condensing vapors. It is based on the over-all resistance (I / UA) being equal to the sum of the individual resistances,

ΣR +¹/UA = R_c + R_c + R_d = R_L

where R_c = condensate resistance R_w = wall resistance R_d = scale or dirt resistance R_L = liquid-side resistance

The vapor-side resistance depends upon the tempera­ture driving force and upon the temperature of the condensate. The dirt resistance and tube-wall resistance also depend upon their respective temperatures.

Slope = 1/C₂

Double-Pipe Heat Exchangers. The simplest type of heat exchanger is the double-pipe heat exchanger as the basis for others. The double-pipe heat exchanger is essentially two concentric pipes with one fluid flowing through the

center pipe while the other fluid moves cocurrently or countercurrently in the

annular space. The length of each section is usually limited to standard

pipe lengths, so that, if an appreciable heat-transfer surface is required, banks of sections are frequently used. If the required area is too large, a double-pipe exchanger is not recommended. The use of a double-pipe heat exchanger is not limited to liquid-liquid heat exchange but may also be used for gas-liquid exchange and for gas-to-gas exchange. Materials of

construction may vary, depending upon the fluids being handled. Either fluid may be moved through the tube or annulus at relatively high velocities,

thereby aiding in the heat-transfer process.

Shell-and-Tube Exchangers. When the required heat-transfer surface is large, the recommended type of exchanger is the shell-and-tube variety. In this type of heater or cooler, large heat-transfer surface can be achieved

economically and practically by placing tubes in a bundle; the ends of the

tubes are mounted in a tube sheet. This is very commonly accomplished by ex­panding the end of the tube into a close-fitting hole in the tube sheet by a process called "rolling." The resultant tube bundle is then enclosed by a cylindrical casing (the shell), through which the second fluid flows around and through the tube handle.