Text №11 Shell and tube heat exchanger

Shell and tube heat exchangers¹ consist of a series of tubes. One set of these tubes contains the fluid that must be either heated or cooled. The second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required. A set of tubes is called the tube bundle² and can be made up of several types of tubes: plain³, longitudinally finned⁴, etc. Shell and tube heat exchangers are typically used for high-pressure applications (with pressures greater than 30 bar and temperatures greater than 260 °C).^[2] This is because the shell and tube heat exchangers are robust⁵ due to their shape. Several thermal design features must be considered when designing the tubes in the shell and tube heat exchangers:

• Tube diameter: Using a small tube diameter makes the heat exchanger both economical and compact. However, it is more likely for the heat exchanger to foul up⁶ faster and the small size makes mechanical cleaning of the fouling difficult. To prevail over the fouling and cleaning problems, larger tube diameters can be used. Thus to determine the tube diameter, the available space, cost and the fouling nature of the fluids must be considered.

• Tube thickness⁷: The thickness of the wall of the tubes is usually determined to ensure:

• There is enough room for corrosion

• That flow-induced vibration has resistance

• Axial⁸ strength

• Availability of spare⁹ parts

• Hoop¹⁰ strength (to withstand internal tube pressure)

• Buckling strength (to withstand overpressure in the shell¹¹)

• Tube length: heat exchangers are usually cheaper when they have a smaller shell diameter and a long tube length. Thus, typically there is an aim to make the heat exchanger as long as physically possible whilst not exceeding production capabilities.

1.Shell and tube heat exchanger – кожухо-трубчатый теплообменник

2.Tube bundle – трубный пучок

3.Plain - гладкий

4.Finned - ребристый

5.Robust - прочный

6.Foul up – засорять, загрязнять

7.Thickness - толщина

8.Axial - осевой

9.Spare parts – запасные детали

10.Hoop - хомут

11.Shell – кожух

Text №12 Selecting optimal heat exchangers

Due to the many variables involved, selecting optimal heat exchangers is challenging. Hand calculations are possible, but many iterations¹ are typically needed. As such, heat exchangers are most often selected via computer programs, either by system designers², who are typically engineers, or by equipment vendors.

To select an appropriate heat exchanger, the system designers (or equipment vendors³) would firstly consider the design limitations for each heat exchanger type. Though cost is often the primary criterion, several other selection criteria are important:

• High/low pressure limits

• Thermal performance

• Temperature ranges⁴

• Product mix (liquid/liquid, particulates or high-solids liquid)

• Pressure drops⁵ across the exchanger

• Fluid flow capacity⁶

• Cleanability⁷, maintenance⁸ and repair⁹

• Materials required for construction

• Ability and ease of future expansion

Choosing the right heat exchanger (HX) requires some knowledge of the different heat exchanger types, as well as the environment where the unit¹⁰ must operate. Typically in the manufacturing industry, several differing types of heat exchangers are used for just the one process or system to derive the final product. For example, a kettle HX for pre-heating, a double pipe HX for the ‘carrier’ fluid and a plate and frame HX for final cooling. With sufficient knowledge of heat exchanger types and operating requirements, an appropriate selection can be made to optimize the process.

1. Iteration – итерация, повторение, шаг

2. System designer - системотехник

3. Vendor – фирма-поставщик

4. Range - диапазон

5. Pressure drop – падение давления

6. Flow capacity – пропускная способность

7. Cleanability - очищаемость

8. Maintenance - техобслуживание

9. Repair - ремонт

10. Unit – установка