Thermal Unit Operation (ChEg3113)

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1 Thermal Unit Operation (ChEg3113) Lecture 9- Shell and Tube Heat Exchanger Design Instructor: Mr. Tedla Yeshitila (M.Sc.)

2 Today Review Shell and tube heat exchanger Steps in Shell and tube heat exchanger

3 Review Double pipe heat exchanger design Thermal design, and Hydraulic design Example

4 Many industrial service requires use of large number of double pipe hairpins, but it consumes considerable ground area and also entail a large number of points at which leakage may occur. It also result high pressure drop. So shell and tube heat exchanger are best choice when large heat transfer surface is required. Shell and tube equipment involves expanding a tube into tube sheet and forming a seal which does not leak under reasonable operating condition.

5 Stationary Tube Sheet Exchanger The simplest type of shell and tube heat exchanger is fixed or stationary tube-shell exchanger. The essential parts are a shell (1) equipped with two nozzles and having tube sheets (2) at both ends, which also serve as flanges for the attachment of the two channels (3) and their respective channel covers (4). The tubes are expanded into both tube sheets and equipped with transverse baffles (5) on the shell side. The baffle are held securely by means of baffle spacer (6).

6 Main components of shell and tube heat exchangers are: Shell: is the case for the heat exchange Tube bundles Tube sheet (to support the tube bundles) Head Baffles

7 The area between inside the shell and outside the tube is called shell side of heat exchanger. It has its inlet and outlet. Each end of tube opens to the head, so one head into the tube and the other out of the tube. The area inside the tube and head called tube side of heat exchanger. The tube side supported inside by partition called baffle. It also direct flow to the shell side of the HX which increases the efficiency of the unit.

A tube hole is drilled in a tube sheet with slightly greater diameter than the outside diameter of the tube, and two or more grooves are cut in the wall of the hole.

8 A tube hole is drilled in a tube sheet with slightly greater diameter than the outside diameter of the tube, and two or more grooves are cut in the wall of the hole. The tubes are placed inside the tube hole, and a tube roller is inserted into the end of the tube. The roller is rotating mandrill having a slighter taper (thickness reduce at the end). It is capable of exceeding the elastic limit of the tube metal and transforming in to a semi plastic condition so that it flows into the grooves and form extremely tight seal. Tube rolling need great care to prevent tube damage.

9 In some industry, the tubes are actually packed in the tube sheet by means of ferrules (ring or cup) using a soft metal packing ring, so that the tubes can be easily removed easily.

10 Heat exchanger tubes (condenser tubes): They are different from steel pipes or other types of pipes which are extruded to iron pipe size. The outside diameter is the actual outside diameter in inches with a very strict tolerance. Can be made of steel, copper, aluminum, aluminum bronze, copper-nickel, and stainless steel. They are obtained in a number of different wall thickness defined by the Birmingham wire gage (BWG) or gage of the tubes. The sizes of tubes are listed in Table 10 of Appendix of which the ¾ outside diameter and 1in. OD are the most common in heat exchanger design.

11 Tube pitches: Tube holes can not be dilled very close together, since too small a width of metal between adjacent tube structurally weakens the tubes walls. The shortest distance between two adjacent tube holes called clearance (ligament). Tube pitch (P T ) is the shortest center to center distance between two adjacent tubes. Tubes are laid out either in square or triangular pattern. The advantage of square pitch is that the tube accessible for external cleaning and cause a lower pressure drop when a fluid flow in the direction indicated.

Square pitch can be rotated 45 0 or if the tubes are spread wide enough, triangular pitch can include mechanically cleanable modification. The common pitches for square layouts are: ¾ in.

12 Square pitch can be rotated 45 0 or if the tubes are spread wide enough, triangular pitch can include mechanically cleanable modification. The common pitches for square layouts are: ¾ in. OD on 1 in. square pitch 1 in. on 1 ¼ in. square pitch The common pitches for triangular layouts: ¾ in. OD on 15 in. triangular pitch 16 ¾ in. OD on 1 in. triangular pitch 1 in. on 1 ¼ in. triangular pitch

13 Shell: They are fabricated from steel pipe with nominal IPS diameter up to 12in. as given in the Table 11. Above 12in. and including 24 in. the actual outside diameter and the nominal pipe diameter are the same. The standard wall thickness for shells with inside diameters from 12 to 24 in. inclusive is 3/8 in. which is satisfactory for shell-side operating pressure up to 300 psi. Pressure drop at a given thickness is to be used. Greater wall thickness may be obtained for grater pressure. Shells above 24 in in diameter are fabricated by rolling steel plate.

14 The calculation of the effective heat-transfer surface is frequently based on the distance between the tube sheets instead of the overall tube length. Baffles Higher heat transfer coefficient results when a liquid is maintained in a state of turbulence. To induce turbulence outside the tube, baffles are used which cause liquid to flow through the shell at right angles to axes of the tubes.

15 The center to center distance between baffles called baffle pitch or baffle spacing. Baffle spacer

The baffle spacing is usually not greater than a distance equal to the inside diameter of the shell or

16 Since the baffle may be spaced close together or far apart, the mass velocity is not entirely dependent upon the diameter of the shell. That means, it depends also on the baffle distance. The baffle spacing is usually not greater than a distance equal to the inside diameter of the shell or closer than one-fifth the inside diameter of the shell. Common types of baffles are: Segmental baffle Disc and doughnut baffle Orifice baffle

17 Segmental baffles are dilled plates with heights which are generally 75 percent of the inside diameter of the shell. These are known as 25 percent cut baffle. Even though there are different percent cut baffle, we use 25 percent cut baffle in the next lectures. The baffle pitch, not the 25 percent cut of the baffles determines the effective velocity of the shell fluid.

18 1-2 exchanger means one pass in the shell and two pass in the tubes. Figure. Temperature relationship in 1-2 exchanger

19 The assumptions in 1-2 exchanger: 1. The shell fluid temperature is an average isothermal temperature at any cross section. 2. There is an equal amount of heating surface in each pass. 3. The overall coefficient of heat transfer is constant. 4. The rate of flow of each fluid is constant. 5. The specific heat of each fluid is constant. 6. There are no phase changes of evaporation or condensation in a part of the exchanger. 7. Heat losses are negligible.

20 Steps in Shell and tube heat exchanger design (1-2 exchanger) Process conditions required: Hot fluid: T 1, T 2, W, C, s or ρ, μ, k, ΔP, R d Cold fluid: t 1,t 2, w, c, s or ρ, μ, k, ΔP, R d For the exchanger the following data must be given: Shell side ID Baffle space Passes Tube side Number and length OD, BWG, and pitch Passes

21 1. Heat balance 2. True temperature difference Q=WC(T 1 -T 2 ) = wc(t 2 -t 1 ) For a heat exchanger with countercurrent flow, the mean temperature difference is known as the log mean temperature difference, ΔTLM or LMTD. LMTD = Δt 2 Δt log (Δt 2 /Δt 1 ) The log mean temperature difference is the maximum mean temperature difference that can be achieved in any geometry of heat exchanger for any given set of inlet and outlet temperatures.

22 For any other type of heat exchanger, the mean temperature difference can be expressed as Δt = LMTD* F T (F T from figure 18) Where F T is always less than or equal to 1. Estimating the mean temperature difference in a heat exchanger by calculating the log mean temperature difference and estimating F T is known as the F factor method. F T varies with geometry and thermal conditions. The thermal conditions are defined by parameters such as the overall heat transfer coefficient, U, the area available for heat transfer, A, the mass flow rates of the two steams and, the specific heat capacities of the two streams c 1 and c 2, and the temperature change in each stream (T 1 - T 2 ) and (t 2 t 1 ).

23 For any given geometry, F T is often presented as a function of two non dimensional parameters: R, the ratio of the thermal capacities of the two streams, and P (sometimes known as effectiveness, E), the ratio of the achieved heat transfer rate to the maximum possible heat transfer rate. R= wc WC =T 1 T 2 t 2 t 1 achived heat transfer rate, S= maximum heat trassfer rate = t 2 t 1 T 1 t 1

24 S Figure. Typical relationship between F and P for various values of S (Based on Single pass shell with any even number of tube passes).

25 3. Caloric temperature (T C and t C ): T C =T 2 +F C (T 1 -T 2 ) t C =t 1 +F C (t 2 -t 1 )

26 At the end of this class: You will have more know how about shell and tube exchanger design You will have the concept of true temperature difference and caloric temperature calculation You will be able to do the first 3 steps in shell and tube exchanger design

27 End of lecture -9

Lecture 2: Thermal Design Considerations

Lecture 2: Thermal Design Considerations The flow rates of both hot and cold streams, their terminal temperatures and fluid properties are the primary inputs of thermal design of heat exchangers. 1.2.