Question

A shell and-tube heat exchanger is required for the following service: Hot stream Cold Stream Aromatic...



A shell and-tube heat exchanger is required for the following service:

Hot stream Cold Stream
Aromatic Stream Cooling Water
inlet Temperature (oC) 85 20
outlet Temperature (oC) 40 35
Mass Flowrate x heat Capacity (kW/oC) 85.2
Hot Stream cold stream (Cooling water)
Heat Capacity (J/kg K) 2840 4193
Density (kg/m3) 750 999
Viscosity (cP) 0.34 1.016
Thermal conductivity (W/m.K) 0.19 0.594
Fouling Factor (m2.oC/W 0.00018 0.000176


? The cooling water is allocated to the tube-side of the exchanger.
? It is preferable to employ a fixed-tubesheet heat exchanger for the service.
? 25.4 mm OD tubes (2.1 mm wall thickness) on a 1.25 x OD triangular pitch are available. The
thermal conductivity of the tube material is kw = 51 W/m?K.
? Consider four (4) tube passes.
? The maximum tube length that can be employed is 6.098 m i.e. L < or = 6.098 m
? Assume 1-shell pass. The shell diameter should not exceed 1.5 m i.e. Ds < or = 1.5 m
? Baffle cut is set at 25% and the baffle spacing (lb) is assumed to be 0.5 x Ds
? The maximum allowable tube-side and shell-side pressure drops are 70 kPa and 50 kPa
respectively.
? Total annual cost (TAC) is given as follows:
TAC = 123 A^(0.59) + 1.31 [ delta Ptube . mtube/ densitytube + delta Pshell . mshell/ densityshell ]
where A is the area of the exchanger in m2, TAC is in $/year, ?delta P is in Pa, m in kg/s and density? in
kg/m3

2.2 Provide a mathematical expression for the following constraints:
o the tube side velocity,
o the shell-side velocity, and
{Hint: Note: cooling water in the tubes}


2.3 Derive an expression for the number of tubes in terms of the tube-side velocity, number of tube
passes, mass flowrate and tube inside diameter.

2.4 Calculate the number of tubes at the minimum and maximum tube-side velocities
2.1). Show the calculations and summarise your answers in a table.
{It is convenient to use a spreadsheet for this calculation}


2.5 For each of the number of tubes calculated in 2.4, determine the shell diameter Ds. (you can fit a straight
line to the data if you would rather like to use an equation). Show the calculations and summarise
your answers in a table.

2.6 Plot a graph of tube length vs shell diameter for the minimum and maximum velocity as well as
the maximum shell diameter. Discuss your findings.


2.7 Select various shell diameters between the limits found in Coulson. {4 to 5 values should be sufficient}.
For each shell diameter, determine the tube length at the maximum tube-side pressure drop.
Show the calculations and summarise your answers in a table.


2.8 Using the same shell diameters selected in 2.7, determine the tube length at the maximum shellside
pressure drop. Show the calculations and summarise your answers in a table.
2.9 For each shell diameter, determine the tube length based on the heat transfer duty i.e. Q=UA?Tm
{You need to determine the heat transfer coefficients on the tube and shell-side}. Show the
calculations and summarise your answers in a table.
2.10 Plot your results from 2.7, 2.8 and 2.9 on a graph of tube length (x-axis) vs shell diameter (yaxis).
{Please use a spreadsheet to do this. Do not submit hand-drawn plots}. Identify the feasible
region.
2.11 Determine the exchanger with the minimum cost. Discuss your approach and findings.
2.12 How does the above approach of exchanger design compare/contrast with the standard
method?

Homework Answers

Answer #1

A-2.4)

Ovrall HTC 600 W/m2K Taken from R c vol. 6
Tube ID (di) 19 mm
No. of Tube pass 4
Mass flow rate 30 kg/sec
Heat Transfer Area 260 m2
Tube Length Tube Side velocity No. of Tubes
1.83 0.237085 2381.437
2.44 0.316113 1786.077
3.66 0.474170 1190.718
4.88 0.632227 893.039
6.1 0.790283

714.431

A-2.5

Tube Length Tube Side velocity No. of Tubes Tube Bundle Dia Clearance Shell Dia
1.83 0.237085 2381.437 1636.5790 NA -
2.44 0.316113 1786.077 1442.9763 80 1522.9763
3.66 0.474170 1190.718 1208.3568 77 1285.3568
4.88 0.632227 893.039 1065.4116 73 1138.4116
6.1 0.790283 714.431 966.2867 71.5 1037.7867

WHere k1=0.175 & N1=2.285 for 4 passes at tube side

FOr Tube bundle shell clearance refer graph 12.10 and for tune bundle dia refer table no. 12.4 of RC vol.6

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