# Introduction Thermodynamic theory is at the core of heat exchanger design

Introduction

Thermodynamic theory is at the core of heat exchanger design; it incorporates a range of necessary fluid and solid parameters, flow dynamics and required heat duties to achieve specific temperature changes. The fundamental background of heat-transfer theory can be found in Coulson et al. (1999) which discusses energy relationships and heat transfer in the chemical engineering industry. Many of the basic laws of thermodynamic behavior can be derived from this source to provide additional contextual background to heat exchanger principles. This design process uses the design procedure outlines in Sinnott (2005) for a shell and tube heat exchanger. The fundamental features of a shell and tube heat exchanger can be seen in Figure 1. A tube side fluid enters the heat exchanger through an inlet on the heat exchanger shell. Its flow is directed through the heat exchanged by baffles, allowing axial and perpendicular flow across the tubes allowing for heat transfer. The baffles are also responsible for holding the tube bundle together and in the correct shape. Additional shell passes can also be achieved by axial baffles direction shell side fluid flow. The tube side fluid enters the unit through a front-end stationary head. This head piece is mounted to the shell and to plant piping. A rear end head piece is also attached, although its design can also be fixed (hence no allowing the tube bundle to be removed) or floating. The number of tube side passes is determined by the arrangement of the tube bundle and determined the shells diameter. TEMA nomenclature is used to describe the head and shell types of heat exchangers. This same nomenclature is used in this report and refences can be found in Sinnott (2005).

SWOT Analysis

SWOT analysis of heat exchanger can summaries the key takeaways behind the selection of the heat exchanger.

Table 1: SWOT analysis

Strengths

• Fulfills the criteria and constraints given

• Good heating and cooling capacity

• Fully completed heat exchanger design Weaknesses

• Is not optimized completely and extra pressure drop and low residence time

• Show linear velocities

• Heat losses

• Based on assumptions that thermal conductivity along the length remain same

Opportunities

• Using Aspen Plus or Aspen HYSYS the exact thermal and transport properties can be calculated

• Conducting more research related to thermal conductivities of the wall

• Pressure drop calculations can be carried out using different process simulators for accurate rigorous calculations Threats

• No simulation was conducted to improve the design and rating performance

• Minor error in calculations are possible

Fig. 1, Typical Shell and Tube Heat Exchanger (Shah, 1981)

According to the TEMA design and through evaluation it was found that the most appropriate design for the heat exchanger was AFT heat exchanger with a pull-through floating head will be used as it was the best for cleaning and it is the most common configuration for chemical processes (Mukherjee, 1998).

The recommended design is to have two shell passes and four tube passes as outlined in graphs in (Sinnot, 2013). Whilst this report has been extensive in calculating the various design specifications found in the heat exchanger, it is advised that further study take place whether it be simulating or removing assumptions as these will lead to a more valid and reliable heat exchanger. The principle parameter of the calculations is the overall heat transfer coefficient of the system, which relies on shell and tube side fluid properties and heat exchanger sizing and materials. This coefficient is used to size the heat exchanger and perform calculations to determine the required shell and tube parameters. Although since an approximated overall heat transfer coefficient is required to start the design process, an iterative methodology is used through the design until the calculated heat transfer coefficient reached an acceptable deviation from the assumed heat transfer coefficient.

Fig. 2 Construction based heat exchangers

Fig. 3 Parts of different heat exchangers TEMA

Classification of Heat Exchanger by Construction Type

Heat exchangers also can be classified according to their construction features. For example, there are tubular, plate, plate-fin, tube-fin, and regenerative exchangers. An important performance factor for all heat exchangers is the amount of heat transfer surface area within the volume of the heat exchanger. This is called its compactness factor and is measured in square meters per cubic meter.

Tubular exchangers are widely used, and they are manufactured in many sizes, flow arrangements, and types. They can accommodate a wide range of operating pressures and temperatures. The ease of manufacturing and their relatively low cost have been the principal reason for their widespread use in engineering applications. A commonly used design, called the shell-and-tube exchanger, consists of round tubes mounted on a cylindrical shell with their axes parallel to that of the shell. The principle components of this type of heat exchanger are the tube bundle, shell, front and rear end headers, and baffles. The baffles are used to support the tubes, to direct the fluid flow approximately normal to the tubes, and to increase the turbulence of the shell fluid. There are various types of baffles, and the choice of baffle type, spacing, and geometry depends on the flow rate allowable shell-side pressure drop, tube support requirement, and the flow-induced vibrations. Many variations of shell-and-tube exchanger are available; the differences lie in the arrangement of flow configurations and in the details of construction.

The character of the fluids may be liquid-to-liquid, liquid-to-gas, or gas-to-gas. Liquid-to-liquid exchangers have the most common applications. Both fluids are pumped through the exchangers; hence, the heat transfer on both the tube side and the shell side is by forced convection. Since the heat transfer coefficient is high with the liquid flow, generally there is no need to use fins.

Some basic types of baffles has been shown in the figure.

Fig. 4, Classification of heat exchanger

Fig. 5 Baffles types

Methodology / Deign Steps

Here are the steps for rating of Heat Exchanger

1. Perform energy balance

2. Select heating and cooling media

3. Calculate the steam flowrate

4. Determine or extract physical and transport properties of fluid

5. Allocate heating and cooling fluids side

6. Decide the heat exchanger type

7. Calculate the LMTD

8. Guess the value of overall heat transfer coefficient

9. Estimate the provisional area requirement

10. Estimate the actual area from the given information

11. Calculate individual shell and tube side heat transfer coefficients

12. Calculate the overall coefficient

13. Calculate heat transfer rate

14. Keep on iterating to reach exact solution

?

Assumptions

1. Negligible heat loss to the surroundings.

2. Negligible kinetic and potential energy changes.

3. Constant properties

4. Steady state operation

5. Velocity of both fluids entering and leaving the heat exchanger are same over the cross section

6. Counter current heat exchanger is assumed

7. There are no thermal sink or heat source attached to the heat exchanger

8. Thermal resistance of wall remains same over the whole length of tube

9. Saturated steam is assumed to be a heating media

10. Saturated liquid is leaving the heat exchanger (condensing steam)

11. Individual and overall heat transfer coefficients remains same along the length

12. Ratio of viscosity of fluid and viscosity at the wall is assumed to be 1

13. No extended surfaces are being used

14. Fluid flow is uniformly distributed

15. Specific heat of fluids remains same throughout the heat exchanger

?

Equations and Calculations:

Basis data is as follows:

1. Compute Energy balance

Using the below equation, we can calculate the heat transfer for the heat exchanger;

Q = 12539442.08 kJhr

2. Select heating medium (Given)

Saturated steam as a heating media at 300 kPa

3. Compute utility flowrate

Steam flowrate = Q / (Lambda )

Steam flowrate = 5794.57 kg/hr

4. Compute physical properties

Stream ID Water_In (1) Water_Out (2) Steam_In (3) Steam_Out (4)

T ? 10 70 134 134

Mass Heat Capacity kJ/kg-C 4.1798 4.1798 2.199 2.199

Lambda kJ/kg 2164 2164 2164 2164

Mass Density kg/m3 992.0203 992.0203 1.6509 1.6509

Viscosity cP 0.6514 0.6514 0.0133 0.0133

Thermal Conductivity W/m-K 0.6315 0.6315 0.0272 0.0272

5. Allocate fluids side

Condensing steam is allocated in the shell side due to volume change and water is in the tube side.

6. Decide the exchanger type

The most used shell and tube type is chosen.

7. Determine LMTD

LMTD = 30.85 C

8. Select value of overall coefficient, U

Assumed value of overall heat transfer coefficient is 800 W/m2-C

9. Estimate the tentative area requirement

Area = 508.07 m2

10. Estimate the actual area (based on given geometry)

Actual Area = 29.48 m2

11. Calculate the shell and tube side heat transfer coefficient

Re = 5360; Pr = 1.08

ho = 104 W/m2-C

12. Calculate the overall coefficient

Uo = 8000 W/m2-C

13. Calculate heat transfer rate

14. Pressure drop for the tube side

f = 0.00708

Pressure Drop = 6.03 psia

15. Pressure drop for the shell side

Pressure drop = 4.65 psia

Fig. 6: Counter-Current Heat Exchanger

Result and Discussion

The result indicated that the heat exchanger is under-designed due to which it is impractical to use such heat exchangers. Usually, heat exchangers are overdesigned to incorporate leakage, losses, and other factors. This heat exchanger can be used to process part of flowrate but to meet the heat exchange requirement, it is required to increase the area of heat transfer. Other way is to install a parallel heat exchanger to meet the requirement.

?

Takeaways and Learning

This problem helped us to apply the heat transfer fundamentals to deal with a pragmatic problem. Moreover, this problem ease to develop deep understand of heat exchanger and to explore different types of heat exchangers along with application in real life. It facilitated to incorporate different concepts to analyze the practical problem and imply some valid assumption to made calculations simpler to build a solid base for further improvements.

?

Nomenclature

Symbol Meaning Units

? Density kg/m3

µ Viscosity

?T Change in temperature ?

?Tlm Log mean temperature difference ?

?Tm Mean temperature difference ?

kw Wall thermal conductivity W/ m °C

kf Fluid thermal conductivity W/ m °C

Q Heat Duty kj/hr

U Overall heat transfer coefficient W/m2-C

A Area m2

Nt Number of tubes

?? Mass flow rate kg/hr

Cp Heat capacity kJ/kg-?

ut Tube side velocity m/s

us Shell side velocity m/s

jh Tube side heat transfer factor

jn Shell side heat transfer factor

?Pt Tube side pressure drop kPa

Re Reynolds number

Pr Prandlt number

Nu Nusselt number

do Outer tube diameter mm

di Inner tube diameter mm

de Equivalent shell diameter mm

L Tube length mm

Db Bundle diameter mm

Ds Shell diameter mm

lb Baffle spacing mm

As Shell side cross section area m2

hi Tube side heat transfer coefficient W/m2-C

ho Shell side heat transfer coefficient W/m2-C

hid Tube side fouling factor

hod Shell side fouling factor

jf Friction factor

Np Number of tube side passes

?Ps Shell side pressure drop psia

?