Heat Exchangers: Types, Design, And Construction Features

I will briefly discuss this project at the start of the Lecture on Wednesday 17th October. See QReview video.

The Figure below shows part of an airconditioning plant. In order to replace stale air in an office block, 86 m3 /min of air is discharged to the outside and replaced by an equal amount drawn into the building.

To save on the cost of heating this cold air a “run-around” system is installed to use heat from the discharged air to pre-heat the fresh air.

The system works on water flowing through the pipes which form two finned tube heat exchangers, one to extract heat from the outgoing air and the other to preheat the incoming air. The total heat transfer from one air stream to the other is 10 kW and it can be assumed that the pipes connecting the two heat exchangers are perfectly insulated.

Determine suitable sizes for the two heat exchangers (ie their heated length and outside surface area under the fins)

You will need to make reasonable assumptions about the flow rate of water and its temperature rise and fall while passing through the two heat exchangers.

You will also need to make realistic estimates of the inside and outside heat-transfer coefficients for the exchangers in order to calculate their overall heat transfer coeffcients.

Marks will be awarded for full explanation of your methodology, justification of all assumptions and full referencing of all data sources.

A heat exchanger is a equipment which enables movement of energy from a hot fluid to a cold fluid. The transfer occurs at maximum rate with minimal expense and investment. Each fluid temperature varies as it moves across the exchangers. Thereby the temperature of the partition wall also varies along the length of the heat exchanger. On the basis of a type of heat exchange process, design and specifications, corresponding direction of fluid motion and physiological state of fluids, heat exchangers can be categorized in many ways (Furman and Sahinidis 2002).

1. Type of heat exchange process:

On the basis of heat exchange process heat exchangers can be classified as

• Direct contact heat exchangers.
• Indirect contact heat exchangers.

In this type of heat exchangers, heat is exchanged by directly mixing of hot and cold fluids and the transfer of mass and heat takes place. Assumptions are made such that where mixing of such fluidsare harmless and desirable. The following assumptions are made while mixing of such fluids (Shah and Sekulic 2003).

1. The overall coefficient of heat transfer should not vary.
2. The conditions of flow must be constant.
3. The heat exchanger is to be considered as insulated thereby it does not dissipate heat to the surroundings.
4. It should be considered that no change of phase occurs during heat transfer.
5. Assume that the specific heats and the mass flow rate of both hot and cold fluids are not vary.

 The construction of a direct heat exchanger is such that the steam combines with cold water where it gives its latent heat energy to the water and then condenses. The hot water and other gases which are not condensed leave the container

Fig.1.1 DIRECT CONTACT HEAT EXCHANGERS

Indirect contact heat exchangers

          The heat exchange between two fluids is possible by transmission through a wall which separates the hot and cold fluid. They can be classified as

1. Regenerators
2. Surface exchangers or Recuperators

Regenerator

A regenerator functions periodically where the solid matrix absorbs the heat from the hot fluid and then sends it to the other fluid which is cold significantly. The hot and the cold fluid are allowed to pass through a unit containing solid particles. These solid particles are called the matrix which provides a sink and a heat flow source alternatively.  The following parameters affect the efficiency of a regenerator (Xuan and Li 2003).

• The heating ability of a regenerating material
• The absorption amount
• The emission of heat

Recuperators

 In this heat exchanger type the fluids employed for heat exchange are available on either end of the partition wall as tubes and pipes. These type of heat exchangers are employed when the cold and hot fluids are not allowed to mix with each other i.e. when the conditions are undesirable.  

The following are the failures that commonly occur in heat exchangers.

1. Due to the accumulation of deposits chocking of tubes is possible and thereby may cause failure of the heat exchanger.
2. Heat exchanger might fail when there is an excessive transfer of heat.
3. Failure may arise if the pump pressure is increased throughout.
4. If the temperature of the fluids is either extremely hot or extremely cold, this may lead to failure of the heat exchanger.
5. Lack of control over the heat exchanger may cause failure.
6. If the temperature exceeds the safe design limit failure may occur.
7. Radiations from refractory surfaces may result in failure.
8. Improper heating throughout the circumference of the exchanger may result in failure.

 On the basis of the direction of fluid flow, the heat exchangers can be classified as follows

• Parallel flow on the unidirectional flow
• Counterflow
• Crossflow

Parallel flow heat exchangers

 A parallel flow heat exchanger involves the flow of hot water and cold water in the same direction. The two fluids running in steam enter the exchanger from one end and leave from the other end. The temperature of the fluid decreases as the fluid approaches the outlet from the inlet in case in of a parallel flow heat exchanger. It requires a great amount of area for heat transfer thereby it cannot be employed in real time. It involves the employment of a partition wall thereby parallel flow heat exchangers can also be called as parallel flow recuperator or surface heat exchanger. Recuperator is a type of heat exchanger in wich the fluids employed for heat exchange are available on either end of the partition wall in the form of tubes and pipes. These type of heat exchangers are employed when the cold and hot fluids are not allowed to mix with each other i.e. when the conditions are undesirable (Kim and Sin 2006).

Design and Constructional Features

Fig.1.2 PARALLEL FLOW HEAT EXCHANGERS

Counterflow heat exchangers.

 A counterflow heat exchanger is entirely different to a parallel flow heat exchanger. As the name suggests, in a counter flow heat exchanger the hot water and the cold water flow in completely opposite directions unlike in parallel flow heat exchanger where both the fluids flow direction is same In counter flow heat exchangers, the fluids enter the heat exchanger from two opposite ends. Unlike in parallel flow where the temperature difference varies from the inlet to the outlet, in counterflow heat exchangers the temperature difference is negligible i.e. it does not very much and remains not vary. These type of heat exchangers due to counterflow provide maximum heat transfer for a definite surface area. Thereby counter-flow heat exchangers are the most preferred ones for heating and cooling of fluids (García-Valladares and Velázquez 2009).

Fig.1.3 COUNTER FLOW HEAT EXCHANGERS

Cross flow heat exchangers

 Cross flow heat exchangers include the exchange of heat between hot and cold fluids by crossing each other in space mostly perpendicular to each other i.e. at right angles. The temperature of the fluid remains uniform throughout but the flow direction only varies.

Heat exchangers can be classified as:

Concentric Tubes

Two concentric tubes are employed where each tube carries either hot or cold fluid. The direction in which the flows can be either parallel or counterflow based on the application. The performance of the heat exchanger can be enhanced by using swirling flow.

Fig.1.4 CONCENTRIC TUBES HEAT EXCHANGERS

Shell and tube

 One of the fluid i.e. either hot or cold fluids flow through a number of tubes enclosed by a shell while the other fluid is forced out of the shell and flows on the outer surface of the tubes. This type of heat exchangers is utilized where high efficiency and reliability of heat transfer is required. The use of multiple tubes is enhanced by increasing the surface area.

Fig.1.5 STRAIGHT OR SHELL TUBE HEAT EXCHANGERS

Multiple shell and tube passes

Multiple shells are employed to enhance the overall heat transfer. It is possible where the shell is re-connected i.e. interlinked, which allows the fluid to flow to and forth across the tubes while multiple tube passes are employed where the tubes are re-routed in opposite direction.

Compact heat exchangers

 These exchangers comprise of large volume for a unit surface area. Compact heat exchangers are special purpose heat exchangers which are generally employed where coefficient of convective heat transfer of one of the fluids is very much smaller than the other fluid. The temperature of each fluid varies as it passes through the exchangers. Thereby the temperature of the partition wall also varies along the length of the heat exchanger (Lee 2010).

The physiological state of fluids

 On the basis of the physical state, heat exchangers can be categorized as condensers and evaporators.

Condensers

 In this type of heat exchanger. The cold fluid temperature gradually ascends as it approaches the outlet from the inlet and the cold fluid also accepts the latent heat lost by the hot fluid. Throughout the heat exchanging phenomenon, the temperature of condensed fluid remains not vary.

Evaporators

This type of heat exchanger is entirely opposite to the condenser such that the cold fluid maintains its temperature while the hot fluid dissipates heat where its temperature descends gradually as it approaches the outlet.

The following governing parameters are to be considered for analyzing, designing and estimating the performance of a heat exchanger.

• Overall heat transfer coefficient (U)
• The entire surface area where  heat transfer was carried out and
• The inlet and the outlet fluid temperatures  (t₁,t₂)

While analyzing a heat exchanger the following assumptions must be made

1. The overall heat transfer coefficient should be not vary.
2. The flow conditions must be constant.
3. The heat exchanger is to be considered as insulated thereby it does not dissipate heat to the surroundings.
4. It should be considered that no change of phase occurs during heat transfer.
5. Assume that the specific heats and the mass flow rate of both the fluids i.e. hot and cold fluids are not vary.
6. If there is any change in potential energy or kinetic energy it is assumed to be negligible.
7. It is assumed that the axial conduction for the tubes employed in a heat exchanger is considered to be negligible.

The overall heat transfer coefficient of a heat exchanger in which the hot and cold fluids are separated by a plane wall is given by

If a tube wall is employed as the means for separation of fluids then the overall heat transfer coefficient is given by                                                          

A_i = 2r_i L; A _o = 2r_o L

The overall heat transfer coefficient is considered based on the following factors.

• The rate of fluid flow.
• The physical, mechanical, chemical, biological and various other fluid properties.
• The thickness of the tube material employed in the heat exchanger must also be taken into consideration.
• The geometric dimensions and configurations of the heat exchanger.

It should be noted that the overall heat transfer coefficient U will gradually descend if the fluid flowing on side of the exchanger has low values of heat transfer coefficient. Tars, oils and other gases have extremely low values of coefficient  of heat transfer. Water and liquid metals have high values of heat transfer coefficient. Thereby they are mostly employed to attain high values of U during boiling and condensation processes. The thermal resistance in heat exchanger must be low for attaining maximum efficiency and to obtain an effective design (Lei et al., 2008).

 Fig.1.6 OVERALL HEAT TRANSFER COEFFICIENT

 During operation, the heat exchanger is exposed to large amount of ash, soot, dirt and scale etc. where the whole heat exchanger gets covered entirely with the above-mentioned impurities mainly on the outer of the tube surface. This phenomenon of corrosion and deposition of such impurities is termed as fouling. As a result of such deposits, the resistance of the heat exchanger towards heat increases and thereby it affects the performance of the heat exchanger. It is difficult to predict the thickness of the deposits and also to determine the thermal conductivity of scale deposits. The adverse effect of such deposits can be reduced by specifying an equivalent scale transfer coefficient h_s. The scale deposited on the inner and outer surfaces are denoted as h_is and _so respectively. The corresponding thermal resistance to the formation of scales on the inner (R_si) and outer surfaces (R_io) are

The fouling factor is defined as the reciprocal of the scale heat transfer coefficient. It is denoted by R_f.r

Considering the thermal resistance due to scale formation the heat transfer is given by

The overall coefficient of heat transfer based on the inner and outer sides of the inner tube

1. The velocity of the fluids is an important parameter while considering fouling.
2. The temperature plays a pivotal role in affecting fouling.
3. The composition of water is one of the major factors to be considered for fouling.
4. The material of the tube which the heat exchanger employs also plays a significant role in fouling.

The following are the processes that involve fouling

1. crystallization fouling
2. Particulate or sedimentation fouling
3. Polymerization
4. Rusting fouling
5. Biotic fouling
6. Freeze fouling

The following are the ways to prevent fouling

1. By proving a proper design for the heat exchanger fouling can be prevented.
2. By providing proper treatment to the process system fouling can be prevented.
3. By employing a proper cleaning system to the heat exchanger fouling can be prevented.

The following are the properties to be considered for selecting materials for a heat exchanger.

1. The material physical properties are to be considered.
2. The mechanical properties such as the amount of stress it can withstand, strain, Young's modulus etc. are to be considered.
3. The material should also be considered on the basis of it reacts to the climatic conditions.
4. It should be considered on the basis of its alkalinity.
5. The surface finish of the material also plays an important role in selection.
6. The durability and service life of the material also plays a significant role.
7. The amount of noise it can resist at the time of operation is also an important factor to be taken into consideration.
8. The reliability of the material is one of the major criteria for selection.

The following are the failures that commonly occur in heat exchangers.

1. Due to the accumulation of deposits chocking of tubes is possible and thereby may cause failure of the heat exchanger.
2. Heat exchanger might fail when there is an excessive transfer of heat.
3. Failure may arise if the pump pressure is increased throughout.
4. If the temperature of the fluids is either extremely hot or extremely cold, this may lead to failure of the heat exchanger.
5. Lack of control over the heat exchanger may cause failure.
6. If the temperature exceeds the safe design limit failure may occur.
7. Radiations from refractory surfaces may result in failure.
8. Improper heating around the circumference of the exchanger or along the length of the tubes may result in failure.

 LMTD abbreviated as the Logarithmic mean temperature difference is defined as the change in temperature between the two fluids i.e. hot and cold fluids and if the temperature difference is not vary it would give the same amount of heat transfer that it would actually give under variable conditions for different temperatures.

The following conditions are to be assumed to the LMTD for various types of heat exchangers.

1. The overall heat transfer coefficient should be not vary.
2. The flow conditions must be constant.
3. The heat exchanger is to be considered as insulated thereby it does not dissipate heat to the surroundings.
4. It should be considered that no change of phase occurs during heat transfer.
5. Assume that the specific heats and the mass flow rate of both the fluids i.e. hot and cold fluids are not vary.
6. If there is any change in potential energy or kinetic energy it is assumed to be negligible.
7. It is assumed that the axial conduction for the tubes employed in a heat exchanger is considered to be negligible.

The logarithmic mean temperature for parallel flow

Consider a very small area of the heat exchanger. The rate of flow of heat through this area is given by

 Due to the transfer of heat  through the area  the hot fluid is cooled by  while the cold fluid is heated by  On integrating the above equations and on solving we get

Logarithmic mean temperature difference for a parallel flow heat exchanger:

Given

Total heat transfer Q = 10 KW = 10 KJ/s

The system works on water flowing through the pipes which form two finned tube heat exchangers, one to extract heat from the outgoing air and the other to preheat the incoming air.

Assumptions

Consider the heat exchangers to be parallel flow heat exchangers in which air gets heated. The two fluids running in steam enter the exchanger from one end and leave from the other end. In a parallel flow heat exchanger, the temperature of the fluid decreases as the fluid approaches the outlet from the inlet. It requires a large amount of area for heat transfer thereby it cannot be employed in real time. It involves the employment of a partition wall thereby parallel flow heat exchangers can also be called as parallel flow recuperator or surface heat exchanger. Recuperator is a type of heat exchanger where the fluids employed for heat exchange are available on either end of the partition wall in the form of tubes and pipes. These type of heat exchangers are employed when the cold and hot fluids are not allowed to mix with each other i.e. when the conditions are undesirable.

Fig.1.7 HEAT EXCHANGERS

1. Total heat transfer Q = 10 KW = 10 KJ/s
2. Inside heat transfer coefficient                      ………. 120 W/m^{2 o}C
3. Outside heat transfer coefficient                   ……….. 195 W/m^{2 o}C
4. Inlet and Outlet temperatures of the hot fluid………. 450 ^oC and 250^o C
5. Inlet and Outlet temperatures of the cold fluid ………60⁰ C and 120^oC
6. Inside and Outside diameters of the tube……………..50mm and 60 mm

To find:

1. The length of the tube
2. The surface area under the fins

The length of each tube for the heat exchanger is calculated by:

Q = U A

Where A  is the surface area under the fins

           U is the overall heat transfer coefficient

 is the Logarithmic mean temperature difference ( LMTD )

We know that Logarithmic mean temperature difference (LMTD) is given by

= 236.66⁰ C

The overall heat transfer coefficient is given by

            U = 66.09 W/m^{2 o} C

Total Heat transfer coefficient is given by

         10000= 66.09  236.66

Surface area under the fins A =

A = 0.639 m²

Therefore the length of each tube is 3.39m and the corresponding areas of the two heat exchangers are 0.639 m².

Conclusion

Thus the design and calculation of heat exchanger for the installation of the run around system which will pre-heat the inlet air utilizing the discharged air is done. The heat exchanger is designed and the calculations are performed and the final results are listed.

References

Boomsma, K., Poulikakos, D. and Zwick, F., (2003). Metal foams as compact high-performance heat exchangers. Mechanics of materials, Vol. 35, no. 12, pp.1161-1176.

De Paepe, M. and Janssens, A., (2003). Thermo-hydraulic design of earth-air heat exchangers. Energy and buildings, Vol. 35, no. 45, pp.389-397.

Furman, K.C. and Sahinidis, N.V., (2002). A critical review and annotated bibliography for heat exchanger network synthesis in the 20th century. Industrial and Engineering Chemistry Research, vol. 41, no. 10, pp.2335-2370.

García-Valladares, O. and Velázquez, N.,( 2009). Numerical simulation of parabolic trough solar collector: Improvement using counterflow concentric circular heat exchangers. International Journal of Heat and Mass Transfer, Vol. 52, no. 34, pp.597-609.

Kim, N.H. and Sin, T.R.,( 2006). Two-phase flow distribution of air-water annular flow in a parallel flow heat exchanger. International Journal of Multiphase Flow, vol. 32, no. 12, pp.1340-1353.

Lee, H.S.,( 2010). Thermal design: heat sinks, thermoelectrics, heat pipes, compact heat exchangers, and solar cells. John Wiley and Sons.

Lei, Y.G., He, Y.L., Chu, P. and Li, R.,( 2008). Design and optimization of heat exchangers with helical baffles. Chemical Engineering Science, vol. 63, no. 17, pp.4386-4395.

Shah, R.K. and Sekulic, D.P., (2003). Fundamentals of heat exchanger design. John Wiley and Sons.

Xuan, Y. and Li, Q.,( 2003). Investigation of convective heat transfer and flow features of nanofluids. Journal of Heat transfer, vol. 125, no. 1, pp.151-155.