Essay: Heat Exchanger Performance w/ Hexagonal and Semi-Circular Fins



Abstract

 A Heat Exchanger is a device use for the heat transfer from one fluid to another, whether the fluids are separated by a solid wall so that they never mix or the fluids are directly in contact. The heat exchanger is widely used in different industries such as condensing process, petroleum refining, chemicals and paper, power generation, chemical processing, A.C, refrigeration, and a food processing applications. Etc. Various Enhancement methods are used to increase performance of heat exchanger such as treated surfaces, rough surfaces etc.

Chapter 1

1.1 INTRODUCTION

Double pipe heat exchanger is used in chemical industry. When to construct this type of heat exchanger, the size of material that is considered since it affected the overall heat transfer. Basically the heat exchanger has two types that are parallel flow heat exchanger, counter flow heat exchanger and efficiency of counter flow heat exchanger is high than the parallel flow heat exchanger. So it is widely used. After few years of research the fins has introduce in heat exchangers for improve performance.

The fins increase the effective area of a surface than heat transfer will increase. past few years lot of modification are implemented in fin design for increase heat transfer rate in the heat exchanger. The reason for implementation of fin is increase the   pressure drop. While the pressure drops increases that affect mass flow rate and make it low due to low mass flow rate the amount of heat transfer is increased.

Various research papers were studied for checking effect of different fins on heat transfer performance.

   Fig 1: Double-pipe heat exchanger

1.1.1 CLASSIFICATION OF HEAT EXCHANGER

CLASSIFICATION ACCORDING TO TRANSFER PROCESS

Direct type

Indirect type

Direct transfer type

Storage type

CLASSIFICATION ACCORDING TO SURFACE COMPACTNESS

Compact

Non compact

CLASSIFICATION ACCORDING TO CONSTRUCTION

Tubular

Double pipe

Shell and tube

Plate baffle

Rod baffle

Spiral tube

Plate

Gasketed

Spiral

Lawella

Extended surface

Plate fin

Tube fin

Regenerative

Rotory

Disk type

Drum type

Fixed matrix

CLASSIFICATION ACCORDING TO FLOW ARRANGEMENT

Single pass

Parallel flow

Counter flow

Cross flow

Multi pass

Extended surface heat exchanger

Cross counter flow

Cross parallel flow

ACCORDING TO NUMBER OF FLUIDS

Two fluid

Three fluid

N-fluid

ACCORDING TO HEAT TRANSFER MECHANISM FLOW ARRANGEMENT

Single phase convection

Single phase convection on one side , two phase convection on other side

Two phase convection on both side

1.3 Problem Definition

Theoretical analysis has been carried out in different studies on effectiveness, overall heat transfer coefficient and friction factors are analyzed by using heat transfer enhancement method .the numerical and analytical investigation for hexagonal fin, semicircular fin, tabulator, Semi Circular Disc Baffles, helical fins are carried out for review study.

1.4   Objective of Project

  Main objective of our project is to reduce following problem:

Our main aim of the project is to increase the performance of heat exchanger.

Based on this concept the semi circle , hexagonal shape fins are placed over the inside copper tube so the cold fluid are passed over the fins in that time pressure drop of the fluid are increased  so the heat transfer rate from hot fluid to clod fluid  are increase.

 

 

Chapter 2

Literature review

K.saravanakumar4, et.al studied on the performance of shell and finned tube heat exchanger   using hexagonal and semi circular fins. In experimental analysis result was completed and the effect on temperature reduction and the pressure drop in the finned tube and shell side was observed. In present the heat transfer rate and efficiency is low by using finned tube. Compare both existing and new model type heat exchanger over all heat transfer rate is increased and  also the efficiency of the heat  exchanger for  semi  circle fins is increase d by 3%, and for hexagonal fins is increased by 6% of finned tube heat exchanger. So the hexagonal fin is used than the semicircle fin.

Shewale omkar m1,et.al studied on the helical fins over the inner tube results into the increase in the heat transfer area and reduction in the hydraulic diameter of the flow channel. In addition to this the rotation of inner tube improve the turbulence and mixing of fluid molecules which is necessary to improve the heat transfer rate. For the present work the nusselt number for the inner tube with helical fins is 4 times higher than that of the plain inner tube for stationary condition. The nusselt numbers at the speed 50 rpm and 100 rpm are 36% and 64% more than that of stationary inner tube. The nusselt numbers at 100 rpm are 21 % higher than that of 50 rpm.

Sarmad a. Abdal hussein2,et.al studied on experimental investigations of heat transfer and friction factor of double pipe heat exchanger fitted with inserted semicircular disc baffles with spacing of 15cm and 45 cm carried out for turbulent flow. The following conclusion could be made:

The heat transfer coefficient and friction factor increases with the decrease in baffle spacing compared with smooth tube.

Inserted semicircular disc baffle (15 and 45) cm increases the heat transfer rate by 1.9 and 1.3 times that of smooth tube respectively.

C. K. Pardhi1 , et.al studied on performance improvement of double pipe heat exchanger by using turbulator . After investigating different heat transfer augmentation techniques it has found that:

As compared to conventional heat exchanger the augmented has shown a significant improvement in heat transfer coefficient by 61 % for twisted tape and 78% for twisted tape ii.

On equal pressure drop basis the smooth tube is better to twisted tape (1.3 to 1.7 times).

Twisted tape of lower twist ratio (p/d =3.5) gives higher heat transfer coefficient (by 1.39 times) than higher twist ratio of p/d = 7.

Chapter3

Methodology

3.1 Different types of heat transfer enhencement tecniques

3.1.1Types of Fin

3.1.1.1 Hexagonal Fin and Semicircle Fin

   Fig .3 Hexagonal fin   Fig.4 Semicircle fin

The small shell and tube heat exchanger is modeled with sufficient details to resolve the flow and temperature fields. In existing model the heat transfer rate is low by using finned tube. Compare  both existing and new model type heat exchanger  over all  heat  transfer  rate  is  increased  and  also the efficiency of the heat  exchanger  for  semi  circle fins is increase d by 3% , and for hexagonal fins is increased by 6% of finned tube heat exchanger. So the hexagonal fin is more efficient than the semi circle fin.

Heat exchanger efficiency:

Without  fin = 47%

With  hexagonal fin = 53%

With  semicircle fin = 50%

3.1.1.2Helical Fins on the Inner Rotating Tube

the helical fins over the inner tube results into the increase in the heat transfer area.In addition to this the rotation of inner tube improve the turbulence and mixing of fluid molecules which is important to improve the heat transfer rate . the nusselt number for the inner tube with helical fins is 4 times higher than  the plain inner tube at rest condition. The nusselt numbers at the speed 50 rpm and 100 rpm are 36% and 64% more than that of stationary inner tube. The nusselt numbers at 100 rpm are 21 % higher than that of 50 rpm

Fig.4.  Helical Fins

3.1.1.3Aluminium Fins with a Star-Shape Cross-Section

the effects of star-shape fin inserts on the heat transfer rate and pressure drop in a double-tube heat exchanger. Based on the review, the following conclusions can be drawn:

1. The overall heat transfer coefficient in a concentric-tube heat exchanger was enhanced with a star-shape fin insert by as much as 51% at a constant pumping power.

2. The rate of increase in the pressure drop was larger than the rate increase in the heat transfer rate, in general. We believe that a better heat transfer enhancement and lesser pressure drop could be achieved by making the fin thickness smaller so as to reduce flow restriction while maintaining a large surface area.

3. The benefit of a twisted fin may be obtained in a cross flow heat exchanger, where the twisted fin forces hot fluid to move circumferentially, while moving along the axial direction.

   Fig.5. Star-Shape Cross-Section fin

3.1.1.4 FIN MATERIAL

Metal Thermal conductivity   (W/(m•K))

Silver 429

Copper 399

Gold 316

Aluminium 235

Yellow brass 120

Cast iron 80.1

   Table 1

3.2 NANOFLUID

A nanofluid is a fluid containing nanometer-sized (<100nm) particles, called  nanoparticles. These fluids are engineered colloidal suspensions of nanoparticles in a base fluid .the nanoparticles used in nanofluids are typically made of metals, oxides, carbides or carbon nanotubes. Common base fluids include water, ethylene glycol  and oil. Nan fluids have novel properties that make them potentially useful in many applications in heat transfer.

3.2.1Need of Nan fluids:

Due to size of  nano particles, pressure drop is minimum.

Higher thermal conductivity of nano particles will increase the heat transfer rates.

Nan fluids are most suitable for rapid heating and cooling systems.

3.2 .2Types of Nano fluid

SERIAL NO. MATERIAL THERMAL CONDUCTIVITY (w/mk)

1 Engine Oil 0.15

2 Ethylene Glycol 0.25

3 Water 0.61

4 Silver 405

5 Alumina 200

6 Gold 319

   Table .2

3.3TWISTED TAPE

Based on the review results, key findings of this study could be summarized as follows:

The nusselt number obtained for the tube with twisted wire brush inserts varied from 1.55 to 2.35 times in comparison to those of the plain tube.

The inner convective heat transfer coefficient for twisted wire brush inserts is approximately 9-11 % higher than that for plain tube.

The pressure drop for twisted wire brush inserts is 4- 5 % higher than that obtained for plain tube.

  Fig.6 Twisted wire brush inserts

   Fig.7 Twisted tape

3.4 Experimental Setup

Fig.8. Line Diagram of counter and parallel flow heat exchanger

Description:-

The apparatus consists of a concentric tube heat exchanger. The hot water flows through inner tube and cold water flows through outer tubes. Direction of cold or hot fluid flow can be changed from parallel or counter to hot or cold water so that unit can be operated as parallel or counter flow heat exchanger. For flow measurement rotameters are provided a magnetic drive pump is used to circulate the hot water from a recycled type water tank, which is fitted with heaters and digital temperature controller.

Experimental procedure:

1 . Starting procedure (For Parallel Mode)

Close all the valves V1 – V8.

Open the lid of hot water tank, fill the tank with water and put the lid back to its position.

Ensure that switches given on the panel are at OFF position.

Connect electric supply the set up.

Open by pass valve V3 and switch ON the pump.

Switch ON the heater and wait till desired temperature achieves.

Connect cooling water supply to the set up.

Connect the outlet of cooling water to drain.

Open the valve V4 – V5 for circulation of cold water and adjust the flow rate by V1.

Allow hot water to flow through heat exchanger and adjust the flow rate by valve V2-V3.

At steady state record the temperatures and flow rate of hot and cold water.

Repeat the experiment for different flow rate of hot and cold water.

Repeat the experiment for different bath temperature.

Closing Procedure

When experiment is over switches OFF the heater.

Switch OFF the pump.

Stop cooling water supply by close the valve V1 and V4.

Drain the hot water tank by open the valve V8.

2.Starting Procedure (For Counter Mode)

Close all the valves V1-V8.

Open the cap of hot water tank, fill the tank with water and put the cap back to its position.

Ensure that switches given on the panel are OFF position.

Connect electric supply to the set up.

Open by pass valve V3 and switch ON the pump.

Switch on the heater and wait till desired temperature achieves.

Connect cooling water supply to the set up.

Connect the outlet of cooling water to drain.

Open the valve V6-V9 for circulation of cold water and adjust the flow rate by valve V1.

Allow hot water to flow through heat exchanger and adjust the flow rate by valve V2-V1.

At steady state record the temperature and flow rate of hot and cold water.

Repeat the experiment for different flow rate of hot and cold water.

Repeat the experiment for different bath temperature.

Closing procedure (For counter mode)

When experiment is over switches OFF the heater.

Switch OFF the pump.

Stop cooling water supply by close the valve V1 and V6.

Drain the hot water tank by open the valve V8.

3.5 CALCULATION

3.6  Method

Logarithmic mean temperature difference is defined as that temperature difference which , if constant , would give the same rate of heat transfer as actually occurs under variable conditions of temperature difference.

3.6.1 LOGARITHMIC MEAN TEMPERATURE DIFFERENCE FOR “PARALLEL FLOW”

The hot and cold fluid temperature distributions associated with a parallel-flow heat exchanger are shown in Figure. The temperature difference ∆T is initially large but decays with increasing x, approaching zero asymptotically.

For parallel flow, it follows thatT_hi=T_h1,T_h0=T_h2,T_ci=T_c1  and T_c0=T_(c2.)

The energy balances are subject to the following assumptions.

1. The heat exchanger is insulated from its surroundings, in which only heat exchange is done between the hot and cold fluids.

 2. Axial conduction along the tubes is negligible.

3. The change in kinetic energy and potential energy are negligible.  

4. The fluid specific heats are constant.

5. The overall heat transfer coefficient is constant.

The specific heats may change as a result of temperature variations, and the overall heat transfer coefficient may change because of variations in fluid properties and flow conditions.

Applying an energy balance to each of the differential elements of Figure.

dq = -m¬hcphdTh = -ChdTh – – – – – – – – – (1)

and  dq = mccpcdTc = Cc dT¬c   – – – – – – – – –  (2)

and its very costly process and consumed more time. So, this process is also not applicable. So we think about new process.

 

WhereCh and Cc are the hot and cold fluid heat capacity rates, respectively. These expressions may be integrated across the heat exchanger to obtain the overall energy balances Equations. The heat transfer across the surface area dA may also be expressed as

dq = U ∆T dA  – – – – – – – – – – – (3)

Where ∆T = Th – Tc is the local temperature difference between the hot and cold fluids. To determine the integrated form of Equation (3), we begin by substituting Equations (1) and (2) into the differential form of Equation ∆T = Th – Tc

d(∆T) = dTh – dTc

d(∆T) = -dq(1/Ch+ 1/Cc)

Substituting for dq from Equation (3) and integrating across the heat exchanger, we obtain

ln((∆T_2)/(∆T_1 ))= -UA(1/C_h +1/Cc) – – – – – – – – – – – – (4)

Substituting value of C¬h and Cc, it follows that

ln((∆T_2)/(∆T_1 ))= -UA/q[(Thi – Tci) – (Th0¬– Tc0)]

Recognizing that, for the parallel-flow heat exchanger of Figure 1 ,∆T1 = (Thi – Tci) and

∆T2 = (Th0¬– Tc0), we than obtain

    q=UA (∆T_2-∆T_1)/(ln⁡((∆T_1)/(∆T_2 )))

Comparing the above expression with Equation q=UA ∆Tm, we conclude that the appropriate average temperature difference is a log mean temperature difference, ∆Tlm. Accordingly, we may write

    Q=UA∆T_lm – – – – – – – – – – – – – (5)

Where  

∆Tlm = (∆T_2-∆T_1)/(ln⁡((∆T_2)/(∆T_1 ))) = (∆T_1-∆T_2)/(ln⁡((∆T_1)/(∆T_2 )))   – – – – – – – – – – – – – (6)

3.6.2 LOGARITHMIC MEAN TEMPERATURE DIFFERENCE FOR “COUNTER FLOW”

    

The hot and cold fluid temperature distributions associated with a counterflow heat exchanger are shown in Figure 2. In contrast to the parallel-flow exchanger, this provides for heat transfer between the hotter portions of the two fluids at one end, as well as between the colder portions at the other. Than the change in the temperature difference,∆T = Th – Tc, with respect to x. Note that the outlet temperature of the cold fluid may now increase the outlet temperature of the hot fluid.

It may be shown that Equations 5 and 6 also apply. However, for the counterflow heat exchanger the endpoint temperature differences must now be defined as

(∆T_1≡T_h1-T_c1=T_hi-T_co)¦(∆T_2≡T_h2-T_c2=T_ho-T_ci )

  ……………………. (7)

Note that, for the same inlet and outlet temperatures, the log mean temperature difference for counterflow exceeds that for parallel flow,∆T_(lm,CF)>∆T_(lm,PF). Hence the surface area required to effect a prescribed heat transfer rate q is smaller for the counterflow than for the parallel-flow arrangement, assuming the same value of U. Also note that Tc,o can exceed Th,o for counterflow but not for parallel flow.

3.6.3 NTU METHOD

If only the inlet temperatures are known, the LMTD method requires iteration. In these cases (and in some others), the effectiveness-NTU method should be used instead ( ε − NT U ). REMEMBER: Q˙ = q.

We want to so that   we’ll first show that this is true for  parallel – flow, where Ch – Cmin.

So, what is q equal to, in terms of the temperature and heat capacity rate of the hot fluid?

q = Ch (Thi –  Tho)

Now, returning to the derivation of the LMTD, we know that for a parallel-flow heat exchanger.

Now we have three expressions for temperature that relate to ∈, NTU, and Cr and we can eliminate temperature altogether to find ∈ only in terms of NTU and Cr. the derivation is as follows:

Tco = Cr ( Thi – Tho ) + Tci