Final Mini p Report Akarsh Soni and Amit Ranjan

8/12/2019 Final Mini p Report Akarsh Soni and Amit Ranjan

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HEAT TRANSFER ANALYSIS IN FINNED TUBE

HEAT-EXCHANGER WITH DESICCANT TABLET

A Project Report for

MINI PROJECT

Submitted by:

AKARSH SONI (2011ME10648)

AMIT RANJAN (2011ME10653)

Under the esteemed guidance of

PROF. P.M.V. SUBBARAO

MECHANICAL ENGINEERING DEPARTMENT

INDIAN INSTITUTE OF TECHNOLOGY DELHI

HAUZ KHAS, NEW DELHI-110016



ACKNOWLEDGEMENT

I would like to express my gratitude to everyone who contributed, in

different ways, in completion of this project. My first thank has to go to my

supervisor- Prof. P.M.V. Subbarao for his keen interest, invaluable guidance

and timely supervision throughout the project work. Working with him was

more of a learning process which generated a lot of interest inside me

towards the project.

I also wish to extend my thanks to faculty members of MechanicalEngineering Department for their support and continuous inspiration. I would

like to express my sincere regards to Mr. Raj for providing immense help in

completing this project and their assistance during various phase of project.

Finally thanks to my parents and friends for their direct or indirectsupport and constant encouragement.

April, 2014 Akarsh Soni & Amit Ranjan



INTRODUCTION AND THEORY

Adsorption refrigeration and heat pump have been thought as environment benign and costeffective in waste recovery heat system. Adsorption working pair of calcium chloride-ammonia has been proposed and developed for cooling.

Phase I : Initially reactor is cooled by a heat transfer fluid at Tm (ambient temperature). This

causes a drop in pressure in the reactor. When the pressure in the reactor reaches the

evaporator pressure the gas valve-1 connected to the evaporator is opened. The ammonia

gas at low pressure and temperature from evaporator is supplied to reactor and is adsorbed

by CaCl2 adsorbent.

The valve-1 is closed when there is equilibrium between evaporator and reactor pressure.

Phase II : After completion of phase I, reactor is heated by a heat transfer fluid because of

which adsorbed ammonia is released by CaCl2 in the form of ammonia vapours. The

heating is continued till the ammonia gas pressure reaches the condenser pressure. (The

temperature increase due to heating of ammonia and formation of ammonia vapours leads

to an increase in pressure inside the reactor.)

The valve-2 is opened when the ammonia gas pressure in reactor exceeds thecondenser pressure.





OBJECTIVE AND MOTIVATION

Objective:

Do heat transfer analysis in finned tube heat exchanger (solidified compoundadsorbent made by CaCl2 and activated carbon) and experimentally-

1. Compare the heat transfer through HX using "CaCl2" and "CaCl2 with Activatedcarbon" as desiccant.

2. Compare the heat transfer through HX by varying the amount of activated carbonin CaCl2.

3. See the effect of mass flow rate of hot fluid, different temperature of hot fluidon the heat transfer through the HX.

Motivation:

Although some work has been done on problems related to swelling and smashing of theCaCl2 adsorbent particles. Heat transfer analysis of the available system has not yet beenconducted. Use of the system without any proper heat transfer analysis can be termed as

‘brutal engineering’ because of the resources that would be wasted in doing so.

We intend to study the heat transfer analysis to extract maximum possible energy from

the waste energy (we intend to provide heat to HX through low grade heat sources) by

considering the factors affecting heat transfer and hence maximising the performance of

finned tube HX. This might also open the doors for use of low grade heat source for the

device.



SCHEMATIC



LITERATURE REVIEW

Following works had been studied for a better understanding of the project at hand andthe fundamental principles related to it.

Marcel Lacroix, University of Sherbrooke: He presented a theoretical model for

predicting the transient behaviour of a shell-and-tube storage unit with the PCM on theshell side and the HTF circulating inside the tubes. A series of numerical experimentswere done to assess the effects of:

(1) The shell radius: On increasing radius, melting temperature of PCM is not reached,only sensible heat is stored.

(2) The mass flow rate and inlet temperature of the HTF: Stored energy in PCM varieslinearly with inlet temperature and with an increasing slope for augmenting mass flow rate.

(3) The presence of fins attached to the inner tubes on the thermal behaviour of thethermal unit: Annular fins were most effective for moderate mass flow rates and small inlettemperatures.

L.W. Wang, R.Z. Wang, J.Y. Wu, K. Wang: Adsorption performances of three types of adsorbents, CaCl2 with different expansion space, simple compound adsorbent andsolidified compound adsorbent made by CaCl2 and activated carbon, were tested, in whichammonia was used as refrigerant.

The solidified compound adsorbent showed best performance for adsorption ice makerson fishing boats for the larger filling quantity of adsorbent. The mass transfer performanceis improved by the addition of activated carbon in solidified compound adsorbent at thecondition of low evaporating temperature.

L. Wang, L. Chen, H.L. Wang, D. L. Liao: Study was conducted on the adsorption

characteristics of the adsorption refrigeration working pairs using alkaline-earth metalchlorides as adsorbents and ammonia as refrigerant. The adsorption refrigerationexperiments of composite adsorbents were investigated. The study indicated that therefrigeration capacity could be enhanced by compositing the adsorbents whichindicates that composite adsorbents can perform better in adsorption refrigeration, andcan be employed in adsorption refrigeration system using low-grade heat source.

Aytunc Erek, Dokuz Eylul University: He studied the heat transfer enhancement in the thermal energy storage system by using radially finned tube. The solution of the systemconsisted of solving the equations of the heat transfer fluid (HTF), the pipe wall and fin,and the phase change material (PCM) as one domain. Fully developed velocity distributionwas taken in the HTF.

The results showed that solidification fronts can be significantly increased if the fin height isincreased. These results gave some knowledge for the design of the thermal energy storagesystem.



FABRICATION OF HX WITH DESICCANT AND PROCEDURE

1. Manufacturing of finned tube heat exchanger:

The finned tube of following dimensions was used to conduct the

experiment: Length of finned tube = 25 inch = 63.5 cm

Inner diameter of tube = 16 mm

Outer diameter of tube = 17 mm

Diameter of fins = 35 mm (from the outer dia. Of

tube) Fin spacing = 5 mm

Fins per unit length = 29 per 5 inch = 2.29 per 1 cm

Thickness of fin = 0.5 mm

Manufactured finned tube were obtained from the industry directly and were usedto conduct the experiment.

2. Preparing mixture of “CaCl2 + Water” or “CaCl2 + Activated Carbon

+ Water”:

Amount of water was calculated for the amount of CaCl2 used to fill the finnedtube considering CaCl2.3H2O

54 gm water (wt. of 3 molecule of water) in 111 gram of CaCl2 (wt. of 1 molecule of

CaCl2) Mixture was prepared by carefully mixing water in CaCl2 (or activated carbon

if used) as the reaction is exothermic in nature and large amount of heat is liberated

in preparing small amount of mixture. 2 + 3 2 → 2.3 2 + .



3. Calibration of thermocouples:

Thermocouple wires (8 wires) were prepared (beads were made using Welder).After this, they were dipped into ice (0C) and hot water (100C) respectively andrespective temperature were noted down from digital analogue.

For eg: Wire 1 showed temperature of ice and hot water--> -3C and 96C respectively

While they were at a temperature of 0 and 100 C respectively. So, error of thermocouplewire 1 is [(-3)+(-4)]/2 = -3.5 Hence, 3.5 was added to the temperature shown bythermocouple while doing the experiment. (Tactual = T thermo. + 3.5)



4. Filling of finned tube with desiccant and removing water from it

After preparing the mixture of CaCl2, activated carbon and water. The mixture was filledin the finned tube uniformly.

As it is desiccant so it adsorbs water the surrounding and hence there is a need to heatit in a furnace to desorb extra water molecules from the filled fin tube.



Fig: Drying the paste after filling in finned tubes

5. Preparing experimental set-up

Experimental set-up consisted of:

Heat Exchanger:

It was a long tube made of mild steel with radial fins on the outer surface of it. Length

of finned tube was approx. 25 inch or 63.5 cm. Inner and outer diameter of tube were

16 and 17 mm respectively. Fins were of diameter 35 mm (from the outer dia. Of tube)

and 0.5 thickness with spacing of 5 mm in between.

The spacing between the fins was filled with desiccant mixture i.e. “CaCl2 + Activatedcarbon + Water” uniformly.



Fig: Heat Exchanger with high pressure tube Fig: Duct around HX

Duct: Heat exchanger was enclosed by a pipe to see the effect of natural and forcedconvection. Used to guide the air flow over the heat exchanger. The ambient airenters from the bottom and the heated air leaves the duct at the top.

High pressure tube: It was connected to the ends of the finned tube with m-seal

ensuring that there is no leakage from the ends. Hot flow from the water flow system

was passed through this high temp and high pressure pipe to the finned tube HX.

Water flow system: It consisted of tubes connecting tank, high pressure tube and pump.

To measure mass flow rate- Manometer was connected across the orifice plate toobtain the pressure difference across orifice. Hence, calculating the mass flow rate ofhot water through the HX.

Heater- Heaters were used to vary the temperature of water flowing through the HX(i.e. 60, 70, 80 C) And an electrical control system was provided to control thetemperature and maintain a steady value.

Pump: To circulate hot water through the Heat Exchanger.



Digital Analogue: Thermocouples from the different points of HX (along the HX at 0, 5, 10,

15, 20, 25 inch points from one end) were connected to the digital analogue and reading

were taken from it to get the temperature variation along the length of HX.

Blower & Anemometer:

Blower was used to provide air velocity in the duct parallel to the HX i.e. along thelength of finned tube and test the HX for forced convection.

To measure the air flow velocity in the duct in which HX is placed. Hence, mass flowrate of air can be calculated i.e.

=̇ ∗ ∗



6. Conducting experiment for different parameters

After achieving steady flow through the HX and constant hot water temperature, wetook observation i.e. temperature reading from the digital analogue for different sets:

1) CaCl2 2) CaCL2+Activated carbon (.125)* 3) CaCl2+Activated carbon (.2)*

*weight fraction of activated carbon in the desiccant mixture of CaCl2, activatedcarbon and water.

All above HXs were tested for different parameters i.e. different temperature of hotwater, mass flow rate of hot water, free and forced convection.

Figs: Full system including HX, Pump, Heater. Water Flow system,Digital Analogue, High pressure tube, Manometer.





Pipe inlet 80.8 82 81.6

Air velocity 0.1 0.1 5.7m/s

Ambient Temp.

© 36 36 36

Air outlet temp

© 40.1 41 39

Dia. Of

external pipe

(cm) 18 18 18

b) At 70 C

Mass

Flow

rate

(Kg/m3) 5 2.5 5

type Free Free Forced

Points along the Temperature Temperature Temperature

length of pipe (C) (C) (C)

Pipe outlet

0 68 68.3 67.4

5 68.3 68.6 67.9

10 68.6 68.8 68.2

15

68.9

69.1

68.8

20 69.3 69.4 69.1

25 69.6 69.8 69.5

Pipe inlet 70 70 70

Air velocity 6.2

Ambient Temp. © 36 36 36

Air outlet temp © 40 41 39

Dia. Of external

pipe 16 16 16

c) At 60 C

Mass

Flow

rate

(Kg/m3) 5 2.5 5



Flow

type Free Free Forced

Points along the Temperature Temperature Temperature

length of pipe (C) (C) (C)

Pipe outlet

25 58.3 58.5 58

20 58.6 58.8 58.4

15 58.9 59.2 58.9

10 59.3 59.5 59.3

5 59.6 59.7 59.6

0 60 60 60

Pipe inlet

Air velocity 5.9

Ambient Temp. ©

36

36

36

Air outlet temp © 40 41 39

Dia. Of external

pipe 16 16 16

d) Desiccant = CaCl2 + Activated Carbon

a. At 60 C

Mass Flow Rate

(kg/s) 5 2.5 5Flow type FREE FREE FORCED

Points along pipe

TEMP

©

TEMP

©

TEMP

©

Outlet 25 59 59 58.5

20 59.3 59.2 58.8

15 59.5 59.4 59.1

10 59.5 59.8 59.4

5 59.7 60 59.7

Inlet 0 60 60 60

Duct diameter=17cm

Air outlet temp 42 43 42

a) At 70 C

Mass Flow Rate

(kg/s) 5 2.5 5

Flow type FREE FREE FORCED



Points along

pipe

TEMP

©

TEMP

©

TEMP

©

Outlet 25 69 69 68

20 69.2 69.1 68.2

15 69.4 69.3 68.7

10 69.5 69.5 69.1

5 69.8 69.7 69.6

Inlet 0 70 70 70

Duct diameter=17cm


a) At 80 C

Mass Flow Rate

(kg/s) 5 2.5 5

Flow type FREE FREE FORCED

Points along

pipe

TEMP

©

TEMP

©

TEMP

©

Outlet 25 79 79 78

20 79 79.2 78.4

15 79.3 79.3 78.6

10 79.6 79.6 79.15 79.7 79.8 79.6

Inlet 0 80 80 80

Duct diameter=17cm


SAMPLE CALCULATION

Calcium chloride desiccant at full opening valve forced convection:

Water inlet temp. = 80.8ᵒ C

Water outlet temp. = 77.6ᵒC

Orifice diameter= 18cm

Pipe diameter =65cm



Pressure difference in orifice section = 5mm of mercury

=5*13600*9.81/1000

=667.08 Pa

Volume flow rate through tube = Q

=Cd*A2*√2*(P1-P2)/ρ*(1D22/D2

1)

=.75*∏*.182/4 *√2*667.08/1000(1-.182/.652)

=.229*10-3 m3/s

Mass flow rate of water through tube = ρ*Q

=1000*0.229*10-3

=0.229 kg/s

Heat transferred = mw* Cp*(Tw2 – Tw1)

=0.229*4.186*(80.8-77.6)*1000

=3080 W

Air intlet temp = 36ᵒC

Air outlet temp.= 39ᵒC

LMTD = [(80.8-36)-(77.6-39)]/ln[(80.8-36)/(77.6-39)]

=42.42ᵒC

Therefore, UASTln = 3080

Hence, U = 3080/(.06982*42.42)

=1039.92 W/m2 ᵒC



RESULT TABLE 1: CaCl2 only

Type of

flow

Inlet

temperature

©

Outlet

Temperature

©

MassFlow

Rate

(kg/s) ∆Tlm

U(W/m2

ᵒc)

Air inlet

temperature

©

Air inlet

temperature

©

Free 81.6 78.9 0.23 2021 41.25 701.7 36 41

Free 82 80 0.115 962.7 45.41 303.64 36 40

Forced 80.8 77.6 0.23 3080 42.42 1039.92 36 39

Forced 70 67.4 0.23 2503 30.32 1190 37 39

Forced 60 58 0.23 1925 20.93 1317.3 37 40

RESULT TABLE 2: CaCl2 + Activated Carbon

Type of

flow

Inlet

Temperature

©

Outlet

Temperature

©

Mass

Flow

Rate

(kg/s)

Air inlet

temperature

Air outlet

temperature ∆Tln

U(W/m2

ᵒC)

Free 80 79 0.115 481.39 40 43 17.92 181.63

Free 80 79 0.23 1925 40 42 38.48 716.49

Forced 80 78 0.23 1925 40 42 37.96 726.31

Forced 70 68 0.23 1925 40 42 27.95 986.43

Forced 60 68.5 0.23 1444.17 40 42 18.19 1137.11



DISCUSSION

If we increase the mass flow rate of the hot water flowing throughtube then overall heat transfer co-efficient value of air side increases.

It can be explained as Cmin/Cmax ratio decreases while we increase

the value of mass flow rate and effectiveness value will not change

much more by changing the flow rate so we get a higher value of

Nusselt number implies higher value of overall heat transfer co-

efficient.

In case of blower we get higher value of overall heat transfer co-efficient as it increases the convective heat transfer co-efficient which

depends strongly on air speed. Also increasing air speed will decrease

the value of Cmin/Cmax ratio which leads to increase in value of NTU

and so overall heat transfer co-efficient.

As we increase the hot water temperature flowing through tube we get

lower value of overall heat transfer coefficient. We are expecting it ti be

high as due to higher temperature difference between watertemperature and ambient temperature heat transfer magnitude should

be higher and so overall heat transfer coefficient.

It happens because of stability of desiccant. At higher temperature the

desiccant particles is not bounded together and it lose contact with the

finned tube and increasing the thermal resistance. So we get lower

value of overall heat transfer coefficient.

The overall heat transfer coefficient for desiccant with activatedcarbon is lesser than with only calcium chloride as a desiccant. In ourexperimental temperature range its value is around 8-20% lesser thanthat of calcium chloride desiccant. But this heat exchanger is used in

adsorption refrigeration cycle where ammonia gas is adsorbed indesiccant and by using calcium chloride with activated carbon its mass

transfer performance is improved appreciably which compensates forits lower value of overall heat transfer co-efficient.



SOURCES OF ERROR

Calcium chloride as a desiccant starts adsorbing water from surrounding.So its property will be altered. So some of the heat transfer fraction will be

used by water molecules latent heat of fusion and we get lower value of air

outlet temperature implies high value of LMTD and so lesser value of

overall heat transfer co-efficient. To improve this we can conduct

experiment in an evacuated chamber and passing dry air to the duct.

Thermocouples are not welded along the different sections of the

tube. During heating of the finned tube desiccant becomes softer and itthermocouple may get misplaced.



REFERENCE

1.

The adsorption refrigeration characteristics of alkaline-earth metal chlorides andits composite adsorbents.

By- L. Wang, L. Chen*, H.L. Wang, D.L. Liao, The School of Chemistry and ChemicalEngineering, South China University of Technology, Guangzhou 510640, GuangDong, China.

2. Compound adsorbent for adsorption ice maker on fishing boats.

By- L.W. Wang, R.Z. Wang*, J.Y. Wu, K. Wang, Institute of Refrigeration andCryogenics, Shanghai Jiao Tong University, Shanghai, 200030, China.

3. Study of the heat transfer behaviour of a latent heat thermal energy storage unitwith a finned tube.

By- Marcel Lacroix, Department de genie mecanique, Universite deSherbrooke, Sherbrooke, Canada.

4. Phase change around a finned tube.By- Aytunç EREK, Dokuz Eylül Üniversitesi, Mühendislik Fakültesi, MakinaMühendisliği Bölümü, 35100-Bornova/İzmir

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Final Mini p Report Akarsh Soni and Amit Ranjan