HEAT TRANSFER ANALYSIS IN FINNED TUBE HEAT-EXCHANGER WITH DESICCANT TABLET A Project Report for MINI PROJECTSubmitted by:AKARSH SONI (2011ME10648)AMIT RANJAN (2011ME10653)Under the esteemed guidance ofPROF. P.M.V. SUBBARAOMECHANICAL ENGINEERING DEPARTMENTINDIAN INSTITUTE OF TECHNOLOGY DELHI HAUZ KHAS, NEW DELHI-110016
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8/12/2019 Final Mini p Report Akarsh Soni and Amit Ranjan
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.
8/12/2019 Final Mini p Report Akarsh Soni and Amit Ranjan
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.
8/12/2019 Final Mini p Report Akarsh Soni and Amit Ranjan
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.
8/12/2019 Final Mini p Report Akarsh Soni and Amit Ranjan
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)
8/12/2019 Final Mini p Report Akarsh Soni and Amit Ranjan
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.
8/12/2019 Final Mini p Report Akarsh Soni and Amit Ranjan
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.
8/12/2019 Final Mini p Report Akarsh Soni and Amit Ranjan
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:
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.
8/12/2019 Final Mini p Report Akarsh Soni and Amit Ranjan
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