SUPERVISOR:-K. MANJUNATHAssistant Prof., Department Of Mechanical Engineering,Delhi Technological University

TEAM MEMBERS:-Aman Kumar(2K10/ME/019)Ankit Gupta(2K10/ME/027)Ankit Himmatramka(2K10/ME/028)Anurag(2K10/ME/033)

SECOND LAW BASED OPTIMISATION OF REFRIGERATOR

AND AIR CONDITIONER EVAPORATOR

REFRIGERATION“The American Society of Refrigerating Engineers” defines refrigeration as “THE SCIENCE OF PROVIDING AND MAINTAINING TEMPERATURES BELOW THAT OF SURROUNDING TEMPERATURE”.

PURPOSES OF REFRIGERATION:- To reduce the temperature of substance. To transform a substance from one state to another. To maintain substance at desired state.

INTRODUCTION

The evaporator is one of the four basic and necessary hardware components of the refrigeration/AC system.

It is basically a heat exchanger.

In the evaporator, the refrigerant is evaporated by the heat transferred from the heat source.

The heat source may be a gas or a liquid.

Used in : Refrigerators, Air-Conditioners, Cold Storage, Industrial Waste Disposal etc.

EVAPORATOR

HOW AN EVAPORATOR WORKS

Domestic Refrigerator/Industrial RefrigeratorWindow AC/Split AC

Natural and Forced Convection Type Refrigerant Flow Inside or Outside Tubes Flooded and Dry Type

Shell-and-Tube EvaporatorShell-and-Coil EvaporatorDouble pipe type Evaporator Baudelot type EvaporatorDirect expansion fin-and-tube EvaporatorPlate Surface Evaporator

TYPES OF EVAPORATOR

REFRIGERATION CYCLE

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OPTIMISATION OF EVAPORATORENTROPY GENERATION MINIMISATION(EGM)EXERGY ANALYSISTHERMAL DESIGN ANALYSISTO ANALYZE PERFORMANCE FACTORSIDENTIFY & MINIMISE THE PROBLEMSANALYTICAL CALCULATIONSSOFTWARE SIMULATION – MATLABGENETIC ALGORITHM

OBJECTIVE

Review of research papers and journals

LITERATURE REVIEW

ADRIAN BEJAN,1975

Book – “ENTROPY GENERATION MINIMIZATION”(1995)

Book - “THE METHOD OF THERMODYNAMIC OPTIMIZATION OF FINITE-SIZE SYSTEM AND FINITE-TIME PROCESSES”(1995)

EGM combines the basic principles of Heat Transfer, Fluid Mechanics, Thermodynamics to modeling and optimization of real systems and processes that are governed by finite size and finite time constraints, and are limited by heat and mass transfer and fluid flow irreversibilities.

ENTROPY GENERATION MINIMISATION(EGM)

This work applies the second-law analysis of thermodynamics to quantify the exergy destruction of the components of screw liquid chiller, and to identify the potential for each component to contribute to improve the overall energy efficiency of the system.

A component associated with a higher proportion of exergy destruction may more dramatically contribute to improve energy efficiency.

Second-law analysis is demonstrated to be a feasible and effective means in improving the energy efficiency of a screw liquid chiller.

TZONG-SHING LEE(2010)

An extensive EGM-based optimization analysis of the peripheral finned-tube extended surface geometry has been conducted in this paper

There is an optimum NTU associated with a minimum entropy generation number for both types of heat transfer boundary condition.

The heat exchanger effectiveness is not a suitable performance parameter to evaluate the heat exchanger performance because it increases monotonically with the NTU and does not take into account the fluid pumping power through the heat exchanger.

BRUNO F. PUSSOLI, JADER R. BARBOSA JR.(2012)

First law of thermodynamicsBased on energy conservationEnergy can neither be produced nor destroyed, it can only change it’s form. Second law of thermodynamicsClausius statement of second law of thermodynamics: “It is impossible to have a device that while operating in a cycle produces no effect other than transfer of heat from a body at low temperature to a body at higher temperature.”Kelvin-Planck statement of second law of thermodynamics: “It is impossible for a device operating in a cycle to produce net work while exchanging heat with bodies at single fixed temperature”.

LAWS OF THERMODYNAMICS

aos.wisc.edu

Fundamental data on two-phase flow boiling pressure drop are essential for the design and operation of heat exchangers.

The experimental pressure drop results of these study show that the pressure drop along the test section increases with mass flux and exit quality but decreases with system pressure. The pressure drop gradients in 2.01 mm tube are about 3 times higher than those in the 4.26 mm tube.

X. Huo, D. Shiferaw, T.G. Karayiannis, Y.S. Tian and D.B.R. Kenning(2008)

Analytical expressions for optimum flow rates in evaporator of refrigerating system

The expression derived can be used for,

Design purposes

Checking suitable flow velocities in existing plants.

Incorporating in algorithms for optimal operation of systems with variable speed compressors.

E. Granryd(2010)

Experimental investigation on 2 phase flow heat transfer of 5 refrigerants in horizontal small tubes of various inner diameter

The effects of mass flux, heat flux, saturation temperature and inner tube diameter on the heat transfer coefficient are reported.

Jong-Taek Oh, A.S. Pamitran, Kwang-Il Choi, Pega Hrnjak(2011)

Any substance capable of absorbing heat from another substance.

Liquid/gas used in evaporator tubes.

Source of cooling the gas (refrigerant) or air (air conditioning).

High latent heat of vaporization.

Ammonia, CO2, SO2, Ethane, Ethylene, Freon(fluorinated hydrocarbons) family like Freon-11,12,13,14,etc, Azetropes, Water Vapor, Air, NO2 etc.

R600a (isobutane), R290 (propane), R134a, R22, R410A, and R32 etc.

REFRIGERANT

An Optimized Design of Finned-Tube Evaporators Using the Learnable Evolution Model

We described an experimental system, ISHED1, developed to assist a design engineer in optimizing finned-tube evaporators. Specifically, given input parameters and technical constraints, the system optimizes the refrigerant circuitry in the evaporator. The novelty of this approach is in applying the recently developed learnable evolution model, which integrates knowledge-based evolutionary computation with symbolic learning that guides the process of generating new designs. Generated designs are evaluated using the EVAP evaporator model, which simulates the designs.

Piotr A. Domanski, David Yashar, Kenneth A. Kaufman, Ryszard S. Michalski(2004)

Domanski, Yashar, Kaufman and Michalski (2004)

A FUNCTIONAL ARCHITECHTURE OFISHED1

This paper presents a comparable evaluation of R600a (isobutane), R290 (propane), R134a, R22, R410A, and R32 in an optimized finned-tube evaporator, and analyzes the impact of evaporator effects on the system coefficient of performance(COP).

They optimized evaporator circuitry for each refrigerant using a non-Darwinian evolutionary scheme, and performed simulations of the optimized evaporators.

better COP for low-pressure refrigerants (having a high critical temperature) versus high-pressure refrigerants (having a low critical temperature).

The high-pressure refrigerants provided higher evaporator capacities than the low-pressure refrigerants.

Piotr A. Domanski, David Yashar, Minsung Kim(2005)

Domanski, Yashar and Kim (2005)

Comparison ofCOP of variousRefrigerantsWith R22

This paper presents an optimal design of a micro evaporator to maximize the heat transfer coefficient for an active micro cooler

They designed optimal parameters to maximize the heat transfer coefficient. Selected parameters are the number of gaps, the channel width, the lateral gap size.

The optimal parameter set is determined as 3, 0.5 mm and 1.25 mm respectively. The maximized heat transfer coefficients are 0.465, 0.457 and 0.430 W/cm2K for the heater powers of 40, 60 and 80 W, respectively.

Taijong Sung, Daesik Oh, Taewon Seo and Jongwon Kim(2007)

This paper presents experimental performance analysis of a window air conditioner(capacity, 1TR) with and without heat exchanger(IHE), along with performance comparison of R22 and R407C in the same AC.

Use of IHC has increased coefficient of performance(COP) when running on R22(5.86%) and R407C(6.3%). When retrofitting R22 with R407C, COP is found to drop with(6.26%) and without(6.64%) IHE.

R Vijayan and P S S Srinivasan(2009)

This paper presents experimental results of the heat transfer performance of New Tri-tube type evaporators

The low temperature evaporator test facility was developed to closely simulate refrigerator-freezer conditions.

The overall heat transfer coefficient of tri-tube type evaporator is increased about 120% comparing of finned tube type evaporator.

The defrosting heater power of tri-tube type evaporator is decreased about 50% comparing of finned tube type evaporator.

Jang-Seok Lee, Tae-Hee Lee, Seong-Hun Ham, Se-Yoon Oh, Kwan-Shik Cho(2010)

This paper describes a steady state model of a multiple effect evaporator system for simulation and control purposes.

Steady state simulation is done successfully by using ‘fsolve’ solver in MATLAB source code for three feeding sequences backward, mixed and splitting.

The effect of variation of various input parameters on steam economy is studied.

They concluded that mixed feed is always optimal for entire range of each parameters.

Deepak Kumar, Vivek Kumar, and V. P. Singh(2010)

Primary Factors :

Refrigerant used

Air Flow Distribution

Turbulence

Area in contact with refrigerant

Conduction Path Resistance and Piping Geometry

Refrigerant Pressure Drop and Superheating Value

FACTORS AFFECTING PERFORMANCE OF AN EVAPORATOR

Refrigerant Used

It should be,

Non-corrosive.

Safe (nontoxic, nonflammable, ecofriendly).

Having favourable thermodynamic properties.

Examples – Ammonia, CFC, Freons, R22, R410A, R744 etc.

Air Flow Distribution

The air flow distribution should be uniform for maximum efficiency.

Liu & Chen (2012) found that for equal flow rates, efficiency of a multi-circuit evaporator decreased by 7.78% in case of non-uniform distribution of air flow as compared to uniform one.

The decrease in performance can be attributed to decreased heat transfer in case of non-uniform distribution of air flow.

REFRIGERANT & AIR FLOW DISTRIBUTION

TurbulenceTurbulence in evaporator should be minimised to prevent unwanted losses (in vibration etc.) and maintain system stability.

To minimise turbulence, harsh angles or rough surfaces in coils should be avoided as they disrupt the normal flow pattern.

Area in contact with refrigerantArea in contact with refrigerant vapour should be large.

Large contact area allows more heat transfer per unit time for a given temperature difference between the evaporator coils and refrigerant.

TURBULENCE & SURFACE AREA

Conduction Path Resistance

Conduction path resistance should be low.

Overall resistance of conduction path depends on thermal resistance of material used and the width & length of the conducting channel.

So, they should be accordingly varied to get lowest resistance.

Pressure drop and Superheating valueIn case of refrigerant pressure drop along coil length (Hermes, 2004),

Without Superheating, flow is purely parallel. So, performance of parallel flow evaporator is more than the counter flow arrangement.

With Superheating, flow is part counter, part parallel. So, the performances of above mentioned coil arrangements depend on predominant manner of flow.

PATH RESISTANCE & PRESSURE DROP

Major problems in the working of an evaporator :

Frost Formation

Fouling and unpleasant odour

Corrosion and leakage

Dirt accumulation

Foaming

PROBLEMS ENCOUNTERED

The evaporator coils tend to be naturally cold, and generally hover right around the freezing point of water. If moisture stays on the fins too long, the water can freeze and begin to coat the blades. This is typically caused by blocked coils or failed expansion valves, and the frost layers can seriously inhibit the performance of the air conditioning system.

(www.howstuffworks.com)

FROST FORMATION

Fouling is the accumulation of unwanted material on solid surfaces to the detriment of function. The fouling material can consist of either living organisms or a non-living substance (inorganic or organic).

Foods, proteins and polysaccharides can create such deposits that reduce the efficiency of heat transfer.

Due to fouling there is a build-up of mold and bacteria on the evaporator fins which produce foul odour.

Air conditioning systems are flushed to remove these odours, and sometimes the fins are coated with a moldicide protectant.

(Gerlach and Newell, 2001)

FOULING AND PECULIARODOUR

Corrosion can occur when acidic solutions such as citrus juices are concentrated. The surface damage caused can shorten the long-life of evaporators.

Brass tube with corrosion traces in it (wikipedia.org).

Excessive corrosion can lead to leakage of the refrigerant from the evaporator coil, thus hampering its operation and performance because of loss of heat circulation.

(Shen, Su & Wang, 2000)

CORROSION AND LEAKAGE

In the normal process of operation, the evaporator coil and fins can become coated with normal dust, dirt and lint, which is often attracted by water. These layers of dirt are not as dangerous as frost, but they can still stop the evaporator from performing correctly and inhibit the efficiency of the entire system.

Foaming can also create a problem since dealing with the excess foam can be costly in terms of time and efficiency. Antifoam agents are to be used, but only a few can be used when food is being processed.

(Byuan and Lee, 2010)

DIRT ACCUMULATION AND FOAMING

The following conclusions can be drawn from literature review :

The heat transfer between refrigerant and air/gas is two phase heat transfer.The pressure drop between refrigerant and air/gas is two phase pressure drop.Refrigerant undergoes phase change with different flow patterns.Performance parameters can be optimized using Entropy Generation Minimization (EGM) Technique.Common problems in evaporator operation have been identified and thus, efforts to lessen them can be made.Exergy analysis can be further carried out to optimize the performance parameters.

STILL THERE IS A SCOPE TO OPTIMIZE AN EVAPORATOR ON THE BASIS OF SECOND LAW OF THERMODYNAMICS

WORK DONE SO FAR

The first law analysis does not consider the irreversibilities occurring in the system which may lead to poorer thermal design of evaporators.

By using second law analysis we will be able to analyse the irreversibilities taking place in the system easily. By the entropy generation analysis of thermal system, we will obtain the geometric and operating parameters which improve the performance of thermal systems and reduce operating cost.

Exergy or entropy generation analysis provides easier way for optimizing the geometric and operating parameters.

FUTURE SCOPE

In this project, initially we have worked to choose the area of field, topic of project work and method of analysis.

As a part of the initial step, we carried out literature survey and came to know some of scope for further work.

Based on the conclusions of literature survey, we have defined the problem and started working on it.

In the future, work that we will be carrying out is problem formation and simulation.

After doing parametric study of the system we will taking up the optimization procedure.

CONCLUSION

Bejan, A. (1977) ‘The concept of irreversibility in heat exchanger design: counter flow heat exchangers for gas-to-gas applications’, Journal Heat Transfer, Vol. 99, pp.374–380.

Bejan A. (1995) Entropy Generation Minimization, CRC Press, Boca Raton, NY.

Bruno F. Pussoli, Jader R. Barbosa Jr (2012) ‘Optimization of peripheral finned-tube evaporators using entropy generation minimization’, International Journal of Heat and Mass Transfer 55 (2012) 7838–7846.

CE Vincent and MK Heun (2006) ‘Thermoeconomic Analysis & Design of Domestic Refrigeration Systems’, Domestic Use of Energy Conference 2006.

Deepak Kumar, Vivek Kumar, and V. P. Singh(2010) ‘To Study the Parametric Effects on Optimality of Various Feeding Sequences of a Multieffect Evaporators in Paper Industry using Mathematical Modeling and Simulation with MATLAB’, International Journal of Chemical and Biological Engineering 3:3 2010.

REFERENCES

E. Granryd(2010) ‘Analytical expressions for optimum flow rates in evaporators

and condensers of heat pumping systems’, international journal on refrigeration

33,2010.

Jang-Se Jang-Seok Lee, Tae-Hee Lee, Seong-Hun Ham, Se-Yoon Oh, Kwan-Shik Cho(2000) ‘A STUDY ON NEW TRI-TUBE TYPE EVAPORATORS IN DOMESTIC REFRIGERATOR/FREEZER, International Refrigeration and Air Conditioning Conference(2000).

Jong-Taek Oh, A.S. Pamitran, Kwang-Il Choi, Pega Hrnjak (2011) ‘Experimental investigation on two-phase flow boiling heat

transfer of five refrigerants in horizontal small tubes of 0.5, 1.5 and 3.0 mm inner

diameters’, International Journal of Heat and Mass Transfer 54 (2011) 2080–2088.

Piotr A. Domanski, David Yashar, Minsung Kim (2005), ‘An Optimized Design of

Finned-Tube Evaporators Using the Learnable Evolution Model’, VOLUME 10,

NUMBER 2 HVAC&R RESEARCH APRIL 2005.

R Vijayan and P S S Srinivasan (2009) ‘The influence of internal heat exchanger on performance of window AC retrofitted with R407C’, 2009.

S.C. Kaushik and K. Manjunath (2011) ‘Second law analysis of condenser by using new heat transfer and pressure drop model based on flow regimes’, Int. J. Exergy, Vol. 9, No. 3, 2011.

Taijong Sung, Daesik Oh, Taewon Seo and Jongwon Kim (2007) ‘Optimal design of a micro evaporator to maximize heat transfer coefficient’, Asian Symposium for Precision Engineering and Nanotechnology 2007.

X. Huo, D. Shiferaw, T.G. Karayiannis, Y.S. Tian and D.B.R Kennin (2008) ‘BOILING TWO-PHASE PRESSURE DROP IN SMALL DIAMETER TUBES’, Aspen Tech Inc., Berkshire, UK,2008.

Douglas T. Reindl and Todd B. Jekel (2009) ‘Frost on Air Cooling Evaporators’, published in ASRAE Journal

Xinzhou Song, Dong Huang, Xiaoyu Liu, Qun Chen (2012) ‘Effect of non-uniform air velocity distribution on evaporator performance and its improvement on a residential air conditioner’, published by Elsevier Ltd.

B. Shen, Z. Shu and Y. Wang (2000) ‘Research on characteristics of Double Evaporators in VRV Air Conditioner’, International Refrigeration and Air Conditioning Conference.

Christian J.L. Hermes, Marco E. Marques, Claudio Melo and Joaquim M. Goncalves (2004) ‘Effect of coil Geometry on Frost-Free-Finned-Tube Evaporator Performance’, International Refrigeration and Air Conditioning Conference.

Websites used – google.com, wikipedia.org, howstuffworks.com, sciencedirect.com