Heat Exchangers in Refrigerator Systems

INTRODUCTION

The intend heat exchangers, such as condensers and evaporators, very much influences energy effectiveness of the refrigeration system. A properly designed heat exchanger will not simply helps decrease energy consumption but in addition reduce cost and increase storage volume. Recovering energy efficiency and reducing cost essentially poses a multi objective optimization problem. As these objectives are differing, no single design will satisfy both at the same time.

Generally, heat exchanger optimization can be classified into system-level optimization and component-level optimization as in system-level optimization. a complete system is analyzed while component level analysis is performed by separating the heat exchanger from the system by providing proper junction boundary conditions. Previously it has been used design optimization methods, which are merely thermodynamic in nature, for example entropy generation minimization (EGM) The main concept of EGM is that if entropy, which measures the irreversibility of the system and minimized followed by system’s performance improvement the technique is used to optimize heat-exchanger list between the condenser and evaporator. The resultant system from the analysis is the good  energy efficient, but  may not be good for an economic point of view a new approach was projected which integrates economics with thermodynamic optimization, which means  the ‘‘thermo-economics” approach. Thermo-economics blends two very dissimilar fields’ thermodynamics and economics, making it predominantly suitable to design thermal systems with efficiency and capital cost. (Gholap et.,2007 ) . Condensation heat transfer, equally inside and outside horizontal tubes, plays a main role in refrigeration the science of condensation heat transfer has been adopted a substitute working fluids and new improved surfaces for heat exchangers. (Cavallini, A etal.,2003)

Literature review

Literature review and discussion for heat exchange in refrigerator system

There are many  publications focus on heat exchangers in an refrigerator system  has have quite a lot of design variables such as geometry, surface finish, Material In this work. These variables are inhibited by various reasons such as manufacturing capability and space limitation. Heat exchanger has many variables, e.g. number of rows, width, fin density, tube diameter, tube spacing, fin spacing, etc.

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Dependent variables are the characteristics of the design household refrigerators, energy consumption, cost, and internal volume can serve as independent variables. (Gholap,,2007 ) describe heat optimizing system in an refrigerator system the study was proposed considering overall energy-consumption and heat-exchanger with low cost material which was multi-objective optimization problem. Thus he tried to calculated over all energy-consumption  by adding the energies consumed by the compressor, fan motors, and defrost heater for a 24 hours period and heat-exchanger  costs of material were calculated by means of  individual weight of the coil multiplied through the rate per unit and weight of the correspond metal in the market. His researched  was focus on various kinds of design of the condenser for heat optimizing, most household refrigerators has  wire-on-tube design for the condenser and finned tube

Design for the evaporator which is used extensively according to research heat exchanger is divided into along length, height and depth each of element has refrigerant and air steam. However, heat optimizing system does not show much development Thus, The designer can prefer a resolution based on the manufacturer’s strategic direction regarding cost and the Energy-consumption requirement. A research on wire on tube condenser by (Lee, TH.,2001) observes the correlation on the airside heat transfer coefficient of wire-on-tube type heat exchangers by  using single layer tube  rather than using bundles of tubes.

The improved correlations were based on Zhukauskas correlation which discussed via the numerical analysis. Thus Concluding the ratio of Zhukauskas correlation for a single cylinder to the heat transfer coefficient across to the air flow in the wire-on-tube type heat exchanger is constant and the heat transfer parallel to the air flow in the wire-on-tube type heat exchanger. Hence, Zhukauskas correlation has a more accurate prediction result compared to the existing cases.

Another research was proposed by (Yang, C, 2005) presenting the numerical results of the consequence of dissimilar parameters on the performance of capillary tube-suction line heat exchangers (CT-SLHX), together with condensing and evaporating temperatures, degrees of sub cooling and superheat, tube diameter, tube length, and refrigerant flow inlet quality. The effects of different geometric and operation circumstances on the capillary tube performance and investigated that the best design present capillary tube-suction line heat exchanger exists about 3 K of sub cooling and 1.4-1.6 m of the heat exchanger length. Further found that R-134a performs better in terms of heat transfer rate and evaporator capacity than R-600a and heat transfer rate from the capillary tube to the suction line decreases by about 8-10% when non-adiabatic inlet arrangement is used compared with adiabatic

Wire-on-tube Design

The most extensively used condenser in refrigerant system since many years is wire-on-tube design used for the condenser and finned tube design intended for the evaporator. Wire-on-tube type heat exchanges consists of tube bundles within which a heat transfer medium such as refrigerant is strained to flow, whereas a second heat transfer fluid like air is concentrating transversely through tubes. As the airside thermal resistance of this heat exchanger is much advanced than that of refrigerant side, improved surfaces such as wires straight welded on the sequence of tubes are engaged to successfully reduce the resistance as in the other finned tube heat exchangers. Even though these are widely used in home refrigerator for low cost and simple making, the general design data and correlation for the airside heat transfer be not easily obtainable. This makes additional studies have been focused on the research area for cycle matching and frosting.

The airside heat transfer coefficient of heat exchanger will be able to calculated using the following two methods:

1. The heat transfer coefficient of tubes and wires can be obtained, respectively, by multiplying Zhukauskas correlation by correction factor
2. Results is formulated directly by performing the heat transfer experiment on wire-on-tube heat exchanger. (Lee,TH.,2001)

Capillary Tube-suction Line Heat Exchange

Optimizing a capillary tube-suction line heat exchanger is beneficial designers for vapour compression refrigeration systems and better enhancement of the system performance.

A capillary tube is extensively used in household refrigerators owing to its low cost and effortlessness. It improves the system capability by minimizing the evaporator’s inlet-enthalpy. In addition it act as a shield for the compressor and increases the inlet temperature. (Gholap 2007)

.The capillary tube and the suction line figure a counter-flow heat exchanger. Refrigerant vapour is condensed as of the evaporator to the condenser in process and is packed together in the condenser throughout the process. A non-adiabatic throttling procedure is implicated throughout process where heat is transferred from the capillary tube to the suction line throughout process, while at the same time heat is separate from the evaporator for the duration of process The heat transfer from the capillary tube to the suction line over and over again leads to the upgrading in the thermodynamic efficiency of the system owing to the condensed refrigerant quality and towards the inside of the evaporator. Moreover, the transfer of heat to the suction line ensures the refrigerant vapour enters the compressor by this means of eliminating suction line sweating and preventing slugging of the compressor. The capillary tube has been extensively investigated even though a few numerical models of the non-adiabatic capillary tubes are existing but there require understanding on some flow phenomena within a tube, such as the reverse heat transfer, the re-condensation, and the chocked flow of the condenser in a refrigeration system. Some phenomena possibly will cause unstable flow in the capillaries the two-phase flow heat transfer coefficient has commonly been believed to be infinite. This hypothesis may be unsuitable in situations, where re-condensation or overturn heat transfer possibly will occur from the suction line to the capillary tube under distant operating conditions. This precise hypothesis of infinite heat transfer has been undisturbed in the present model by using suitable two-phase heat transfer correlations to reproduce the actual flow performance inside the capillary tubes. As a result a numerical investigation of refrigerant flow due to non-adiabatic capillary tubes through modifying the computer model developed. (Gholap 2007)

Numerical Model

Restricted element approach and suitable heat transfer correlations are integrated into the model to calculate the performance of the capillary tube-suction line heat exchanger. This model is of two modes:

• Design mode: to calculate the capillary tube length for the specified refrigerant flow rate,
• Simulate mode: to estimate the refrigerant flow rate for the particular tube length. . The finite-element

The model comprises the operating surroundings of the refrigeration system, geometrical Parameters of the capillary tube-suction line heat exchanger and refrigerant properties. This capillary tube separated into three sections.

Adiabatic inlet region (connecting the condenser), the heat exchange region and the adiabatic

Outlet region (connecting the evaporator).refrigerant flow inside the capillary tube comprises of two regions: the single-phase region and the two-phase region. Numerical solution is conceded out and separating the flow domain into small essentials by the side of the capillary tube and the suction line. The length of each control volume model is 1 mm. Homogeneous flow theory analyzes the two-phase flow inside the capillary tube. The model is based on the conservation of mass, momentum and energy; these equations are solved at the same time all through an iterative process. Within the design, the mass flow rate is an input and the sum of total length of the capillary tube is an output, whilst in the replicate case, the capillary tube length is an input and the mass flow rate is an output, which is evaluated iterative process, The calculated length is compared among the given capillary tube total length, thus the supposed mass flow rate can be adjusted. (Yang, 2005)

Enhanced tubes of condensers

Air-cooled in addition to water-cooled condensers with in-tube condensation of refrigerants are extensively used in water chillers, air-conditioners .Trying to replicate the physics of the condensation procedure analysis on condensation of refrigerants believe to be the transition faced next to the air-conditioning and refrigeration industry in recent years, An introduction of latest ozone friendly refrigerants as substitute for CFCs and HCFCs. Condensation with horizontal tubes, the two point flow is conquered by vapour shear or gravity forces. At the same time annular flow pattern has high vapour shear, wavy and slug flows become visible when gravity is the controlling force.

In refrigeration applications with halogenated working fluids, better tubes are frequently used in bundle condensation. Tubes with two-dimensional or three-dimensional external fins are used in few cases they can also be internally amplified to increase the water-side heat transfer coefficients, when fouling is not a problem Bundle condensation has investigated, for reliability in heat transfer. The use of geotropic mixtures combination with bundle condensation should be dejected as it can give rise to relevant refrigerant composition shift. (Cavallini A et al., 2003)

Research question

HOW TO HEAT OPTIMIZING TECHNIQUE AND EFFICIENCY IN HOUSEHOLD REFRIGERATOR SYSTEM?

AIMS

The study of the project proposal is based on optimizing heat exchangers in the house hold refrigerator systems. A multi-objective optimization procedure is implements and finding out optimal design which minimise energy consumption and material cost for the household refrigerator systems. The study considers various design of optimization of heat exchangers.

Objective

• To research on properly designed heat exchanger that will help to decrease energy consumption.
• To carried out a literature review on reducing the energy consumption  refrigerator systems and also in low cost and increase the volume storage

Research method

Research method is a process of getting better knowledge and understanding of work from peoples those who previously experienced and in some of the research is to take action on previous peoples understanding (Marshall and Rossman, 1999). In a research data can be collected from two sources primary research and secondary research. The primary research is to conducting interviews. These interviews help get knowledge and data for the key research issues and also to get different views of peoples .according to (silverman, 2004) interviewing play a key role in getting an large amount data in quick time .lot of researches make use of interviews to get a clear structure .and in secondary research that data is collection by means of document, previous research, articles, publication and text book. For this research the secondary source has been used.

Research design 

Research design play a very important role in research to know the topic is researchable or not. The research design has to consider the three elements philosophical assumptions, specific research method and strategies of inquiry and also research question (John, 2009) .For this research. The first task is to find out the research topic which is related to the field and researchable. The choose topic was heat optimizing technique and efficiency in household refrigerator system. Because this topic related to the field and find interesting.After finding the topic the next task was to find literature reviews. After finding all the resource for the topic outline all the important points and start writing.

Data collection

Data collection involved collecting data by the source of primary and secondary. Collecting data involved in selecting data from different source and the techniques of data, that will collected for the purpose of final research (Silverman, 2004) according to (Sapsford, Roger and  Jupp, Victor;2006) Research doesn’t involves in first hand data collection, but it can  also based on the made material available made by other peoples. In this assignment the secondary source has been used to collect data for the research and all the material in the net treated with caution because it can be false. Therefore the data has been collected from the source NORA which is available in Northumbria University and the other source is Northumbria library for referencing text book for this research.

Data analysis

Data analysis is a process of brining all the collected data and structure for the research.

Overview of research

Heat exchangers can have several design variables which are controlled by various reasons such as width, fin density, tube diameter, tube spacing, fin spacing, and so on. Energy-consumption and heat-exchanger which form a multi-objective optimization difficulty, Optimization was stopped after 50th generation ever since no additional development was observed. The explanation is divided into two regions.  Region one represent energy efficient and cost-efficient compared with the baseline design. Region two represents both energy and cost is better than for the baseline design. In the case of cost efficient solutions, condenser conductance stays close to its minimum value. For energy-efficient solutions, it stays close to its maximum value with an enhancement in the evaporator conductance thus; region two is most desired optimization.

RESOURCES/CONSTRAINTS

The mainly used resources need for a research is websites, journals and text books for getting the information and previous research papers and the constraints are

• Time management for the research
• Find the research work for writing a literature review.
• Access to the websites which is held by an engineering organisation.
• Finding books related to the research topic.
• Collecting data from different resources.
• For designing the software used is solid works 2010.