Refrigeration is an important aspect in the preservation of food before it is prepared, served or transported. Having this aspect in mind, the need of having an energy efficient refrigerator is a talk of the day in the food industry. However the current designs of commercial chest freezer are not energy efficient as they consume 5KVA/day which is costly to business operators. The aim of this project is to redesign the condensing unit of a 20 cubic foot chest freezer to increase efficiency. Simulations of air circulation in the condenser, heat transfer from the refrigerant into the pipe then fins and compressor load in the condenser will be examined to have an optimum point. This will seek to improve the cool down time and reducing compressor load. The electrical control circuit of the chest freezer will be redesigned to enable lower power consumption on the system Findings/Results?
Table of Contents
List of Figures
Several companies in different trades in Zimbabwe have been faced with economical challenges which lead to their closure. For those companies which managed to sail through and remained operating, they were using old equipment and methods. This has made it difficult for them to compete regionally in producing energy efficient chest freezer and aesthetical appealing products. These economic challenges gave birth to SMEs industry in Zimbabwe which operates in different sectors of the industry from primary, secondary to tertiary industry. These SMEs include supermarkets, bottle stores and butchery which are faced by high operating cost emanating from energy bills. The equipment which is used by the business operators varies from tills, lights, refrigeration systems and the PA system. Among the equipment used by the SMEs, refrigerators are ranked high in energy consumption. Refrigerators consume more than 60% (source) of the energy used by the SMEs which translate to over half of the energy cost. This gives rise to the notion of energy management in try to look for ways in which energy cost can be reduced in order to make business viable.
Refrigeration is defined as any process of removing heat from the body being refrigerated and transferring it to another body whose temperature is below that of the refrigerated body, (Da-Wen Sun 2001). R.J Dossat defines Refrigeration “as any process of heat removal”. This process of moving heat from a cooler body to a warmer body involves the input of external energy. There are several systems which are used to achieve refrigeration, but mainly vapour compression system and Absorption system are used mostly.
- Vapour compression cycle
In this cycle there’s is the compressor, the condenser, the expander and the evaporator. The operation of the system is a closed system whereby refrigerant (heat absorbing liquid) is circulating in the system. The compressor changes the refrigerant vapour from low pressure to high pressure during the compression cycle. This pumping transfers heat from the inside of the cabinet to the outside. The compressor transfers heat from one place to another. The refrigerant is pumped into the condenser where it loses heat and changes to liquid and enters the expander where it is reduced its pressure before entering the evaporator. In the evaporator the refrigerant vaporises and absorbs heat then it flows back to the compressor, (Andwer D.Althouse, Carl H. Turnquist, Alfred F. Bracciano 2004).
- Absorption cycle
This cycle is based on the liquid evaporation and it is mainly made up of evaporator, a condenser and an absorber. An absorber is filled with adsorbent replaces the compressor of a conventional vapour compression system. The absorber is heated by thermal energy discharges the refrigerant vapour from the adsorbent bed. Then the refrigerant is delivered to the condenser and condenses. The liquid refrigerant flows to the evaporator and is stored in it. During the charging period, the cooled adsorbent bed absorbs the refrigerant vapour from the evaporator and induces the refrigeration effect there by completing the cycle, (Da-Wen Sun 2001).
In this project we are concerned by the vapour compression cycle which employee’s compressor to provide compression. The vapour compression cycle systems are the ones which are mostly used in the Zimbabwean market because of their higher COP and because they are powered by alternating current. These units are highly energy in efficient which is costly at the end of the day. Due to high cost associated in operating refrigerators business operators have derived ways in which they take as alternatives which is buying refrigerators regionally or using domestic refrigerators. Although the refrigerator bought regionally are cheaper and aesthetically appealing, but most of them are not energy efficient. On the other hand those that have resorted to the use of house hold chest freezer are facing high operating costs due to short Mean Time Before Failure, (MTBF) of 8 months. This is so because these chest freezers cannot operate in conditions where there is high volume of goods traffic.
More so, the locally produced commercial chest freezers in Zimbabwe are energy inefficient which makes it costly for business operators who rely on refrigeration in preserving or storing their products at desired temperatures. This creates a need to investigate on ways in which locally produced refrigerators could be improved. It is against this background that this research centres on the redesign of the high side of the chest freezer to improve energy efficiency.
The high side of a 20 cubic foot commercial chest freezer is highly energy inefficient.
To redesign the high side of a chest freezer to increase energy efficiency
- To select a compressor which is able to reduce energy consumption
- To select an appropriate ozone friendly refrigerant which can reduce energy consumption
- To design the heat exchange media of the condensing unit of a 20 cubic foot chest freezer
- To develop a control circuit of a 20 cubic chest freezer which will help to reduce energy consumption
- How can the cool down time be improved?
- What are the current condensing units and condenser designs available are like?
- What are the key factors to consider when designing condensing unit?
- What type of material is used for constructing condensing units and why?
- How can we determine the type of refrigerant to use in the system?
- How to control refrigerant cycles and improve energy management
This section will outlines the research techniques employed in carrying out the research of research design project. This shows vital methods used in the research formulates the idea generation stage which satisfies the research design process that the researcher used.
- Conducting field studies on high pressure side to see how best we can redesign it to work in Zimbabwean environment while saving energy
- Literature survey on similar problems, analyze their merits and see how best to improve them.
- Collecting empirical data on a survey on how a condensing unit works and its effect to cool down time.
- System design and analysis using simulation software which are CFD and Proteus
High energy consumption of the existing 20 cubic foot commercial chest freezer has given a green light to the redesign of the existing condensing unit. The other alternative source of using domestic chest freezer has failed to yield result. This is due to high frequency of traffic of volumes of products entering and leaving the refrigerator, which makes it unable to copy up with prevailing heat loads. This will also mean that the refrigerators do hardly archive their desired set temperature which in turn unhealthy to the compressor as it runs all the trading hours and have a few off cycles in the night. Also due to the nature of most of the business they cannot use absorption refrigeration cycle due to its in efficiency. Absorption refrigeration need longer periods to archive desired temperature which in businesses like butchery is a negative aspect as it affects the quality and texture of the meat. This will justify a huge need to redesign commercial chest freezer condensing unit to reduce energy consumption in order to make business viable.
The redesign of the high side of the commercial chest freezer will enable the users to realise the benefit of the product. By the reduction of operating cost due to reduced energy bills
This chapter will discuss the existing type of condensing unit on how they function. We will get into the different components which built a condensing unit for it to be able to achieve its desired function. A condensing unit consist of the compressor, the heat reject medium and a fan motor. These components provide liquid refrigerant to the evaporator in the right required amounts per design. The compressor compresses vapour refrigerant to high pressure gaseous refrigerant, the high pressure refrigerant circulate in the condenser losing heat and therefore liquefaction takes place. It exits the condenser as high pressure liquid refrigerant being supplied to the expander and into the evaporator then back to compressor to complete the cycle.
A refrigerator is as any process of removing heat from the body being refrigerated and transferring it to another body whose temperature is below that of the refrigerated body. A simple vapour compression refrigeration system consists of the following equipments:
- Expansion valve
The diagram below in Fig 1 shows the flow of refrigerant in a vapour compression refrigerator. The low temperature, low pressure vapour at state B is compressed by a compressor to high temperature and pressure vapour at state C. This vapour is condensed into high pressure liquid at state D in the condenser and then passes through the expansion valve. At the expansion valve the liquid is throttled down to a low pressure liquid and passed on to the evaporator, where it absorbs heat from the cabinet or from the circulating fluid (being refrigerated) and vaporizes into low pressure vapour at state B. The cycle then repeats again and again until the cooled space has reached its desired temperature. (Refrigeration Cycles)
Vapour compression system
The Compressor performs it duty and does requires work
The above figure 3 shows some of the models which are currently available on induced draft air to cool the refrigerant. They have their merits and demerits
- Easy to install.
- Requires no water.
- Condensing medium, air, will not freeze.
- Low fan power consumption
- Adequate supply of fresh air is required.
- Fan may be loud in large operations and it also add noise pollution.
- Changes in air temperature may cause the condensing pressures to fluctuate.
- It can clog at the entering side of the air thereby reducing condensing capacity
This type of condenser uses natural ambient temperature around the condenser to cool down the refrigerant to liquid state. There are also 2 types which are finned tube condenser or plate surface condenser as in figure 4. When it is finned tube type the fins will be widely spaced as to allow little resistance. When it is a plate type it is usual mounted at the back of the fridge.
Natural convection air cooled condensing unit
Figure 4 Source: Natural air cooled condensing unit (www.gstatic.com/images)
- No power is need to cool the fluid in the condenser
- If finned it will clog with time
- They cannot hold huge heat loads
Systems which use water to cool the refrigerant can be divided into two sections which are wasted water and recirculation water. In a system which uses wasted water the water enters the condenser and after exiting the condenser it is disposed into the sewer water. The waste water system are limited to waste of water as many city authority do not allow the use of waste water due to the water recurring shortages around the world. In most cases it might be used in very small systems. Also the recirculation water system it is an important aspect to take not of the power required to circulate the water in the system and also the source of water as to prevent scaling in the condenser. Below is the diagram of water cooled condenser in figure 5.
Water cooled condenser
Figure 5 Source: Water cooled condenser (http://marinersgalaxy.com/wpcontent/uploads/2013/02/Condenser.jpg)
Advantages of water cooled condensers
- When the systems are large they take small space
- They are quiet in operation than air cooled ones
- Usually have a longer life
- Higher efficiency
- Indoor placement is possible even in large capacity
- Larger tonnage capabilities
- Refrigeration containment is easy
Disadvantages of water cooled condensers
- Additional maintenance costs
- Water treatment costs
- Mechanical room needed
Evaporative condenser is a conservation water device which incorporates a condenser and a cooling tower combined into a single unit. In an evaporative condenser the water which in the sump is pumped from the sump to the spray headers. It sprays down over the refrigerant coils and it is returned to the sump. Then the air is drawn from outside at the bottom of the condenser opposing the water which is being sprayed. In order to prevent water lose the water sprayed is restricted to be blown out by the opposing wind by the eliminators. In which the small water drop lets condense and drops back into the sump. The evaporative condenser has a system of makeup water in order to keep the desired level of water in the sump.
Figure 6: Evaporative condenser (www.gstatic.com/images)
Compressors are the driving power of the refrigerant in a vapour compression system. They can be divided into three categories which are:
- hermetic compressor
- semi hermetic compressor
- open type compressors
Hermetic compressors are totally enclosed in a crankcase so that all the process happens in a closed case. In the crankcase there will the winding and the compressing section connected to form a single unit and they are placed in a crankcase where they will be working in. The crankcase is welded as there is no need to access the inside components in this category none of its parts can be accessed to repair.
Figure 7: Hermetic compressor (www.gstatic.com/images)
Its advantage is that the oil in the crankcase is used for both cooling the windings and lubricating the parts of the compressor.
Its disadvantage is that if one of the two winding and the compression part is fault they can be repaired which in turn means discarding the compressor
On semi hermetic the compressor they still have the crankcase but the winding and the compressing parts can be separated to repair either the windings or the compressing parts. This is possible upon opening the crankcase because it is bolted.
Semi hermetic compressor is that they are serviceable in their life span
There is lose of refrigerant when opened each and every time
Semi hermetic compressor
These compressor are separate the compressing part and the winding part are two different components. They constitute an electric motor as a winding which turns the pumping part. They can be connected as direct coupling or using belts
Open type compressor
Advantages of open type compressors
- Easy to repair each components
- When repairing each section it does not affect the other section
Disadvantages of open type compressors
- By use of belt on coupling it adds cost of operation
- They have lower speeds when employing belts.
A compressor is pump which has an inlet and outlet side. The compressor changes the low pressure vapour refrigerant to high pressure during the compression cycle (Modern refrigeration). The pumping of the compressor transfers heat from the inside the cabinet to the outside environment. There are basically three types of compressors which are commonly used for refrigeration duty which are reciprocating, rotary and centrifugal compressors (Dossat). The reciprocating and rotary compressor types are positive displacement compressors which mean that compressing of the refrigerant is done mechanically by means of a compressing member. Whilst in centrifugal compressors it has no compressing members, compression of the vapour is done primarily by action of the centrifugal force that is developed as the vapour is rotating by high speed impeller.
Reciprocating compressors are largely used in refrigerant systems where the refrigerant required is in small displacement and condensing at relatively high pressures. Reciprocating compressors cannot be employed with low pressure refrigerants which require a large displacement per unit capacity (Dossat). They are suitable in applications where evaporator pressure is above one atmosphere their sizes are available from 9watts up to 250 tons in large industrial area. The reciprocating compressors consist of
1. Cylinders where compression takes place
2. Pistons which compress vapour
3. Valve plates which open and closes pressure in the cylinder being formed by up and down of the piston.
4. Bore and stroke are joined with the crankshaft and the piston.
5. Crankshaft it connects the rotor and the stroke
6. Winding which convert electrical current into magnetic flux
7. Rotor which convert magnetic flux into rotating motion
Functionality of reciprocating compressors
When electrical power is switch on it enters the winding which in turn create magnetic flux as shown in figure 10. The rotor which is placed in the central of the windings is force to rotate as it has magnetic material around it. The rotor is connected to the crankshaft the shaft is also attached to stroke and piston, the design of the shaft is causes an up and down movement as it rotate. The movement of the piston up and down tends to open the suction valve when the piston is moving down. when it starts to move up it will force the suction valve to close , with continual rising up of the piston it will create pressure which is then hold in the cylinder up to an extend that the discharge valve cannot hold the pressure then it will open allowing high pressurized vapour to leave the chamber. On reciprocating type we have hermetic semi hermetic and open type compressors (Morden refrigeration)
Rotary compressors are applicable on systems like air conditioners, packed central system and heat pumps. There are basically four general designs of rotary compressors these are:
- Rolling Piston
- Rotating vane
- Helical lobe (screw)
Below are the picture which show their construction
The above figure 11 shows the rotary blade compressor. The black arrows indicate direction of rotation of rotor. Red arrows indicate refrigerant vapour flow.
Operation of rotating vane
There are basically two types which are stationery blade and rotating blade. In all the two types the blades provide continuous seal of refrigerant vapour. The low pressure vapour from the suction line is drawn into the opening. The vapour fills the space behind the blades as it revolves; the trapped vapour in the space ahead of the blade is compressed until it is exhausted into the condenser
Advantages of vane compressors
- They have very small clearance volume
- Provide large size opening into the suction line
Advantages of scroll compressors
- Better energy efficiency ratios
- Better volumetric efficiency (actually, this is a repetition of the above one). Apart from that, this is better at lower temperatures.
- Less noise
- Can handle liquid to some extent (at least there won’t be any damage)
- Less wear and tear
- First cost is very high
They are basically designed for large capacity system ranging from 50 to 5000 tons.
Figure 15 shows a centrifugal compressor where in the compressor vapour moves outwards as it is moved rapidly in a circular path. The process is called the centrifugal force. Vapour is feed into the compressor, and then a disk with radial blades (impellers) spins rapidly this forces vapour against the outer diameter. The pressure generated by one blades is so small so the is need to have several of these blades in series which in turn create a pressure difference and pumps a sufficient volume of vapour.
Advantages of centrifugal compressor
- The compressor is simple to design and low weight
- No piston or valves cylinder which means less friction
- Only wearing parts are the main bearings
- Pumping efficiency increase with speed
- High flow rate than positive displacement
- Generating a higher pressure ratio per stage as compared to axial flow compressor.
Disadvantages of centrifugal compressor
- Large frontal area for a given air flow rate compared to the axial flow compressor.
- Unsuitable for very high compression, limited pressure.
- They are sensitive to changes in gas composition.
- They work at high speed, sophisticated vibration mounting needed.
- Problem of surging, stalling and choking
A research methodology is a systematic way of solving research problem (Kumar 2011).This illustrate different steps which the research will adopt, the theory of how the research was undertaken which including the theoretical assumptions. Which the research was based and the implications they have on the method adopted. This information helped us to make decisions and to clarify the importance of this research project and to fully understand the problem at hand. This will help to adequately solve the issue at hand, also having a greater understanding of the condensing units of chest freezer.
Saunders, Lewis, and Thornhill (2009) defined a research design as a blue print for the research. It is a systematic way of carrying out a research and answering the research question. There are basically three types of research designs which are used and these are exploratory, descriptive and causal (Experimental) research. This research will use exploratory and descriptive research designs as clarified below. The research design also has to specify whether a research is qualitative or quantitative. Quantitative and qualitative research designs will be conceptualized to obtain triangulated results bringing together differing strengths of two methods.
Project flow chart
Data collection techniques employed include
- Direct observations
- Desktop research
- Engineering tools
Primary data was collected through direct observations of the existing condensing units. The observations bring on board a number of factors. The factors include how the product is being used by the business operators, knowing their primary concerning in use, knowing also the heat loads.
The internet has been a vital tool for much of the gathered information, related journals and books were also used to obtain information on best practices of designing the high pressure side of commercial chest freezers. This provides the major design considerations to be taken into account before any alterations of the high side.
The concept of concurrency was employed where appropriate whereby some of the activities in the design process were done at the same time. This significantly reduced the project lead time. Experts in the different fields were consulted for information that was important to this project. MS Excel was used for data analysis and data presentation and Gantt chart. Auto CAD was used for producing concepts and working drawings. Proteus software was used designing the control circuit of the condensing units
Information gathered through the above mentioned processes were of great help to the researcher in compiling this document. The research methodology helped in obtaining the much data that highlighted the areas which can be optimised in designing condensing units of chest freezes.
This chapter presents information gathered from the questionnaires which were prepared to know information of how the users would want the product to perform and what can be improved on a 20 cubic foot commercial chest freezer. The results represent what the users say about the existing commercial chest freezers.
The researcher used different methods to collect data, there were 25 questionnaires which were prepared and this was to find if whether the users would want the chest freezer to be redesigned to reduce power consumption
The bar graph below shows the average current per given time on the days the study was conducted from the respondents which answered the questionnaires. Of the 25 response 4 of them which didn’t mind refused to grant us access to measure current flow in their condensing units at that particular time.
From the above table it show that the average current of the existing units is 2,8 amps per unit which is a high energy up take.
4.3 Condenser type in the market
The pie chart shows the number of each type of condensing units being used in the currently available 20 cubic commercial chest freezer in use. The majority of the units are employing induced air draft condensing units.
4.4 Types of refrigerants in use
The table below has the information collected pertaining the type of refrigerant being used by the same commercial chest freezers which are currently being used. Of the 21 units which were managed to extract information from them
|Type of condensing unit||Type of refrigerant gas||Number of units|
|Induced air draft||R 22||4|
|Natural convection||R 134a||4|
|Water cooled||R 404||1|
|Skin condenser||R 600a||1|
It is seen that the most used refrigerant is R134a on these units by different manufactures
This chapter has given the researcher the insight of what is on the ground thus providing information which shows that there’s is room for improvement, on the current designs of commercial chest freezers to reduce energy consumption.
This chapter focuses on the conceptualization of the high side section of a commercial chest freezer. Different alternative solutions will be discussed to hw best they can help in reducing energy consumption of the 20 cubic chest freezer. Follow will the concept generation, selection and the optimizing stages of the applicable solutions to solve the problem. The decision matrix will be used to help the selection of the design which conforms to technical standards and customer requirements.
The concepts were generated through analysis of information gathered and the review of different scholarly articles. The design concept we developed continually on in the progress of understanding and trying to reduce energy consumption. Three concepts were developed and analyzed to see if they meet the standards and the customer requirement to be viable on the market.
5.2 Concept 1
The 1st concept the refrigerant is being discharged from the compressor into the coil and to the pipe to the door seal then into the evaporating section. The high side section has a fan which is cooling the condenser and an extended pipe to the door peripherals in order to try to remove all the heat from the refrigerant. The need to extend the pipe to the door peripherals is to extend the condenser. As leaving the condenser as an induced draft cooled increases fan motor size.
- Reduced size of fan motor
- Using the cooling effect on the door seal
- There is increase in heat load to the refrigerated cabinet by used of peripheral pipe
- Need to use refrigerant with a lower condensation temperature
- Using a compressor with lower condensation point increase compressor power
- Increase running time to archive required temperature
In this concept the refrigerant is pump from the compressor to the water tray were it is circulate and then flows in the natural air cooled condenser then it is supplied into the evaporator. The water tray has a float valve which controls the entering and the stopping of water into the tray. This method has a greater advantage that it does not employee a fan motor which is the reduction in power. Also there is great challenge in removing heat from the refrigerant to desired conditions especially when the water has heated as the temperature difference would have been reduced significantly.
- No fan motor which is a power consuming device.
- A refrigerant with average condensing temperature can be used.
- When the water tray water has high temperatures the refrigerating effect is reduced
- Has a relatively shorter running time than concept one
In the third concept the refrigerant is compressed from the compressor to the discharge pipe which goes into the water tray. From the water tray the pipe goes into a colied induced draft condenser which is reduced in size which also needs a smaller fan motor. The refrigerant flows to the natural air cooled condenser where it is further cooled to liquefaction point then the refrigerant will be supplied to the evaporator.
High load of heat uptake
Lower power consumption
Can take refrigerants with high liquefaction temperature
Very small fan motor size
Take more material to build
(Ulrich 1898) proposed a concept selection method which is a two stage which will be used in this project give clarity and distinct difference between the proposed concepts giving quantitative evaluations depending on scoring matrix and screening process. The important factors which have an outstanding need to satisfy are feasibility and effectiveness.
Effectiveness is it able to produce the desired outcome of reducing power consumption of commercial chest freezer. What outcome will be realised if the idea is implemented.
Feasibility is to certain that can the program be done with the material, knowledge, time and other resource. An analysis was done and certain factors will be considered examining the proposed concept to see if they can address the current problem. The research devised the concept selection criteria in order to be able to select the concept which can help in archiving the required results. Applying the reference scale of (+ – 0) was implemented were negative rating signals the bad features about the concept. Positive rating signals the good features about a concept. While zero reflects that the concept can be improved. A concept with low marks in ranking will be rejected and the two concepts will be used.
|Criteria||Concept 1||Concept 2||Concept 3|
|1. Cooling loads.||–||–||+|
|4. Power consumption||–||+||+|
|Concept 2||Concept 3|
After all the consideration have been factored in and with the help of concept scoring and concept screening I will develop concept 3, this has been shown by the number of total score as concept 3 has a higher score. The concept can be manufactured and there are less complications. The design will serve our main concern which is to reduce power consumption of commercial chest freezer
The results of the concept will be discussed in the later chapters to see the outcome and the embodiment of the selected concept is presented in appendix.