This project speaks about the design and study of a refrigeration system that operates as a water cooler in the summer and as water heater in the winter by reversing the cycle. A full design and selection of the system's equipment is made for this cycle that operate on an average evaporator pressure of 200kPa, -10ÌŠ C, and an average condenser pressure of 1MPa, 45ÌŠ C. The mass flow rate of the compressor was found to be 0.00163kg/s, COP = 4.5 and COPh = 5.5. It was concluded that the use of an electrical heating element is better than reversing the cycle.
Introduction
The main aim of this project is creating a refrigeration cycle that will work as a refrigeration cycle in the summer and as a heat pump in the winter, it will be used to cool and heat water.
Another aim of this project is to build the ability to of understanding the thermodynamic principles and employ them in a design process.
Overview of this report
In the first section (the previous one), the main goal of the study was mentioned in order to clarify the idea of it. In order to achieve this main goal, three objectives must be accomplished to obtain the required results and data that will show how the main goal is fulfilled.
A literature review will be provided in the second section in order to take a full idea about the cooling systems and see where this science has reached.
The Third section will talk about the objectives of the study and the means of doing them. It will show the methodology of how they are done and accomplished. The first objective is how to calculate the required cooling load, the second one is to show the bases of selecting the equipment and finally how manipulating the existing conditions and circumstances may affect the performance of the system.
Full analysis, discussion design, selection and calculations are conducted in section four.
Recommendations and conclusions are obtained in the fifth section.
Objectives
Calculate heat load depending on the heat load demand of the application.
Select or design the components of the cycle (Refrigeration & Reversed) based on the calculated heat loads.
Design the Refrigeration cycle depending on the heat loads and selected components in order to know the exact location and sizes of the system.
The refrigeration cycle will be studied and monitored, and then it will be reversed and studied to obtain the performance parameters. It will also be modified to increase its functionality and efficiency and these modifications will be studied.
Literature Review
(National Productivity Council, 2006) Refrigeration systems depend on the principle of that pressurizing a fluid at high levels will increase its temperature above the ambient temperature, leading to the rejection of heat from the liquid to the ambient due to the established temperature gradient, and allowing the fluid to expand and decrease its pressure to lower levels will decrease its temperature allowing it to absorb heat for the same reason. There are two main kinds of refrigeration cycles: Vapor compression and absorption cycle.
The vapor compression cycle is the cycle that involves vapor being compresses in a compressor. This cycle consist of four main stages. The first stage occur in the evaporator (space where temperature is desired to be decreased); in this stage the refrigerant absorbs heat from the surrounding until it completely changes it phase from liquid to vapor, this occurs because the low pressure liquid's temperature is much lower than the evaporator outside temperature which will lead to the boiling of the refrigerant inside the tubes and evaporating. The second step is compressing the vapor in order to increase its enthalpy (thus temperature and pressure), which will lead to create a temperature gradient between the refrigerant and the ambient. The third phase of the process is to reject the heat contained in the refrigerant to the outer space (sink); this process will lead to a phase change by which the discharged superheated vapor from the compressor will keep losing heat until it condenses. Usually, forced air convection or water is used to accomplish effective and fast cooling; the vapor is cooled until it completely changes into liquid. The final step is to expand the fluid to reduce it pressure so that its temperature is much cooler than the surroundings (evaporator) and it is ready to absorb heat again.
There are two advantage of vapor compression cycle on the other cycles. The first one is that the phase change process from liquid to vapor or vice versa is relatively high, allowing more heat to be withdrawn from the area of interest which means that the system will have a bigger Refrigeration Effect. The second advantage is related to the fact that when the fluid is vaporizing, its temperature doesn't change, which means keeping a high temperature gradient allowing for the heat transfer process to maintain its fast rhythm, instead of allowing the cooling fluid to raise its temperature as cooling is in process and increasing the operation's time.
The second cycle is the absorption cycle; it consists of an evaporator, absorber, high pressure generator and a condenser. Though the absorption cycle has a low COP (between 0.65 - 0.7) and its initial cost is relatively high, its running cost on the long term is low and it can serve systems with large capacities.
(UFC, 2002) This study sets the standards of designing a refrigeration system. Safety of lives, health, tools and environment should be taken into concern in all steps of design, so, the design must meet the ASHRAE 15 standards in order to be approved. Operation and maintenance considerations must be taken into account from the early stages of the design process to insure safe and easy maintenance; the lead mechanical design should participate in the "Charrette" design because it is a basic component in the design. The design should be economical by providing it with the longest life-cycle, minimum costs, maximumenergy efficiency and low costs maintenance. The chosen refrigerant should be available or have a replacement, economical to install and environment friendly.
(Cengel el. at., 2006) The transfer of energy from cold mediums to hot mediums needs a special machine to do so, this machine is called refrigerator, The other machine that do the exact opposite operation is called a heat pump, both machines are identical, they only differ in objective. The efficiency of refrigerators and heat pumps is measured by the coefficient of performance (COP) which is given by the equations:
and
Theoretically, , but this case is almost not applicable due to losses and extreme low temperatures. The Carnot cycle is the reversible cycle that will give the best efficiency among all other mechanical cycles operating between the same two heat sinks, it consist of two isothermal and two isentropic processes. Carnot cycle serves as a reference to which all cycles are compared to because it has the highest efficiency, the COP's for refrigerators and H.P. according to Carnot are:
and
The Vapor-Compression refrigeration cycle is the most famous and used refrigeration cycle, it consists of four device and four processes occurring among them, they are:
1-2: Isentropic compression in a compressor.
2-3: Constant Pressure Heat rejection in a condenser.
3-4: Isentropic expansion in a capillary tube or expansion valve.
4-1: Constant pressure heat rejection in an evaporator.
The area under the T-s diagram represents the energy transfer processes involved in a refrigeration cycle, area under line 4-1 represents heat absorbed by the evaporator and area under curve 2-3 represents heat rejected by the condenser, these area do not include thermal losses. A general rule states that the COP increases by 3 or 4 percent for each Celsius degree raise in the evaporator or decrease in the condenser.
The actual vapor compression cycle differs from the ideal one because of: unavoidable heat transfer to or from the surroundings, the fluid friction inside the tubes, the discharged vapor from the evaporator is superheated to avoid damaging the compressor by tiny liquid droplets that didn't vaporize in the evaporator, the liquid leaving the condenser is sub-cooled so that all the refrigerant entering the expansion device doesn't include any small portions of vapor that may malfunction the device, the compressing and expansion processes are not totally isentropic and there are pressure losses in all the cycle's parts. All these irreversibilities decrease the overall COP of the cycle but they are needed to ensure proper operating of the system.
Heat pump systems are becoming more popular because it is an efficient way to heat homes and constitutions though its initial cost is relatively high. The main problem that faces heat pumps is the evaporator's coils frosting in moist environments when its temperature drop below 5 ÌŠC, it can be avoided by reversing the cycle (cold evaporator becoming hot condenser) but this action will decrease the system's efficiency, higher COP's can be obtained condenser's coils are placed at deep ground levels or in hot geothermal water but the system will be more complex. For ordinary heat pumps, aiding devices such as electric, gas or oil heaters are used to ensure heating sustainability when HP can't operate at very low temperatures. HP's are designed based on the maximum load.
Sometimes, an ordinary vapor compression cycle is not enough for more complex commercial or industrial applications, so modifications are mad on the original cycle to fulfill these demands. Cascade refrigeration systems are those systems that consist of more than one refrigeration cycle to increase the total system capacity and decrease compression work. Cascade systems employs a heat exchanger that connects between each two refrigeration cycle in a manner that makes the condenser of the first refrigeration cycle be the evaporator cooled space of the second cycle. Usually, there is no mixing between the refrigerants in the two cycles so any fluid that has the required properties can be used. Another modification on the vapor compression cycle is Multistage Compression, which is essentially a cascade system with the heat exchanger being replaced with a mixing chamber when the same refrigerant at the same pressure is circulating in the two cycles because mixing chamber provides better heat transfer characteristics. In this cycle, the liquid flashed from the condenser enters a mixing chamber, the vapor is directed toward vapor mixer and the liquid is flashed again, vaporized in the evaporator and then is directed to the same vapor mixer after it has been compressed by the low pressure compressor, the total vapor coming from the mixing chamber and the first stage is compressed by the second high compression compressor to the condenser's pressure. This multi compression will decrease the amount of compressor work raising the cycle's COP.
(Dossat el. at., 2001) Refrigerants are classified depending on the application or property. They are classifies as Primary for those that are used in the vapor compression cycle and secondary for liquids used in transporting low temperature thermal energy such as chilled water. They are also categorized into three groups depending on their chemical formula: Halocarbon compounds that contain one of the three halogens (Chlorine, Fluorine or bromine), Inorganic compounds such as Water, Ammonia, Air and Carbon dioxide, and Hydrocarbons like Methane, Propane or Ethane. There are other properties that refrigerants are categorized according to such as Toxicity, Flammability and Oil miscibility. There is no ideal refrigerant that can be used for all applications. Depending on the application, when choosing a refrigerant some requirement must be taken into account: The lower the toxicity, flammability and exclusivity the better, it must not react with the lubricating oil or any other system component, must provide the highest efficiency to the cycle, be economically attractive and environmentally safe.
Evaporator is basically a heat exchanger that allows the refrigerant to evaporate by the heat absorbed from the cooled space decreasing the space's temperature. It is categorized depending on: the type of construction, circuit design, refrigerant supplying method type, type of defrosting process and the type of convection heat transfer applied (force or natural). In general, they are classified as Air cooled for evaporators using air cooler and liquid chillers for cooling liquids applications. The heat transfer surface area is increased to improve heat transfer operation, this is done by either installing fins, increase the number of evaporator tube or use vacuum pressure between the tube and the outer plates to make a firm contact between them. Relatively, the best evaporators are the Shell-And-Tube chiller bundles because they have higher efficiency compared with other evaporators, doesn't need a lot of space, easy to maintain and can be used with almost any type of liquid chilling applications.
Condensers are heat exchangers that are used to reject the heat gained in the evaporator and generated in the compressor allowing the refrigerant to cool down and condense. Condensers are classified in the same manner evaporators are, and they have three main categories which are: Air-cooled, Water cooled and Evaporative cooled. Condenser capacity is calculated using the equation , this factor is obtained from tables.
A compressor is a device used to raise the pressure of the refrigerant's vapor from the evaporator pressure to the condenser pressure. Depending on the crank position, the compression cycle stages according to the time-pressure diagram of the pressure inside the cylinder are: Intake, Compression, Discharge and Re-expansion. The higher the volumetric efficiency the better overall performance of the compressor, volumetric efficiency is inversely proportional with the compression ratio. Compressor performance depends directly on the evaporator and condenser pressures, inlet and outlet temperatures, required mass flow rate and the density of the refrigerant. There are three main types of compressors: reciprocating, centrifugal and screw compressors.
There are supplementary devices employed in the refrigeration cycles such as liquid driers and liquid absorbers.
(Maclaine, et. al., 1996) This is a study performed on R600a, R134a and R12. R600A showed higher energy efficiency regarding electricity consumption; it saved 20% over the other refrigerants. The need for non-toxic, non-flammable, Ozone friendly and low green house emissions refrigerant motivated the inventing of R600a and R134a. The study revealed that the thermal conductivity of R600a is better than R12 and R134a which meant higher heat transfer rates and higher COP's. The higher molecular mass of R600a gives it longer life and enhances hermetic compressors because it has lower diffusion rates, it also allows for the use of smaller compressors and thus lower costs. The compressor discharge temperature for R600a is lower than other refrigerant, which means lower materials cost and increased COP since the condenser's operating temperature is reduced. Finally, the pressure of R600a in evaporators and condenser is half of that in other systems using different refrigerant which means lower initial and running costs and longer life cycle.
Main References
- Cengel, Yunus, Thermodynamics An Engineering Approach.
- Cengel, Yunus, Heat and Mass Transfer.
- Dossat, Roy, Principles Of Refrigeration.
- Al-Asaad; Mohammad, Hammad, Heating, Ventilating and Air Conditioning.
Methods
Throughout this section, the scope and methodology adopted in this project are clarified. It will show the principles and assumptions that were made and the mathematical relations that were employed to achieve its goals. The flow of work is also presented in this chapter to direct the reader in the steps that were made.
Theory
Heat load means the amount of heat transferred to or from the enclosed medium that is being considered. This heat load must be considered because it shows how much energy must be removed/added to the system. There are many kinds of heat loads, the ones being considered are as follows:
A. Heat Load due to Conduction:
B. Heat Load due to infiltration
C. Heat Load due to Equipments
D. Product Load
In nature, heat always travels from higher temperature region to lower temperature region in the direction of the temperature profile. In order to reverse the direction of energy flow; a machine must be employed, which is a refrigerator or a heat pump.
The selection of equipment will depend strongly on the heat load calculations. The selection will be made by choosing the appropriate unit that will provide the system with the required effort. For example, evaporator and condenser will be selected depending on the amount of heat required to be absorbed and rejected, respectively, and this will be the case for the rest of the system's components.
These selections will be mad from the manufacturer's brochures. Quality and capacity will be the main factors affecting the selection process.
After ending the previous step, a full operating system will be presented, and some quantities must be obtained (such as COP) in order to know how the system is doing.
Studying of effects will be made to decide what changes must be made in the way these water cooler are built in order to improve efficiency, lower costs and increase capacities. As a way of accomplishing this objective, the effect adding insulation for the storage tank for example will be studied
This project is quite important in that it will show some of the weaknesses in the existing water cooling/heating devices that are heavily used. The system will be designed to serve a facility where 50 employees work, it will cover their needs of drinking.
`Flow of Work
Start
Gather the information
Design the refrigeration system
Check for the performance
Reverse the cycle and check for its performance and make some changes
Fig. 1: Flow of work.
Analytical Model 1
: To Obtain the amount of energy needed to be supplied to or extracted from the system
: Amount of cooling inside the evaporator
: mass flow rate of the refrigerant.
: To obtain the needed surface area of the coil.
: To obtain the coil's length.
Numerical Modelling
The calculation program that was used to obtain results is Microsoft Excel because all the equations are simple mathematical relationships. Also all comparison charts were generated by Excel.
Analysis & Discussion
This chapter will include:
1- Refrigeration cycle: Performance Parameters calculations (like: volume flow rate, refrigeration effect and COP).
2- Reversed refrigeration cycle performance parameters.
3- Detailed design and selection steps.
3- Effects and conditions on the efficiency of the cycle.
Analytical Modelling and Numerical Results
4.1.1 Heat Load Calculations
As stated in section 5, there are 50 employees that this cooler is going to serve. A human body daily needs at least 1 liter of water. Assuming that an average working employee sleeps for 7 hours, spend another 9 hours in work and the rest of the time is consumed by daily activities, this means that the employee at work will consume :
…………….…………………………………………….(12)
Accounting for the 50 employees, the total consumption of water at the facility is:
………….………………………………..……(13)
Where 1.1 is a safety factor to account for excessive usage assumed by the designer. The heat capacity of water is 4.18kJ/kg.KÌŠ. The water inlet temperature equals the outside ambient temperature, setting the outside temperature to be 38ÌŠC in summer and 5ÌŠC during winter and taking the ideal water temperature for drinking purposes to be 17ÌŠC. The density of water is assumed to be equal to 1kg/liter.
Using equation 1, the cooling load of water is:
…………………………(14)
Since the storage tank will have a constant volume, then the heating load calculations will use the same water mass. Notice that water will be warmed up to 55ÌŠC during winter. Heating load is:
……………………………..(15)
4.1.2 Tank Volume
It is assumed that during operation period the tank will be filled and emptied 4 times. Dividing the total volume on 4 we get:
……………………………..(16)
Tank design
The tank is going to be a 230Ã-150Ã-150mm container to form a volume of 5.175liters which almost coincide with the heat load demand. The material of the tank is 304 Stainless steel because it is rust resistant and chemically inert.
4.1.3 Equipment: Background and Specifications
Figure 1: Refrigeration Cycle Chart.1
4.1.3.1 Refrigerant
Refrigerants are the fluids running in the mechanical cycle and they transport heat from one part to the other. Theoretically, any fluid can be used as a refrigerant, but their operating characteristics decide if they are suitable to be used as refrigerants or not. In general; there is no substance that can be used as an ideal refrigerant for all applications, each refrigerant has its own
properties that allows it to be used for a certain application. Refrigerants are usually categorized according to:
Explosiveness and flammability.
Toxicity.
Miscibility to oil and/or moisture.
Chemical formula.
Environmental impact.
Selection of Refrigerant
The most suitable refrigerant for such applications is R-134a (1,1,1,2-Tetrafluorethane). The chemical formula of this refrigerant is CH2FCF3. R-134a was selected because:
Non-explosive and non-flammable which makes it suitable for residential applications.
Environmentally safe with ozone depletion value of zero.
Non-toxic and chemically stable.
It's been available for long time enough for workers in the field to gain the required experience to handle it.
_________________________
1Source: Cengel, (2006), Thermodynamics An Engineering Approach, McGraw-Hill.
Fig. 2: R-134a Thermodynamic chart.1
The only problem that faces R-134a is its affinity for moisture, but this problem could be solved
by good sealing of the system and the using of drying agents
Active Region
The active region is the region where the refrigeration cycle is going to take place on the Ph diagram, in other words; the physical state of the refrigerant at each stage of the cooling process. Determining this region is very important because it will be the base of obtaining other quantities in order to specify the equipment later on.
In this project; the main four states of the refrigerant are as follows:
1 Evaporator exit-Compressor inlet: the refrigerant will leave the evaporator as superheated gas at 4C and 0.25MPa.
2 Compressor exit-Condenser inlet: Refrigerant will enter the condenser at 70C and 1.2MPa.
3 Condenser exit-Throttling device inlet: Refrigerant will leave the condenser in liquid state and enter the throttling device at 34C and 1.25MPa.
4 Throttling exit-Evaporator inlet: the liquid will be throttled to a temperature of -6C and a pressure of 0.25MPa and enter the evaporator as saturated liquid.
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2Source: http://www.nzifst.org.nz/unitoperations/appendix11a.htm, accessed on February, 25th 2010
Fig. 3: Refrigeration cycle of the device.3
4.1.3.2 Evaporator
Evaporator is essentially a heat exchanger that is used to remove heat from on medium to another by allowing one of them to vaporize by the heat extracted by the other one.
Heat is removed from medium 1 (the one heat is removed from) by one of the four heat transfer
methods: conduction, natural convection, forced convection or radiation to the surrounding which is usually filled with a gas or a liquid. After the surrounding fluid caries a certain amount
of heat, it is transported to the evaporator coil where heat exchange occurs between the fluid and the surface of the coil by conduction, in some cases; medium one is in direct contact with the evaporator coil so conduction heat transfer occur directly between medium one and the coil. Heat is then conducted through the coil or tube walls, until it reaches the refrigerant on the inner surface where heat is transferred by conduction or convection. The refrigerant uses the absorbed energy as latent heat to transform state from liquid to vapor then it is transported to the compressor.
In general, evaporators are categorized into five groups depending on:
Shape and construction: plate-surface, bare or finned tubes.
Circuit construction: flow (cross or counter) and type of circuits (single, split)
Refrigerant supply method: dry expansion or refrigerant flooding.
Surrounding fluid transportation: natural or forced convection.
Defrost technique: cycle reversing, hot (gas, air or water), electric heater.
Evaporators must be designed to allow for maximum operation efficiency, easy maintenance and optimum oil return and to prevent freezing and high costs. The material of the evaporator must be highly heat conducting to improve the heat transfer process, such as brass or stainless steel, the material also must be chemically inert with the refrigerant or the surrounding media.
Evaporator design
A) Initial Design
The amount of energy needed to be removed from the water in the 5.175L tank is calculated using equation1:
………(17)
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3Source: http://www.nzifst.org.nz/unitoperations/appendix11a.htm, accessed on February, 25th 2010
The time this water is needed to be cooled to that temperature is 30 minutes, this means that energy must be removed in the rate of:
………………………………………………………….....(18)
The refrigeration effect is calculated simply by subtracting the enthalpy (obtained from the Ph diagram) state from the inlet state; that is:
............(19)
In order to obtain the mass flow rate of the refrigerant that will accomplish this amount of heat removal rate is calculated as follows:
……………………..(20)(Dossat, 2001)
Then, the volume flow rate is calculated:
.........................................................................................................(21)(Dossat, 2001)
Thus the velocity of the fluid running in the pipes is:
………………………………………...(22)
Having the refrigerant parameters obtained the remaining variable that hasn't been calculated yet is the length of the evaporator coil. Since the amount of heat transported to the refrigerant is the same amount extracted from water the following is valid:
…..................................................................................................….........(23)
……………………………………………………......(23-a)
…………………......……(24)
In the previous equation; the total length was multiplied by a factor of safety of 15% in order to ensure that the refrigerant will reach the compressor as superheated vapor in order to prevent damaging it with liquid droplets.
Fig. 4: Evaporator Design.
4.1.3.3 Compressor
A compressor is a machine that increases the pressure of a fluid (usually a gas) to a desired value. This process is performed by placing the gas in smaller space thus reducing the specific volume and increasing the pressure according to the universal gas constant. Compressors are classified as high-pressure devices because there is a significant difference in the value of the inlet and outlet pressures in the opposite of fans which are classified as low pressure devices.
Air compression devices are widely used in our life in many applications such as tire inflating, drills and pneumatics. In refrigeration, compressors are used to increase the pressure of the refrigerant vapor to the condenser pressure, by doing so, the enthalpy content and temperature increase.
Compression cycle consists of four strokes:
1-2: Intake stage.
2-3: Compression stage.
3-4: Discharge.
4-1: Re-expansion.
Depending on the construction and method used for compression, compressors are categorized to rotary, reciprocating, centrifugal, axial flow or jet compressors, in water chillers, piston-cylinder compressors are usually used. They are also categorized as closed if the compressor and driving motor are concealed in the same case and opens if they are not.
Compressor Specifications
Since the variables of the evaporator are now set, the specifications of the compressor may be selected. The suction pressure and temperature of the compressor must equal the pressure of the evaporator, that is, Pin= 234.44kPa and Tin= -6ÌŠC. Now the discharge conditions must be selected depending on the condenser state, but as a first try, the discharge pressure and temperature are: Pout= 1.2MPa and Tout= 45ÌŠ C. The flow rate of the compressor will equal the volume flow rate which is 0.00000124m3/s, these are the variables that compressor will be adjusted in order to meet them.
For this application, QD28H11 LIYOUNG compressor was selected because:
1- Provides high efficiency.
2- Low vibration level.
3- Low voice.
4- Good reliability.
Table 1: Compressor QD83HG technical data
Model
QD83HG
Refrigerant
R134a
Displacement (cm3)
8.3
Capacity(w)
210
Motor input(w)
170
Current(A)
1.25
C.O.P(W/W)
1.23
Oil charge(cm3)
160
Cooling
S
Motor type
RSIR
Power supply
220-240V~50
Table 1 and Fig. 5: QD28H11 data and picture.5
4.1.3.4 Condenser
A device used to extract heat from the refrigerant vapor and rejected out to the heat sink at constant pressure to allow the refrigerant vapor to cool down and go back to its liquid state. The amount of heat that must be rejected by the condenser equals the heat absorbed by the evaporator and the heat generated by compression.
Depending on type of cooling, condensers are categorized as air, water and evaporative cooled, the most common type of condensers are the forced convection air compressors which are used in almost all of the air conditioning devices.
Heat transfer that occur in the condenser obeys the equation, in this equation, ∆T value depends on the direction of the flows of the fluids on both sides of
the condenser wither they were parallel (and thus use normal temperature difference) or cross flowed (where logarithmic mean temperature difference must be used). The condenser operating temperature is a function of the evaporator temperature and thus care must be taken when selecting the condenser. Condensers and evaporators are very similar in everything, in fact they may be assumed to be identical devices on different sides of the cycle, and they have same kinds and same categories in general.
Condenser Design
The process of condenser design is very similar to the evaporator design because essentially they are both the same: heat exchangers. The main difference between them is the heat transfer
method. In the case of this project; the evaporator's heat transfer mechanism was natural convection, the condenser's heat transfer mechanism is forced convection which means that a fan will be employed.
A) Fan selection
The selected fan is V12E12BGB5-01 from the V12 B5 Series fabricated by Nidec. The technical data of the fan are as follow:
Air Flow
Operating Voltage
Operating Power
225 CFM
12V
27.5W
Fan Speed
Sound Level
Operating Temperature
7000rpm
66.5dBA
-10ÌŠC - 70ÌŠC
Dimensions: 120Ã-120Ã-38mm
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5Source: http://www.liyoung-compressor.com/Water-Dispenser-Compressor-19.html, accessed on February, 12th 2010.
Table2 and Fig. 6: V12E12BGB5-01 Technical data and picture.6
B) Condenser design
…………………………………………………………………………………...(25)
The mass flow rate inside the condenser is the same of that in the evaporator. On time basis:
........(26)
The velocity of the air leaving the fan is:
………………...……………………………….......(27)
Since this is a forced convection heat transfer operation, the heat transfer coefficient must be calculated. In order to obtain it, Reynolds number, Nusselt number and Prandtl number must be
obtained. These variables are calculated as follows:
Average temperature is
………………………..…..(28)(Cengel, 2006)
Using interpolation, the parameters values at this temperature are obtained from tables:
Air Density Ρ=1.0788kg/m3 Air Thermal Conductivity k=0.027642
Prandtl number Pr= .72176 Dynamic viscosity µ=0.00001981kg/m.s
Using the same
…………………...(29) (Cengel, 2006)
At this Re value, Nusselt number is calculated using the equation:
Heat transfer coefficient is then obtained:
…...…..(30)(Cengel, 2006)
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6Source: http://www.nidec.com/v12e_b5/v12e_b5_01.htm, accessed on February, 18thth 2010.
required surface area is then calculated by dividing qcondenser on h*ΔT:
……………………….………(31) (Cengel, 2006)
Finally, the length of the condenser's coil is obtained by:
………………………(32)
Fig. 7: condenser Design
4.1.3.5 Metering Device
The job of the metering device is o regulate the flow of the refrigerant from the condenser to the evaporator while maintaining the pressure difference, they control the flow of refrigerant in such that the refrigeration effect equals the heat load. There are many types of metering device such as: Hand expansion, Constant pressure expansion, internally equalized thermostatic expansion, Externally equalized thermostatic valve, pressure balancing thermostatic expansion and Refrigerant distributors and multi-outlet valves. Each type of expansion valves has its own properties and uses.
Metering Device Selection
In reference to (2001, Dossat) table 12-1 page 353 which shows the capacities of Sporlan Valve Company's Throttling Expansion Valves, the metering (throttling) device is selected as follows:
High pressure Phigh= 1.2MPa = 174.04psi, Low pressure Plow = 0.25MPa = 36.26psi, Ph - Pl = 137.78psi → Needed to be provided by the throttling valve.
Inlet temperature Tin = 34C = 93.2F → liquid inlet temperature
System capacity q = 0.252kJ/s = 0.23903Btu/s = 14.3418Btu/min = 0.072ton.
The corrected system capacity is obtained by multiplying the design capacity by the correction factor obtained from the table (using interpolation):
……………………………………………………………..(33)
0.075ton is sufficiently close to 0.125ton → Accepted.
4.1.3.6 Reversing Valve
Valves are tools installed to the piping system in order to regulate the flow by stopping it, allow it to move or partially close the way in front of it to decrease the flow amount. Reversing valves have the same task but it have a different property that allow the valve to reverse the flow of the fluid going through the piping, as an example about this project, reversing valve change the direction of the refrigerant by reversing the inlet and exit of all devices in the cycle making the evaporator to operate as the heat rejection part and the condenser as the cold side heat absorption.
Design of the reversing valve
The reversing valve is a four way valve that converts the flow of the refrigerant at the compressor at converts the outlet to the evaporator and the inlet from the condenser as shown in the figure below.
Fig. 8: Reversing device design.
4.1.3.7 Drier/filter
A drier is the part of the system that is responsible of removing moist and contamination that may exist in the service due to operation conditions or manufacturing. The drier al acts as a filter because it traps the contamination that may adversely affect the system. A drier is made from a small cylinder filled with silica gel (silicon dioxide) which is the drying agent also called as "desiccant".
4.1.4 Reversing the Cycle
The amount of heat needed to be supplied to the heated medium can be calculated in the same manner as in equation (17) which calculated the heat needed to be removed by the evaporator to achieve the required cooling. The amount of supplied heat equals:
……………………………………(34)
Since the same compressor is operating, thus; the same flow rate exist and the heat supplying rate can be obtained as follows:
………………(35)
The time needed to heat water to the required temperature is then easily calculated by dividing Q over q:
…………………………………………..(36)
4.1.5 System Performance
In refrigeration; performance is measured by the coefficient of performance (COP). This concept was obtained in order to forbid obtaining efficiencies greater than one which validates the thermodynamic principles. The COP is measured by dividing the refrigeration effect (the amount of heat removed by the evaporator) by the heat of compression. In the case of reversed cycle (heat pump); the value of the heat supplied by the condenser to the surrounding replaces the refrigeration effect. For this cycle; COP and COPh are:
………..……………….(37) (Dossat, 2001)
……......….(38)(Dossat, 2001)
4.2 Validation modelling
4.2.1 System Study
After the system has been designed and the parameters are all set; the system must be studied and examined in order to reveal the point of weaknesses if they existed and search for applicable modifications to be installed to the system and thus increase its efficiency. The studying process is accomplished by obtaining a graphical representation for variables and then discusses the relationship and then decides if there is a chance for changing or modifying the system.
4.2.1.1 Increasing Refrigeration Effect
Chart 1: COP, COPh versus Refrigeration effect.
As noted from figure; the relationship between the refrigeration effect and the COP is proportional and that is expectable since the value of COP equals R. effect divided by the heat of compression, as heat of compression remains constant with increasing refrigeration effect COP will increase in condition this process occurs inside the evaporator. Even though this seems a good indication, unlikely; increasing the refrigeration effect leading to super heat the saturated vapor, the specific volume of superheated vapor is greater than saturated vapor meaning that the compressor will rotate more in order to maintain the same mass flow rate and thus the same amount of cooling; leading to increasing the input to the system and thus decreasing the efficiency adding to that more heat generated by the compressor will be added to the refrigerant.
Chart 2: Heat rejected by the condenser versus the Refrigeration effect.
Increasing the refrigeration effect will also increase the heat needed to be rejected by the condenser thus extra amounts of condenser tubes must be added to account for this increase. In the case of reversed cycle being operating; this is preferable because the water is being heated now and thus the greater the amount sucked from the surrounding and rejected by the condenser the faster high temperature of water will be accomplished.
4.2.1.2 Outside temperature
Chart 3: Mass flow rate of the refrigerant versus Water inlet temperature.
The previous chart shows the proportional relationship between the outside temperature and the mass flow rate supplied by the compressor. This relationship may be explained by the fact that there is a greater amount of heat to be removed from the water being cooled under the assumption of that cooling must occur in the same time interval so the cycle must be repeated more frequent in order to provide this amount of cooling provided that the temperature difference between the condenser coils and the ambient is smaller thus reducing the temperature gradient and thus the heat transfer operation. This will also decrease the cycles COP because more input is supplied to obtain the same output and thus less efficiency. This fact reveals a fact that there must a reasonable space between the coils of the condenser and the near walls in order to allow surrounding air carrying the rejected heat to be transported in order to maintain the desired temperature difference.
4.2.1.3 Lowering Evaporator Pressure
In reference to figure 3, and as known in the thermodynamic cycles the amount of work extracted by the cycle equals the area enclose between the processes lines. Reducing the evaporator's pressure will increase the work extracted by the cycle, the refrigeration effect and the overall COP of the cycle. The following chart represents the relationship between the evaporator pressure, compressor work and the COP.
The following char proves that the relationship between the compressor work and the COP of the refrigeration cycle is inversely proportional at decreasing evaporator pressure and this result is quite reasonable because when the pressure is reduced, the refrigeration effect increase and in the same time; the compressor must provide more work per unit mass in order to reach the condenser's pressure and the overall result of dividing refrigeration effect by compressor work decreases. In the case of increasing the condenser's pressure; the case is reversed but the same conclusion is obtained. However, the two lines intersect nearly at 215kPa. This point represents the optimum value of the evaporator pressure to obtain the highest COP at the lowest compressor work.
Chart 4: Compressor work, COP versus Evaporator's pressure7
.
4.2.1.4 Other Considerations
The design didn't take into account the fact that there are unavoidable pressure losses in the system. It also assumed that the enthalpy at evaporator's inlet equals the enthalpy of saturated liquid at condenser's pressure which is not valid in reality. These short comes will be accounted for in the modified design keeping the general layout of the system the same.
_________________________
7Data of this chart were taken from tables (a-15 and a-16) from the book: Thermodynamics An Engineering approach and they are based on different set point that is different from the data on which previous designs were made. COP value was multiplied by 25 to clarify the optimum range by visual inspection
4.3 Final Design Statement
This is the final cycle selection that will represent the cycle of this project's water cooler/heater. In this cycle; the short comes stated in 7.4 will be accounted for. Also the optimum evaporator pressure obtained in chart 4 will be used instead of the one existing.
Fig. 9: Final Refrigeration Cycle.3
Q= 454.26kJ as calculated in equation 17, the mass flow rate is not changed and kept at 0.00163 as calculated in equation 20. In the new cycle; the enthalpy of the refrigerant leaving the evaporator equals approximately 412kJ/kg while the inlet enthalpy remains the same here, this leads to:
…...…..(39) (STATEMENT)
…………………………………………(40) (STATEMENT)
In the new cycle, the average refrigerant temperature and thus evaporator's surface equals -10ÌŠ C
…………………(41)
……………………………………………………......(42)
………………………(43) ________________________
3Source: http://www.nzifst.org.nz/unitoperations/appendix11a.htm, accessed on February, 25th 2010
(STATEMENT)
Regarding the condenser; the calculations are as follows:
.…............................................................................................................ (44)
(STATEMENT)
Here, the condenser's inlet pressure is decreased by nearly 3kPa in order to account for pressure losses inside the condenser, and the exit temperature is lowered this time by an amount that equals the super heating increase (according to Dossat, 2001).
Repeating equation 31 with q= 0.326 the result becomes
………………………….........………………..…(45)
Finally, the length of the condenser's coil is obtained by:
………………………….................................................................………(46)
(STATEMENT)
The new cycle COP's are:
…………………………….…...........................................…………………….(47)
(STATEMENT)
……………….......…....(48)
(STATEMENT)
Chapter 5 Conclusions & Recommendations
After all the calculations and modifications conducted, final conclusions about the work are made and based on them; specifications are made. The obtained conclusion and recommendations are listed in this chapter.
5.1 Conclusions
The coefficient of performance of the refrigeration cycle depends proportionally on the refrigeration effect. However; increasing the refrigeration effect means lowering the evaporator pressure and this is limited by the compressor and throttling valve capabilities, also lowering evaporating pressure below certain values will increase the compressor work in a manner that will decrease the overall COP.
The material selection is quite suitable. The dimension of the tube may be raised in order to increase the cross sectional area and reduce the total length.
In the case of refrigeration cycle operating; the performance of the system will decrease with increasing outside temperature because the temperature difference between the condenser's coil and the ambient which will decrease the temperature gradient and thus the heat transfer operation. The opposite is true for heat pump status.
Even though the COP of the reversed cycle is higher than that of the refrigeration cycle; the amount of energy and time needed to achieve the required temperature is relatively high, so, the use of an electric heating element will be more efficient and faster that reversing the cycle, it will also save the expenses of reversing valve and will save space.
Recommendations
It is recommended to do the following:
Isolate the container tank in order to prevent losses.
Use electrical heating element instead of reversing the cycle.
The length of the evaporator and condenser may be reduced by using larger tube diameter.
