1.1 BACKGROUND OF THE STUDY
Heat is defined in physics as the transfer of thermal energy across a well-defined boundary around a thermodynamic system. The thermodynamic free energy is the amount of work that a thermodynamic system can perform. Enthalpy is a thermodynamic potential, designated by the letter “H”, which is the sum of the internal energy of the system (U) plus the product of pressure (P) and volume (V). Joule is a unit to quantify energy, work, or the amount of heat. Heat transfer is a process function (or path function), as opposed to functions of state; therefore, the amount of heat transferred in a thermodynamic process that changes the state of a system depends on how that process occurs, not only the net difference between the initial and final states of the process. (NJIT, 2011)
Thermodynamic and mechanical heat transfer is calculated with the heat transfer coefficient, the proportionality between the heat flux and the thermodynamic driving force for the flow of heat. Heat flux is a quantitative, vectorial representation of the heat flow through a surface. (NJIT, 2011)
In engineering contexts, the term heat is taken as synonymous to thermal energy. This usage has its origin in the historical interpretation of heat as a fluid (caloric) that can be transferred by various causes(Lienhard, 2008) and that is also common in the language of laymen and everyday life.
Modern refrigeration has many applications, such as preserving medicine, blood, and the most important application, the preservation of food. Most foods kept at room temperature spoil rapidly. This is due to the rapid growth of bacteria. Refrigeration preserves food by keeping it cold, which greatly slows down the growth of bacteria. In days past, blocks of ice were used in iceboxes to refrigerate food and other items. These iceboxes were small and not very practical. Today, mechanical refrigeration systems make transportation, storage, and use of refrigerated goods easy and practical.
The refrigeration system consists of four basic components namely the compressor, the condenser, the evaporator, and the thermal expansion valve. These components are essential for any refrigeration system to operate.
Condensers and evaporators are heat exchangers with fairly uniform wall temperature employed in a wide range of HVAC-R products, spanning from household to industrial applications. In general, they are designed aiming at accomplishing a heat transfer duty at the penalty of pumping power. There are two well-established methods available for the thermal heat exchanger design, the log-mean temperature difference (LMTD) and the effectiveness/number of transfer units ( ε-Ntu) approach (Kakaç and Liu, 2002; Shah and Sekulic, 2003). The second has been preferred to the former as the effectiveness, defined as the ratio between the actual heat transfer rate and the maximum amount that can be transferred, provides a 1st -law criterion to rank the heat exchanger performance, whereas the number of transfer units compares the thermal size of the heat exchanger with its capacity of heating or cooling material. Furthermore, the ε-Ntu approach avoids the cumbersome iterative solution required by the LMTD for outlet temperature calculations.
The thermal expansion valve is a metering device which can also be called a TX Valve (Thermostatic Expansion Valve).This valve has the capability of controlling the refrigerant flow. If the load on the evaporator changes, the valve can respond to the change and increase or decrease the flow accordingly.
Expansion valves are flow-restricting devices that cause a pressure drop of the working fluid. The valve needle remains open during steady state operation. The size of the opening or the position of the needle is related to the pressure and temperature of the evaporator. There are three main parts of the expansion valve that regulate the position of the needle. A sensor bulb, at the end of the evaporator, monitors the temperature change of the evaporator. This change in temperature creates a change in pressure on the diaphragm. For example, if the temperature in the evaporator increases, the pressure in the diaphragm increases causing the needle to lower. Lowering the needle allows more of the working fluid into the evaporator to absorb heat. The pressure at the inlet of the evaporator affects the position of the needle and prevents the working fluid from flowing back into the compressor. Since the pressure before the valve is higher than the pressure after the valve, the working fluid naturally flows into the evaporator. The pressure at the inlet of the evaporator acts on the diaphragm. There is also a spring providing a constant pressure closing the valve needle. The spring constantly restricts the amount of working fluid entering the evaporator.
The pressure spring can be adjusted to increase or decrease pressure based on temperature needs. The pressure created by the spring acts on the opening of the valve. When the pressure of the sensor bulb acting on the diaphragm is greater than the combined pressure of the evaporator and spring, the valve opens to increase the flow rate of the working fluid. An increase of flow rate lowers the temperature of the evaporator and allows for more heat absorption. (Hannifin, 2014)
In systems involving heat transfer, a condenser is a device or unit used to condense a substance from its gaseous to its liquid state, by cooling it. In so doing, the latent heat is given up by the substance, and will transfer to the condenser coolant. Condensers are typically heat exchangers which have various designs and come in many sizes ranging from rather small (hand-held) to very large industrial scale units used in plant processes. In terms of a refrigerator, a condenser is used to get rid of heat extracted from the interior of the unit to the outside air. Use of cooling water or surrounding air as the coolant is common in many condensers. (Hindelang, Maureen; Palazzolo, Joseph; Robertson, Matthew, 2012)
The purpose of the condenser model is to estimate the parameters which characterize condenser performance. Figure below is a top view of the condenser. Complications of modeling the condenser include air exits at the rear and bottom of the condenser region and recirculation of air from the grille outlet region to the grille inlet region.
Condenser heat exchanger geometry
Heat transfer in evaporators is governed by the equations for heat transfer to boiling liquids and by the convection and conduction equations. The heat must be provided from a source at a suitable temperature and this is condensing steam in most cases. The steam comes either directly from a boiler or from a previous stage of evaporation in another evaporator. Major objections to other forms of heating, such as direct firing or electric resistance heaters, arise because of the need to avoid local high temperatures and because of the high costs in the case of electricity. In some cases the temperatures of condensing steam may be too high for the product and hot water may be used. Low-pressure steam can also be used but the large volumes create design problems.
The evaporator is essentially a counter-flow heat exchanger. However, there are a few distinct differences. One difference is that between the end of the evaporator and the outlet temperature probe is about 15 inches of suction line and an accumulator. The additional suction line passes through the air which has already been cooled. In some cases the superheated refrigerant is hotter than the exit air of the evaporator. In these cases the superheated refrigerant is re-cooled. This re-cooling region of the superheated refrigerant uses up evaporator space and can cause lower exit temperatures than predicted by a model that does not account for this section. Unfortunately no thermocouples were installed in this refrigerator to measure what the temperature of the refrigerant is before it passes through the accumulator region.
Evaporator heat exchanger geometry
As the compressor starts, it draws in the cold refrigerant gas in liquid form as it leaves the evaporator. Refrigerators use a type of refrigerant gas that turns into a liquid at very cold temperatures: 15.9 degrees Fahrenheit (3 degrees Celsius). The compressor then puts pressure on the gas compressing it. As the gas is compressed, its temperature goes up.
The compressor pushes out the hot, compressed gas through the outside metal coils (tubes) on the back or bottom of the refrigerator. These coils allow the heat to dissipate into the surrounding air. Because it is under pressure, the gas changes into a liquid as it cools
A thermal expansion valve (often abbreviated as TEV, TXV, or TX valve) is a component in refrigeration system that controls the amount of refrigerant flow into the condenser thereby controlling the superheating at the outlet of the evaporator. Thermal expansion valves are often referred to generically as “metering devices”.
Flow control, or metering, of the refrigerant is accomplished by use of a temperature sensing bulb, filled with a similar gas as in the system, which causes the valve to open against the spring pressure in the valve body as the temperature on the bulb increases. As the suction line temperature decreases, so does the pressure in the bulb and therefore on the spring causing the valve to close. An air conditioning system with a TX valve is often more efficient than other designs that do not use one. (Parker Hannifin, 2014)
Geometry of a thermal expansion valve
1.2 STATEMENT OF THE PROBLEM
The efficiency of refrigerators is relatively average and this has required the need to know the heat transfer occurring in the basic components of a refrigerator. The need for improvement of is required and this can be achieved if the heat transfer in these components are known and compared.
1.3 AIM AND OBJECTIVES OF THE STUDY
The aim of this study is to evaluate how heat transfer occurs in refrigerator basic components.
The objectives are as follows;
- To design a refrigerator rig in which gauges are attached to these basic components
- To examine the heat transfer in refrigerator evaporators, condensers, compressors and thermal expansion valves
- To carry out a test on the refrigerator rig designed.
- To compare the results of the test gotten from the rig.
1.4 SCOPE OF THE STUDY
This study focuses mainly on an experimental investigation on how heat transfer occurs in refrigerator evaporators, condensers, compressors and thermal expansion valves.
1.5 SIGNIFICANCE OF THE STUDY
This report describes an extensive experimental and analytical effort aimed at predicting the performance of evaporators, condensers, compressors and thermal expansion valves when designed as a model.
2.0 LITERATURE REVIEW
This chapter gives a review of previous research works carried out on design and performance of refrigerator basic components. Series of reports, term papers and articles have been written on the subject of heat transfer in these components.
Bullard (1993) worked on a report which describes an extensive experimental and analytical effort aimed at predicting the performance of evaporators and condensers using alternative refrigerants. Heat exchanger models are also expressed in a form where heat exchanger tube diameters and lengths are explicitly specified to help analyze new configurations. The models designed were useful for assessing the applicability of refrigerant heat transfer correlations to refrigerator models. The correlations that were used in the models were developed under ideal conditions in long straight tubes. The accuracy of these models will provided insight into how well the heat transfer correlations work in actual modeling applications. Results of the evaporator and condenser models show that variable conductance models are more accurate than simple constant-conductance models. Variations in heat transfer resistance resulting from changes in refrigerant flow properties can be accounted for by the variable conductance model. Therefore, variable conductance models have the advantage of being versatile and can be used to predict the behavior of alternative refrigerants, changes in tube diameters, etc. The evaporator model was able to predict evaporator loads within 4 %. This indicates that the evaporator heat transfer resistance is known within about 4 %. Using a simple constant conductance model it was estimated that a 10% error in the heat transfer resistance causes only a 1 % error in the calculation of COP (Bullard, 1993). Therefore, a 4 % error in the estimated heat transfer resistance would be expected to produce only a 0.5 % error in the calculation of COP. Similarly, the heat transfer resistance of the condenser was estimated within about 5 %, allowing condenser loads to be predicted within 5%, except in cases where subcooling was excessive. This could also lead to a 0.5 % error in the estimation of COP. Bullard and Porter (1992) showed that such small uncertainties in parameters such as heat exchanger conductances tend to cancel one another and combine with other parametric uncertainties in ways that permit quite accurate prediction of COP and system energy use. The condenser model provided several insights about how performance can be improved, the volumetric air flow rate across the condenser coils can be improved by eliminating places where the air can escape from the condenser region without removing heat. Our model indicates that if the volumetric air flow rate were increased by 20 cm (from 110 cm to 130 cm) the condenser size could be reduced by 10 % while providing the same amount of heat transfer. Eliminating recirculation of outlet air to the grille inlet can result in even better performance. For example, when all of the recirculation was eliminated in our condenser model it was found that the condenser size could be reduced by as much as 40 %. By eliminating only the recirculation that occurred inside the condenser region our model predicted a possible reduction in condenser size of 25 %. It is quite clear from these figures that recirculation and regions where unheated air can leak: are both undesirable. In order to eliminate them, however, it is necessary to provide an unimpeded exit path at the back of the refrigerator and eliminate or seal any holes that were punched in the floor of the condenser compartment during the manufacturing process. For both the evaporator and the condenser the heat transfer resistance of each heat transfer zone is the sum of three components: the air-side heat transfer resistance, the constant part of the refrigerant-side heat transfer resistance, and the variable part of the refrigerant-side heat transfer resistance.
Hasanuzzaman et al (2008) carried out a project presenting the heat transfer analysis of a refrigerator-freezer during the open door operation. Conduction, convection, radiation, sensible and latent heat impose in the cabinet of the refrigerator-freezer. The Rayleigh number varied from 4.2×108 to 5.4×108 in the freezer cabinet and from 3.1×108 to 4.9×108 in the fresh food cabinet that is proved the laminar flow. Energy loss due to the heat transfer was about 46.2% through the walls conduction, 10% convection and radiation that was more affected by the number of door opening and cabinet load. The refrigerator-freezer used was volume 460 liter, top mount freezer section, two doors and both doors hinged at right hand side. T-type thermocouples were used to measure temperature. Humidity sensor was used to monitor the relative humidity with range of 3% to 95% and accuracy of ± 5%. Heat pump was used to maintain the required temperature inside controlled chamber of range from 16 ºC to 32 ºC. Dehumidifier was used to maintain relative humidity with range from 0% to 90%. Electronic balance was used to weight the mass of evaporated water with weighing capability of 0.01g up to 2 kg. Data logger was used to store the data from the test unit to personal computer. Energy consumption was measured by power meter with the accuracy ± 0.2% of the reading. Automatic door opening mechanism was used to open and close the doors of the fresh food and freezer cabinet and also programmable Logic Controller was used to control the door opening and closing.
Melo et al (2000) presented an experimental analysis of heat transfer paths from the surroundings to the interior food compartment of a 230 liters refrigerator under closed door conditions. The overall refrigerator thermal conductance was determined from conventional reverse heat loss rate measurements as well as from unconventional measurements taken with specially manufactured heat flux sensors. The heat transfer rate through each of the various pathways (walls, door, gasket, etc.) was also quantified. It was noted that the heat fluxes through the bottom wall and through the compressor compartment walls were quite different from the values given in. This is directly related to the lower power released by the electric heaters during this test. The largest heat transfer rate was the one through the side walls (44%) and the smallest was the heat conduction through the bottom wall of the compressor compartment (1.9%). Comparing the total heat transfer rate measured by the heat flux meters (41.93 W) with the wall heat transfer rate calculated from equation 3 (47.00 W) it may be also seen that there are heat transfer paths that were not taken into account. These heat transfer paths account for 5.0 W or 10.7% of the total heat transfer rate and are also related to the conduction heat transfer along the door and wall steel frames.
Orzechowski & Stokowiec (2014) presented a paper on the measurement of heat transfer coefficient for the refrigerator’s condenser in free convection regime. In order to improve the heat transfer, a heat exchanger with cross steel ribs was used. The measurement was conducted in stationary conditions with a constant water flow rate. Temperature distribution on the outer surface of the heat exchanger was measured using an infrared camera. Basing on the thermogram for a single rib and temperature read from it, the temperature coefficient of the rib was calculated. The method allowed estimating the heat transfer coefficient. The approximation presents a very good match proved by low standard deviations. It makes the mathematical model possible to be adopted to determine the heat transfer coefficients for similar exchangers. Exceptions result from an industrial welding of individual elements. In the paper the heat dissipated to the ambient by the condenser coils and cross ribs as well as the heat flux transferred to the condenser by flowing water were calculated. The difference in both values is low (absolute error do not exceed 6%), which means that the assumed method was suitable. The heat transfer coefficients were calculated using the experimental data approximation. The calculated values for the presented data were 13.1 Wm-2 K-1 for an upper rib and 17.8 Wm-2 K-1 for a lower rib. Applying the same procedure, the heat transfer coefficients were calculated for all the condenser elements. Their values do not show any significant changes. The average value of heat transfer coefficient for the condenser is 17 Wm-2 K-1 for an upper rib and 18.2 Wm-2 K-1 for a lower rib. Loss of heat rate dissipated by a single rib is calculated to be0.016 W.
Sim et al (2000) in this study, analytical model was developed using the lumped parameter formulation to find out temperature distribution of metal, oil, and refrigerant of a hermetic reciprocating compressor. Correlations of heat transfer coefficients in the literatures were applied to the present model. Motor and mechanical loss and heat generation during compression process were included in the present model. Parametric study was performed as the following parameters; heat transfer area, air velocity over the compressor, mixing parameter of the suction muffler, heat conductivity, heat transfer coefficient of metal in the suction and the discharge systems. Experiment was conducted to measure thermal characteristics of the compressor at steady state. Pressure, temperature, and power of the system were measured with refrigerating load. The predicted result of the analytical model has a good agreement with experimental result.
Hermes (2012) presented a paper which assesses the approach introduced in a prior publication for the optimum design of condensers and evaporators aimed at balancing the heat transfer and pressure drop tradeoffs. Case studies carried out with different heat exchanger configurations for light commercial and household refrigeration applications are reported. The analysis indicated that a heat exchanger design with a high aspect ratio is preferable to a low aspect ratio one as the former produces a dramatically lower amount of entropy. In addition, since the refrigeration system COP obeys the Te/(Tc–Te) scale, it was found that the heat exchanger design that presents the best local (component-level) performance in terms of minimum entropy generation also leads to the best global (system-level) performance. The results showed that the temperature difference of suction chamber gas is 7.8°C and increment of volumetric efficiency is 3.1%. Therefore it is known that insulation of suction system is very effective to improvement of performance of the compressor.
This study explored an analytical formulation proposed elsewhere (Hermes, 2012) that conflates two different heat exchanger design methodologies, the Kays and London’s (1984) ε-Ntu approach and the Bejan’s (1987, 1996) method of entropy generation minimization. It was shown that there exists a particular ε-Ntu design for condensers and evaporators that minimizes the dimensionless rate of entropy generation. To this observation follows the conclusion that a high effectiveness heat exchanger has not necessarily the best thermal-hydraulic design, as the effectiveness does not account for the pumping power effect.
Case studies considering a tube-fin condenser for light commercial refrigeration applications and an evaporator for frost-free refrigerators were also carried out. In case of the condenser coil, where the entropy production due to viscous fluid flow is of the same order of that due to finite temperature difference, the analytical formulation of Hermes (2012) showed to be suitable for thermodynamic optimizations. The analysis also indicated that a heat exchanger design with a high aspect ratio is preferable to a low aspect ratio one as the former produces a dramatically lower amount of entropy. In addition, it was found that in case of a “no-frost” evaporator working under dry coil conditions, the pressure drop influence on the dimensionless entropy production is negligible in comparison to the finite temperature difference.
Corberan (2000) designed a model for heat exchangers working as evaporator or condenser of a refrigeration system is presented, including a comparison between calculated and measured results for a plate heat exchanger and a tube & fins coil. In their paper, the main characteristics of the model and a comparison between experimental and calculated results was presented and discussed. The flow inside the pipes or channels is considered to be one-dimensional, and discretized in as many elements as required. The basis of the numerical procedure is to de-couple the calculation of the fluid flows from each other. Then, both fluid flows evolution along the heat exchanger are calculated through the integration of the 1-D conservation equations. For the refrigerant, the two fluid (liquid & vapour) separated model under equilibrium is considered. Specific correlations for evaporation and condensation heat transfer in pipes and plate heat exchangers have been implemented into the model. Concerning the air, both sensible heat transfer and dehumidification are considered with appropriate correlations for the heat transfer coefficient and friction factor.
Kelman and Bullard (1999) worked on a thesis which focused on the performance advantages of a dual-temperature evaporator refrigerator system over a conventional design through extensive use of computer aided modeling, it is demonstrated that energy savings of at least 8-10% was achieved since the design was coupled with variable speed compressor technology. In addition to the validation study, various optimization techniques was used to reduce energy consumption of sequential cooling systems are also introduced. These methods were subsequently applied to a particular experimental prototype design. The thesis also explored ways of reducing overall charge requirements for a refrigerator system by means of introducing parallel circuits in the heat exchangers. Other topics include evaporator de-superheating phenomenon, a series of studies on flow instabilities in capillary tubes, and a dual-speed compressor performance analysis. While a dual-temperature refrigerator is running in the fresh food mode, the lower temperature lift can raise the steady state COP by as much as a factor of two. This could improve the overall system COP by up to 20%. Also, using fresh food compartment air to defrost the evaporator during the off-cycle can be more energy efficient thanemploying auxiliary power sources. If a single-speed compressor were used in a dual-temperature evaporator system, it would pump nearly twice as much mass flow in the fresh food operating mode than in the freezer mode. Unless there is a way to reduce capacity by slowing down the compressor while cooling the fresh food compartment, larger heat transfer areas or faster fan speeds would be required for efficient operation. However, shorter runtime fractions associated with larger evaporator and condenser could lead to higher transient losses. Also, increasing the size of either heat exchanger would have only a marginal effect on the system performance but could be quite costly. On the other hand, slowing the compressor down by a factor of four or five will result in energy savings approaching the theoretical maximum. However, with a current limitation of only a factor of turndown, about 22% fresh food mode COP boost is expected. Over the entire range of typical operating conditions the overall energy savings are anticipated to be around 8-10%.
This chapter presents the methodology that will be used to carry out the study. The steps involved are;
- Design and fabrication of a refrigerator rig.
- An experimental investigation on the heat transfer occurring in the evaporator, condenser, compressor and thermal expansion valve.
- Analysis of result data obtained from the experiment.
- Comparison of the results obtained from each component.
3.1. CONDENSER GOVERNING EQUATIONS
The refrigerant-side of the condenser was divided into four regions. The four regions include the sub-cooled zone, the part of the two-phase zone upstream of the condenser fan, the part of the two-phase zone downstream of the condenser fan, and the superheated zone. The value of the area ratio, α, is 2.58 for every region of the condenser.
Equations 3.1 through 3.12 give the conductance of each condenser regions.
For the superheated region:
For the sub-cooled region:
And For the two-phase region:
For modeling the condenser both laminar and turbulent flow cases will be investigated so that the proper heat transfer coefficient is chosen.
In the two-phase region of the condenser the Chato/Dobson (Dobson et. aI., 1993) correlation will be put to use. The Chato/Dobson correlation takes the form:
In order to solve this equation for the heat transfer coefficient the liquid and vapor properties of the refrigerant must be found. The liquid and vapor properties are easily determined since the saturation temperature of the refrigerant is known. The parameter that is not so easy to determine is the difference in temperature between the refrigerant and the condenser wall, ∆T. The difference in temperature between the refrigerant and the wall is interdependent with the heat transfer coefficient. However, as long as we know the heat flux through the condenser wall the temperature difference is easily determined using equation below
q = htp∆T
3.2. EVAPORATOR GOVERNING EQUATIONS
The evaporator is modeled as three consecutive counter-flow heat exchanger. The first region of the heat exchanger is the two-phase region of the evaporator, the second is the superheating region, and the third is the de-superheating region. The variable conductance model will be used to calculate the conductance of each evaporator zone.
The Equation below can be used for each of the three regions. The value of α for each of the regions is equal to the ratio of the region’s external surface area to its internal surface area.
For the two-phase and superheating region α is 5.0 and for the de-superheating region it is 1.13 (because it has no fins). The value of Rair for each of the three regions is equal to the air-side heat transfer resistance, and it is dependent on the conductance of the evaporator tubing, fin efficiency, and the air-side heat transfer coefficient. It is assumed to be the same for each of the three regions. The resulting conductance equations are shown below.
For the two-phase region:
For the superheating region:
And for the de-superheating region:
Equations 3.21 through 3.23 were used to calculate the overall conductance of each of the evaporator regions.
Since refrigerant mass flow rates are small htp is calculated using the Chatol/Wattelet correlation, hsp and hdsp are both calculated using the Gnielinski correlation.
The chato/wattelet is given by the equation;
Gnielinski correlation is used for calculating the heat transfer coefficient of the evaporator’s superheated region. This equation was developed by Gnielinski (Incropera and De Witt, 1990).
The Gnielinski equation is given as:
The friction factor “f” can be determined for smooth tubes using the equation:
COMPRESSOR AND THERMAL EXPANSION VALVE GOVERNING EQUATIONS
The overall heat transfer coefficient “U” and heat transfer coefficient “k” for both the compressor and thermal expansion valve cannot be determined because both components are not heat exchangers. However, temperature gauges will be attached to the inlet and outlet of both components to compare the temperature of the refrigerant at the inlet to the temperature of the refrigerant at outlet.
4.0 EXPECTED RESULTS
This chapter summarizes the results that will be gotten from the tests to be carried out on the designed model.
- Results of the evaporator and condenser models should show that the models are more accurate heat exchangers.
- The overall heat transfer coefficient (U) and the heat transfer coefficient (h) of both the evaporator and condenser for each region should vary when calculated and since the tests will be carried out considering the environment temperature at morning and during the day.
- The condenser model should provide several insights about how performance can be improved.
- The temperatures at inlet and outlet of both the compressor and thermal expansion valve should vary when taking readings from the gauge.
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