Analyzing The Relationship Between Coefficient Of Performance (CoP) And Evaporation/Condensation Temperatures

Refrigeration and Coefficient of Performance

Dsicuss about the Coefficient Of Performance Of Vapour-Compression.

Cooling of buildings is one of the major factors which construction of building materials will have to consider [1]. To achieve the best vapor compression cycle should be able to achieve the best coefficient of performance during the cooling process. Better vapor compression cycle is able to indicate the effectiveness of the refrigerator systems. Refrigeration machines are always used in cooling of buildings [2]. These machines are able to employ vapors-compression cycles to achieve the cooling effect. A volatile fluid is mostly used to achieve the cooling effect. This fluid is able to change phase at low temperature in order to absorb heat and cool the loads. In addition, in order to achieve the cooling effect, the refrigerator is able to change the fluid pressure and achieve the temperature at which phase-change point. Two major key factors which are considered for the cooling effect is the energy supply to the cooling load and the size of the cooling load. The Coefficient of Performance (CoP) is defined as the ratio between these two quantities and thus able to measure the efficiency of the systems [3]. Therefore the evaporation and condensation temperatures will have a close relation with the CoP. This report will be able to investigate the relationship between CoP and evaporation/condensation temperatures at which the cycle operates [4].  Theoretically, any cooling system, refrigeration systems and condensers temperature are guided by the size and type of the system components and temperature required for cooling load and heat sink available.

Refrigeration is defined as a process of heat removal from a region or state and substance in order to maintain low temperature, by transferring the heat to another region. The heat transfer on the refrigerator is based on the vapor compression cycles. The performance of the heating and refrigerator systems is measured in terms of CoP [5]; which is expressed as follows;

CoP_R  = {Desired output/ required input} = (cooling effect/work input)

= Q_L/W_{net, in}

The power input is depended on situations of the equipments. The power can be measured at different points and therefore this means that the CoP will depend on the location where the power is measured at. Under this experiment, different condenser designs with varying evaporator loadings are used to analyze the efficiency of CoP [6]. Most importantly, the CoP is depended on the evaporator cooling load, Q_L and the power input cycle to the system, W [9]. The power used in calculation of CoP may be different depending on the location it is measured. For instance, shaft power may be used, and this can be calculated from data involving the torque T and angular velocity ω.

Therefore:                   W_S = T ×ω

                        W_S =(F × r) where motor speed n_m = n_c3 R

In the R713 rig, the torque arm radius, r = 0·165 m and the belt pulley ratio, R = 3.08

Thus:        W_S =(0⋅165F)                                                                  

The power in the systems can therefore be derived from different sections [7]. After getting the power, the theoretical limit point of the CoP of the system operating a reversed Carnot is given as;

Experimental Analysis

CoP_Carnot =

The main objective of this report will be to analyze the relationship between CoP and evaporator temperature for the condenser which is recorded at different temperatures. 

Hilton R715 Refrigeration Unit 

1. First a suitable condenser was selected with saturation temperature adjusted to 30^oC. The corresponding saturation pressure for the Hilton R715 was determined from thermodynamic property table.
2. Next, the unit was connected and ran and the power of the electrical heater was set in the evaporator to about 200W.
3. Condenser water flow rate was adjusted to achieve the required condenser saturation pressure. The conditions were then allowed to settle.
4. The following details were recorded;

t₄ refrigerant temperature entering evaporator          (?C)

P_E evaporator pressure                                           (kPa)

P_C condenser pressure                                           (kPa)

F dynamometer spring balance force                          (N)

Q_E evaporator load                                                  (W)

W_e motor input power                                              (W)

n_c compressor rotational speed                              (RPM)

5. Next, the evaporator load was increase in 200W steps and the above data recorded. In addition, when the condenser pressure drifted as the evaporator load was increased, minor adjustments on the cooling water flow to steady the pressure at its initial value was carried out.
6. In addition, the experiment was repeated with an increased condenser temperature of 40 ^oC.

In the experiment, Hilton R715 Refrigeration Unit was used. The following diagram  represent the different parts and combination of the unit;

Provide discussion of discrepancies with actual process, compare between the theoretical forecast and the observed experimental behavior. Make clear assumptions and identify possible errors.

After the experiment, the following results were achieved.

The change of power input is able to influence the CoP and other outputs of the refrigerator [8]. The motor input is able to increase with the increase on the power input in the system. Regardless on the evaporator temperature, the motor input is able to increase when power input is increased. In addition, another factor which affects the motor input is the temperature change for the system. Operating the system at 36 ^oC is able to yield a lower motor input each moment. The increase of the temperature to operate the system at 44 ^oC is able to show an increase of the motor inputs at each step. In addition, another factor which is affected by the change in the power input is the tachometer revolutions per minute. Increasing the power input has an inverse impact on the tachometer [8]. The tachometer is decreasing in each set of the experiment carried when the input power is increased. The tachometer is important since it is able to influence the angular velocity of the system. This means that the angular velocity, which will be increasing. And therefore since torque multiplied by angular velocity gives the shaft power, it is justified that the motor output is increasing.

Moreover, from the experiment it is clear that the torque is as well affected by the change on power input. The torque force is increasing with the increase in the power input in both experiments. Therefore, it can be conclude that the power input increase requires more torque force to operate the system.  Change in the evaporator temperature has little or no change on the torque. This can be noted from the trend of the torque values in the first set of experiment and the second one. The changes in the two experiments are following the same trend and are almost the same. In addition, increasing the input power has direct influence on the water flow. This is because the power increases the power which moves the water around. In addition, the increase in flow means that the cooling influence is enhanced [9]. The increase in water flow is a positive indicator since it helps the refrigerator to increase its effectiveness. The power input therefore makes the water flow to increase and therefore enhance the reduction of the temperature. In addition, the refrigerant flow is also affected by the change on the power input. The increase in power input is able to lead to an increase on refrigerant flow within the system. This can be related to the increase on the power which drives the fluid through. The increased flow is a good aspect of the refrigerator since it increases the rate of cooling the system. In conclusion, it is clear that the increase on the power input has a positive impact on the CoP. This is because increasing the power is able to increase the effectiveness of the refrigerator. The CoP is able to measure the effectiveness of the system. The increased flow of the coolant fluids is able to increase the effectiveness of cooling effect.

The change on the power input is able to affect the different refrigerator inputs. This included the effectiveness of the refrigerator which is measured in terms of CoP. Therefore through the change of the input power, the analysis of the CoP was clearly carried out and the aim of the experiment was achieved. In addition, the change of the evaporator temperature is seen to have impact on the CoP. The effectiveness of the refrigerator is affected by this change in temperature and the analysis of the impact was achieved through the experiment.

The effectiveness of the refrigerator system is largely depended on the cooling system [10]. The analysis of the CoP is important but relying is on the input power might not be such effective. Future research needs to be based on the changes done on the coolant fluid and its influence 0on CoP. In addition, the further researches must be able to look at the other factor which affects the effectiveness of the refrigerator. In addition, change on the evaporator density should be analyzed to determine whether it is able to influence CoP. This will ensure that high effectiveness of the refrigerating materials is achieved and enhance CoP.

References

 [1] Radermacher, Reinhard, and Yunho Hwang. Boca Raton, FL: Taylor & Francis, (2005). Vapor Compression Heat Pumps with Refrigerant Mixes. New Haven and London: Yale University Press.

[2] Arora, Ramesh C. (2011). Refrigeration and Air Conditioning, New Delhi: PHI Learning,. Print

[3] James W. C (2005). Thermodynamic Analysis of Subcooling and Superheating Effects of Alternative Refrigerants for Vapour Compression Refrigeration Cycles. John Wiley & Sons, Ltd,. Internet resource.

[4] Hundy, G H, A R. Trott, and T Welch. (2016).Refrigeration, Air Conditioning and Heat Pumps. , New Haven and London: Yale University Press.

[5] Nikolaidis, Christos. (1997). A Computer Study of Multi-Stage Vapour Compression Refrigeration Cycles by the Exergy Method. , London: Icon Books.

[6] Kaushik, Shubhash C, Sudhir K. Tyagi, and Pramod Kumar. Cham: Springer, (2017). Finite Time Thermodynamics of Power and Refrigeration Cycles, Internet resource.

[7] Grace, Iain N., (2000). Modelling the Performance and Dynamics of Vapour Compression Refrigeration Systems, Print.

[8] Wayne, C. (2011). Real-time Simulation of Vapour Compression Cycles. , New Haven and London: Yale University Press.

[9] Sheppard, P A, and Franc?ois N. Frenkiel.New York: Academic Press, (2008) Atmospheric Diffusion and Air Pollution: Proceedings of a Symposium Held at Oxford,.. London: Icon Books.

[10] Hundy, Guy. (2008). Refrigeration and Air-Conditioning, Burlington: Elsevier, Internet resource.