CONCLUSION

 

            As the conclusion, we can say that, refrigerator consists with two compartments - one for frozen items and the other for items requiring refrigeration but not freezing. It can throw all the heat from the loads inside the compartments and make it cooled and long lasting life. It suitable for application in food and medical industry.

            We can also said that the objective of this experiment have been completely achieved as required and at the same time, all the parameters required to be solved have been calculated and solved accordingly. In addition, all of the experiments have eventually being done according to the procedures given systematically and appropriately. 

DiSCUSSION 5

1)      Outline at least 3 measures to increase the COP_ref of a refrigeration system.

           

            To increase the COP_ref of a refrigeration system, some consideration must be taken. Such as:

                                       

                     i.            The Work input, W_net,in for the whole refrigerator.

                   ii.            Temperature difference between actual and before the process started.

                  iii.            Secondary working fluid. Such as ammonia.

                 iv.            The fluctuating of pressure that affect the refrigeration systems.

 

 

DiSCUSSION 4

1)      Give examples with appropriate diagrams and explanations of actual loads in refrigeration practice in a factory.

 

 

 

 

Figure 1: Batch continuous air blast freezer with counter flow air circulation

 

 

 

Figure 2: Batch continuous air blast freezer with cross flow air circulation

Figure 3: Batch continuous air blast freezer with cross flow air circulation

(Also constructed with counter current series flow air circulation

           

            As we know, the refrigerator is use to keep all the things cold by lowering their original temperatures; instead, an evaporating gas called a refrigerant draws heat away, leaving the surrounding area much colder. Refrigerators work on the principle of cooling through evaporation.

            A refrigerator consists of two storage compartments - one for frozen items and the other for items requiring refrigeration but not freezing. These compartments are surrounded by a series of heat-exchanging pipes. Near the bottom of the refrigerator unit is a heavy metal device called a compressor. The compressor is powered by an electric motor. More heat-exchanging pipes are coiled behind the refrigerator. Running through the entire system is pure ammonia, which evaporates at -27 degrees Fahrenheit (-32 Celsius). This system is closed, which means nothing is lost or added while it is operating. Because liquid ammonia is a powerful chemical, a leaking refrigerator should be repaired or replaced immediately.

            The same goes with the refrigeration practice (with actual loads) in a factory. As the example we can see from Figure 1, 2, 3 that the refrigerator fills with fish. The refrigeration process begins with the compressor. The process start when Ammonia gas is compressed until it becomes very hot from the increased pressure. This heated gas flows through the coils behind the refrigerator, which allow excess heat to be released into the surrounding air. This is why users sometimes feel warm air circulating around the fridge. Eventually the ammonia cools down to the point where it becomes a liquid. This liquid form of ammonia is then forced through a device called an expansion valve. Essentially, the expansion valve has such a small opening that the liquid ammonia is turned into a very cold, fast-moving mist, evaporating as it travels through the coils in the freezer. Since this evaporation occurs at -27 degrees F (-32 degrees Celsius), the ammonia draws heat from the surrounding area. This is the Second Law of Thermodynamics in effect. Cold material, such as the evaporating ammonia gas, tends to take heat from warmer materials, such as the water in the ice cube tray.

            As the evaporating ammonia gas absorbs more heat, its temperature rises. Coils surrounding the lower refrigerator compartment are not as compact. The cool ammonia still draws heat from the warmer objects in the fridge, but not as much as the freezer section. The ammonia gas is drawn back into the compressor, where the entire cycle of pressurization, cooling and evaporation begins anew. 

DiSCUSSION 3

1)      Explain the tem of COPref and its effect in rating refrigeration systems against economic considerations  

 

The energy efficiency or Coefficient of Performance (COP) of a refrigerating system can be defined as the ratio between the refrigerating capacity of the plant, which is given by Q (cooling/freezing capacity, kW) as well as the power or electricity consumption, P (kW) of the compressors and pumps. In terms of the COP, it is basically and primarily depending on the working cycle and the levels of temperature at evaporating or condensing temperature and the same goes with the properties of the refrigerant and system design as well as the size factor. In a way or another. The efficiency of a refrigerator is expressed in terms of the coefficient of performance (COP) and it denoted by the COP_ref as equation below:

For the refrigerator, the important thing is the quantity of the heat supply to the system from the surrounding. The power input is important because from its quantity, we can know how much must be paid for and constitutes the main item of the running cost. In a way or another, if the usage of electricity is high, certainly it may result to a higher cost and be very costly. Whereas, at lower electricity power consumption, it will certainly be more economical and lower down the cost factor.

DISCUSSION 2

1)      Fill in the parameters from a set of experimental data into the refrigeration system diagrams

           

 

DISCUSSION 1

 

1)      Plot the graph of COPref against evaporator load at constant condenser saturation     temperature. From the graph, discuss the effect on the COPref as the evaporation load is increased at a constant condenser temperature.

 

 

3

2

QC = 15.05

 

 

 

 

 

 

 

 

 

 

 

 

                                                    

	Qe = 13.7

 

 

From the experiment that we have done, it can be said that the maximum value of COP_ref obtained is given by the value of 10.94 which the load used is given by the value of 60N. While, the minimum value of COP_ref obtained is 8.00 which where the load is at 20N value. As for the graph obtained, it can be noted that, the graph tends to decrease at the value of load from 0N until 40N. Whereas, at the value between 40N to 60N, the graph line starts to increase and it actually reaches its maximum point (highest peak) at the value of 60N. However, the graph begins to decrease after it reaches its highest peak until to the value of 80N (final load).

 

Sample calculation

 

Load = 0

 

Evaporator

 

Q_4-1 = m_r (h1- h₄)

  m_r = 0.07441,                         h₁= 317.4,       h₄ = 132.6

Q_{4-1 =} 0.07441(317.4 – 132.6)

        = 13.7

 

Condenser

 

Q_2-3 = m_r (h₂- h₃)

    m_r = 0.07441,                       h₂ = 334.8,      h₃ = 132.6

Q_2-3 = 0.07441 (334.8 – 132.6)

       = 15.05  

 

Compressor

 

W_1-2 = P =    m_r (h₁- h₂₎            

         m_r = 0.07441,                  h₁= 317.4,    h₃ = 132.6      

W_1-2 = P =  0.07441(317.4- 132.6)        

              = 1.29             

 

COP_ref  =  

            =   317.4-132.6

                  334.8-317.4   

            =    10.62         

 

RESULT

 

Load	Evaporator Temp(0C) T3	Condenser Exit temp (0C) T1	Refrigerant Flow rate (kg/s)	Cooling Water Flow rate (kg/)	Q4-1	W1-2	Q2-3	COPref
0	22.17	26.38	0.07441	0.01484	13.75	1.29	15.05	10.62
20	24.54	17.60	0.07723	0.01328	16.18	2.02	18.13	8.00
40	31.45	17.86	0.07677	0.01328	16.02	1.83	17.86	8.73
60	29.65	18.28	0.07718	0.01328	16.53	1.50	18.03	10.94
80	29.08	18.57	0.07690	0.01289	16.47	1.76	18.22	9.38

EXPERIMENTAL PROCEDURE

 

1. The experiment is started at a condenser saturation temperature of 20^oC.
2. Program 1 is entered and the evaporator load is increased approximately 10%.
3. Return to the main menu and Program 2 is entered. “no print out” is selected and    the three parameters; 5. Condensing Temperature; 2. Refrigerant Flow Rate; and 14. Cooling Water Flow Rate is displayed.
4. The condensing temperature of 20^oC may be maintained by small adjustments of the cooling water flow rate. The system is stable when all three parameters show generally horizontal lines (approximately 1 minute).
5. Return to the main menu and Program 1 is selected with print-out option (raw and calculated data) when the system is stabilized.
6. Evaporator load is increased (by 10%) and the results are printed out. Repeat until evaporator load is 60%.

 

ENERGY TRANSFERS ANALYSIS

 

Compressor

If compression is adiabatic, q_1-2 = 0 and w = h₁- h₂

Power requirement, P = m (h₁- h₂ ), where m is the flow rate of working fluid per unit time.

 

Condenser

q_{1-2 =}   h₃- h₂+ w

w = 0, therefore q_2-3 = h₃- h_2, and rate of heat rejection Q_2-3 = m (h₃- h₂ )

 

Expansion valve

Q_{3-4 =}   h₄- h₃+ w

w = 0 at the expansion valve and process is adiabatic,

therefore h4= h₃

 

Evaporator

Q_4-1 =   h₁- h₄+ w

w = 0, therefore q_4-1 = h₁- h_4, and rate of heat rejection Q_4-1 = m (h₁- h₄)

 

Coefficient of Performance (COP)

 

The Vapor Compression Cycle

Hot Region

1

2

3

Compressor

W 1-2

Expansion valve

EVAPORATOR

CONDENSER

4

q4-1

Source Cold Region

q2-3

Ideal refrigeration systems follow the theoretical Reversed Carnot Cycle process. In practical problems in the compression and expansion of a gas and vapor mixture present practical problems in the compressor and expander. Therefore in practical refrigeration compression usually take place in the superheated field and a throttling process is substituted for the isentropic expansion.  

 

 

 

 

The cycle:

 

1-2       Isentropic compression of the vapor, from the evaporating to the condensing                  pressures                        

 

2 -3      Condensation of the high pressure vapor during which heat is transferred to the high temperature region.

 

3 -4      Adiabatic throttling of the condensed vapor from condensing to the evaporating             pressure.

 

4 -1      Evaporating of the low pressure liquid during which heat is absorbed from the    low temperature source.

THEORY

 

A refrigeration cycle is work to lower and maintain the temperature of a controlled space by heat transfer from a low to high temperature region.

 

High temperature reservoir, TH

 

 

 

 

Q_H

 

                                                                   W_net  

 

 

 

 

Q_L

 

	Low temperature reservoir, TL

 

 

 

 

 

Refrigeration is another term for the cooling effect of the refrigeration system, which is the rate of heating being removed from low temperature region with specified evaporator and condensation temperatures. The unit “duty” measurement is in the watts (for 1 ton of refrigeration = 351 W)

Variation in Refrigeration Coefficient of Performance at Various Process Temperatures

Variation in Refrigeration Coefficient of Performance at Various Process Temperatures

 

INTRODUCTION:

 

Refrigeration is used widely in various applications from industrial to domestic situation, mainly for the storage and transport of perishable foodstuffs and chemical substances. It has the prime function to remove heat from low temperature region and it can also be applied as a heat pump for supplying heat to a region of high temperature.

 

OBJECTIVE:

 

The objective of this experiment is to investigate the variation in Coefficient of Performance (COP_ref) of a vapor compression refrigeration system.