Introduction
At times, industries require the temperatures to be kept below the ambient for some processes to be effective. To do so, they must design ways to eliminate the excess heat generated by machines and introduce an artificial room temperature lower than the ambient. They must effectively eliminate temperatures above the ambient. A refrigerator is thus required in these operations. This refers to a device that can maintain the temperature below the ambient. The primary objective of a refrigeration process is to lower the temperature through heat removal. Since its invention, refrigeration has seen used in many industrial and household sectors (Coquelet & Richon, 2009). Some of the applications include preservation of foods at low temperatures to inhibit mold and bacteria that are poisonous for human consumption. Another end user is the industrial chillers used to cool industrial processes, cold storage go-downs such as meat and dairy processing plants, commercialized ice machines used in large hotels and mortuaries as well as motor vehicle air conditioning in trucks and trains. All the systems, however, share the same principle in that they compress a refrigerant (liquid or gas used in the refrigeration system) vapor to high pressures that make its condensing temperature be above the ambient. In turn, it loses heat and condenses to liquid after cooling, thus making the cycle continues. In this experiment, we assess the performance of a vapor compression refrigeration system.
Objectives
Theory
The foundation of the refrigeration system is a thermal cycle that works between two temperature sources. In the cycle a fluid (refrigerant) passes via various states in a specific sequence and then returns to the initial state, thus completing a cycle (Aprhornratana& Eames, 1995). During the cycle, heat is removed from the low-temperature region and dissipated to the surrounding. This keeps the temperature inside the system as low as possible (usually according to the user’s settings). This follows the law of energy that energy cannot be destroyed, but transferred from one state to the other. Using the second law of thermodynamics (heat will not pass from a colder region to a warm one without spending energy or doing work), then it can be seen that the refrigerator requires some energy input for it to operate. Notably, the refrigeration cycles and the heating system are the same. However, for the pump, the heat released is utilized in the system, but for a refrigeration system, it is lost to the surroundings.
A vapor compression system is one of the common refrigeration systems used currently. As shown in the diagram below, it has the following components:
A condenser- this absorbs heat from the refrigerant (at a constant pressure) and then transfers it to the higher temperature supply (Halm & Suen, 2002).
The compressor usually runs by a motor. It compresses the vaporous refrigerant fluid and provides ample mechanical energy, W, to the system
A throttling or expansion system that helps to expand the fluid working medium when a constant enthalpy process takes place
An evaporator that assists in the evaporation of the refrigerant as it absorbs heat from the lower temperature tank.
Analysis of such a refrigeration cycle requires the use of several diagrams such as the p-h and T-s. For an ideal refrigeration cycle, the pressure diagram has the following processes:
An adiabatic or isentropic compression to the ultimate compression temperature accompanied by superheating of the refrigerant or working medium
An isobaric cooling of the condensation temperature.
Release of the condensation enthalpy usually referred as isobaric condensation
The expansion of the wet vapor
And an absorption of the evaporation enthalpy regarded as isobaric evaporation.
An isentropic compression or expansion takes place when no heat flows into or out of the gas takes place. Using the ideal gas law it is expressed as ppk= constant where the value of k is expressed as k=cpcv. If we combine the two equations, then we find out that pVk=constant.
cp and cv are the specific heat at constant pressure and specific heat at constant volume respectively.
Taking into consideration the effects of temperature (T), pressure (p), entropy (s) and volume (v) then the entropy of the gas can be calculated from the algebraic equation
s2-s1=cplnT2T1-Rlnp2p1
However, given that expansion and compression are reversible, then s2-s1=0
This means that cplnT2T1=Rlnp2p1
Hence, the effect of pressure on temperature is represented as
T2T1=p2p1[1-1γ]
On the other hand, the effect of volume on pressure is represented as a ratio
p2p1=v1v2γ
There is a difference between the ideal cyclic and real cyclic process in that compression is not on the latter later, compression is not isentropic. This means that more work is done at the compressor to achieve the same final pressure. The greatest mistake that happens is when droplets get into the compressor thus damaging it. A sub-cooling liquid is placed at the vapor portion at the inlet before it gets to the evaporator (Park, Ahn, & Kim, 2012).
Figure 2: p-h diagram of a real refrigeration cycle
For the refrigeration capacity such as heat transfer from low temperature sources, QL, the mass flow rate (m) must be known.
m=V/v. consequently, the refrigeration capacity QL is calculated as
Qi =mql = m (h1-h4). This value is equivalent to the heat in the water cooled evaporator.
Thus QL = mwC (Tin-Tout) where mw = water flow rate in evaporator, c= specific heat capacity, Tin= inlet temperature and Tout= outlet temperature. From the plotted p-h diagram the work done by the compressor (W) is calculated as W=m (h2-h1). The coefficient of performance (COP)( ratio of useful energy) is then calculated as
COP= QL/ W. since the compressor work is a difference between emitted and absorbed heats then COP can be greater than 1. However, for a real cycle, the value of energy consumption by the compressor is equal to the energy consumption of the motor. On the other hand, the compression ratio for the cyclic process (indication of increase in pressure in the compressor) is a ratio of the upper and lower isobars given by φ= p23p14
Apparatus
Hilton R715 laboratory demonstration unit
Hermetic compressor
Air-cooled condenser
Capillary tube / Thermostatic expansion valve
Water cooled evaporator
Shut-off valves
Sight glass
Drier
Filter
Solenoid valve
Pressure gauges
Thermometric wells
Rotameter
Procedure
The Hilton R715 laboratory demonstration unit was set up (it has a twin cylinder compressor)
R134a was set as the working fluid to replace the R12 (CFC) previously used in commercial and domestic air conditioning and refrigeration system.
All the valves, indicators, and transducers were checked to be operational
Hand valves 2 and 4 were closed and 2 and 3 opened.
The water flow system was started
The compressor was started and its operation observed.
Sufficient time was allowed for each reading to stabilize before taking the readings
Results
Temperatures and pressures at pressures at the locations 1,2,3,4 as indicated in Figure 1 above were read and recorded in the corresponding columns in the table below
Also the motor input power, compressor speed, inlet and outlet temperatures, water mass flow rate and spring force were measured and recorded.
When the compressor was turned on, water passed through the condenser
Data:
Bore of each cylinder = 38 mm
Stroke of each cylinder = 19 mm
Torque arm = 165 mm Speed ratio: - Motor: Compressor = 1.98:1
The states in a refrigeration cycle exists as either liquid or vapor states. Most of the refrigerants have low freezing point and, therefore, they rarely freeze to solid state.
Three different COP’s can be found depending on where the compressor work is measured:
the chart COP using the work input found from the p-h chart
This can be defined as the theoretical coefficient of performance which is calculated by taking (measuring) the values of the enthalpies at the given points in the PH chart of the liquid refrigerant. It is the ratio of the refrigeration effect and the compressor work Theoretical COP=qEWC=h1-h4h2-h1=316417=0.7577
Where qE=evaporator's cooling capacity
wc=Compressor power
h1=Enthalpy on sunction compreesor
h2=enthalpy on discharge compressor
h3=enthalpy on condenser exit
h4=enthalpy on evaporator inlet
the system COP using the mechanical shaft power input from the motor;
This is the actual COP calculated from the equation
(T2-T1-(T4-T3)) / (T2-T1)
(56.7+0.1-(-15.4-26.3))/(56.7+01.)=1.736
where T2is the outlet temperature of the condenser
T1is the inlet temperature of the condenser
T3 is the inlet temperature of evaporator
T4 is the outlet temperature of the evaporator
the overall COP using the electrical power inputs to the evaporator and the motor
This is the average COP and is plotted against the different evaporator temperatures obtained.. This is shown on the graph 1.0
Graph 1.0
Determine the volumetric and isentropic efficiencies of the compressor.
The isentropic efficiency can be calculated as follows
ηT=h2-h1h3-h1
Observations
Why the chart, the system and the overall COP are different
In this experiment, everything flowing in the system is per KG. The input energy is multiplied to the main evaporator. The longer horizontal line on the top of the graph represents the rejected heat to the ambient air. The difference between the two lines is a result of the energy input by the drive motor. If the lower bottom line is lowered, the evaporation temperature and pressure are lowered. This lowering increases the energy required for rotating the motor to one kG of the refrigerant around the refrigeration system and, therefore, reduce the COP. If the line is raised, the opposite happens.
What is considered to be the optimum condition of a refrigerant at the entry to the compressor? What was the actual condition? What is the reason for this?
It can be said that optimum conditions are saturated vapor ideally. But in reality, the used compressor compresses only the dry gas. Therefore, at least 1.5k of the superheat is required to acquire dry vapor. The reduced heat transfer reduces the evaporation pressure. When the fan goes off, air cooling is foiled on a water chiller.
Explain how the thermostatic expansion valve works
This is a precision valve. It regulates the rate at which the liquid refrigerant enters and flows inside the evaporator. The controlled flow is to minimize the evaporator’s efficiency while at the same time preventing excess backflow of the liquid refrigerant to the compressor. One of its design features of this device is to separate the low pressure from high pressure sides of the air conditioning system. The liquid refrigerant enters goes to the valves at a very high-pressure through the system’s liquid, but the pressure is reduced when the device limits the amount of the liquid refrigerant going into the evaporator (Hultén & Berntsson, 2002).
The suitability of a refrigerant for a particular application using a vapor compression refrigeration cycle, is subject to various thermodynamic and practical considerations, discuss what desired properties for the refrigerant to have
Vapor density
This enables the use of smaller compressors and other equipment
Enthalpy of vaporization
High enthalpy of vaporization ensures maximum heat absorption during refrigeration
Thermal conductivity
This should be high enough to enable faster heat transfer in the process of condensation and evaporation
Other factors include strong dielectric, critical temperature, minimum specific heat, minimized leak tendency, non-toxic liquid, low-cost refrigerants, and readily available liquid refrigerant.
References
Coquelet, C. & Richon, D. (2009). Experimental determination of phase diagram and modeling: Application to refrigerant mixtures. International Journal of Refrigeration, 32(7), 1604-1614. http://dx.doi.org/10.1016/j.ijrefrig.2009.03.013
Aprhornratana, S. & Eames, I. (1995). Thermodynamic analysis of absorption refrigeration cycles using the second law of thermodynamics method. International Journal of Refrigeration, 18(4), 244-252. http://dx.doi.org/10.1016/0140-7007 (95)00007-x
Halm-Owoo, A. & Suen, K. (2002). Applications of fault detection and diagnostic techniques for refrigeration and air conditioning: a review of basic principles. Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering, 216(3), 121-132. http://dx.doi.org/10.1243/095440802320225338
Hultén, M. & Berntsson, T. (2002). The compression/absorption heat pump cycle—conceptual design improvements and comparisons with the compression cycle. International Journal of Refrigeration, 25(4), 487-497. http://dx.doi.org/10.1016/s0140-7007 (02)00014-2
Park, S., Ahn, J., & Kim, T. (2012). Off-design operating characteristics of an open-cycle air refrigeration system. International Journal of Refrigeration, 35(8), 2311-2320. http://dx. doi.org /10.1016/j.ijrefrig.2012.08.005
