Refrigeration Cycle

Refrigeration cycle produces hot part and cold part (compare to ambient temperature). It needs at least four components, they are compressor, condenser, expansion device, and evaporator. Work fluid (refrigerant) goes around from one component to other component again and again, so we call it cycle. Refrigerant experiences all process in components

Figure 1. Refrigeration cycle schema

Figure 1 show schema of refrigeration cycle which consist of four main components and connected pipes (showed by line), and also the flow direction of refrigerant is showed at that figure. At the figure, there are two types of line, red and blue. The red one shows that the pressure is high and the blue shows the pressure of refrigerant is low.  As known, pressure is proportion by temperature, then the color of line also show the temperature condition of refrigerant.

1 to 2, Compression process. Output from evaporator, refrigerant is gas phase. Then compressor sucks it from evaporator and discharge it to condenser.  Before through the compressor the refrigerant pressure  is low and after through compressor refrigerant pressure is high. This process also makes the refrigerant temperature increase.

2 to 3, Condensation process. Output from compressor, refrigerant phase is still gas. As its name condenser is a component to make refrigerant phase change  from gas into liquid. This process needs to release heat. Usually condenser is put outside, because the temperature of ambient outside is lower than temperature of refrigerant in condenser, then heat transfer can occur from refrigerant to outdoor air temperature.

3 to 4, Expansion Process. Output from condenser, refrigerant phase  is already liquid and has  high pressure and temperature. Expansion device makes pressure of refrigerant drop into low pressure, this process also is followed by drop temperature. Most of refrigerant phase after through expansion valve is still liquid and a few gas.

4 to 1, Evaporation Process. As its name, the function of this component is to make refrigerant phase changes from liquid into gas. This process need heat, and heat is got from the objects which want to be cooled. The temperature of refrigerant in evaporator is lower than the objects, then heat transfer will occur from objects to refrigerant, then heat is used for evaporate refrigerant. After the refrigerant is already in gas phase, refrigerant  will experience the process from number   1 again. It happen repeat and repeat again.

Air Conditioner and refrigerator

Air conditioner and refrigerator have same work principle, it is refrigeration cycle. Refrigeration is reverse of Power cycle (rankine cycle) which use heat for produce some work (Power), refrigeration use  work to move the heat. If we read some books, maybe we will find that refrigeration cycle is a system which allow heat transfer from cold area to warm area (figure a) whereas heat transfer is spontaneously from warm area to cold area. Actually, if we focus on each component of refrigeration cycle, heat transfer from cold area to warm area is not happen. But because of we assume the refrigeration cycle as one system, we can say like that.

Fugure (a). refrigeration system as one system

Figure (b). refrigeration system produce cold and hot part

In fact, refrigeration cycle produce the difference of temperature, there are cold part and hot part (figure b). We use cold part to cool the objects. Object in this content is defined as all things that we want to make it cold. In refrigerator, the objects are foods, water, etc. But, in air conditioner system the object is the room air.

Refrigeration system at least needs four components, there are compressor, condenser, expansion valve, and evaporator. Refrigeration system also need work fluid, it is called refrigerant. There are many types of refrigerant, some of them are forbidden because of the environment issue. Refrigerant  always goes around from one component to another component, and at each component refrigerant experiences the process based on the function of component. 

Direct Current Motors

Motors are used in many equipment, such as fan, blender, pump, etc. How motors work?

Motor is the equipment which convent from electricity to rotation. Before discuss about motor, let us review relation between current, magnetic field and force. See figure 1, Force can be generated by current and magnetic field. If current flow at a wire and it is in the magnetic field, then they are generated some force to the wire. We can see the direction of magnetic field is from North (N) to south (S). By right hand rules, direction of current is showed by I, direction of magnetic field is showed by B, and force is showed by F. Then they are generated force at wire to down direction. We can determine their direction by right hand rule. 

Figure 1. (a)Force, current, and magnetic field; (b) Right hand rule of direction; (c) simple construction DC motor

Now, let us consider the construction of simple direct current motor. The current flows from positive along the wire to negative, the magnetic field direction is from North (N) to South (S). Based on the right hand rules, loop will make rotation like the arrow direction (clock wise).

Brushes are static, but commutators  move (make rotation movements). Commutators will connect to the different pole of electricity (battery) alternately . Then, when the current flows, there will be a force to down direction at the right part of loop and force to up direction at the left part of loop. The construction of simple DC motor make a rotation at one direction (clock wise or counter clock wise) depend on the current flows.

Source of figure: Halliday, Resnick, Walker: Fundamentals of Physics; 
Douglas Giancoly: PHYSICS PRINCIPLES WITH APPLICATIONS

How to produce electricity

Electricity can be generated by several ways. One of them is by using faraday law which explained relation between electromotive force and magnetic flux.  Electromotive force (electrical voltage) will occur if there are changes of magnetic flux.

Magnetic flux is magnetic field which through perpendicular on some area.  If the area is parallel with magnetic field direction, the flux magnetic will be zero. If the area is perpendicular it will be maximum. See figure 1.

Figure 1. Generate the electromotive voltage.

We can see at the figure 1, magnet have magnetic field which have direction from north to south pole of magnet. Magnetic field through the circle of wire (it is magnetic flux). If we move the magnet, thus there are changes of magnetic flux at the circle of wire. Based on faraday law the electromotive force will occur.  So, as we can see the electrical voltages are detected at galvanometer. If we move the magnet by the opposite direction, electromotive force also have the opposite value. For example at the figure we move the magnet into the circle of wire, then galvanometer show the positive voltage. if we move the magnet away from circle of wire, then galvanometer will show the negative voltage.

Besides that, the changes of magnetic flux also can occur if the circle of wire is moved. Based on this principle, electrical generator can be built. There are several types of generator, but generally generators are divided in two types: Alternate current generator and direct current generator.


Source of figure: Halliday, Resnick, Walker: Fundamentals of Physics

GENERATOR Alternate Current (AC)

Generator is equipment which can converts from mechanical energy (kinetic energy) into electricity. Generally, generator can be divided into two types: AC Generator and DC Generator. Generator can produce electricity based on the faraday which law which said that change of flux magnetic will produce electromotive force (electrical voltage).

Based on faraday law, to build generator we need magnetic field.  Flux magnetic is magnetic field which through at surface area.  We can get the change of flux magnetic by rotate/drive the surface. Figure 1. Is construction of Simple Generator

Figure 1. (a) construction simple generator. (b) its electrical voltage

There are two kinds of poles magnet: N and S, so there will be magnetic field from N to S direction. Field magnetic which is through the loop area, it is called flux magnetic.  Loop is wire which is made like a loop.  Flux magnetic will be maximum when loop is vertical and will be zero when loop horizontal. By rotate the loop, we can get the changes of magnetic flux and it will produce electromotive force/emf (Electrical voltage). greater number of loops will produce greater voltage. Loops is connected by wire to slip rings. Slip rings are driven as external rotator rotation. Slip rings are  also connected to brushes which is static (not rotate as slip rings). These brushes are made from conductor which can transfer electricity that we use it.

We can connect external rotator to turbine, thus we can simplify above:
Rotation of external rotator --> Rotation of loop --> changes of flux magnetic --> electromotive force (emf)

loop is connected to slip rings by wire, so electrical voltage can move from loop to slip rings and we can use it to our need.

Source of figure: Halliday, Resnick, Walker: Fundamentals of Physics

STEAM POWER PLANT (PART 2: RANKINE CYCLE TURBINE)

Rankine cycle is one of the thermodynamic cycles which convert from heat into power (kinetic energy). We say it is a cycle because the process in this cycle always repeat and repeat again. To convert from heat energy into kinetic energy, it has several processes  and needs several components to support those processes. Those are: Boiling, expansion, condensation, and pumping. Figure 1. Is schema of those processes.

               Figure 1. Schema Rankine Cycle

At figure 1 there are two kinds of lines, ref and blue. Red is sign for high pressure and blue is sign for low pressure. Processes from 1 to 4 will be repeated again and again, so the rotor of turbine will be rotated continuously.

1 to 2, pumping  the water into boiler. At point 1 the pressure of water is low and at point 2 the pressure is high. The phase at point 1 and 2 is water (liquid).

2 to 3, Boiling. This Process needs energy especially heat energy, it can be from kerosene, coal, etc. Water will be boiled and water phase changes from water (liquid) into steam (gas). This steam (at point 3) has high pressure and temperature, so it has high energy (enthalpy).

3 to 4, Turbine Expansion. In this process occur the conversion into work (kinetic energy).  The high energy whose steam at point 3 is used to produce work (kinetic energy). This process causes drop pressure and temperature of steam (steam at point 4).

4 to 1, Condensation. To change phase from steam (gas) into water (liquid) need remove heat or in other word it has to be cooled. This process usually need heat exchanger, so that it can remove heat then the steam phase will change.

Those processes from 1 to 4 will be repeated again and again, so the rotor of turbine will be rotated continuously. 

STEAM POWER PLANT (PART 1: TURBINE)

Steam power plant  maybe is the most common power plant in the world. This technology converts from heat into electricity. Steam Power Plant has at least two parts, Turbine and Generator. Turbine is the system which converts from heat energy into kinetic energy (Rotation) and Generator is the system which use kinetic energy to produce electricity.

TURBINE

To produce rotation, turbine needs heat source. It can be coal, gasoline, kerosene, etc. This heat energy is used to boil the water. Because of that, phase of water will be changed from water (liquid) to vapor (gas/steam). This steam is used to rotate the rotor of the turbine. The figure.1 bellow explain about that.

Figure 1. (a) Schema of turbine; (b) Illustration from heat into rotation

If the system  is like that figure,  it will needs water more and more because after the steam rotate the rotor, steam is thrown away. The solution of this problem is Rankin cycle.

In rankine cycle, steam will be used again. Figure 2 is explanation about that.

Figure. 2

After rotate the rotor, steam enter to condenser. In condenser steam phase will be changed from steam (gas) into water (liquid) and all the water will be collected in a tank. From tank, water is pumped to boiler again, and the process is repeat again and again.    

That is the principle of simple turbine system which allow convert from heat into rotation.