Refrigeration and Air Conditioning Equipment Cooling Episode 8

14. How is the pump primed? (Sec. 27, Par. 1) 15. Explain what you should do after the pump is primed and before it is stared. (Sec. 27, Par. 1) 16. List at least four causes for failure of a newly installed pump to prime. (Sec. 27, Par. 3) 17. A pump that uses a stuffing box takes liquid in for sealing at ___________________. (Sec. 28, Par. 2) 18. When is it necessary to pipe water from a clean water source to the stuffing box? (Sec. 28, Par. 3) 23. Name the four types of bearings commonly found in centrifugal pumps. (Sec. 28, Par. 17) 24. What occurs when a bearing is lubricated too often? (Sec. 28, Par. 17) 25. What type of grease is recommended for grease-lubricated bearings? (Sec. 28, Par. 19) 26. Why aren’t vegetable and animal greases used to lubricate pump bearing? (Sec. 2, Par. 19) 27. The maximum operating temperature for grease-lubricated bearings is __________________. (Sec. 28, Par. 20) 19. Why is exact packing tightening important? (Sec. 28, Par. 4) 28. The maximum operating temperature for an oil-lubricated babbitted sleeve bearing is ___________________. (Sec. 28, Par. 22) 20. How would you stagger the packing joints in the stuffing box that uses five rings? (Sec. 28, Par. 5) 21. The first step to perform when dismantling a mechanical seal is to _________________. (Sec. 28, Par. 10) 29. What are the four drilled recesses in the bushing of a “Magic-Grip” coupling used for? (Sec. 28, Par. 24) 30. (Agree)(Disagree) During installation of a “Magic-Grip” coupling, the recessed holes should be facing the pump. (Sec. 28, Par. 26) 22. Which item shouldn’t you disturb when dismantling a mechanical pump unless it is to be replaced? (Sec. 28, Par. 11) 102 CHAPTER 6 Fundamentals of Electronic Controls A MISSILE STREAKS across the sky. The missile’s flight is controlled electronically from a command post. The success of the launch and flight of the “bird” depends largely upon how well the electronic technicians performed their tasks. 2. Let us compare the missile launch to an electronic control system. The missile can be compared to the controlled variable-humidity, temperature, airflow, etc. The movable rocket motor is the controlled device. The controlled device is the component within the system that receives a signal from the control to compensate for a change in the variable. Last, but not least, we have the guidance system. Our controllers thermostats, humidistats, etc. -perform in much the same way as a guidance system. A change in the controlled variable will cause the controller to respond with a corrective signal. 3. In this chapter we will discuss vacuum tubes, amplification, semiconductors, transistor circuits, bridge circuits, and discriminator circuits. We will relate amplifier, bridge, and discriminator circuits to electronic controls. Electronic controls are becoming popular in the equipment cooling area of your career field because of their sensitivity and reaction time. 29. Vacuum Tubes 1. Electricity is based entirely upon the electron theory--that an electron is a minute, negatively charged particle. Atoms consist of a positively charged nucleus around which are grouped a number of electrons. The physical properties of any atom depend upon the number of electrons and the size of the nucleus; however, almost all matter has free electrons. The movement of these free electrons is known as a current of electricity. If the movement of electrons is in “one” direction only, this is direct current. If, however, the source of voltage is alternated between positive and negative, the movement of electrons will also alternate; this is alternating current. 2. The vacuum tube differs from other electrical devices in that the electric current does not flow through a conductor. Instead, it passed through a vacuum inside the tube. This flow of electrons is only possible if free electrons are somehow introduced into the vacuum. Electrons in the evacuated space will be attracted to a positively charged object within the same space because the electrons are negatively charged. Likewise, they will be repelled by another negatively charged object within the same space. Any movement of electrons under the influence of attraction or repulsion of charged objects is the current in a vacuum. The operation of all vacuum tubes depends upon an available supply of electrons. Electron emission can be accomplished by several methods--field, thermionic, photoelectric and bombardment-but the most important is thermionic emission. 3. Thermionic Emission. To get an idea of what occurs during thermionic emission you should visualize the Christmas sparkler. When you light the sparkler it burns and sparks in all directions. The filament in a vacuum tube reacts the same way when heated to a high temperature. Millions of electrons leave the filament in all directions and fly off into the surrounding space. The higher the temperature, within limits, the greater the number of electrons emitted. The filament in a directly heated vacuum tube is commonly referred to as a cathode. Refer to figure 86 for the symbol of a filament in a vacuum tube with heating sources. 4. The cathode must be heated to a high temperature before electrons will be given off. However this does not mean that the heating current must flow through the actual material that does the emitting. You can see in figure 87 that the part that does the heating can be electrically separate from the emitting element. A cathode that is separate from the filament is an indirectly heated cathode, whereas an emitting filament is a directly heated cathode. 5. Much greater electron emission can be 103 Figure86. Thermionicemission. obtained, at lower temperatures, by coating the cathode with special compounds. One of these is thoriated tungsten, or tungsten in which thorium is dissolved. However, much greater efficiency is achieved in the oxide-coated cathode, a cathode in which rare-earth oxides form a coating over a metal base. Usually this rare-earth oxide coating consists of barium or strontium oxide. Oxide-coated emitters have a long life and great emission efficiency. 6. The electrons emitted by the cathode stay in its immediate vicinity. These form a negatively charged cloud about the cathode. This cloud, which is called a space charge, will repel those electrons nearest the cathode and force them back in on it. In order to use these electrons, we must put a second element within the vacuum tube. This second element is called an anode (or plate), and it gives us our simplest type of vacuum tube, the diode. 7. Diode Vacuum Tube. Each vacuum tube must have at least two elements or electrodes: a cathode and an anode (commonly called a plate). The cathode is an emitter of electrons and the plate is a collector of electrons. Both elements are inclosed inside an envelope of glass or metal. This discussion centers around the vacuum tube diode from which the air as much possible has been removed. However, it should be understood that gaseous diodes do exist. The Figure87. Indirectlyand directlyheatedcathodes. 104 Figure88. Electronflowin a diode. term “diode” refers to the number of elements within the tube envelope (di meaning two) rather than to any specific application, as shown in figure 88. 8. The operation of the diode depends upon the fact that if a positive voltage is applied to the plate with respect to the heated cathode, current will flow through the tube. When the plate is negative with respect to the cathode, current will not flow through the tube. Since current will pass through a vacuum tube in only one direction, a diode can be used to change a.c. to d.c. 9. Diode as a half-wave rectifier. Experiments with diode vacuum tubes reveal that the amount of current which flows from cathode to plate depends upon two factors: the temperature of the cathode, and the potential (voltage) between the cathode and the plate. Refer to figure 89, a diagram of a simple diode rectifier circuit. 10. When an a.c. source is connected to the plate and cathode such a circuit, one-half of each a.c. cycle will be positive and the other half will be negative. Therefore, alternating voltage from the secondary of the transformer is applied to the diode tube in series with a load resistor, R. The voltage varies, as is usual with a.c., but current passes through the tube and R only when the plate is positive with respect to the cathode. In other words, current flows only during the half-cycle when the plate end of the transformer winding is positive. When the plate is negative, no current will pass. 11. Since the current through the diode flows in one direction only, it is direct current. This type of diode rectifier circuit is called a half- Figure89. Simplehalf-waverectifiercircuit. 105 Figure90. Outputof a half-waverectifier. wave rectifier, because it rectifies only during one-half of the a.c. cycle. As a result, the rectified output will be pulses of d.c., as shown in figure 90. You can see from figure 90 that these pulses of direct current are quite different from pure direct current. It rises from zero to a maximum and returns to zero during the positive half-cycle of the alternating current, but does not flow at all during the negative half-cycle. This type of current is referred to as pulsating direct current to distinguish it from pure direct current. 12. In order to change this rectified alternating current into almost pure direct current, these fluctuations must be removed. In other words, it is necessary to cut off the humps at the tops of the half-cycles of current and fill in the gaps caused by the negative half-cycle of no current. This process is called “filtering” ‘ 13. Look at the complete electrical circuit of figure 91. Filtering is accomplished by connecting capacitors, choke coils (inductors), and resistors in the proper manner. If a filter circuit is added to the half-wave rectifier, a satisfactory degree of filtering can be obtained. Capacitors C1 and C2 have a small reactance at the a.c. frequency, and they are connected across the load resistor, R. These capacitors will become charged during the positive half-cycles as voltage is applied across the load resistor. The capacitors will discharge through R and L during the negative half-cycles, when the tube is not conducting, thus tending to smooth out, or filter out, the Figure91. Filternetworkaddedtoa half-waverectifier. 106 ... - tailieumienphi.vn