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SUBCOURSE OD1750 REFRIGERATION AND AIR CONDITIONING IV (EQUIPMENT COOLING) EDITION A REFRIGERATION AND AIR CONDITIONING IV (EQUIPMENT COOLING) Subcourse OD1750 Edition A United States Army Combined Arms Support Command Fort Lee, VA 23801-1809 14 Credit Hours INTRODUCTION This subcourse is the last of four subcourses devoted to basic instruction in refrigeration and air conditioning. The scope of this subcourse takes in unit components of the absorption system, including their functions and maintenance; water treatment methods and their relationship to centrifugal systems; centrifugal water pumps and electronic control systems, including the relationship of amplifier, bridge and discriminator circuits to electronic controls. The subcourse consists of three lessons. Lesson 1. Direct Expansion and Absorption System. 2. Centrifugal Systems and Water Treatment. 3. Centrifugal Water Pumps and Electronic Control Systems. Unless otherwise stated, whenever the masculine gender is used, both men and women are included. CONTENTS Page Preface.................................................................................................................................................................... ii Acknowledgment.................................................................................................................................................... iii 1 Direct Expansion Systems ..................................................................................................................................... 1 2 Absorption Systems ............................................................................................................................................... 26 Lesson 1 Chapter Lesson 2 Chapter 3 Centrifugal Systems ............................................................................................................................................... 46 4 Water Treatment ................................................................................................................................................... 77 Lesson 3 Chapter 5 Centrifugal Water Pumps)...................................................................................................................................... 96 6 Fundamentals of Electronic Controls..................................................................................................................... 103 7 Electronic Control Systems ................................................................................................................................... 132 Answers to Review Exercises................................................................................................................................. 139 The passing score for ACCP material is 70%. Preface YOU HAVE studied the fundamentals and commercial refrigeration and air-conditioning systems. This final volume deals with another phase of your career ladder-equipment cooling. Since the principles of equipment cooling are common to all refrigeration systems, your mastery of the subject should be easy. All of the systems covered in this volume can be applied to commercial refrigeration and air conditioning. To qualify you in equipment cooling, we will present the following systems in this volume: (1) (2) (3) (4) (5) (6) (7) Direct expansion Absorption Centrifugal Water treatment Centrifugal water pumps Fundamentals of electronic controls Electronic control Keep this memorandum for your own use. ii ACKNOWLEDGMENT Acknowledgment is made to the following companies for the use of copyright material in this CDC: Honeywell, Incorporated, Minneapolis, Minnesota; Carrier Air Conditioning Company, Carrier Parkway, Syracuse, New York; Terry Steam Turbine Company, Hartford, Connecticut; Koppers Company, Incorporated, Baltimore, Maryland iii CHAPTER 1 Direct Expansion Systems JUST WHAT DO we mean when we say "direct expansion"? In the dictionary we find that the word "direct" means an unbroken connection or a straight bearing of one upon or toward another; "expansion" relates to the act or process of expanding or growing (in size or volume). Now we can see that a direct expansion system for equipment cooling is one in which the controlled variable comes in direct contact with the single refrigerant source, thereby causing the liquid refrigerant to boil and expand. The centrifugal and absorption systems differ in that that they us a secondary refrigerantwater or brine-to cool the variable. are ready to install the coil. The next problem is where to install it. The preheat coil is installed in the outside air duct, before the mixing of outside and return air. Now we are ready to discuss a few applications of a preheat coil. 2. We will cover various components peculiar to large direct expansion systems, normally of 20 tons or more in capacity. Remember, the window- and floormounted air-conditioning units are also considered direct expansion systems. Before we discuss the installation of a semihermetic condensing unit-the most commonly used unit for direct expansion systems-we will cover the various coils that are used in a direct expansion system. The application of the water-cooled semihermetic condensing unit will concern us in the second section, and we will conclude the chapter with system servicing and troubleshooting. 5. The damper on the face of the preheat coil closes when the fan is turned off and opens when it is turned on. This damper is normally closed when the fan is off or if the fan fails to operate. This prevents preheat coil freezeup. 4. Thermostatically controlled water or steam valve. Figure 1 shows a system that uses a narrow range temperature controller. The temperature of the incoming air is sensed by the thermostat feeler bulb. The thermostat is calibrated to modulate the valve open when the temperature is 35° F. 6. Thermostatically controlled face and bypass dampers. The mixed air temperature remains relatively constant until the outside air temperature exceeds the desired mixed air temperature. The use of the face and bypass damper, illustrated in figure 2, makes it possible to control mixed air temperature without endangering the preheat coil. The damper is controlled by a temperature controller in the mixed air duct while the preheat coil is controlled by a valve which is modulated by a narrow range temperature controller in the outside air duct. The face and bypass damper will close and the return air opens when the supply fan is turned off. 1. Coil Operation 1. There are three coils used in the typical system. From the outside in, the coil sequence is: (1) preheat, (2) direct expansion (D/X), and (3) reheat. We will discuss the application of these coils, their use and control, and the valves and dampers which control the flow of water and air. 7. D/X Coil. In equipment cooling systems, the D/X coil is located after the preheat coil. It serves two primary functions-cooling and dehumidification. 2. Preheat Coil. You must consider three things before installing a preheat coil in an equipment cooling system. These are: (1) Is preheat necessary? (2) Will the coil be subjected to subfreezing temperature? (3) What size preheat coils are needed? 3. After you have determined a need, provided for freezing temperatures, and correctly sized the coil, you 8. Simple on-off control. The compressor is controlled by a space thermostat in an on-off manner. Figure 3 shows a system using this type of control. This system is best suited for use on small compressors and where large variations in temperature and humidity are not objectionable. 1 Figure 1. Control of preheat with outdoor air thermostat. Figure 3. On-off compressor control. two-position thermostat opens the refrigerant solenoid valve when the space temperature rises and closes it when the temperature drops below the set point. This control action will cause large swings in temperature and relative humidity. The nonrestarting relay prevents short cycling of the compressor during the off cycle. It allows the compressor to pump down before it cycles "off." 13. Multiple D/X coil solenoid valves. The system shown in figure 6 is similar to that previously discussed (fig. 5) except that it now has two D/X coils and two solenoid valves. The two-stage space thermostat operates D/X coil 1 in an on-off manner when the cooling load is light. It also holds the valve to coil 1 open and operates the valve to coil 2 in an on-off manner during heavy load conditions. The nonrestarting relay functions the same as the one in figure 5. 14. The supply fan starter circuit must be energized, in both applications, before the control circuit to the solenoid valves can be completed. 15. Two-position control and modulating control of a face and bypass damper. This system uses a face and bypass damper (shown in fig. 7) to bypass air around the D/X coil during light load conditions. The space thermostat opens the refrigerant solenoid valve when the face damper opens to a position representing a minimum cooling 9. The differential adjustment on the thermostat should be set relatively wide to prevent short cycling under light load conditions. The control circuit is connected to the load side of the fan starter so that turning on the fan energizes the control systems. 10. Two-speed compressor. Figure 4 shows a typical two-speed compressor installation. A two-stage thermostat (space) cycles the compressor between low speed and off during light load conditions and cycles the unit between high and low speed during heavier loads. The thermostat also shuts off the compressor if the space temperature falls below the set point. 11. The humidistat cycles the compressor from low to high speed when space humidity rises above the high limit set point. It can do this when the compressor is on low speed. This system is best suited for use on reasonably small compressors where large swings in temperature and relative humidity can be tolerated. 12. Solenoid valve installation. Figure 5 shows a system which uses a space thermostat to operate a solenoid valve and a nonrestarting relay. The Figure 2. Preheat control with bypass and return air dampers. Figure 4. Two-speed compressor control. 2 Figure 5. On-off control with a solenoid valve. Figure 7. Two-position control of a D/X coil solenoid valve and modulating control of a face and bypass damper. load. It also modulates the face and bypass dampers to mix the cooled air with the bypassed air as necessary to maintain the correct space temperature. A capacity controlled compressor must be used if short cycling, under light load conditions, is to be avoided. 16. It is necessary to adjust the face damper so that it does not close completely. This will help prevent coil frosting under light load conditions. The control circuit to the solenoid valve is wired in series with the supply fan motor. When the fan is shut off, the solenoid valve will close. 17. Two-position control and modulating control of a return air bypass damper. This system, shown in figure 8, is similar to the system we have just discussed. The only difference is that we bypass return air instead of mixed air under light load conditions. 18. Reheat Coil. The reheat coil is used to heat the air after it has passed through the D/X coil. It expands the air, thus lowering the relative humidity. A D/X coil and reheat coil are used to control humidity. 19. Simple two-position control. Figure 9 shows a system which uses a space thermostat to control a reheat coil and a D/X coil. It opens the solenoid valve to the heating coil when the space temperature falls below the set point temperature, and opens the D/X coil solenoid valve when the temperature is above the set point. A two-position humidistat is provided to open the cooling coil solenoid valve when the space relative humidity exceeds the set point of the controller. When a humid condition exists, the humidistat will override the thermostat. The thermostat senses the reduced air temperature and opens the reheat coil solenoid valve which will lower the relative humidity. The D/X coil solenoid valve will close when the supply fan is shut off. 20. Control of dehumidification with a face and bypass damper. We discussed the use of face and bypass dampers when we discussed D/X coils. Now we will apply this damper system to humidity control, as shown in figure 10. A space humidity controller is used to open the D/X coil valve when a predetermined minimum dehumidification load is reached. It also modulates the face and bypass damper to provide the mixture of dehumidified and bypass air necessary to maintain space relative humidity. 21. The space thermostat modulates the reheat coil valve as needed to maintain space temperature. If the space humidity drops below the set Figure 6. On-off control of multiple D/X coil solenoid valves. Figure 8. Two-position control of a D/X coil solenoid valve and modulating control of a return air bypass damper. 3 Figure 9. Dehumidification control in a two-position D/X system. Figure 11. Dehumidification control in a D/X return air bypass system. point of the humidity controller, and the space temperature rises because the discharge air is too warm to cool the space, the thermostat will open the D/X coil valve and modulate the face and bypass damper to lower the space temperature. The reheat coil must be controlled by a modulated valve so that the thermostat can position the valve within its range. This will prevent large swings in temperature and relative humidity. This system also provides a method of closing the D/X coil valve when the supply fan is shut off. 22. Control of dehumidification with a return air bypass system. Figure 11 shows a system which uses a return air bypass damper to control airflow across the D/X coil for dehumidification. The space humidistat opens the D/X coil valve when a predetermined minimum cooling load is reached and positions the bypass damper to maintain space relative humidity. 23. The space thermostat acts in a way that is similar to that of the thermostat in figure 10. The control circuit to the D/X coil valve is connected to the supply fan so that the valve will close when the fan is shut off. This arrangement helps prevent coil frosting and reheat coil freezeup. 24. We have discussed the three coils that you will find in a typical equipment cooling system. Now we will discuss a complete system which maintains temperature, relative humidity, and air changes. 25. Typical D/X Equipment Cooling System. Figure 12 shows a system which may be used to condition air for electronic equipment operation. Thermostat T1 senses outdoor (incoming) air and modulates the preheat coil valve to the full open position when the temperature falls below the controller set point. A further drop in temperature will cause the thermostat T1 to modulate the outside and exhaust air dampers shut and the return air damper open. 26. The space thermostat (T2) operates the reheat coil valve as necessary to maintain a predetermined space temperature. The space thermostat (T2) will modulate the cooling coil valve when the space humidity is within the tolerance of the humidistat. The space humidistat opens the cooling coil valve when a minimum cooling load is sensed. It has prime control of this valve. The outside and exhaust air dampers are fitted with a stop so that they will not completely close. This procedure allows for the correct amount of air changes per hour. 27. There are many other direct expansion systems. The blueprints for your installation will help you to better understand the operation of your system. Most of the system components are similar to those previously discussed. 2. Application of Water-Cooled Condensing Units 1. Water-cooled semihermetic condensing units are rated in accordance with ARI Standards with water entering the condenser at 75°F. 2. Condensing units are available for different temperature ranges. We are interested in the "high temperature" unit, as it is used for air conditioning Figure 10. Dehumidification control in a D/X face and bypass system. 4