Third class Air conditioning and Refrigeration Department. Lecturer Amir Kh. Ali

Transcription

1 1 Third class Air conditioning and Refrigeration Department Lecturer Amir Kh. Ali

2 2 Chapter 1 - Strategy of Maintenance What is Maintenance?

3 3 1-1-Why Maintenance HVAC? Increases equipment life & reliability Reduces size & scale & number of repairs Lowers maintenance costs through better use of labor & materials Reduces emergency repairs & overtime Improved parts control Reduces energy costs by 5% to 20% Eliminate the potential problems 1-2-Types of Maintenance Emergency At some point an HVAC system will experience a major component failure that results in a repair that the technician cannot justify based on the repair cost and the age or condition of the unit. At this point the equipment must be replaced with whatever equipment is obtainable quickly. Periodically The AHU Evap. Cond. Should be inspected periodically for air leaks, rusting condensate drainage problems, and dirt accumulation Routine Even the best installed, most efficient equipment requires routine maintenance Preventive The preventive maintenance process can include anything from a simple visual inspection annually to proactive replacement of components based on run time, regardless of whether they have failed. Most programs for retail stores fall somewhere within this range Predictive maintenance is important. By tracking different system indicators Such as oil temperature RPM speeds. And other factors Annually Air handlers coils need to be cleaned periodically to keep heat transfer at maximum amount.boilers need to be cleaned annually, level and condition of oil of centrifugal compressors need to be checked,cooling towers need to be cleaned

4 HVAC Maintenance Maintaining an HVAC (Heat, Ventilation and Air Conditioning) unit is important to prevent major problems from affecting your home or business. HVAC maintenance will not only lower your utility costs, but it will help increase the service life of your unit. Longer life means less money out of your pocket spent on repairs or new systems. Below is a look the parts of an HVAC system and why they need to be cleaned or replaced periodically Parts of the System need Maintenance A HVAC system consists of several parts that function together to heat and cool your home or apartment building. The main components of an HVAC system include A- filters B- evaporator C- condenser coils D- air intake E- dampers F- supply ducts. An HVAC unit also contains fans, bearings, and belts that must be regularly cleaned to keep your unit running smoothly. Dirt, debris or other matter found in any of those components can result in the reduced functioning of your HVAC unit. A- Filter Replacing filters is perhaps the easiest part of HVAC maintenance, filters should be replaced every service time during charging, evacuation or even replacing any component in the system. B-Coils Coils have a tendency to grow mold, due to being damp and wet. They are also in contact with humid, moist air, a breeding ground for mold and bacteria. Coils can also easily degrade if left dirty for extended periods of time. Left untreated for too long, coils will grow a sticky film on them that is extremely difficult to clean. The cleaning chemicals needed to treat this mold have a tendency to damage and pit the coils, requiring them to be replaced. C- Dampers Dampers keep the HVAC unit operating efficiently by keeping compressors running when the temperature dips below 60 degrees Fahrenheit. If they are not kept properly cleaned and lubricated, they will begin to stick, creating a loss of cooling and heating efficiency in your HVAC unit. Proper HVAC maintenance should include keeping the dampers well serviced, as this is the most common problem seen in HVAC systems. D- Fans and Belts Fans and belts are of the least concern during HVAC maintenance, but should still be maintained and replaced at least twice per year. Poorly operating fans or belts result in less cooling and heating efficiency, excessive noise coming from the unit, or constant vibration

5 5 while running. Well-kept fans and belts mean a smoother running HVAC unit that requires less maintenance and will function longer without repairs Things to Check 1- Thermostat Check the Thermostat settings to make sure the system will keep a home or building cool or warm, depending on the season. Set the thermostat to an energy-saving temperature when the house is empty to keep utility costs lower. A good starting points is 78 F ( 25.5 C ) in the summer and 68 F ( 20 C ) in the winter. 2- Condensate drain Inspect condenser drain to make sure there are no blocks or leaks, which could cause water damage in a home or affect the air humidity levels. 3-Overallcheck on the system controls to be sure everything is properly operating, starting up, and shutting off. 4- Cooling Unit Clean the evaporator and condenser coils when performing maintenance on a system. Dirty or damaged coils reduce the ability of an HVAC system to cool a home, and can cause the machine to work harder. This results in more money spent on energy costs and repairs. 5- Heating Unit Check all the connections for gas or oil, depending on what a unit runs on. Check the gas pressure on a regular basis to be sure it is not too high or too low, and observe the burner combustion and heat exchanger. Problems or issues with any of these components cannot only result in less efficiency, but can be a fire hazard. For safety purposes all heating components should be checked on a regular basis or any time there is a suspected problem> 6- Other Important Lubricate all the moving parts of an HVAC system, such as bearings, fans and belts. Be sure all these components are properly installed to prevent dirt from getting lodged between them and to prevent friction from creating wear and tear. Change or clean the air filters on a monthly basis to prevent a decrease in the air quality and an increase in overall energy costs 7- Insulation Make sure your home is properly insulated. This is the single most important step in conserving energy, including the insulation of the pipes of the split units. Why? Insulation the e suction line is done to prevent condensate from developing and dripping off.this condensate drip can cause water damage to the material under it or cause a slip hazard for people walking below it. Another very important reason to insulate the suction line, which is sometimes overlooked, is to limit the amount of heat added to the refrigerant as it travels back to the compressor. This additional heat will cause the system to reject unnecessary heat without any benefit. In addition, if too much heat is added it may cause the compressor to run too hot. A well insulated suction line can save money by reducing both energy consumption and equipment breakdowns. More over the superheated refrigerant will affect the density of the oil causing a poor lubrication for the moving parts.

6 6 Chapter 2 Classification of air Conditioners 2.1 Air conditioning Air conditioning is defined as the process of treating air so as to control simultaneously its temperature, humidity, cleanness and distribution to meet the requirements of the of the conditioned space. 2.2 What is comfortable air? The heat and coldness that the man feels depend not only on air temperature (dry bulb temperature), but also on humidity and distribution of air,although comfort differs with the distinction of Gender, Age, and Work. 2.3 Classification of air conditioners There are many kinds of classifications of air conditioners, but the representative classifications are: a- Classification by expansion methods b- Classification by heat rejection methods c- Classification by types Classification by expansion methods Expansion methods are largely classified in two types: a- Direct expansion b- Indirect expansion The direct expansion method is that heat is directly exchanged between air to be conditioned and the refrigerant for example the air conditioners, refrigerators -----etc.

7 7 Direct Method the indirect expansion method is the heat is exchanged indirectly between air to be conditioned and the refrigerant by means of water or brine. For example in chillers or centrifugal water chillers with fan coil units Indirect Method

8 Classification by heat rejection methods Heat rejection methods are largely classified in two types: a- Water cooled type (cooling towers) b- Air cooled type (residential air conditioning units)

9 Classification by the units type An HVAC designer will recommend different types of air conditioning systems for different applications. The most commonly used are described in this article. The choice of which air conditioner system to use depends upon a number of factors including how large the area is to be cooled, the total heat generated inside the enclosed area, etc. 1. Window Air Conditioner Window air conditioner is the most commonly used air conditioner for single rooms. In this air conditioner all the components, namely the compressor, condenser, expansion valve or coil, evaporator and cooling coil are enclosed in a single box. This unit is fitted in a slot made in the wall of the room, or more commonly a window sill. 2. Split Air Conditioner The split air conditioner comprises of two parts: the outdoor unit and the indoor unit. The outdoor unit, fitted outside the room, houses components like the compressor, condenser and expansion valve. The indoor unit comprises the evaporator or cooling coil and the cooling fan. For this unit you don t have to make any slot in the wall of the room. Further, present day split units have aesthetic appeal and do not take up as much space as a window unit. A split air conditioner can be used to cool one or two rooms. 3. Packaged Air Conditioner An HVAC designer will suggest this type of air conditioner if you want to cool more than two rooms or a larger space at your home or office. There are two possible arrangements with the package unit. In the first one, all the components, namely the compressor, condenser (which can be air cooled or water cooled), expansion valve and evaporator are housed in a single box. The cooled air is thrown by the high capacity blower, and it flows through the ducts laid through various rooms. In the second arrangement, the compressor and condenser are housed in one casing. The compressed gas passes through individual units, comprised of the expansion valve and cooling coil, located in various rooms. 4. Central Air Conditioning System Central air conditioning is used for cooling big buildings, houses, offices, entire hotels, gyms, movie theaters, factories etc. If the whole building is to be air conditioned, HVAC engineers find that putting individual units in each of the rooms is very expensive making this a better option. A central air conditioning system is comprised of a huge compressor that has the capacity to produce hundreds of tons of air conditioning. Cooling big halls, malls, huge spaces, galleries etc is usually only feasible with central conditioning units Classification by installation methods of

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11 11 Chapter Installing split unit Select an unobstructed location on your interior wall to mount the indoor air conditioning unit Avoid direct sunlight and heat sources. Avoid locations where gas may leak or where oil mist or sulphur exists. The indoor unit requires at least 6" (15 cm) of open space surrounding its top and sides. The unit should also be mounted at least 7 feet (2.13 m) above the ground. Install the unit at least 3.3 feet (1 m) away from antenna, power or connecting lines that are used for television, radio, home security systems, intercoms or telephones. The electrical noise from these sources could cause operational problems for your air conditioner. The wall should be strong enough to hold the unit's weight. You may need to construct a wood or metal frame to provide added support 1- Secure the mounting plate to the interior wall Hold the mounting plate against the wall where you want to install the indoor unit. Use a level to make sure the plate is both horizontally and vertically square. Drill holes into the wall at the appropriate spots to affix the plate to the wall. Insert plastic anchors into the holes. Secure the plate to the wall with tapping screws. 2- Create a hole in the wall to fit the piping Find the best spot for the hole to the exterior based on the opening in the mounting bracket. You should also consider the length of the pipe and the distance that it needs to travel to reach the outside unit. Drill a hole that is 3" (7.5 cm) in diameter through the wall. The hole should slope downward toward the exterior to ensure adequate drainage. Insert a flexible flange into the hole 3- Check the electrical connections. Lift the unit s front panel and remove the cover. Be sure the cable wires are connected to the screw terminals. Also, make sure that they match the diagram that comes with the unit. 4- Connect the pipes

12 12 Run the piping from the indoor unit toward the hole drilled through the wall. Minimize bending to ensure that the unit performs well. Cut a length of PVC pipe 1/4" (6 millimeters) shorter than the length between your interior and exterior wall surfaces. Place the pipe cap on the interior end of the PVC pipe. Insert the pipe into the hole in the wall. Bind the copper pipes, the power cables and the drain pipe together with electrical tape. Place the drain pipe on the bottom to ensure a free flow of water. Secure the pipe to the indoor unit. Use 2 wrenches, working in opposite directions, to tighten the connection. Join the water drainage pipe to the indoor unit s base. Run the bound pipes and cables through the hole in the wall. Make sure that the drainage pipe allows water to drain in an appropriate place. 5- Secure the indoor unit to the mounting plate by pressing the unit against the mounting plate 6- Install the Outdoor Condenser The outdoor unit s location needs to be away from any heavily trafficked, dusty or hot areas. The outdoor unit needs 12" of space surrounding its perimeter to ensure proper functioning Lay the concrete pad on the ground and make sure that it is level. The pad should be high enough so that the condenser will sit above the level of winter snows. Set the outdoor condenser on top of the pad. Use rubber cushioning under the unit's feet to minimize vibration. Make sure that no antenna of a radio or television is within 10 feet (3 meters) of the outdoor condense Connect the electrical wires. Remove the cover. Refer to the unit s wiring diagram and make sure the cable wires are connected as the diagram suggests. Following the manufacturer's instructions for wiring is crucial. Fasten the cables with a cable clamp and replace the cover. Bleed the air and humidity from the refrigerant circuit Remove the caps from the 2-way and 3-way valves and from the service port. Connect a vacuum pump hose to the service port. Turn the vacuum on until it reaches an absolute vacuum of 10mm Hg. Close the low pressure knob and then turn off the vacuum. Test all of the valves and joints for leaks. Disconnect the vacuum. Replace the service port and caps Wrap the joints of the piping with insulating covering and insulating tape. Affix the piping to the wall with clamps Seal up the hole in the wall using expanding polyurethane foam.

13 13 All the work can be shortcut in the following 1- Sit e the Installation 2- Unpack and Inspect 3- Install Exterior unit 4- Wire the power 5- Check for leaks 6- Evacuate and Charge 3-2 Drain piping Provide the drain piping short with a downward inclination, and do not make any air trap through it. 3-3 Finishing work Provide the finishing work accurately so that rain water does not invade into the room 3-4 Final check Remove the shipping plate. Check the ground connection. Check that screws are not loosened Completely open the stop valves in the gas and liquid lines. 3-4 Test run Check the following items. 1- Check the temperature difference between the suction air and the discharge air is over 8 C 2- Check that the power source voltage is correct. 3- Check the running voltage is correct. Calibration Some instruments be calibrated. Calibration means to change the instrument's output or reading to correspond to a standard or correct reading. for example, if a speedometer shows 55 mph for an automobile actually traveling at 60 mph, the speedometer is out of calibration. If the speedometer can be changed to read the correct speed, it be calibrated. Some instruments are designed for field use and will stay calibrated longer. The electronic instruments with digital read-out features may stay in calibration better than the analog (needle type) and be more appropriate for field use.

14 14 Chapter.4 Refrigeration System Mechanical Components Mechanical components of the system are classified into four major parts,condenser, evaporator, compressor and metering device are known as basic parts of the system or these are the four major mechanical devices to complete the process in order for the heat absorption and extraction to be performed. In order to increase the refrigerating capacity of the system and to maximize its performance, there are some more devices that are installed in the refrigerant circuit. These are called auxiliary (accessories) parts that also play a very important role in any refrigeration and air conditioning system in the process of heat absorption and extraction. Some auxiliary components are used to protect compressor or the system from any critical condition. The application of these components may vary based on the kind of unit and its application, and are seldom used in domestic refrigeration and air conditioning. 4.1 Basic components 1- Compressor The factors which are important when selecting a compressor. Efficiency at full, partial and no load Noise level Size Oil carry-over Vibration Maintenance Capacity Pressure

15 Types of Compressor Based on Compression Motion a. Reciprocating Compressor Reciprocating compressor compresses refrigerant by back and forth motion. This compressor uses valve at its inlet and outlet to seal the compression chamber. It uses piston, cylinder or the suction and compression chamber, connecting rod and crankshaft connected to the source of force that rotates the shaft to push and pull the piston inside the cylinder. Used for air and refrigerant compression Works like a bicycle pump: cylinder volume reduces while pressure increases, with pulsating output Many configurations available Single acting when using one side of the piston, and double acting when using both sides A-1 Classification of Reciprocating Compressor Hermetic This compressor is driven by an electric motor inside the compressor housing. Compressor and the motor that rotates the shaft are sealed together Semi Hermetic This compressor is also using electric motor inside the compressor housing to drive the mechanical parts. Unlike hermetic compressors, this compressor could be overhauled and some internal parts could be replaced during field servicing. Semi hermetic compressors are using gaskets to seal some parts that will be opened when accessing internal parts. This compressor also has sight glass to see the level of lubricant inside. Semi hermetic compressors are widely used in container refrigeration and some other commercial

16 16 Open Type Compressor Open type compressors are externally driven, this could be either driven by external engine or electric motor. Compressor used in car air conditioning system is a good example of open type compressor where in a clutch is used to engage and disengage the compressor while engine is running. Open type compressor is purely composed of mechanical parts without motor winding inside. This is completely serviceable in the field and could be overhauled. This uses shaft seal to prevent from refrigerant and oil leak from its housing as the shaft extends outside.. Open type compressors are most commonly used in transport refrigeration manufactured by Thermo king Corporation Open type has a separate housing for both compressor and its prime mover. It is flexible in terms of repairs and in case of motor burnout, the refrigeration system remains unaffected. Its prone to leakages through input shaft. Hemetic type has completely sealed housing with its motor also sealed in same housing. It is leak proof but cannot be repaired. Semi-Hermetic compressors are also housed with motor in the same housing but the casing is bolted type and can be repaired easily and is leak proof also. Rotary Compressor This compressor compresses refrigerant by a rotating motion. This is commonly used in some domestic and commercial air conditioning units. Most of the small rotary compressors are hermetic and could no longer be repaired when internal parts such as electric motor and some moving parts become defective. Electric motor is installed inside along with the rotating vane. Rotary compressors are using high speed electric motors to rotate and drive the vane to be able to compress refrigerant into high pressure vapor. Rotors instead of pistons: continuous discharge

17 17 Benefits: low cost, compact, low weight, easy to maintain For this reason they have gained popularity with industry. Sizes between hp TYPES Lobe compressor Screw compressor Rotary vane / Slide vane Centrifugal Compressor This type of compressor is using an impeller to draw and compress vapor refrigerant in a rotating motion. As the impeller rotates it sucks vapor refrigerant along with its axis and squeezes it into perpendicular direction. The direction of flow of vapor using centrifugal compressor resembles the direction of flow of cooled air in window type air conditioning units that are mostly using radial fan. This type of compressor is also driven by high speed electric motor. The centrifugal air compressor is a dynamic compressor, which depends on transfer of energy from a rotating impeller to the air. The centrifugal air compressor is an oil free compressor by design. The oil lubricated running gear is separated from the air by shaft seals and atmospheric vents. The centrifugal is a continuous duty compressor, with few moving parts, that is particularly suited to high volume applications-especially where oil free air is required. These compressors have appreciably different characteristics as compared to reciprocating machines. A small change in compression ratio produces a marked change in compressor output and efficiency. Centrifugal machines are better suited for applications requiring very high capacities Scroll Compressor Screw Type Compressor Maintenance of Compressor The core of a refrigeration system is the compressor, which is designed to pump cool refrigerant gas from the evaporator into the condenser. To accomplish this task the compressor raises the temperature and pressure of the low superheated gas, forcing it into the condenser. The compressor should never pump liquid. This will not only damage the compressor, but can create a potential safety hazard.

18 18 Similar to the human heart, the refrigeration compressor needs to be properly maintained and requires periodic inspection and testing. Unfortunately, the compressor is often ignored until it malfunctions or stops running altogether, at which time it gets replaced and the system is back up and running - temporarily. Oftentimes the culprit is not the compressor, but a system failure or design problem with accessory equipment which killed the compressor prematurely. 1. MEASURING THE COMPRESSOR SUCTION AND DISCHARGE PRESSURE 2. TROUBLESHOOTING THE COMPRESSOR DISCHARGE LINE TEMPERATURES Use an infrared thermometer to do a quick survey of: Compressor head temperatures Compressor oil sump temperatures Evaporator coil and suction line temperatures Discharge line temperatures Condenser coil and liquid line temperatures Fan motor temperatures. 3. RECORDING A TEMPERATURE OVERNIGHT To check refrigeration system performance, it is often useful to record temperatures in the refrigerated space. This allows you to detect problems that may go unnoticed with a single system check. 4. COMPRESSOR VALVE PERFORMANCE TEST To test small hermetic and semi-hermetic compressors used for medium and low temperature applications, the following method can be used to test for internal valve leakage. Attach a pressure/vacuum module to a DMM and set the module to cm/in. Hg. Connect the module at the suction line service port. Close the compressor off from the low side of the system by front seating the suction service valve. Run the compressor for two minutes. Turn off the compressor and observe the reading. The compressor should have pulled down to at least 16 inches (410 mm) of Hg. If the vacuum reading starts weakening toward 10 inches (254 mm) of Hg vacuum, the discharge valves of the compressor may be leaking and will probably need to be replaced. If the compressor doesn't pull a vacuum below 16

19 19 inches Hg, the suction valves are weakening and may need to be replaced. If the compressor is welded or hermetically sealed and these conditions exist, a new compressor is the only possible remedy. TROUBLESHOOTING COMPRESSOR ELECTRICAL MOTOR FAILURES CAUSED BY REFRIGERATION SYSTEM PROBLEMS Occasionally defective compressors with electrical winding failures are diagnosed by a service technician as caused by an electrical system problem. However, mechanical system failure or inferior installation and service practices often cause compressor electrical problems. These problems include: 1. Poor piping practices resulting in oil not adequately returning to the compressor during the run cycle. 2. High discharge temperatures creating acids in the oil. 3. Insufficient airflows across the evaporator and condenser coils. 4. Extremely low suction pressures. 5. Liquid refrigerant flooding back into the compressor. Diagnosing these refrigeration system problems and avoiding compressor failure can be done effectively using DMMs, clamp meters, digital thermometers, pipe clamps, infrared thermometer, and pressure/vacuum modules Procedures to diagnose these refrigeration problems 1. Compressor bearings can fail or lock up due to poor piping practices, which causes oil clogging in the system and results in insufficient oil return to the compressor. If the bearings don't lock-up and continue to wear during these conditions, the rotor will lower into the stator housing, shorting out the windings. To diagnose this problem, measure the compressor amps. They should not exceed the manufacturer's full load ratings. Worn bearings will cause higher than normal amps. Inspect the oil level via the compressor sight-glass. If there is no sight- glass, use your infrared thermometer to measure the sump of the compressor housing. The oil level can be detected with the temperature probe. The sump temperature will be different on the compressor housing at the oil level. 2. High discharge temperatures are caused by high head pressures or high superheat. The compressor discharge line can be measured quickly using the infrared thermometer on a dull section of pipe. Measure the discharge pressure using a pressure/vacuum module. Convert the refrigerant pressure to temperature and compare it to the ambient air temperature. If there is a temperature difference greater than 20-30Â F (11-17Â C), there is either noncondensible gases in the system or restricted airflow across the condenser. 3. Check for insufficient airflows across the evaporator using a digital thermometer. (See Figure 3.) Place a bead thermocouple on the discharge side of the coil and on the return side of the coil. Record the temperature difference on the air conditioning unit. Expect about 18-22Â F (10-12Â C) temperature difference. On refrigeration units, expect about 10-15Â F (5-8.5Â C) temperature difference 4. Extremely low suction pressures can be checked using the pressure/vacuum module and your DMM. Install it at the compressor and record your suction pressure. Convert the refrigerant pressure to temperature using a pressure-temperature (PT) chart. Measure the return air temperature before the evaporator. Compare the refrigerant temperature to the desired

20 21 evaporator return air temperature. On air conditioning units, expect about 35-40Â F (19-22Â C) temperature difference and, on refrigeration units, expect about 10-20Â F (5-11Â C) temperature difference. 5. Check for liquid refrigerant flooding back to the compressor by determining the superheat using your pressure/vacuum module and pipe clamp, along with your DMM. Check suction pressure and convert the refrigerant pressure to temperature, using your PT chart. Measure the suction line pipe temperature. Compare the difference of the two temperatures. If there is no temperature difference, then you are bringing back liquid to the compressor. If there is a temperature difference between 10-20Â F (5-11Â C), then you have normal superheat and you are not slugging the compressor with unwanted liquid. SUMMARY Troubleshooting and servicing refrigeration, air conditioning, and heat pump systems is a challenge for any technician, entry level or experienced. Regardless of the size or location of the system, it is imperative that the service technician understands the principles and the tools needed to perform proper troubleshooting efficiently Chapter 5 Condenser Condenser Introduction Condensers and Evaporators are heat exchangers in which the refrigerant undergoes a phase change. In condensers the refrigerant vapor condensers condenses by rejecting heat to an external fluid, which acts as a heat sink In a typical refrigerant condenser, the refrigerant enters the condenser in a superheated state. It is first de-superheated and then condensed by rejecting heat to an external medium. Temperature of a pure refrigerant remains constant during condensation, when the refrigerant side pressure drop is negligible. The refrigerant may leave the condenser as a saturated or sub-cooled liquid, depending upon the temperature of the external medium Classification of Condensers Based on the external fluid. Condensers can be classified as: 1. Air- cooled Condensers 2. Water cooled Condensers 3. Evaporative Condensers 1) Air cooled Condensers In these condensers air is the external fluid i.e. the refrigerant rejects heat to air flowing over the condenser Air-cooled condensers can be further classified into : Natural convection type, or Forced convection type

21 21 Natural Convection type Heat transfer from the condenser is buoyancy induced natural convection and radiation Since the flow rate of air is small and the radiation heat transfer is also not very high. The combined heat transfer coefficient is small Hence a relatively large condensing surface is required to reject a given amount of heat Hence these are used for small capacity refrigeration systems like household refrigeration and freezers Natural convection type can be further classified based on construction as - Plate Type - Fin Type Plate type The finned type condensers are mounted either below the refrigerator at an angle or on the backside of the refrigerator The fin spacing is kept large to minimize the effect of fouling by dust and to allow air to flow freely with little resistance In some designs, the condenser tubes are attached to a slotted plate, which acts as a fin In another common design, thin wires are welded to the condenser tubing For all natural convection condensers refrigerators should be placed such that air can flow freely over the condenser surface

22 22 o In forced convection type condensers, the circulation of air over the condenser surface is maintained by using a fan or a blower o These condensers normally use fins on air side for good heat transfer o The fins can be either plate type or annular type o Forced convection type air cooled condensers are commonly used in window air conditioners, water coolers and packaged air conditioning plants

23 23 1) Water Cooled Condensers In water cooled condensers water is the external fluid Depending upon the construction, water cooled condensers can be further classified into: I. Double pipe or tube-in tube type II. Shell and coil type III. Shell and tube type a) Double Pipe or tube-in-tube type Normally used up to 10 TR capacity In these condensers the cold water flows through the inner tube, while the refrigerant flows through the annulus The refrigerant in the annulus rejects a part of its heat to the surroundings by free convection and radiation The heat transfer coefficient is usually low because of poor liquid refrigerant drainage if the tubes are long b) Shell and coil type Used in systems up to 50 TR capacity The water flows through multiple coils, which may have fins to increase the heat transfer coefficient The refrigerant flows through the shell In smaller capacity condensers, refrigerant flows through coils while water flows through the shell

24 24 When water flows through the coils, cleaning is done by circulating suitable chemical through the coils c) Shell-and-tube type This is the most common type of condenser used in systems from 2 TR up to thousands of TR capacity In these condensers the refrigerant flows through the shell while water flows through the tubes in single to four passes The condensed refrigerant collects at the bottom of the shell The coldest water contacts the liquid refrigerant so that some subcooling can also be obtained Cleaning of the coil this system done by circulating 40 per cent water and 1 HCL 3) Evaporative Condensers In evaporative condensers, both air and water are used to extract heat from the condensing refrigerant Evaporative condensers combine the features of a cooling tower and water-cooled condenser in a single unit

25 25 Water is sprayed from top part on a bank of tubes carrying the refrigerant and air is induced upwards There is a thin water film around the condenser tubes from which evaporative cooling takes place

26 26 Chapter 5 Evaporator Evaporator It s a heat exchanger like the condenser, except the evaporators capture heat from the space where it dose use (like sponges when they soaks a limited quantity of water in a full of water tray), but the condenser reject the previous caught hot in the evaporator (like sponge when you squeeze it, it will spill all of the water) The function of the evaporator is: a- Absorb heat from the space where located b- Maintain or remove the humidity of the air in space Types of Evaporator Evaporators are divided into two main categories according to their operating conditions Dry expansion (DX) evaporators: In these evaporators the amount of refrigerant fed by the flow control device is just enough so that it all evaporates before it leaves the evaporator.the vapor leaving the evaporator is usually slightly superheated to ensure that there is no risk of any refrigerant liquid reaching the compressor. Dry expansion evaporators exist in two types a- DX cooling coil uses air for cooling b- DX chillers for cooling water or other liquid. Flooded evaporators: these evaporators evaporate only a part of the refrigerant volume delivered. The outlet from these exchangers, therefore, has to be fitted with a liquid separator or a surge drum, the upper part of which contains only vapor which is drawn off by the compressor suction. The unevaporated liquid is fed back to the evaporator inlet. These evaporators have very high heat exchange efficiency because the entire internal exchange surface is essentially in contact with the liquid refrigerant. The expansion devices used with flooded evaporators are low side valves which maintain the liquid volume in the separator a t a fairly constant level

27 27 According to the functions, the evaporators can be classified as: a- Air cooling evaporator which have finned pipe coils, in small coolers, there will be fans to blow the air over the coil. Aluminum fins on copper tube are the most common for halocarbons with stainless steel or aluminum tube for ammonia. b- Liquid cooling evaporators are mostly of shell-and-tube or shell-and-coil type evaporators. Evaporators of this general type with dry expansion circuits will have the refrigerant within the tubes, in order to maintain a suitable continuous velocity for oil transport, and the liquid in the shell. In both side baffles are needed on the water side to improve turbulence, and the tubes may be finned from outside.

28 28 Flooded shell-and-tube evaporators or coolers are commonly used in water chilling applications as in air conditioning. Water passes through the inside of the tubes and is chilled by heat transfer to the boiling refrigerant on the outside of the tubes. A valve usually controls the flow of the refrigerant into the evaporator. c- Plate surface evaporators there are several types, some are construction of two flat sheets as metal so embossed and welded together as to provide a path for refrigerant flow between them. This type of plate surface evaporators has the advantages of easy cleaning and low cost in manufacturing, it can be readily formed into the various shapes required to serve as structural components for examples, the walls of a household refrigerator.

29 29 Defrosting When an evaporator operates at a temperature below 0 0 C, the exchanger surface progressively becomes covered with a layer of frost, produced by moisture condensation on the cold surfaces, the moisture being deposited by circulating air on the heat exchanger surface. Frost is undesirable from operational standpoint for two reasons: a- Thick layers of frost act as insulation greatly reducing the heat transfer capability of the evaporator and this condition becomes critical when the frost bridges the gap between adjacent fins, thus also greatly reducing the surface area available for heat transfer, b- In force convection coils the frost reduces the air flow rate. The following are some of the common methods of defrosting: Manual defrost method(shutdown) Time shutdown defrost method Electric defrosting (Heaters) Water defrosting Hot gas bypass defrost system. Reverse cycle Among the above methods, the most popular ones are hot-gas defrost and water defrost. In the hot gas method the discharge gas from the compressor is sent directly to the evaporator and the evaporator performs temporarily a condenser. The heat of condensation melts off the frost which then drains away. The compressor must still be working on another evaporator to make hot gas available. In water method a stream of water is directed over the coil until all the frost is melted.

30 31 Chapter 6 Metering Devices Metering Devices The purpose Metering devices or so called Expansion valves are: 1- Reduce pressure from condenser to evaporator pressure 2- Regulate the refrigerant flow from the high pressure liquid line into the evaporation rate in the evaporator The expansion devices used in refrigerating systems can be divided into a- Fixed opening type b- Variable opening type Fixed opening type the flow area remains fixed, while in variable opening type the flow area changes with changing mass flow rates Classification of Metering devices There are basically seven types of refrigerant metering devices, these are: 1- Hand operated valves 2- Capillary tubes 3- Automatic expansion valves (AEV)

31 31 4- Thermostatic expansion valves (TEV) 5- Float valves : a- Low side float valve b- High side float valve 6- Electronic expansion valves Capillary tube A capillary tube a long narrow tube of constant diameter typical inner tube diameters off refrigerant capillary tube range from 0.02 to 2mm and the length ranges from 0.6 to 6m, the pressure reduction in a capillary tube occurs due to the following two factors: a- Pressure drop due to frictional resistance offered by the tube walls b- Pressure drop due to acceleration of refrigerant as it flows through capillary tube Capillary tube is Critical charge, which is defined as the definite amount of refrigerant that is put into the refrigeration system so that in eventuality of all of it accumulate in the evaporator, it will just fill the evaporator up to its brim and never overflow from the evaporator to compressor, for this reason critically charged system doesn't use liquid receivers, and use hermetic compressors to minimize refrigerant leakage from the system Advantage of capillary tube 1- It is simple and inexpensive 2- It doesn't have any moving parts hence it dozen' t require maintenance 3- Provides an open connection between condenser and the evaporator during off-cycle, pressure equalization occurs. This reduce the starting torque can be used. Disadvantages of capillary tube 1. Cannot adjust itself efficiently to changing load conditions 2. Susceptible to clogging because of narrow bore of the tube 3. Danger of evaporator flooding and compressor slugging during low load and off-cycle conditions. Hence only with hermetic compressors (refrigerant leakage cannot be tolerated) Automatic Expansion Valve An Automatic Expansion Valve (AEV) valve maintains a constant pressure and thereby a constant temperature in the evaporator, It consists of an adjustable spring and a follow-up spring, The valve acts in such a manner that the evaporator pressure remains constant as long as the refrigeration load is constant At this point, the mass flow rate through the valve is the same as that through the compressor (balanced condition) Increase in refrigeration load When load increases, evaporator pressure tends to increase, to keep the evaporator constant, the needle moves up and the refrigerant flow rat reduces

32 32 The evaporator starves due to the narrow opening The valve counteracts in a manner opposite to what is required But when the load decreases the evaporator pressure tends to decrease and Needle moves down to allow more refrigerant so that the evaporator pressure remains constant and this may lead to flooding the evaporator hence, the valve is suitable only for critically charged systems During off-cycle, valve remains closed as pressure in evaporator increases Applications and salient features of AEV 1. Relatively simple and economical 2. Used in small systems up to 10 TR capacity with Hermetic compressors 3. Used in critically charged system to avoid flooding of evaporator 4. Used where constant evaporator temperature is required, e.g. milk chilling units, water coolers etc.., where freezing of the liquid is disastrous Thermostatic Expansion Valve (TEV) Thermostatic expansion valve is the most versatile expansion valve and is most commonly used in refrigeration systems. A thermostatic expansion valve maintains a constant degree of superheat at the exit of evaporator preventing the slugging of the compressors since it does not allow the liquid refrigerant to enter the compressor A TEV consists of a feeler bulb attached to the evaporator exit tube so that it senses the temperature at the of evaporator,the feeler bulb is connected to the top of the bellows by a narrow tube,the feeler bulb and the narrow tube contain some fluid that is called power fluid,the power fluid may be the same as the refrigerant used in the system or different under normal conditions the pressure of the power fluid P p is the saturation pressure corresponding to the temperature at the evaporator exit Effect of load variation If the load on the plant increases, the evaporation rate of refrigerant increases, the area available for superheating the vapor increases, as the degree of superheat increases, pressure of power fluid increases the needle stand is pushed down and the mass flow rate of refrigerant increases When refrigeration load decreases, less refrigerant boils, degree of superheat decreases, pressure applied by the power fluid above the bellows decreases and the needle moves up causing the mass flow rate of refrigerant decreases causing the evaporator s temperature to increases and consequently the superheat degree increases and pressure of power fluid increases,thus a TEV always establishes balanced flow condition between compressor and itself meaning flow rate is always proportional to the load

33 How can read superheat 33

34 34 Bulb superheat location The reason for this position is because it is the coolest part of the pipe. the top is the warmest part, and the bottom is where the oil collects. The oil acts as an insulator, which prevents the bulb from sensing the proper suction line temperatures. Float valve Float valves are normally used in large systems with flooded evaporators The valve opens or closes depending upon the liquid level as sensed by a buoyant member, the float The float valve always maintains a constant liquid level in the float chamber The float chamber may be placed on the low pressure side and called (Low side pressure valve) or on the high pressure side and called (High side pressure valve) Low side pressure valve This valve maintains a constant level of liquid refrigerant in the evaporator, this type of valve is used with a flooded type of evaporator, when refrigeration load increases liquid level falls momentarily, and float valve opens wider allowing more refrigerant to flow in thus restoring the level, the opposite happens when the refrigeration load decreases High side pressure valve Maintains a constant liquid level in high side float chamber placed at the exit of condenser,when refrigeration load increases more vapor is generated in evaporator,condensation in the condenser increases, which increases level of liquid in float chamber, Valve opens more to allow more liquid into evaporator reverse happens when load falls

35 35 Electronic metering device Thermal electric controlled expansion valve depends on the use of thermostats, directly exposed to refrigerant in the suction line to control the expansion valve needle opening. He does not use elements of pressure, as in the valve. Resistance electrical flow in thermostat changes with temperature. The increase in temperature reduces resistance. Therefore, given the voltage rise of temperature also increases the speed of the current. This increased the current heats and bends bimetallic valve body, opening the valve. Fig. illustrates a typical thermo-electrical expansion valve installation. Thermostat, C, is in direct contact with the refrigerant vapor inside the suction line from the evaporator. Compression refrigeration system accessories and

36 36 1- Liquid Receiver : Liquid receiver is installed between the condenser and the metering device as shown below, and temporarily holds the refrigerant which has been liquefied by the condenser before being sent to the expansion valve. As a result, only the refrigerant completely liquefied can be supplied to the metering device. The liquid receiver is also used as a container in which surplus refrigerant is stored since amount of the refrigerant circulated differs with following condition : Length of the connection pipe between the condensing unit (outdoor unit) and the fan coil (indoor unit) Changes in operation condition. Note : the liquid receiver must not be used in with the capillary tube system because during off cycle, liquid flows to the evaporator through the capillary tube and when the compressor starts again, there is a fear of liquid compression 2- Accumulator A storage tank placed in the suction line to collect any liquid refrigerant that may remain in the evaporator discharge and prevent it from flowing on to the compressor until it has been turned into vapor. An accumulator is installed in the suction line so that it can accumulate any liquid in the suction line, vaporize it, and send it on to the compressor The accumulator may also store any refrigerant not in use in the system and meter it back in appropriate quantities to the compressor

37 37 Chapter 4 Electric Wiring 4-1 Types of Fans The fans are classified according to the direction of the airflow through impeller The blower, of fan as it is sometimes called, can be described as a device that produces airflow or movement. Several different types of blowers produce this movement, but all can be described non- positive-displacement air movers. Remember, the compressor is a positive displacement pump.. when the cylinder is full of refrigerant (or air for an air compressor). The compressor is going to empty that cylinder. The fan is not positive displacement, it cannot build the kind of pressure that a compressor can, the fan has other characteristics that have to be dealt with The two fans that we discuss are : a- Propeller Fan (Axial ) b- Forward curve centrifugal fan (squirrel cage fan wheel ) Propeller fan This kind of fan is used in exhaust fan and condenser fan applications. It will handle large volumes of air at low pressure differentials. The propeller can be cast iron, aluminum, or stamped steel and is set into a housing called venture to encourage airflow in a straight line from one side of the fan to the other figure below. the propeller fan makes more noise than the centrifugal fan so it is normally used where noise is not a factor.

38 38 Propeller fans are often directly connected to their motors, avoiding losses associated with a drive belt. they also have a central hub the allows the motor to fit neatly behind the fan with little penalty in efficiency. The weight distribution of their blades allows for low starting torque. Therefore they must attached with run capacitor. In axial flow, air enters and leaves the fan with no change in direction Centrifugal fan centrifugal van has characteristics that make it desirable for ductwork. It builds more pressure from the inlet to the outlet and moves more air against more pressure. This fan has a forward curved blade and a cut off to shear the air spinning around the fan wheel. this air is thrown by centrifugal action to the outer perimeter of the fan wheel. See the figure below curved or inclined, radial) Centrifugal fans with forwarded blades are suited for application with higher air flow volumes and pressures. Axial propeller fans are more suited for applications with lower volumes and pressures In centrifugal flow, airflow changes direction twice - once when entering and second when leaving (forward curved, backward