COMPRESSORS Modern vapor-compression systems for comfort cooling and industrial refrigeration use one of several types of compressors: reciprocating, rotary, helical (screw type), centrifugal, and scroll. In some systems, the compressor is driven by an external motor (called open-drive or open drive system). The open drive compressor systems are easier to service but use of a seal on the compressor crankshaft drive end can be a source of leaks. Open drive systems commonly use "V" belts or flexible "Coupling" to transmit power from the motor to the compressor. The second major category is the hermetic system, in which the motor is placed inside a housing with the compressor. In hermetic systems, the motor is cooled by refrigerant vapor rather than outside air, the crankcase serves as the intake manifold, and intake valves need not to be directly connected to the suction line. Hermetic systems have fewer leak problems than open systems because they have no crankcase seal. However, hermetic compressors are more difficult to service, although some components subject to failure are usually placed outside the housing. These components are connected to the compressor and motor by leak proof devices. Motors in hermetic systems must not emit any electrical arcs (so they cannot use brushes) as they would pollute the refrigerant oil, and cause a motor burn out. Hermetic systems are classified as 1) full hermetic or 2) serviceable hermetic (semi-hermetic). Many hermetic compressors have welded housing that are not serviceable. If the motor or compressor fails, the entire unit must be replaced. Semi-hermetic systems are commonly used in large reciprocating, centrifugal, screw and scroll compressors. The housing in a semi-hermetic system is bolted and gasket together and may be dismantled for major service operations. COMPRESSOR COOLING Compressors build up considerable heat in the course of compressing refrigerant vapor. Most of travels with high-pressure vapor to the condenser, but the compressor head must also dispose of unwanted heat to remain within safe operating temperatures. This is normally accomplished with either fins or water passages. In hermetic and semi-hermetic systems, the suction line feeds a stream of cool refrigerant to the cylinder heads. Thus, the temperature and pressure of the suction gas are critical to maintaining proper compressor body temperature. Suction gas entering the compressor should not be above 65 deg. F (18 deg. C) on a low temperature installation, or 90 deg. F (32 deg. C) on a high temperature system. A hotter gas is less dense and will pick up less heat in the compressor because there is less of a temperature differential between the compressor motor and the suction gas. The low-pressure cutout control should protect the motor from inadequate suction line pressure. Air–cooled open drive compressors may be cooled by placing them directly in the blast of the condenser fan. An alternative is to dedicate a fan to compressor cooling. Water-cooled compressors may, employ a jacketed heads allowing water to circulate through the head. CENTRIFUGAL COMPRESSOR Centrifugal compressors use impellers, which spin rapidly and fling the refrigerant away from the center intake, using the force called centrifugal force. Centrifugal force is using principle that for example that allows you to swing a bucked overhead without spilling the water in it. Because each impeller adds relatively little pressure, several impellers are often ganged together to create the necessary pressure on the high side (discharge pressure). Centrifugal compressors are used in large systems, often in semi-hermetic or open configurations. The compressor may operate in a system with a positive suction pressure or in vacuum, depending on refrigerant used and operating evaporator temperature desired. Large centrifugal systems may be shipped ready- charged with refrigerant and oil. The centrifugal compressor has no connecting rods, pistons and valves; so the shaft bearings are the only points subject to wear. The compressor discharge pressure is a function of gas density, impeller diameter and design, and impeller speed. Centrifugal compressor impellers rotate very rapidly: Low speed 3,600 RPM Medium speed 9,000 RPM High speed above 9,000 RPM Power is supplied by an electric motor or steam turbine. Vapor enters the center of impeller around the shaft and is directed through the impeller blades. As the impeller accelerate the gas, by the kinetic energy of the impeller is converted to the kinetic of fast moving gas. As the gas enter the volute, it is compressed, and the kinetic energy is converted to the potential energy of compressed gas. The velocity of the gas leaving the impeller is extremely high. The inlet vanes that regulate the amount of supply and the direction of refrigerant vapor from an evaporator may control capacity. Large compressors, with over three stages may omit the inlet vanes. Refrigerant flood back on centrifugal compressors is dangerous due to high speed of the impellers. To prevent flood back, the refrigerant charge must not be excessive and superheat must be adequate. Many centrifugal compressors especially those operating in a vacuum have a purge device built in to allow disposal of unwanted air from the system. The purge unit is a condensing unit with a compressor and condenser that draws vapor from the highest point of the system condenser and compressor and condenses it. Because only refrigerant will condense at the pressure created by the purge unit, the air and other non-condensable that collects on top can be purged manually or automatically through valve to the atmosphere. Purged liquid refrigerant flows through a float-operated valve in the purge unit condenser back to the main system. If a filter-drier is installed in a centrifugal system, it can be placed in a bypass around the float valve. Placing the filter-drier in the main output would impair the compressor operation. Even though the by-pass only takes a portion of the liquid flow, it will eventually remove enough moisture from the refrigerant to control system acidity.  REFRIGERATION SYSTEM CHILLER COMPONENTSFigure 6-1: Two-stage centrifugal compressor. 1-Second-stage variable inlet guide vane. 2-First-stage impeller. 3-Second-stage impeller. 4-Water-cooled motor. 5-Base, oil tank, and lubricating oil pump assembly. 6-First-stage guide vanes and capacity control. 7-Labyrinth seal. 8-Cross-over connection. 9-Guide vane actuator. 10-Volute casing. 11-Pressure-lubricated sleeve bearing. Note that discharge opening is not shown. Figure 6-2: A hermetic centrifugal liquid chiller, single-stage compressor. Using HCFC-22, 300 to 600 nominal tons; using HFC-134a, 200 through 530 nominal tons. The system can utilize either R-22 or R-134a, which allows for conversion from R-22 to R-134a if needed. The unit has a microprocessor to control the system. Cutaway view showing the refrigeration cycle. HELICAL SCREW COMPRESSORS Screw-type compressors are generally and efficiently used in system with capacity above 20 tons of refrigeration. These compressors use a pair of helical screws, or rotors, which rotate together inside a chamber and force refrigerant from intake, low side of chamber toward the end high side of Figure 6-3: Cross-section of screw compressor. A-Male rotor. B-Female rotor. C-Cylinder. Vaporized refrigerant enters at one end and exhausts at other end. As the gas is forced forward, it is compressed into shrinking gaps between the screw lobes, creating the compressing action. No valves are needed, other than service at the intake and exhaust ports. Because the rotors spin continuously, there is les vibration than with refrigeration and air conditioning book the chamber, reciprocating compressors. Helical (screw) compressors are made in open drive or hermetic configurations. The rotors are termed “male” for the drive rotor, and “female” for the driven rotor. The male rotor, with more lobes, spins more rapidly than the female lobe. Capacity control is accomplished by a slide valve which opens in the compressor chamber and allows vapor to exit without being compressed some units are able to operate efficiently at only at only 10% of rated capacity. Figure 6-4: Basic operation of screw compressor. Revolving rotor compresses vapor. A-Compressor interlobe spaces being filled. B-Beginning of compression. C-Full compression of trapped vapor. D-Beginning of discharge of compressed vapor. E-Compressed vapor fully discharged from interlobe spaces. RECIPROCATING COMPRESSORS Reciprocating compressor use a piston sliding inside a cylinder to compress refrigerant vapor. Figure 4-29 shows the principle of operation of a reciprocating compressor. In figure 4-29A,the piston has moved downward in the cylinder, A. It has moved refrigerant vapor from the suction line through intake valve. From there the refrigerant vapor has moved into cylinder space. In Figure 4-29B, the piston has moved upward. It has compressed the vaporized refrigerant into a much smaller space (clearance space). The compressed vapor has been pushed through the exhaust valve into the condenser. Figure 6-5: Basic construction of reciprocating compressor. At he top of the stroke, the piston must come very close to the cylinder head. The smaller the clearance space, the greater the pressure that the piston stroke will create. This clearance may range between 0.010 and 0.020 inches (0.254 mm to 0.508 mm). Small system may use two-piston compressor, while large industrial systems use multi-cylinder multi piston compressors. The compressor crankcase must be designed to dispose of the heat of compression. Compressor crankcases are usually make of cast iron and have fins for dissipating heat to air or in some cases water jackets, to dissipate the heat of compression to the water. In semi-hermetic and hermetic compressor, the cooling is provided by refrigerant from the suction line. Pistons in large reciprocating compressors have separate oil and compression rings. Oil rings, lower on the piston, are used to reduce the amount of oil entering the cylinder from the crankcase. In small systems, oil rings may be omitted and oil grooves used instead to control the oil flow. Compression rings are used to make tight seal against cylinder walls, ensuring that each stroke pumps as much refrigerant as possible. CRANKCASE SHAFT AND CONNECTING RODS Figure 6-6: Cutaway view of small, external-drive, two-cylinder reciprocating compressor. The body is a lightweight alloy casting, Cast-iron cylinder liners are permanently cast into crankcase body. In reciprocating compressors, the crankcase shaft converts rotary motion from the motor to reciprocating motion for the pistons. Crankshaft rotate within main bearing, which must firmly support the crankshaft and resist end loads placed on the shaft by the motor and connecting rods. The exact amount of the endplay should by specify in manufacturer literature. Several types of linkages my be used to connect the connecting rod to the crankshaft:
CRANKCASE SEAL In open – drive systems, the seal between the crankshaft and the crankcase is common source of problems. The seal is subjected to a great deal of pressure variations and must operate and must operate and seal whether the crankshaft is rotating or is stationary. Clearance must be accurate (to.000001 inch or .0000254 mm) between the rotating and stationary surfaces, and lubrication fills that tiny gap. The seal is commonly made of hardened steel a bronze, ceramic, or carbon. The absence of the crankshaft seal is the major advantage of the hermetic design. The rotary-type seal is a simple, common seal that r0tates on the shaft in operation. A spring, in combination with internal pressure, forces the seal face against a stationary seal face. The major source of problem with crankcase seals is leakage due to misalignment. Care must be taken when aligning the motor shaft to the compressor shaft so seal will be not stressed during the operation. The close tolerance as specified by manufacture of the compressor must be observed in both horizontal and angular directions. In most cases, the seal is lubricated by compressor oil pump. Make sure that the compressor is operated occasionally during the long shutdowns to keep the seal lubricated. A slight leakage after start up, during which a dry seal is lubricated with oil, may be normal. A leaking seal can be detected with a refrigerant leak detector. To inspect a leaking seal:
RECIPROCATING COMPRESSOR HEADS AND VALVE PLATES Compressor cylinder heads are generally made of cast iron and are designed to hold the gaskets in place to provide positive seal between the valve plate, the cylinder block and head. Cylinder heads must have passages to admit suction gas into the cylinder. The head is generally affixed to the block with cap screws. Intake valves are designed to admit refrigerant during the intake stroke and close during the compression stroke. Discharge valves are closed during the intake stroke and open at the end of the compression stroke. The valve plate is the assembly holding both valves tightly in place. Valves are usually made of spring steel and designed to make a tight seal until the pumping action of the piston opens them. The mating surfaces of valves must be perfectly flat, and defects as small as 0.001 in. (0.0254 mm) can cause unacceptable leaks. In service, the valve must open about 0.010 in. (0.254 mm). Large openings will cause valve noise, while smaller openings will prevent enough refrigerant from entering and exiting the cylinder. Operating temperature has great effects on valves durability. Intake valves operate in a relatively cool environment and have constant lubrication from oil vapors. Discharge valves are the hottest component in a refrigeration system, operating as much as 50 deg. F to 100 deg. F hotter than the discharge line, so the are more commonly a source of trouble than intake valves. Discharge valves must be fitted with special care. Heavy molecules of oil tend to accumulate on them, causing carbon buildup and interfering with valve performance. Discharge valves and oil will be damaged by temperature hotter than 325 deg. F to 350 deg. F (163 to 177 deg. C). In general, the discharge line temperature should be kept blow about 225 deg. F to 250 deg. F. (107 to 121 deg. C). Figure 6-7: Reciprocating compressor valve plate assembly. Discharge valves may have relief springs to enable them to open abnormally wide if slug of liquid refrigerant or oil enter the compressor piston from the suction line or the compressor crankcase. Figure 6-8: A commercial, hermetic reciprocating compressor. It has four banks of two cylinders each (four connecting rods on each crankthrow) and is bolted for ease in servicing. ROTARY COMPRESSOR Rotary compressors use one or more blades to create the compressing action inside a cylinder. Unlike the reciprocating compressor, no piston is used. There are two basic types of rotary compressors:
In both types, the blade must be able to slip within its housing to accommodate the motion of the rotor, which rotates off center within the cylinder. Inlet (suction) ports are much larger than discharge ports. There is no need for intake (suction) or discharge valves; however, check valves are desirable are desirable in the suction line to prevent oil and high-pressure vapor from entering the evaporator when compressor is not operating. ROTATING BLADE (VANE) COMPRESSOR In a rotating vane design, a rotor (shaft) rotates inside a cylinder, but the center axes of the cylinder and the shaft are not identical. The rotating rotor (shaft) has several precision-machined grooves that accept the sliding vanes. As the shaft spins, these vanes are forced against the cylinder by centrifugal force. As gas enters the compressor from the suction line, the vanes sweep it around. Because the rotor is not centered in the cylinder, the space containing the gas decreases as the vanes force the gas around the cylinder. The result is gas compression. When the gas reaches minimum volume and maximum compression, it is forced out the discharge port. The clearance volume of this system is very low and the compression efficiency very high. Rotating vane compressors are commonly used for the first stage of cascade system. Rotary vane compressors may have between two and eight vanes; large systems have more blades. The edge of the blade where it meets the cylinder wall must be accurately ground and smooth or leakage and excess wear will result. The blade must also be precisely fit into the slot in the rotor. Figure 6-9: A rotary blade compressor. Black arrows indicate direction of rotation of rotor. Red arrows indicate refrigerant vapor flow. STATIONARY BLADE (DIVIDER BLOCK) ROTARY COMPRESSOR In the stationary blade system, a sliding blade in the cylinder housing separates the low-pressure vapor from the high-pressure vapor. An eccentric shaft rotates an impeller in a cylinder. This impeller constantly rubs against the outer wall of the cylinder. As the impeller revolves, the blade traps quantities of vapor. The vapor is compressed into a smaller and smaller space. The pressure and temperature builds up. Finally the vapor is forced through the discharge port. Figure 6-10: Rotary compressor. Stationary blade or divider block is in contact with an impeller. Figure 6-11: Hermetic, single stationary-blade rotary compressor. SCROLL COMPRESSOR In a scroll compressor, compression is performed by two scroll elements an orbiting scroll and a fixed scroll. One scroll “the fixed scroll” remains stationary. The other scroll “the orbiting” scroll rotates through an offset circular path around the center of the fixed scroll. This movement creates compression pockets between the two scroll elements. Low pressure, suction gas is trapped in each peripheral pocket as it is formed; continued motion of the orbiting scroll seals the pocket, which decreases in volume as the pocket moves toward the center of the scroll. Maximum compression is achieved when a pocked reaches the center where the discharge port is located and the gas is discharged. During this compression process, several pockets are being formed at the same time. Figure 6-12: Compression in the scroll is caused by the interaction of an orbiting scroll mated within a stationary scroll. 1-Gas is drawn into an outer opening as one of the scrolls orbits. 2-As the orbiting motion continues, the open passage is sealed off and the gas is forced to the center of the scroll. 3-The pocket becomes progressively small in volume. This creates increasingly higher gas pressures. 4-Discharge pressure is reached at the center of the pocket. Gas is released from the port of the stationary scroll member. 5-In actual operation, six gas passages are in various stages of compression at all times. This creates nearly continuous suction and discharge. Figure 6-13: Cross section through a swash plate reciprocating compressor. As drive shaft and swash plate revolve, double-end piston is moved back and forth in cylinder. The suction process from the outer portion of the scroll and the discharge from the inner portion are continuous. This continuous process gives the compressor very smooth operation. Compression is a continuous process without conventional suction and discharge valves. To prevent the compressor from running backwards after the power has been switched off, a check valve is located directly above the fixed scroll discharge port. A: Cutaway diagram of a scroll compressor. B: Basic compression representation of a scroll compressor. Orbiting scroll orbits the fixed scroll creating a smooth, constant compression inward towards the discharge port at the center. OIL SYSTEMS FOR COMPRESSORS Reciprocating compressors use generally two types of lubricating systems:
Rotary compressors Require a film of oil on the cylinder, blades and roller. Some machines propel the oil by the sliding action; others use an oil pump. Centrifugal compressors Operate at high speed and may have elaborate oil control systems, with a pump, oil separator, reservoirs to lubricate bearings during cast-down, oil filter, relief valve, and oil cooler. Helical screw compressors Need oil to cool, seal, and silence the rotors; they generally have a forced lubrication system. A positive displacement pump may operate independently of the compressor, ensuring complete lubrication at the compressor start up. Oil is separated, piped to an oil sump (reservoir). Cooled and delivered to the bearings and ports for injection into the compression chamber. The oil sump (reservoir) has a heater to prevent oil dilution by refrigerant during the off-cycle. Scroll compressors Require oil to cool and seal between orbiting and stationary scroll. Oil is driven to the scrolls by centrifugal action through hole in a shaft of the motor and orbiting scroll. Three devices are generally used in industrial refrigeration system to control system oil: an oil separator, an oil level regulator, and oil reservoir. Other elements, such as oil strainers, solenoid and isolating valves, may be needed to complete the system. A regular system oil test should be perform, to detect damaging acidity in the refrigeration compressor oil. Promoting oil return Oil in direct expansion or dry evaporator systems must be swept back to the compressor by the flow of refrigerant. The velocity in the evaporator tubes must be sufficient to carry the oil back. The velocity of about 700 feet (214 m) per minute are required in horizontal lines and about 1500 feet (457m) per minute in vertical lines are needed. Several additional measures will help to assure proper oil return to the compressor. Slope the refrigeration lines toward the compressor. Ensure adequate refrigerant velocity in the suction line by making it proper size, not oversize. High viscosity oil (as measured in evaporator condition) is more resistant to return by refrigerant flow. Oil that readily dissolves refrigerant remains more fluid than oil without refrigerant. The amount of refrigerant dissolved in the oil varies according to pressure and temperature conditions in various parts of the evaporator, and the nature of the two fluids. Oil return is more difficult in low-temperature evaporators, because oil becomes more viscous as the temperature and pressure of the refrigerant becomes low. High compression ratio also decreases oil return, because the suction gas is less dense. Thus adequate suction line velocity is especially important in low-temperature evaporators. Oil will not be swept back to the compressor in a flooded evaporator, so an oil return line is required. In some systems, a special chamber is connected to the evaporator to allow refrigerant to be boiled from the oil before the oil is returned to the compressor. DISCHARGE LINE The discharge line on the high side of the system, connect the compressor to the condenser. The line is commonly copper tubing connected by brazing. The discharge may contain; Vibration absorber, Muffler, Oil separator, pressure control valves, and by pass or service valves. Vibration Absorber Both the suction and discharge lines transmit vibration from the compressor to other cooling system components. This vibration can cause unwanted noise and deterioration of refrigerant tubing leading to leaks of refrigerant. On small system with small-diameter soft copper tubing, the vibration absorber may consist of a coil of tubing. Flexible metallic hose, with ID at least as large as the connected tubing, is preferable for larger systems. This section of tubing may be terminated by OD socket, threaded male ends or flanges. Refrigerant traveling with high velocity along the convoluted inner diameter of the absorber may cause a whistling sound. Vibration absorbers are not designed for compression or extension, so they mast be oriented parallel to a compressor crankshaft, not at right angles to it. Muffler A muffler is used to reduce the transmission of reciprocating compressor discharge pulsation and noise to the piping system and to the condenser. A muffler is a cylinder with baffle plates inside. In general, mufflers, which create a large pressure drop, are more effective than those with less restriction. Both the volume and density of the gas flow through the muffler affect muffler performance. Oil separator An oil separator is a container with a series of baffles and screens placed in the discharge line. The discharge vapor with oil fog entering the oil separator is forced to turn and collide against the baffles and screens, allowing droplets of oil to combine into large drops, which drip to the sump at the bottom. The sump allows sludge and contaminants to settle out and may have a magnet to attract ferrous particles. When sufficient oil has accumulated in the sump, it lifts a float and flows back to the compressor crankcase, propelled by the oil pressure in the oil separator. Oil separators are most often found on large and low temperature systems. They are mandatory on ammonia systems. CONDENSER The condenser is the refrigeration circuit high side component, which allows the hot high-pressure refrigerant gas to loose its latent heat of condensation to the environment. This loss of heat causes the gas to condense into high-pressure liquid that can piped to the metering device. The heat rejected by the condenser enters the system in the evaporator and the compressor. Due to inefficiencies and other heat gains, a condenser in an open system must dispose of about 1.25 times the heat gained in the evaporator. Condensers in hermetic systems must also dispose of heat from the motor windings. Many different types of condensers are in use, depending on the function, and the means of disposing of the heat. The two basic categories “water cooled” and “air cooled” are classified by the medium used to remove heat. The basic design goal of a condenser is to remove the most heat at the lowest cost, and space requirements. Water and air are usually plentiful and economical condensing media. Water can remove large amounts of heat quickly and efficiently, which allows the condenser to be relatively small and makes water-cooled condenser more economical when suitable is available. However, water may be scarce or chemically unsuited for condenser cooling use. In addition, water-cooled condensers are subject to scale, fouling, freezing, and corrosion. Air-cooled condensers must be large than water-cooled units, but are not subject to freezing or water problems. Air-cooling is used when water is unavailable, expensive or chemically unsuitable. Fins, wires, or plates may be fastened to condenser tubing to increase the surface area and the ability to dispose of the heat of condensation. Fans or pumps are commonly used to increase the flow of the condensing medium. Such enhancements increase the sub-cooling of the refrigerant, increase the rate of heat transfer, and decrease the oval size of the condenser. AIR COOLED CONDENSER Air-cooled condensers relay on fans to move air across the tubes and fins to remove heat from the refrigerant. Shrouds are used to increase fan efficiency by directing all the airflow across the condenser tubes. Different type of fins can be used to increase the surface area of the condenser. Proper heat transfer in air-cooled condensers can be achieved only if the condenser surface is clean. The air-cooled condenser must be designed to work in the hottest ambient conditions, when the heat transfer will be slowest and the cooling load is likely to be the greatest. Outdoor air-cooled condenser operating in cold weather presents a special system design challenge. Special precautions are needed to protect an outdoor, air-cooled condenser from low ambient temperatures. The major problem is that the refrigerant will not flow through the metering device unless head pressure is sufficient, and the cold ambient temperatures reduce head pressure. For air-cooled condenser to operate in cold ambient temperatures the system may require any of the following devices or combination of:
Disclaimer - While Berg Chilling Systems Inc. ("Berg") makes reasonable efforts in providing accurate information, we make no representations or warranties regarding the accuracy of any content therein. We assume no liability or responsibility for any typographical, content or other errors or omissions. We reserve the right to modify the content of this documentation without advance notice. by Oldrich Bocek(1939-2003) Thermal Management Expert Berg Chilling Systems Inc. |
What type of device in refrigeration cycle which divides the high pressure side and the low-pressure side of a refrigerating system?
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