COMPRESSOR HANDBOOK phần 6 potx

11.14 CHAPTER ELEVEN FIGURE 11.21 Typical gas flow curve for 30:1 ratio gas booster. To find the gas flow, refer to a flow chart similar to Fig. 11.21 of the selected booster. The gas flow depends on gas supply pressure (p O ) and the air drive pressure (p A ). The flow chart above is only for 6 bar air drive. For 4 or 8 bar air drive, another flow chart would be used. Example: Outlet Pressure ϭ 140 bar and Air Drive ϭ 6 bar Gas Supply Pressure: 20 bar Gas Flow: 80 L /min N 11.2.13 Selection of a Booster Boosters are selected based on gas outlet pressure required, gas supply, and the air drive pressures available. Flow capacity should be checked from a flow chart, as the example in Fig. 11.21. If the flow capacity is not sufficient, a double acting booster will be required. If the supply pressure is not sufficient, it would be nec- essary to go to a two stage booster. Example: Air drive pressure p A ϭ 6 bar (88 psia) Working pressure p O ϭ 140 bar (2058 psia) GAS BOOSTERS 11.15 Gas supply pressure p S ϭ 15 bar (218 psia) Gas flow required F ϭ 40 L N /min (1.41 SCFM) Using technical data, a 30:1 ratio unit is the booster that fits these conditions. Refer to flow curve to verify flow capability. 11.2.14 Filling a Storage Tank (in a specific time) Medium Shop Air Air Drive Pressure (p A ) 6 bsar (88 psi) Supply Pressure (p S ) 6 bar (88 psi) Working Pressure (p O ) 80 bar (1176 psia) Tank Volume 20 Liter (.71 Cu Ft/Min) Filling Time (t) 20 min 1. Pre-selected booster according to the pressure ratio p R ϭ p O /p A ϭ 80/6 ϭ 13 2. Required volume in tank at 80 bar. V N ϭ V ϫ p O ϭ 20 Liter ϫ 80 bar ϭ 1600 L N 3. Volume in tank at supply pressure. V N1 ϭ V ϫ p S ϭ 20 Liter ϫ 6 bar ϭ 120 L N 4. Volume to be supplied by booster. V N Ϫ V N1 ϭ F Fill ϭ 1600 L Ϫ 120 L ϭ 1480 L N 11.2.15 Average Gas Flow F ␾ ϭ V/t ϭ 1480 L n /20 min ϭ 74 L N /min A booster would be selected with this capacity, based on the performance curves. 12.1 CHAPTER 12 SCROLL COMPRESSORS Robert W. Shaffer President Air Squared, Inc. Initial patents for the scroll concept date back to the early 1900’s. Unfortunately the technology to accurately make scrolls did not exist and the concept was for- gotten. In 1972, the scroll concept was re-invented. The potential and advantages of the scroll compressor over reciprocating com- pressors were immediately recognized by the refrigeration industry. Because of the tremendous pressure for better efficiency of refrigeration compressors in the early ’70s, there was a strong incentive to pursue the scroll: the balanced rotary motion reduced noise and vibration; there were no valves to break; and valve noise and valve losses were eliminated; fewer parts were needed; and rubbing velocities, along with associated frictional losses were lower. Not only did the scroll com- pressor offer improved efficiency, it also had the added benefit of greater reliability, smoother operation and lower noise. Today, scroll compressors are used extensively for residential and automotive air conditioning by many well known companies. The development of scroll type compressors for air has not been as rapid. Air is much more difficult to compress than refrigerant, especially when oil is not used for sealing and cooling. By the ’90s, machine tool technology had progressed to the point where scrolls could be accurately made and the first dry, oilless scroll compressor was introduced in January, 1992. The oilless scroll air compressors had the same inherent features as the scroll refrigeration compressor when compared to reciprocating oilless air compressors, durability, reliability, lower noise and vi- bration. Currently scroll refrigerant compressors are well established as the standard of the industry. Scroll air compressors are extending from the initial three and five horsepower models into larger and smaller sizes from one to ten horsepower. Figure 12.1 shows typical scroll air compressors ranging in size from 1/8 to 1.0 hp. Recently introduced technology is expected to make scroll air compressors prac- tical in the fractional horsepower sizes. 12.2 CHAPTER TWELVE FIGURE 12.1 Scroll air compressors from 1 / 8 to 1.0 HP. 12.1 PRINCIPAL OF OPERATION The fundamental shape of a scroll is the involute spiral. The involute is the same profile used in gear teeth. An involute is a curve traced by a point on a thread kept taut as it is unwound from another curve. The curve that the thread is unwound from, that is, used for scrolls, is a circle. The radius of the circle is the generating radius. A scroll is a free standing involute spiral which is bounded on one side by a solid flat plane, or base. A scroll set, the fundamental compressing element of a scroll compressor, vac- uum pump or air motor, is made up of two identical involutes which form right and left hand components. One scroll component is indexed or phased 180 degrees with respect to the other to allow the scrolls to mesh, as shown in Fig. 12.2. Crescent shaped gas pockets are formed bounded by the involutes and the base plates of both scrolls. As the moving or orbiting scroll is orbited about the fixed scroll, the pockets formed by the meshed scrolls follow the involute spiral toward the center and diminish in size (the motion is reversed for an expander or air motor). The orbiting scroll is prevented from rotating during this process to maintain the 180 degree phase relationship of the scrolls. The compressor or vacuum pump’s inlet is at the periphery of the scrolls. Air is drawn into the compressor as the inlet is formed as shown in Fig. 12.2. b, c, and d. The entering gas is trapped in two diametrically opposed gas pockets, Fig. SCROLL COMPRESSORS 12.3 FIGURE 12.2 Scroll compressor operation. 12.2 a, and compressed as the pockets move toward the center. The compressed gas is exhausted through the discharge port at the center of the fixed scroll. No valves are needed since the discharge is not in communication with the inlet. Figure 12.2 shows the scroll positions as the line connecting the centers of the two scrolls is rotated clockwise, illustrating how gas pockets diminish in size as the orbiting scroll is orbited. 12.2 ADVANTAGES Scroll refrigerant compressors, air compressors and vacuum pumps have the fol- lowing advantages: • Scroll compressors can achieve high pressure. The pressure ratio is increased by adding spiral wraps to the scroll. Pressures as high as 100 to 150 psig can be achieved in a single-stage air compressor. • Scroll compressors are true rotary motion and can be dynamically balanced for smooth, vibration-free, quiet operation. • There are no inlet or discharge valves to break or make noise and no associated valve losses. • Scroll compressors can be oil flooded, oil lubricated, or oil free. 12.4 CHAPTER TWELVE TABLE 12.1 Typical Performance Data of Scroll Air Compressors Nom. power (HP) Speed (RPM) Disch. press. (PSIG) Air flow (ACFM) Sound power (dBA@ 1 m) 5.0 3050 120 15.0 59 3.0 2630 120 9.0 59 1.0 3450 100 4.0 NA 0.3 1750 30 3.0 49 0.02 3000 10 0.3 39 • Due to the unique orbital motion, the rubbing velocities of the sliding seals are significantly less than piston rings or vanes for comparable speeds. Rubbing ve- locities are typically 30 to 50% less, resulting in greater durability. • Air is delivered continuously, therefore there is very little inlet or discharge pul- sation and associated noise. • The scroll compressor has no clearance volume that gets re-expanded with as- sociated losses. The compression is continuous. • Noise levels 3 to 15 dBA lower than other compressor technology are typical. Table 12.1 gives some typical performance for scroll compressors operating on air. 12.3 LIMITATIONS Although scroll compressors continue to expand into larger and smaller sizes, there are limitations. Since the scroll has a leakage path at the apex of the crescent shaped pockets, there are limits to how small a scroll compressor can be as a function of discharge pressure. Large displacement scroll compressors become large in diam- eter and the moving or orbiting scroll becomes massive. The maximum centrifugal force generated by the orbiting scroll gives a practical maximum size in a single- stage scroll. 12.4 CONSTRUCTION Minimizing leakage of compressed gases within the scrolls is the key to perform- ance in a scroll compressor. There are two primary leakage paths in a scroll compressor. There is a leakage path at the apex of the crescent shaped air pockets where the scroll involves are SCROLL COMPRESSORS 12.5 FIGURE 12.3 Section through involute showing tip seal. in closest proximity. This leakage is minimized by running the scrolls with a very small gap at these points. The size of the gap at the apex of the air pockets is a function of scroll geometry, and the scroll geometry is a function of the scroll manufacturing process. There is also a leakage path between the tip of the involute and the opposite scroll base. Since the involute is relatively long if stretched out, this path is of primary importance. This leakage path is sealed by either, running the scrolls very close together and using oil to seal the remaining gap or using a floating tip seal as shown in Fig. 12.3. The floating tip seal acts much as a piston ring in a piston type compressor and bridges the running gap between the scrolls. For oil-free compressors, the tip seals are made of self lubricating materials. Driven by a demand from the refrigeration industry, machine tool builders have improved the speed and accuracy of scroll manufacture. These new machine tools can produce finished scrolls in one to five minutes with involute accuracy of 0.0002 to 0.0005 inch and with good surface finish. Spindle speeds as high as 30,000 RPM are typical machining scrolls made of aluminum or cast iron. Most of the major machine tool manufacturers have standard scroll machining centers. 12.4.1 Lubricated Scroll Compressors Typically scroll compressors used as refrigerant compressors are oil lubricated. Lubrication greatly simplifies the compressor design. Design features include: 12.6 CHAPTER TWELVE FIGURE 12.4 Typical scroll showing idler shafts. • Cast iron or aluminum scrolls with no special coatings or surface treatment re- quired • A simple eccentric drive at the center of the orbiting scroll • A flat plate thrust bearing to support and locate the orbiting scroll axially Since refrigerant compressor are hermetically closed systems, no special oil clean up is needed at the discharge. The oil can simply circulate through the re- frigeration system and return to the compressor to seal and lubricate. SCROLL COMPRESSORS 12.7 12.4.2 Oilless Scroll Compressors Oilless or oil-free scroll compressors are typically used for air and other gases where the cost of oil clean up is a factor, or where zero oil can be tolerated in the discharge. Design features include: • Cast iron or aluminum scrolls coated to improve corrosion and wear resistance • Tip seals are required for good performance and are made of a self lubricating material. • Idler shafts supported by sealed rolling element bearings are used to support the axial thrust load, locate the orbiting scroll axially and maintain the 180 degree scroll phase relationship. See Fig. 12.4. 12.5 APPLICATIONS Scroll compressors can primarily be used in those applications where its advantages are of benefit, specifically low vibration and noise, and durability. Although scroll compressors can be cost competitive, if cost is the most important factor, alternative technology should also be considered. Some possible applications are given below. • Residential air conditioning • Automotive air conditioning • Process controls • Pneumatic controls • Laboratory • Home health care • Medical and hospital • Computer peripherals • Optical equipment Scroll compressors can be used where vane or reciprocating compressors are used. They can be dry or oil lubricated. 13.1 CHAPTER 13 STRAIGHT LOBE COMPRESSORS A.G. Patel, PE Roots Divison Divison of Dresser Industries Inc. 13.1 APPLICATIONS Straight lobe compressors are used for pneumatic conveying of materials, aerating liquids, extracting gases and vapors, providing low pressure air/gas, supercharging engines and drying materials, etc. 13.1.1 Operating Characteristics Capacity range: 5 cfm to 60,000 cfm Pressure range: 15 psi Vacuum range: 15 in. hgv for conventional compressor 27 in. hgv for externally aspirated compressor or liquid sealed 0.5 micron as a vacuum booster Higher pressure and vacuum levels could be achieved through staging. 13.2 OPERATING PRINCIPLE The more prevalent straight lobe compressors usually have rotors with two or three lobes. The operating principal for a two-lobe compressor is described below. In the two-lobe compressor, two figure eight rotors are mounted on parallel shafts within an elongated cylinder. A set of timing gears keeps the rotors in syn- chronization. In Fig. 13.1, the lower rotor is presumed to be the drive rotor. As it rotates clockwise, the inlet is created on the right hand side and the discharge is created on the left hand side. The driven rotor turns counter clockwise through the action of the timing gears (not shown). In position 1, the drive rotor is delivering volume A to the discharge, while the driven rotor is trapping the volume B between the housing and itself. In position 2, the driven rotor has sealed off volume B from the inlet and the discharge. Volume B is basically at inlet conditions. In position 3, volume B is being discharged by the driven rotor, while the drive rotor is in the process of trapping volume A. The two-lobe compressor discharges four equal [...]... compressor ϭ 6 ϫ rotational speed in rev/sec for three-lobe compressor k ϭ specific heat ratio g ϭ gravity constant ϭ 32. 16 ft/sec2 R ϭ gas constant in lb.ft/ ЊR T ϭ gas temperature in ЊRankine For positive displacement compressors, use of relief valves is very important Any piping blockage could overload the compressors beyond design pressures CHAPTER 14 THE OIL-FLOODED ROTARY SCREW COMPRESSOR Hasu... maintenance Low maintenance costs Long compressor life Full use of driver horsepower Low operating expense Low purchase price High compression ratios (Rc) up to 16 Rc per stage Operation at low suction pressure up to 26 inches of vacuum Light weight Compactness TYPES OF COMPRESSORS (see Fig 14.2) Rotary compressors may be either positive displacement or dynamic compressors The positive displacement... 14 .6 shows efficiency change with variable volume ratio 14.8 CHAPTER FOURTEEN FIGURE 14.5a Adiabatic efficiency (pressure ratio to 7) FIGURE 14.5b Adiabatic efficiency (pressure ratio to 15) THE OIL-FLOODED ROTARY SCREW COMPRESSOR FIGURE 14 .6 Efficiency improvement with variable volume ratio 14.9 CHAPTER 15 DIAPHRAGM COMPRESSORS G Reighard Howden Process Compressors, Inc 15.1 INTRODUCTION A diaphragm compressor. .. small rotary compressors are driven by electric motors, while the larger rotary compressors are usually turbine driven Reciprocating compressors may be driven by electric motors, turbines (gas or steam) or engines (gas or diesel) In some types of reciprocating compressors, the power cylinders and compression cylinders are integrated into one unit, and share the same frame and crankshaft These compressors... plant (Roots Division, Dresser Industries Inc.) FIGURE 13.8 dustries Inc.) 1 760 CFM, 250PSI acetylene product blower (Roots Division, Dresser In- STRAIGHT LOBE COMPRESSORS 13 .6. 4 13.7 Bearings Antifriction bearings are generally the bearings of choice 13 .6. 5 Seals The head plate seals are generally labyrinth or piston ring In air compressors, the area between the headplate seals and the oil seals is vented... is a positive displacement compressor, like its better known relative, the reciprocating compressor In a comparison between the two, the rotary screw gleans honors for its simplicity, low cost, easy maintenance and almost pulsation-free flow It takes a back seat to the reciprocating compressor, however, in handling high pressure (see Fig 14.1.) Benefits offered by rotary screw compressors include: • •... driver 13 .6 CONSTRUCTION (FIG 13.2) The main components of a straight lobe compressor are the rotors ➁, the casing ➀, the timing gears ➆, the bearings ➄, and the seals ➃ 13 .6. 1 Rotors The rotors (2) are nothing more than a set of two toothed gears The common profile for the rotor lobes is involute; cycloidal profile is also used sometimes The FIGURE 13.2 Cross section through straight lobe compressor. .. static pressure FIGURE 14.2 Compressor types THE OIL-FLOODED ROTARY SCREW COMPRESSOR 14.3 Reciprocating compressors consist of a piston acting within a cylinder to physically compress the gas contained within that cylinder They may be either singleacting or double-acting, and can be designed to accommodate practically any pressure or capacity For this reason, the reciprocating compressor is the most common... common type found in the gas industry Each compressor is designed to handle a specific range of volumes, pressures, and compression ratios Compared to the rotary compressor, the reciprocating compressor is more complex, and may cost more to maintain However, its higher efficiency and ability to handle greater pressures outweigh these disadvantages In the selection of a compressor unit, one of the primary considerations,... could be achieved by staging the compressors where discharge gas from the each stage is cooled before sending it to the next stage 13.7.2 Reduction of Power The straight lobe compressors do not have any internal compression The power required by a single-stage compressor is represented by a rectangle 1-2-4-3 on PV diagram as shown in Fig 13.3 The air is drawn into the compressor at a constant inlet pressure . Lubricated Scroll Compressors Typically scroll compressors used as refrigerant compressors are oil lubricated. Lubrication greatly simplifies the compressor design. Design features include: 12 .6 CHAPTER. Inc.) STRAIGHT LOBE COMPRESSORS 13.7 13 .6. 4 Bearings Antifriction bearings are generally the bearings of choice. 13 .6. 5 Seals The head plate seals are generally labyrinth or piston ring. In air compressors,. rotational speed in rev/sec for two-lobe compressor ϭ 6 ϫ rotational speed in rev/sec for three-lobe compressor k ϭ specific heat ratio g ϭ gravity constant ϭ 32. 16 ft/sec 2 R ϭ gas constant in lb.ft/ЊR T ϭ