Compressors
The main difference between pumps and compressors is that the fluid delivered by compressors -- air -- is compressed and under pressure at the time it is delivered, even if there is no load on the system. Most devices used to compress air are very similar in concept and -- perhaps even in hardware -- to hydraulic pumps, and selection considerations are similar.
The only other substantive difference is that most hydraulic systems are powered by a single pump that is actually a part of the system, whereas a host of pneumatic systems are often powered by a single compressor, which is almost a "utility" in the plant like water or electric service. Nevertheless, many small compressors are available for specific, discrete jobs; typically they are positive-displacement compressors. Dynamic, or nonpositive-displacement compressors are typically larger, facility-type units.
Compressors are fairly simple devices, capable of long periods of maintenance-free operation if properly integrated into pneumatic systems. Yet time and again they suffer from early failures because obvious precautions were ignored during system design. Four basic rules can provide substantial improvement in compressor life with only moderate design effort:
? Pumps and compressors should be sized to provide at least the required pressure and flow, and preferably 10 to 25% more.
? Filters should be selected to protect the pumping unit, and sometimes to protect downstream components or products as well.
? Relief valves should be selected to keep pressure or vacuum at appropriate levels.
? Pumping units should be placed in a clean, cool, dry environment.
Bellows compressors consist of a welded metal bellows connected to inlet and outlet ports with check valves. These compressors typically cover the pressure range up to 10 psig, and are used in pollution detecting and measuring devices, gas-sampling instruments, and medical applications. Lubrication is not needed, allowing high purities to be maintained.
Vane compressors are simple machines with few moving parts. Like their hydraulic counterparts, vane pumps, the compressors are inexpensive, with low operating cost, and low starting-torque requirement. They are compact and relatively vibration free, with little pulsation in the compressor output. The sliding vanes are closely fitted in the rotor slots and wear very little during operation. These compressors are available in power ranges from 10 to 500 hp, at pressures to 150 psi.
Reciprocating compressors consist of a piston moving within the cylinder to trap and compress the gas. In principle, such a unit is like an automobile engine, with the pistons compressing the gas and valves controlling its inlet and outflows. Sizes range from less than 1 to over 5,000 hp. Reciprocating compressors have good part load efficiencies and are useful for wide variations in operating conditions.
Diaphragm compressors are a modification of the reciprocating compressor. Compression is performed by the flexing of a metal or fabricated diaphragm which is caused by the motion of a reciprocating piston in a cylinder under the diaphragm. The space between the diaphragm and the piston is usually filled with liquid.
Lobed-rotor compressors have two rotating elements that revolve in opposite directions in a chamber. In most compressors, the rotors do not actually touch and do not drive each other, being driven instead by timing gears. Because the rotors do not actually touch, air leaks between them at a small but constant rate. This leakage, called "slip," is constant for a given compressor at a given pressure. For highest efficiency, these compressors should be operated at maximum speed. They are available in power ranges from 7 to 3,000 hp, delivering pressures to 250 psi. Because the internal lobes do not contact, they need no lubrication.
Liquid piston compressors have no moving parts in wearing contact. A rotor with multiple forward-curved blades rotates in an elliptical casing. Fluid, trapped within the casing, is carried around the inner periphery by the blades. Space between the blades changes volume due to the elliptical fluid path, and the inner surface of the liquid ring trapped between the blades serves as the face of a liquid piston. These compressors accept liquid slugs and fine particles without serious damage. Lubrication is required only in bearings located outside the pump housing. These compressors deliver up to 150 psi throughout the range of 10 to 500 hp.
Centrifugal compressors are best suited to moving large volumes of air at relatively low pressures. Basically, they consist of a high-speed rotating impeller, a diffuser section where velocity is reduced and pressure increased, and a collector section that further reduces velocity and increases pressure. Centrifugal compressors can handle high flow demands well, but when demand decreases much below rated flow and output pressure rises, the compressors can surge. In surge, the pressure field at the compressor outlet varies randomly. If allowed to continue, this condition can damage bearings, blades, and even the housing itself. Centrifugal compressors typically use from two to six stages, supplying from 400 to 3,000 cfm at speeds to 20,000 rpm.
Regenerative blowers (also known as peripheral blowers) use a disclike impeller with blades mounted around its outside edge. As the impeller revolves, air is drawn into the space between the blades. Centrifugal force moves the air in a spiral path outward to the housing, where it slips by the initial blade and returns to the base of the succeeding blade, where the process is repeated. In some models, a flow splitter creates two flow paths, so that the air must make two circuits around the impeller. In other models, the splitter is omitted, and the air makes only one circuit before exiting. Regenerative blowers provide air flows up to 1,000 cfm and pressures to 8 psi.
Helical compressors look like two giant screws meshing together; they work much like hydraulic screw pumps. Maximum pressure from these machines is approximately 125 psi in single-stage configurations. Helical compressors may be either oil flooded or dry.
Dry helical compressors, like lobed units, require timing gears to maintain proper clearance between the rotating elements. These units are most efficiently operated at high continuous speeds.
Flooded compressors do not require any timing gears, because the oil-laden screw surfaces can drive each other. However, oil separators are needed to remove the oil from the air as it leaves the compressor. They are available over a power range of about 7 to 300 hp.
Single-screw compressors are based on the same principle as helical compressors. As the central screw rotates, air trapped between the screw teeth is compressed against the star-shaped rotors. These compressors tend to have low vibration and noise levels, and low discharge pressures. Lubrication is required.
Pumps
vacuum pumps In principle, industrial vacuum pumps are merely compressors run with the inlet attached to the vacuum system and the outlet open to exhaust. In smaller sizes, compressors and vacuum pumps are often identical machines. However, in the large sizes that might power a plant-wide vacuum system, the machines differ in minor ways that are intended to enhance efficiency for one application or the other. Manufacturers strongly advise that the same machine not be used for both vacuum and compression at the same time. The heavy loads will damage it.
Three criteria control pump selection: degree of vacuum produced, rate of air removal, and power requirement. However, applications such as filtration may subject the unit to the ingestion of foreign material.
The first pump performance criterion is the vacuum it produces. Manufacturers provide a maximum vacuum rating expressed as absolute pressure in mm Hg, or vacuum in in. Hg. Larger units are usually rated only for continuous duty, but smaller units may have a higher vacuum rating for intermittent duty. In smaller units, temperature-rise considerations limit the vacuum that can be produced.
Continuous and intermittent vacuum ratings are determined for standard atmospheric pressure: 29.92-in. Hg. Lower ambient pressures reduce the vacuum that can be produced. The rating is determined from:
where Va = adjusted vacuum rating, in. Hg; Vo = original vacuum rating at standard conditions, in. Hg; and Pa = anticipated atmospheric pressure at the application site, in.Hg.
Rate of air removal is the second criterion. Vacuum pumps are flow rated according to the volume of air exhausted with no pressure differential across the pump. Manufacturers provide curves showing free air delivery at rated speed for vacuum levels ranging from 0-in. Hg (so-called "open capacity") to maximum vacuum rating. Some manufacturers also provide curves of capacity at different speeds for a given vacuum.
The last pump criterion is power requirement. Compared with air compressors, vacuum pumps require relatively little power. At low flows, vacuum (or pressure differential) is high; at high flows, vacuum is low. Therefore, power, which is proportional to flow and pressure differential, is generally low.
Power output of the pump can be found from pressure-flow curves provided by manufacturers. Input power and speed requirements are also shown in the data. Overall pump efficiency (including both volumetric and mechanical efficiency) can be evaluated by combining this data. This is done by dividing the free-air capacity of the pump at the required vacuum level by drive power required at that condition. The result is proportional to the product of gage vacuum and air-flow rate and is representative of efficiency.
All three performance criteria -- vacuum, flow and power -- can be affected by pump temperature. At higher vacuum levels, little air flows through the pump, so little heat is transferred to the air. Much of the heat generated by friction must be dissipated by the pump. This heat gradually raises pump temperature and can drastically reduce service life. Temperature excursions are especially important to intermittent-duty pump, which can overheat if on time greatly exceeds off time.
Vacuum pumps are classified as either positive or nonpositive displacement. A positive-displacement pump creates vacuum by isolating and compressing a distinct, constant volume of air. The compressed air is vented out one port, and a vacuum is created at the other port where the air is drawn in. This generates relatively high vacuum, but little flow.
A nonpositive-displacement pump, on the other hand, uses rotating impeller blades to accelerate air and create a vacuum at the inlet port. While nonpositive-displacement pumps cannot produce high levels of vacuum, they provide high flow rates.
Principal types of positive-displacement vacuum pumps include piston, diaphragm, rocking-piston, rotary-vane, lobed-rotor, rotary-screw, and liquid-ring designs.
Reciprocating-piston pumps generate relatively high vacuums -- from 27 to more than 29 in. Hg -- under a variety of operating conditions. Typical pumps of this type have one or more pistons linked to a rotating crankshaft. The alternating piston action moves air past check valves in the cylinder head to create a vacuum at the inlet port. Lubricated piston pumps are quieter, produce less vibration, have a higher capacity, and feature a much longer life than oilless designs, but they are also heavier and more expensive.
Diaphragm pumps offer the advantage of the fluid chamber being totally sealed from the pumping mechanisms. An eccentric connecting rod mechanically flexes a diaphragm inside the closed chamber to create a vacuum. This results in somewhat lower vacuum compared to that produced by a reciprocating piston. However, the diaphragm's lower compression ratio -- low flow, large diameter, and short stroke -- makes for quiet, economical, and reliable operation. The design is available in both one and two-stage versions. Single-stage pumps provide vacuum up to 25.5 in. Hg, while two-stage units are rated to 29 in. Hg.
Rocking-piston pumps combine the compact size and quiet, oilless operation of the diaphragm pump with the high-vacuum capabilities of the reciprocating-piston pump. Here, a piston is rigidly mounted (no wrist pin) on top of the diaphragm unit's eccentric connecting rod. An elastomeric cup skirts the piston and functions both as a seal -- equivalent to the rings on a piston compressor -- and as a guide member for the rod. The cup expands as the piston travels upward, thus maintaining contact with the cylinder walls and compensating for the rocking motion. The absence of a wrist pin is the key to the pump's light weight and compact size.
Single-stage rocking-piston pumps produce vacuum to 27.5 in. Hg; two-stage designs can generate 29 in. Hg or more of vacuum. Rocking-piston pumps are also relatively quiet, operating at sound levels as low as 50 dBA. A drawback to rocking-piston pumps is that they cannot generate a lot of airflow. Even the largest twin-cylinder models have flow rates of less than 10 cfm.
Rotary-vane pumps use a series of sliding, flat vanes rotating in a cylindrical case to generate vacuum. As an eccentrically mounted rotor turns, the vanes slide in and out, trapping a quantity of air and moving it from the inlet side of the pump to the outlet.
Rotary-vane pumps usually have lower vacuum ratings than piston pumps, in the 20 to 28 in. Hg range. However, there are a few exceptions. Some two-stage, oil-lubricated designs have vacuum capabilities up to 29.5 in. Hg. Pumps with recirculating oil systems reach still higher vacuums, in the less than 1-torr range. The pumps offer a number of advantages, including high flow capacities, low starting and running torque requirements, vibration-free operation, and continuous airflow. No valves restrict flow or require maintenance in the rotary design. The compact units are also quiet, generating as little as 45 dBA or sound.
Depending on the application and vacuum level required, an economical alternative to using a high-vacuum pump is two standard, staged rotary-vane pumps. Or, a high-volume, low-duty pump rated for continuous duty of 20 in. Hg sometimes can be operated at restricted airflow or "blanked-off" conditions for short periods of time to provide higher vacuums. As with other types of pumps available in both lubricated and oilless configurations, lubricated rotary-vane pumps are capable of slightly higher vacuum compared to oilless designs.
Liquid-ring pumps feature a multiblade impeller, mounted eccentrically in a cylindrical case that is partly filled with water. As the impeller rotates, liquid is thrown outward by centrifugal force to form a liquid ring concentric with the periphery of the casing. Due to the eccentric position of the impeller, the air space in the impeller cell expands during the first 180° of rotation, creating a vacuum. During the next 180° of rotation, the air space is reduced, discharging compressed air and water. In addition to being the compression medium, the liquid ring absorbs the heat of compression as well as any powder or liquid slugs entrained in the air.
Rotary-screw and lobed-rotor vacuum pumps are two other types of positive displacement pumps. Neither lubricated design is as widely used as rotary-vane and piston pumps, especially in smaller sizes. Due to the size of the gears and rotors, both designs lend themselves to larger installations.
A rotary-screw pump's vacuum capabilities are similar to those of piston pumps, with the added advantage of being nearly pulse-free. Two meshing rotors with helical contours trap air as the screws turn in opposite directions. This action creates chambers of decreasing volume behind and increasing volume in front of the rotor chambers.
Lobed-rotor pumps bridge the gap between positive and nonpositive-displacement units. The pumps have a pair of mating lobed impellers that rotate in opposite directions, trapping air and withdrawing it from the system.
High-speed, multistaged centrifugal blowers and regenerative blowers are the major types of nonpositive-displacement pumps, generally operating at high speeds and attaining moderate vacuum levels.
Centrifugal blowers, for example, are an excellent choice where only intermittent use is required. To keep costs down, a short-life brush-type ac or dc motor powers these blowers, which are widely used in vacuum cleaners.
Regenerative blowers have many advantages because individual air molecules pass through many compression cycles with each revolution compared to the single compression per stage for multistaged centrifugal types. At first glance, regenerative blowers are similar to rotary-vane pumps, but have a special blade and housing configuration.
As the impeller rotates, centrifugal force moves the air molecules from the blade root to its tip. Leaving the blade tip, the air flows around the housing contour and back down to the root of a succeeding blade, where the flow pattern is repeated. This action provides a quasi-staging effect to increase pressure differential capability. The speed of the rotating impeller determines the degree of pressure change.
The end result is not a particularly high vacuum -- approximately 100-in. H2O in single-stage models. But flow capacity is very high, up to several hundred cfm. Multistage versions produce higher vacuum levels, but at lower flow rates.
壓縮機
泵和壓縮機主要區(qū)別是：流體被壓縮機傳送—氣體—在它被傳送的同時被壓縮并處于壓力之下，即使系統(tǒng)在沒有載荷的情況下。大多數(shù)用于空氣壓縮的裝置原理上非常相似--甚至在零部件上-如液壓泵，考慮和選擇是相同的。
它們本質上唯一的不同是大多數(shù)的液壓系統(tǒng)由單一泵供給能量，并且泵實際上是系統(tǒng)的一部分，而大多數(shù)的氣動動力系統(tǒng)往往由單一壓縮機供給能量，像廠區(qū)內的水、電力服務一樣它幾乎是廠區(qū)“公共設施”。不過有許多小型壓縮機用于特殊、不連續(xù)的工作場合。他們通常是可移動型壓縮機。動力型的或非移動型壓縮機通常是大型的單元設備。
壓縮機裝置比較簡單，如果納入適當?shù)臍鈩酉到y(tǒng)則使壓縮機能長期維持運轉，由于在系統(tǒng)設計中忽視了明顯的注意事項導致了壓縮機一次又一次的前期故障。只要適度的努力遵循四項基本規(guī)則可大大提高壓縮機設計壽命：
· 泵和壓縮機應該以最低壓力和流量來分級，最好在10至25%以上；
· 應選擇過濾器來保護泵單元，并且有時也保護下游產品和部件；
· 應選擇安全閥來保持壓力或真空度在適當?shù)乃剑?
· 泵單元應安放在一個干凈、通風、干燥的環(huán)境。
波紋管式壓縮機：
包括金屬焊接波紋管，由截止閥連接在進、出端口。這種壓縮機主要用于各種壓力高達10磅/平方英寸左右，用于污染檢測和測量儀器、氣體取樣工具、醫(yī)療設備。不需要注潤滑油，因而使其保持在較高的純凈度。
葉輪式壓縮機：
由一些簡單的轉動零件構成。相對于它們水力的相似物-葉輪泵-來說，壓縮機價格便宜,操作成本低和低的開車需求。它們結構緊湊，并且在壓縮機輸出時相關的自由顫動、壓力波動小?；瑒拥娜~輪在轉子溝槽中緊密配合并且在操作時很少磨損。這些壓縮機可用在動力范圍在10至500惠普，壓力為150磅/平方英寸。
往復式壓縮機：
由一個活塞在汽缸內移動而吸入并壓縮氣體。原則上，像汽車發(fā)動機一樣的單元由活塞壓縮氣體并且有控制閥控制氣體的吸入與流出。尺寸范圍規(guī)模小于1個調和級數(shù)到5000惠普。往復式壓縮機具有良好的部分負載效率可廣泛用于各種操作條件。
隔膜式壓縮機：
是一種正在改進的往復式壓縮機。壓縮是由一片有彈性的金屬或預制的隔膜它的運動由在隔膜子下的氣缸中的往復運動的活塞所引起的。隔膜與活塞之間的空間通常充滿液體。
葉片-轉子式壓縮機：
有兩個旋轉元件它們在工作腔中以相反地方向進行旋轉。在大多數(shù)壓縮機中轉子實際上并不相互接觸而且并不相互驅動,而是由驅動齒輪來間接傳動。因為沒有實際相互接觸它們之間的空氣泄漏率很小而且保持恒定。這個泄漏量，被稱為“錯漏”因為某一特定壓縮機有一個特定的壓力。為了保持高效率，壓縮機應該以最高轉速運轉。他們可利用在動力范圍為7至3000惠普，傳送壓力達到250磅/平方英寸。由于內部葉片沒有相互接觸，它們不需要注潤滑油。
流體活塞壓縮機：
沒有移動部件處于磨損接觸。一個帶多片向前彎曲葉片的轉子在橢圓形外殼中旋轉。流體被截留在殼體隨著葉片的旋轉被帶到內部殼體的邊緣。葉片之間的空間隨著橢圓形通道的改變而改變，并且被截留在葉片之間的液環(huán)的內表面被看作流體活塞的表面。這些壓縮機可以承受液滴和好的微粒而不產生嚴重的破壞。潤滑油只須注在泵機架外的軸承上。這種壓縮機可傳送壓力高達150磅/平方英寸動力范圍為10至500惠普。
離心式壓縮機：
最適合運送大容量壓力較低的空氣。 基本上，他們包括高速旋轉葉輪,一個擴散節(jié)，在擴散節(jié)中流動速度降低但壓力增高,還有一個收集部件，在其內部進一步降低速度，增高壓力。離心式壓縮機可處理高流量需求的井道，但是當需求量大大低于額定流量和壓力升高，壓縮機將產生喘振。如果產生喘振壓縮機出口的壓力范圍將產生隨意的改變。如果任其繼續(xù)下去，這種情況會破壞軸承、葉片，甚至自己的機架。離心壓縮機通常使用二至六級，可供應量為400至3000立方英尺/秒，轉速可達20,000轉/分。
再生式鼓風機（又名環(huán)形滑道式鼓風機）：
采用了邊緣外安裝了葉片的推動葉輪。由于這個旋轉推動葉輪，空氣被吸入葉片之間的空間。離心力使空氣呈螺旋形路線離開葉輪室，空氣由初始的葉片泄漏并返回低一級的葉片，在這一級這個過程被重復。有些型號一個流程被分離成兩個流程途徑,因此使空氣沿推動葉輪要做兩次循環(huán)。其它型號，分離被省略了，因此空氣在離開時只做一次循環(huán)，再生式鼓風機提供空氣流速多達1000立方英尺/秒和壓力達8磅/平方英寸。
螺旋式壓縮機：
像兩根巨大的螺桿嚙合在一起；它們的操作很像液壓螺桿泵。 這些壓縮機的單級結構最大壓力大約為125磅/平方英寸。螺旋式壓縮機可以是有油潤滑也可以是無有潤滑。
無油潤滑螺旋式壓縮機：
像葉片式單元一樣，需要傳動齒輪使旋轉元件保持適當?shù)那鍧?。這些單元在高速連續(xù)不斷的操作中的效率非常高。
有油潤滑式壓縮機：
不需要任何調整齒輪，因為螺桿表面的石油可以帶動對方相互驅動。不過當石油離開壓縮機時，需要用石油分離器從空氣中分離出石油。他們有較廣泛使用的動力范圍約為7至300惠普。
單螺桿式壓縮機：
與螺旋式壓縮機基于同樣的原則。由于主螺桿旋轉截留在螺桿旋齒中的空氣沿著與星型回轉體相反的方向壓縮。這些壓縮機趨于低振動、低噪音、低排放壓力.需要注潤滑油。
泵
真空泵 原則上，工業(yè)真空泵與壓縮機類似，只是它的入口連接在真空系統(tǒng)而出口直接排入大氣，在小型號中，壓縮機與真空泵往往是相似的機器。不過，在大型號中它們?yōu)槌商椎恼婵障到y(tǒng)提供能量、在中型真空泵方面有所不同它一方面為了提高效率或有其它用途。廠商強烈呼吁在同一機器不能在同一時間同時進行抽真空和壓縮。沉重的負載將會破壞真空泵。
三個標準來控制泵的選擇:抽真空程度，