Main Characteristics of Appliances for Transport and Compression of Air

Transcription

1 Ing. Pavel Dostál, Ph.D., Ostrava University 1 Main Characteristics of Appliances for Transport and Compression of Air Some of the main technical characteristics of appliances for the transport and compression of air are output power and input power (P), volumetric flow ( ) and output pressure (p v ), as well as revolutions (n), number of grades, number of cylinders, temperature of the transported air, dimensions and weight. The input power of appliances for the transport and compression of air can be determined from the general relation: A 1 P = (.1), t η where A work (refers to the surface of the working circulation in the p diagram, see Fig..), t time, η - total efficiency of the appliance. Transform the surface of the working circulation in the p diagram into a rectangle with the same surface and its base equals stroke volume z, its height equals the medium indicated pressure p i. Then we can express the surface of the rectangle (and through this work) as the multiplication of its sides: A = z p i (.). Substituting this relation to the equation (.1): z pi 1 P = (.3), t η and because volume in time is the volumetric flow, we can come up with another equation for the calculation of input power: = p i P (.4), η where theoretical volumetric flow (the real volumetric flow is multiplication of the theoretical volume and transport efficiency). The method of calculation of the theoretical volumetric flow depends on the type of appliance: in piston appliances π D = i S L n = i L n (.5), 4 where i number of active surfaces of first-grade pistons, S piston surface, L stroke of the piston, n revolutions of the crank-shaft, D diameter of the piston; in lamella appliances = 4 π R b l e n (.6), where R inner radius of the rotor, b l width of the lamella, e eccentricity, n rotor revolutions; in appliances with a rolling piston = π ( R r ) b pi n (.7), where R radius of the cylinder, r radius of the piston, b pi length of the piston, n rotor revolutions; in water-circle appliances

2 Ing. Pavel Dostál, Ph.D., Ostrava University = 4 π R b l e n (.8), where R inner radius of the water ring, b l width of the lamella, e eccentricity, n rotor revolutions; in rotational double-rotor appliances = π R b r λ n (.9), where R outer radius of rotors, b r length of the rotors, λ - coefficient of use of the cylinder (0.49 to 0.59), n revolutions of the rotor; in screw appliances = ( S1 z1 + S z ) b r n (.10), where S 1 (S ) surface between the teeth of the first (second) rotor, z 1 (z ) number of teeth of the first (second) rotor, b r length of the rotors, n revolutions of the rotor; in radial pneumatic-dynamic appliances = π D b c m k (.11), where D diameter of an impeller, b width of a channel of an impeller in the diameter D, c m meridian velocity in the diameter D, k coefficient of restriction of flow cross-section; in axial pneumatic-dynamic appliances π = (D D1 ) c m k 4 (.1), where D outer diameter of an impeller, D 1 diameter of the impeller hub, c m speed of meridians, k coefficient of restriction of clear opening. Exercise Calculate how many revolutions a crank-shaft of a single-acting one-cylinder piston compressor with a stroke of 100 mm and piston diameter of 110 mm must have to supply 18 m 3 of air per hour (assume volumetric efficiency of 0.91). Given: i = 1 L = 100 mm = 0.1 m D = 110 mm = 0.11 m = 18 m 3 h -1-1 = m 3 s η = 0.91 n =? The actual volumetric flow of the piston compressor is given by the equation (.5) and volumetric efficiency, thus: π D = i L n η 4 The unknown revolutions will be determined by modification of this relation: 4 n = i π D L η m s n = 1 π ( 0.1m) 0.11m n = 6.4 s The revolutions of the crank-shaft of the given appliance must be 6.4 s -1.

3 Ing. Pavel Dostál, Ph.D., Ostrava University 3 1 Regulation of the appliance for the transport and compression of air The flow of appliances for the transport and compression of air is controlled: by change of revolutions e.g. by means of a gearbox (it is simple, economical, clean); by restriction in displacement e.g. throttle valve embedded in a discharge subcircuit is used (less economical but technically simple); by restriction in suction similar to the previous case, but the restriction element is incorporated in a suction sub-circuit; by depressing the suction valve (in piston appliances with valve mains) after exceeding a given pressure the depressing mechanism opens the suction valve until the pressure drops; valve control - (in piston appliances with valve mains) after finishing the suction stroke, the suction valve is held open during part of the displacement stroke and the air suctioned into the cylinder is discharged back to the suction pipeline (it is similarly possible to hold the discharge valve during the suction stroke, so the compressed air flows back into the cylinder); by change of detrimental volume - (in piston appliances) solved by connecting sufficient detrimental space (economical, more construction difficult); by by-passing into suction the air is by-passed from displacement into suction (by-pass conduit); by discharge into the atmosphere (in turbo-compressors and turbo-blowers) if air is the working medium (simple, not economical); by adjusting the blades (in blade appliances) construction can be solved by adjustment of the guiding, swing or transfer blades (economical, difficult in construction).? 1. What is the difference between compressors, turbo-compressors, blowers and turbo-blowers?. What is an air pump? 3. Complete: entilators are rotational blade ces for port and.ssion of. with working overpressure up to Pa. 4. Which does not belong among pneumatic static appliances?: a) ventilator b) water-circle compressor c) Roots blower 5. Draw a scheme of a lamella compressor and explain the principle of its operation. 6. Draw a scheme of the shape of impellers of radial and axial ventilators and direction of flow of the transported air.

4 Ing. Pavel Dostál, Ph.D., Ostrava University 4 7. What methods of cooling turbo-blowers and turbo-compressors do you know? 8. Explain the principle of operation of a piston compressor and draw a working diagram of one. 9. What methods of flow regulation of turbo-compressors do you know? 10. Find (in a workplace, at home, etc.) an appliance for the transport and compression of air and ascertain its construction workmanship, type, technical parameters and manufacturer. What other type of appliance could be used for the given purpose?

5 Ing. Pavel Dostál, Ph.D., Ostrava University 5 1. Compressors and blowers are pneumatic-static appliances, the value of the head pressure is greater than 0.3 MPa, in blowers between 0.1 to 0.3 MPa. The turbo-compressors and turbo-blowers are pneumatic-dynamic appliances with a working overpressure greater than 10 4 Pa, whereas turbo-blowers are radial appliances without inter-coolers and turbo-compressors are radial appliances with external inter-coolers and all axial appliances.. An air pump is an appliance instrumental to the creation of depression in enclosed spaces. It suctions the air under a lower pressure than the atmospheric pressure and expels it under a pressure slightly higher than (or equal to) the atmospheric pressure. 3. entilators are rotational blade appliances for the transport and depression of air with a working overpressure up to 10 4 Pa. 4. a) 5. See Fig..3 + lamella compressors have an eccentrically embedded rotor in the grooves of which lamellas are embedded that are forced down by centrifugal power to the internal part of cylindrical stator. During rotation the area between the lamellas, stator and rotor gradually increases and decreases, thus suction and compression of air is performed. 6. See Fig Inner cooling, which is performed so that there is a system of channels in the stator case through which cooling water flows. External cooling, when inter-grade coolers (inter-coolers) are incorporated outside the cooler. Cooling by injection of suitable fluid into the compressed air. 8. See Fig.. + When the piston moves from the upper dead point into the lower, the suction valve opens and suction is performed. If the piston is in the lower dead point, the suction valve is closed. During the return movement of the piston the air enclosed in the cylinder is adiabatically compressed at first, when the discharge valve is then opened, discharge is performed. In the upper dead point the discharge valve opens. The piston does not compress all the air from the cylinder, a certain (detrimental) volume remains in the detrimental space at the head of the cylinder. When the piston starts moving from the upper dead point, at first adiabatic expansion of the air is performed from the detrimental space. Afterwards the suction valve opens, suction is performed and the whole cycle is repeated. 9. In turbo-compressor regulation by change of revolutions, restriction in displacement, restriction in suction, by-passing into suction, discharge into atmosphere or by adjusting the blades is used. SKOPAL,., ADÁMEK, J., HOFÍREK, M Stavba a provoz strojů I.. publication. Prague : SNTL, 1987, pp and 16 5 and and

6 Ing. Pavel Dostál, Ph.D., Ostrava University 6 KYSELA, F. Stroje.1. publication. Prague : SNTL, 198, page , and 16.