DEVICE IN AN OIL-FREE ROTARY GAS COMPRESSOR

Abstract: Arrangement in an oil-free rotary gas compressor (2) having a high, built-in pressure ratio and provided with means (13) for injecting a vaporizable liquid, preferably water, into the compressor for cooling the gas during the compression process. It is known in high-speed compressors to utilize water injection which is so restricted that the water is completely vaporized, thereby to obtain a good cooling effect. The efficiency of the compressor is limited, however. High compressor efficiencies are obtained by injecting large quantities of water, although the compressor speed must then be considerably reduced, resulting in a lower compressor capacity. A correspondingly high efficiency can be obtained, however, in a high-speed compressor when, in accordance with the invention, the water is injected into the compressor in a weight quantity which in relation to the weight quantity of gas supplied is greater, although not more than about four times greater than that required for complete vaporization of the water during the compression process.

Description: Device in an oil-free rotary gas compressor The present invention relates to an arrangement in an oil- free rotary gas compressor which has a high, built-in pressure ratio and which is provided with means for injec¬ ting liquid thereinto, preferably water, for the purpose of cooling the gas under compression. By high, built-in pressure ratio is meant here and in the following a ratio which is greater than about 4:1. Oil-free gas compressors are commonly used to compress air from atmospheric pressure to pressures in the region of from 8 to 12 bars. In known compressors of this kind, considerable quantities of water are injected, in order to restrict the terminal temperature of a compression stage to about 50 C, at an incoming air temperature of about 20 C. The rise in temperature is corresponded by a mass ratio, water/air, of 10:1 or thereabove, although it is known to limit this ratio to 1.4:1. The amount of water injected into the compressor per unit of time would, if it were to be consumed, constitute a substantial part of the operating costs. Consequently, the water is remo¬ ved and re-cycled subsequent to being cooled, and optio¬ nally also reconditioned. The water-removal system, which also incorporates a quantity of buffer water and the conditioning system, which protects against, inter alia, the formation of bacteria, lime deposits and acidi¬ fication,is highly space consuming and should be con¬ structed from a corrosion resistive material. The system, when connected to a water injection compressor, is there- fore expensive. Water injection also necessitates a marked reduction in compressor speed, with a subsequent reduction in capacity. In the case of a corresponding dry single-stage compres- sor, outlet temperatures in the order of 350-400°C are reached, resulting in large temperature gradients in the various compressor components, and therewith excessive play therebetween and poor efficiency. In order to over¬ come this, it is necessary to compress the gas in two or more stages and to cool the gas between consecutive stages, This solution, however, results in a compressor of large dimensions, particularly when the cooling arrangements are included in the dimensions of the apparatus as a whole. The advantages and disadvantages encountered with liquid- injection compressors and dry compressors have been detai¬ led in "Mechanical Engineers' Handbook" (1951), McGraw- Hill Book Company, Inc. On page 1879 of this publication there is also proposed a solution which is intended as a compromise between the small dimensions and high speeds of the dry compressor on the one hand and the beneficial cooling effect of the liquid injection compressor on the other. This compromise solution comprises injecting a restricted quantity of water into the inlet of a high ' speed single-stage compressor, so that all the water is vaporized by the heat generated during the compression process. It has been found that this will enable the out¬ let temperature to be restricted to 100-150 C, while reducing temperature gradients and play and improving efficiency to a corresponding degree in comparison with a dry single-stage compressor. The efficiency is lower, however, than that of the initially mentioned water injection low speed compressor. Thus, according to the aforesaid handbook, an ineffective area is found with regard to the quantity of water in¬ jected per unit of time, namely between the limited liquid injection and the injection of considerable quantities per unit of time. This conclusion has pre¬ vailed for approximately 25 years. The object of the present invention is to provide an improvement in oil-free rotary-gas compressors with liquid injection in relation to the total capacity require¬ ment of the compressor. In confliction with the practice documented in the aforesaid handbook, this object has been achieved in accordance with the invention by constructing the liquid injection arrangement in a manner which will enable the liquid to be injected in a weight quantity relative to the weight quantity of the gas supplied which is grea¬ ter, although not more than four times greater, than that required to achieve complete vaporization of the liquid during the compression process. The result of this improvement is that in the final stage of the compression process,water which has not vaporized will lie on the surfaces of the compression chamber, these surfaces being colder than the surroundings, and there seal leakage through the play between the actual rotors themselves and between the compressor housing and the rotors, while the amount of water in the compressor outlet is so small as to produce but small dynamic losses, this water being removed with the aid of a simple condensation separator and either discharged to sewage or recycled through a simple recycling system. If such a system is required, it need only be of simple and in¬ expensive construction,due to the small amount of water concerned and also to the fact that there is less need to clean the system than in the case of conventional water injection systems, while service requirements are, naturally, considerably less. Further characteristics of the arrangement according to the invention are set forth in the following claims. The invention will now be described in more detail with reference to the. accompanying drawing, in which Figure 1 illustrates schematically an embodiment of an arrangement according to the invention; Figure 2 illustrates a simplified construction of the same arrangement; and Figure 3 is a curve illustrating the efficiency achie¬ ved as a function of the mass ratio between the quantity of liquid injected and the quantity of gas supplied. The arrangement illustrated schematically in Figure 1 comprises a screw compressor 2 which is driven by an electric motor 1 and which has connected thereto an in¬ let pipe 3 and an outlet pipe 4. The outlet pipe 4 in- corporates a cooling arrangement 5 and a condensation separator 6. A conduit 7 conducts condensation which has collected in the separator 6 to a buffer container 8, which is provided with an arrangement 11 for main¬ taining a constant level of water in the container 8, said arrangement being connected to a water delivery pipe 9 and a discharge pipe 10. A pipe 12 extends from the bottom of the container 8 to an injection device 13 loca¬ ted in the inlet pipe 3 of the compressor 2. The pipe 12 has a metering pump 14 incorporated therein. When required, a simple arrangement 15 for conditioning the water flowing through the pipe 12 may be connected " to said pipe, primarily for neutralising any acid which forms in the circulating water.- Surplus, non-vaporized water does not contribute to the cooling of the gas to any appreciable extent. Neither does it decrease the amount of water vaporized in any decisive manner. The cooling effect is therefore sub¬ stantially unchanged and is determined by the amount of water that has vaporized. The surplus water has the f nction of seating on the rotor surfaces, which ' are colder than the surroundings, and seal the gaps caused by play between the actual rotors themselves and between said rotors and the rotor housing, therewith to increase efficiency with increasing water supply within the given mass ratio. Regulation of the pump 14 is thus not a critical cooling parameter. When the pressure ratio of the compressor and the temperature and moisture content of the incoming gas are known values, the pump can be controlled in dependence on the mass flow in the inlet pipe 3. Alternatively, the temperature of the gas in the compressor outlet pipe 4 can be detected for the same purpose, or the amount of condensation per unit of time obtained from the conden¬ sation separator 6. This latter control principle provides extremely accurate results, irrespective of variations in the moisture content of the incoming gas. In normal operating conditions, the pressure in the com¬ pressor inlet pipe 3 is about 100 kPa, while the pressure in the compressor outlet pipe is about 800 kPa. Finely divided water is injected from the pipe 12 into the inlet pipe 3 in a quantity per unit of time dependent on the magnitude of the incoming flow. Part of the water injected into the compressor is vapori- zed during compression of the gas in the compressor 2 and the subsequent increase in temperature, until the gas has become saturated with water vapor. The water which remains, this water reaching at a maximum to about four times the amount of water vaporized, including that which accompa- nies the incoming gas, passes through the compressor in a liquid state and seals therewith the gaps formed by the play between the actual rotors themselves and between the rotors and the rotor housing. The water vapor condenses in the outlet pipe during its passage through the cooler 5, and the condensation is collected in the separator β, from where it runs down into the buffer container 8. Initially, the container 8 is filled with water from the pipe 9 by means of the arrange- ment 11 until a desired water level is reached, which is then held constant in a known manner, by supplying water from the pipe 9 and tapping off water through the outlet 10. When the amount of water injected into the compressor is restricted to a value lying in the vicinity of the lower limit value, the amount of water consumed is so small that the costs entailed in recovering and recycling the water of condensation from the condensation separator 6 becomes greater than the costs entailed in drawing a corresponding amount of water from the water mains. Figure 2 illustrates a modified version of the arrangement illu- strated in Figure 1. In the modified version of the arrangement the water is injected into the compressor via valve 31 from the water mains pipe 32, and the water of condensation is conducted from the separator 6 to the discharge pipe 10. Figure 3 illustrates efficiency curves relating respec¬ tively to a conventional, liquid flooded compressor driven at low peripheral speed, curve a, and to a dry compressor driven at high peripheral speeds, curve b. Both curves show the efficiency η as a function of the mass ratio between the amount of liquid injected and the amount of gas supplied. In the case of a compressor according to curve a, the level of efficiency is greatly dependent on the temperature of the water injected into the compressor (This may be due to a high increase in the partial volume of the water when injected into the compressor.) It will be seen from curve a that a high efficiency is obtained when injecting a large quantity of liquid per unit of time, namely about 1.5:1 and thereabove. In the case of a compressor according to curve b the level of efficiency is independent of the temperature of the water injected, within certain limits. When water is injected into a dry compressor, the effi¬ ciency of the compressor will be low both in respect of a mass ratio which is so low that the liquid is vaporized with improved cooling as a result, as previously mentio¬ ned, and in respect of liquid flooding in a conventional manner, which latter is only to be expected since the peripheral speed of the rotors has been adapted for dry operation. What has not previously been observed is that the intermediate part of the curve b, during which no improved cooling is obtained, presents a peak value which is comparable with the efficiency of the conventio- nal liquid-flooded compressor, wherewith it should also be noted that the compressor represented by the efficien¬ cy curve b has a far greater capacity due to the fact that it operates at a peripheral speed which is from 2 to 5 times greater. As a guideline for the mass ratio of liquid to gas can be mentioned 1 :20 with regard to complete vaporization of the water, a pressure ratio of 8:1, dry air at room tem¬ perature and adiabatic compression work. The quantity of water injected, which results in increased efficiency, can then be brought to the mass ratio of 1:4, between which values the arrangement according to the invention operates. Although a relatively high efficiency is obtai¬ ned at greater mass ratios, the increase is obtained at the cost of the expense of apparatus for recycling and reconditioning the circulating liquid, which renders a greater mass ratio less attractive. In order to reduce the axial propagation of heat along the surfaces of metal rotors, the rotors are preferably covered with a heat insulating layer, for example by oxidizing the surfaces or by coating the surfaces of the rotors with a layer of polymeric material. The surface layer is also preferably made as hydrophilic as possible, in order that the water lies on the surfaces of the rotors to the greatest extent possible, so as to improve the sealing function of the water. The water need not be injected into the compressor in the vicinity of its in¬ let, but may alternatively, or in addition, be injected through holes formed in the compressor housing in a manner known per se.

Claims: CLAIMS 1. An arrangement in an oil-free rotary gas-compressor (2) having a high, built-in pressure ratio and provided with means (13) for injecting a vaporizable liquid, preferably water, thereinto for the purpose of cooling the gas under compression, characterized in that the liquid injector means (13) is constructed to inject the liquid in a weight quantity in relation to the weight quantity of gas supplied which is greater than, but not more than about four times greater than the amount required for complete vaporization of the liquid during compression of the gas. 2. An arrangement according to Claim 1 , characterized in that a pressure line extending from the outlet side - of the compressor (2) incorporates a condensation sepa- rator (6) ; and in that the amount of liquid injected is restricted to a value which permits the removal of all liquid including condensation from the compressed gas by means of the condensation separator. 3. An arrangement according to Claim 2, characterized in that the condensation separator (6) communicates, via a buffer container (8) having a means (11) for maintaining a constant level of liquid in the buffer container and a liquid supply line, communicates with a regulator (14) connected to the inlet side of the gas compressor for regulating the amount of liquid charged to the gas compressor (2) per unit of time. 4. An arrangement according to any of Claims 1-3, charac¬ terized in that the weight quantity of liquid injected into the compressor in relation ship with the weight quantity of gas supplied thereto is greater than 1:20, although at most 1:4. 5. An arrangement according to any of Claims 1-4, characterized in that the gas compressor (2) is a single- stage screw rotor compressor and has substantially the same peripheral speed and dimensions as the first stage in a corresponding two-stage dry compressor.

Inventors: TIMUSKA KARLIS, TIMUSKA, KARLIS
Applicants: SVENSKA ROTOR MASKINER AB
Application year: 1986
Main classification: F04C18
IPC codes: F04C18, F04C27, F04C29