METHOD FOR PREVENTING FILTERS FROM BLOCKING

Abstract: The invention is related to a method for preventing the formation of ice and/or white frost in an air filter (2) of a compressor (1) and, possibly, on other surfaces when cold air is conducted through the said filter to the compressor with the intention of increasing the capacity of the compressor, comprising the exertion of radiation (4) on the air to be filtered and/or on the surface of the filter to prevent the formation of ice and/or white frost, the radiation not essentially increasing the temperature of the air conducted to the compressor

Description: Method for preventing filters from blocking Short description of the invention The invention is related to a method for preventing the formation of ice and/or white frost that causes blocking of the air filter of a compressor when cold air is conducted through the filter to the compressor. In the method, radiation is used to prevent the formation of ice and/or white frost in the filter. Prior art Various industrial processes, e.g. nitric acid processes, air distillation, and gas turbine plants, need clean, filtered air. When cold air is filtered in industrial use, the humidity contained by the air is crystallised into ice or white frost on the surface of the filter, blocking the filter, preventing air from flowing, and increasing the flow resistance of the filter. Particularly in industry that uses centrifugal compressors to produce compressed air, the blocking of suction filters as a result of freezing, for example, causes severe disturbances to the operation of the compressors. For other types of machines, such as piston compressors and screw compressors, the blocking of suction filters causes excess build-up of heat which causes disturbances in operation. To ensure that air is clean, the compressor is provided with an air filter on the suction face thereof. A centrifugal compressor has a certain operating coefficient, a pressure ratio, and a pumping limit, characteristic to each compressor, and the compressor should function within the limits of these factors. The pressure ratio is the absolute pressure of the pressure face divided by the absolute pressure of the suction face. If the suction filter is blocked or frozen or white frost is formed on it, the suction head decreases relatively quickly and the pressure ratio increases according to the following formula: the pressure on the pressure face (abs) = the pressure ratio. the suction head (abs) When the pressure ratio increases so that the pumping limit is exceeded, the capability of the compressor to increase pressure deteriorates quickly and the compressed air on the pressure face of the compressor is discharged backwards through the compressor. As a result, the pressure in the suction pipe increases, the pressure ratio decreases, and a quick air flow to the right direction takes place. A situation is created where the air current through the compressor is converted into a reciprocal current which very quickly changes directions. The compressor has now gone into a pumping state. In the pumping state, great changes in the amount of air and the power required take place in the compressor. They are hazardous, not only to the compressor itself, but also to other devices, to the extent where these devices can be broken and disturbances can be caused to the actual process. To prevent suction filters from freezing, manufacturers of compressors have decided that the minimum temperature of the suction air conducted to the filter should be +2 - +5 °C before the filter and, in practice, the suction air should be kept at about +5 °C for safety reasons. It is common knowledge that filters are prevented from freezing by preheating the filtered air to the temperature of +2 - +5 °C, for example, by using steam heaters or hot water heaters; by conducting air which is preheated in an industrial hall from the hall to the filter; by conducting air which is heated in the compressor from the pressure face of the compressor to the suction filter; or by preheating the air in a separate heater. When air which is preheated in an industrial hall is conducted to the suction filter, impurities in the air, such as various gases or solid particles, may cause problems to the compressor or the process. In chemical industry in particular, compounds can be created which stain the compressor and the process devices. In nitric acid factories, for example, there are small amounts of oxides of nitrogen and ammonia in the indoor air, which can form various ammonia salts. The thus formed ammonia salts cause, not only blocking of the suction filter and, as a consequence, increased pressure loss and, furthermore, energy losses, but also an increased need to clean the process devices. The factoiy must be stopped for the time of washing, whereby the mnning degree decreases. All the known methods described above consume relatively large amounts of energy. Particularly, when the suction air is heated by taking air heated in the compressor from the pressure face of the compressor to the suction filter, a very large amount of working energy of the compressor is also consumed and, at the same time, the capacity of the compressor decreases. The above-mentioned technology has been applied in preventing the freezing of the gas turbines of aeroplanes, for example (patent publication US 4 831 819). The heating of the suction air in separate heating equipment by using fuel requires, not only energy, but also separate equipment which, when having disturbances, might in turn cause disturbances in the operation of the compressor and of the process. This technology has been applied, for example, in the method according to patent publication US 4 328 666 to prevent filters from freezing. It is common knowledge that radiation can be used for heating and de-icing. Patent publication US 5 134 266 describes a method for de-icing solid surfaces, such as aeroplanes, rockets, etc. by conducting air heated by radiation to the frozen surfaces. Patent publication JP 8193435 in turn discloses an infrared heat radiator which can be used for de-icing and for preventing the formation of ice on building sites, streets, railroads, etc. The wavelength emitted by the invention in question is 3 to 25 micrometer. Furthermore, patent publication US 5 417 389 describes a device which generates infrared radiation and a method which can be used to de-ice the surfaces of aeroplanes. Description of the invention The invention is related to a method for preventing the formation of ice and/or white frost in an air filter of a compressor and, possibly, on other surfaces when cold air is conducted through the said filter to the compressor with the intention of increasing the capacity of the compressor; in the method, radiation is exerted on the air to be filtered and/or on the surface of the filter to prevent the formation of ice and/or white frost; the radiation does not essentially increase the temperature of the air which is conducted to the compressor. Consequently, the method according to the invention makes it possible to conduct cold air through the filter to the compressor without preheating it and without causing formation of ice and/or white frost in the filter. It is advantageous to use cold air in the compressor because it makes it possible to increase the capacity of the compressor. According to the invention, an increase in the capacity of the compressor is achieved, which is directly proportional to the decrease in the temperature of the air conducted to the compressor. According to the invention, the temperature of the air conducted to the compressor is 0 °C or less than 0 °C. In the method according to the invention, radiation of a wavelength within 800 nm to 300 mm can be used. The radiation can be, for example, infrared radiation or microwave radiation. According to the invention, it is also possible to use various combinations of radiators, such as combinations of infrared radiators and microwave radiators. In the method according to the invention, radiation, infrared and/or microwave radiation in particular, is exerted on the suction air before the filter, and/or on the surface of the filter. The impact of the radiation exerted on the suction air is particularly focused on the humidity, the water molecules or other polarizable compounds in the air. The radiation has no effect on the gas molecules in the air. The radiation is used to increase the amount of thermal energy of the water particles in the air, so that they are not very close to the state of crystallisation into ice and are able to reach the compressor without freezing. When this method is used, radiation does not essentially increase the temperature of the air, and the amount of operating energy needed by the method is minor. The increase in temperature is preferably about 1 to 2 °C. However, it should be understood that the humidity of air and other factors can have an effect on the increase of temperature; therefore, the increase in the temperature can deviate from the above-mentioned, advantageous range. The amount of air conducted to the suction filter each time, as well as other conditions, determine the power requirement of the radiators, their number, quality and installation, such as the distance from the filter and the direction of the radiation downstream or upstream with respect to the air current. In the design, the installation, and the use of all the radiators, it should be seen to that they are not hazardous to people or materials. The radiators are located so that the radiation smoothly meets the air flowing to the compressor and that the retention time is sufficient. In the disposition of microwave radiators in particular, it should be seen to that there are no unsuitable materials in the radiation field and that unwanted electromagnetic radiation is prevented. In the following, the invention is described with reference to the appended drawings in which: Fig. 1 shows schematically a system that uses the method according to the invention, and Fig. 2 shows the capacity of the compressor as a function of temperature. The method according to the invention has been applied in the nitric acid plant of Kemira Agio Oy at Siilinjai'vi (Fig. 1). In this plant, compressor 1 sucks about 50 000 m of air per one hour through suction filter 2 and compresses it into process 3 to a pressure of about 4 bar. The size of the inflow opening of suction filter 2 in this process is about 3 m x 2.5 m, i.e., about 7.5 m 2 . In the method according to the invention, four infrared radiators 4 are installed in the suction duct before the suction filter; the radiators are directed downstream with respect to the air current and locate at a distance of about 1 meter from the filter. The electric power taken by each radiator is 2 kW, totalling 8 kW. Previously, known techniques were used to prevent suction filters from freezing and forming white frost. Cold air was heated to +5 °C by using hot water heaters and warm air was taken from the inside of the factory hall. In the method according to the invention, air of -20 °C is conducted to the compressor through the suction filter. This method can be used to conduct even colder air to the suction filter and the compressor, but the cold resistance of the suction pipage in this plant sets the limit to -20 °C. Fig. 2 shows practical measurement results of the capacity growth of the compressor in the nitric acid plant in question, when air of less than 0 °C is conducted to the compressor. Fig. 2 shows that if the suction temperature of the compressor is about +5 °C, which is the practice according to prior art, the capacity of the compressor is about 51 000 Nm 3 /h. This invention was used to conduct air of -20 °C to the compressor, and the capacity of the compressor corresponding to this temperature is about 58 500 Nm 3 /h, i.e., the growth of capacity in these conditions is 7 500 Nm h, i.e., 14.7%. What is described above is only one preferred embodiment of the invention and it is obvious that the invention can be varied within the appended claims. The infrared radiators can thus be replaced by microwave radiators, or microwave radiators can be used in addition to infrared radiators. In the latter case, four radiators can be installed in the suction duct before the infrared radiators, mainly radiating microwaves. These microwave radiators can be installed so that they radiate in the upstream direction with respect to the air current. The electric power used by each microwave radiator can be, for example, 1 kW, totalling 4 kW. The benefits achieved by the method The technology according to the invention can be used to accomplish various benefits as compared with prior, known technology. The air flowing to the compressor needs not to be heated, which saves heating energy. This method only needs the working energy of the radiators. Since the air going to the compressor does not need to be heated, the capacity of the compressor increases, first, because when the air cools off, its density increases and a larger amount of heavier air can be fitted into the suction space of the compressor. Second, the capacity of the centrifugal compressor increases because the centrifugal force of cold and, thus, heavy air is higher than the centrifugal force of warm and, thus, lighter air. This technology allows a decrease in the energy consumption of the compressor as calculated per a cubic metre of pumped air. According to the equations of the compressor, the power requirement of the compressor is directly proportional to the absolute temperature. On the other hand, the power requirement decreases as calculated per a cubic metre of air that is produced, or as calculated per a product unit of the plant, because the energy needed by extra energy and the auxiliary instruments does not increase a great deal when the capacity of the compressor or of the plant increases, but remains almost the same. This technology can be used to conduct clean outer air to the compressor and thus avoid the disadvantages caused by impure air that is preheated in the factoiy hall. The purchase price of the equipment used according to the invention is low, it is easy to control by automation, the reliability of the equipment is good, there is no risk of freezing, it is easy to use, and it does not require much maintenance. When this technology is applied, higher production is obtained from a unit with the same operation and maintenance costs, because the increase in production obtained by this technology does not increase these costs. By changing over from the old technology to this new one, the capacity of the compressor and, hence, possibly, of the factory unit can be increased by only small investment costs.

Claims: Claims 1. A method for preventing the formation of ice and/or white frost in an air filter of a compressor and, possibly, on other surfaces when cold air is conducted through the said filter to the compressor with the intention of increasing the capacity of the compressor, characterised in that radiation is exerted on the air to be filtered and/or on the surface of the filter to prevent the formation of ice and/or white frost, the radiation not essentially increasing the temperature of the air conducted to the compressor. 2. A method according to Claim 1, characterised in that the radiation increases the temperature of air by about 1 to 2 °C. 3. A method according to Claim 1 or 2, characterised in that the temperature of the air conducted to the compressor is 0 °C or less than 0 °C. 4. A method according to any of the preceding Claims, characterised in that the wavelength of the radiation is 800 nm - 300 mm. 5. A method according to any of the preceding Claims, characterised in that the radiation is infrared radiation. 6. A method according to any of Claims 1 to 4, characterised in that the radiation is microwave radiation. 7. A method according to any of the preceding Claims, characterised in that an increase in the capacity of the compressor is provided, which essentially is directly proportional to the decrease in the temperature of the air conducted to the compressor.

Inventors: LEHTINEN AARO, LEHTINEN, AARO
Applicants: KEMIRA AGRO OY, LEHTINEN AARO, LEHTINEN, AARO
Application year: 1998
Main classification: F04B39
IPC codes: F04B39