PATENT CLAIMS 1. A filter unit for filtering gas, comprising at least one filter element having an outlet opening and an associated filter cleaning nozzle, which nozzle is located in a clean-gas chamber at a distance from the outlet opening and is connected to a source of pressurized primary cleaning gas via a control valve, said nozzle having a nozzle opening for injection of primary cleaning gas and downstream thereof a guide surface for directing the primary cleaning gas and entrained secondary gas towards the outlet opening of the filter, characterized in that the guide surface is part of the outer side of a guide body that is located in the clean-gas chamber and is open to the secondary gas in the chamber. 2. A filter unit according to claim 1, characterizedin that the nozzle opening is an annular slot which is delimited by an annular rim of a feeding pipe for primary cleaning gas and the guide body positioned in front of the feeding pipe. 3. A filter unit according to claim 1 or 2, characterized in that the guide body portion facing towards the feeding pipe is curved, preferably hemispherical, with its central area positioned closest to the feeding pipe. 4. A filter unit according to any one of claims 1-3, characterized in that on the side facing away from the outlet opening of the filter the guide body extends into a housing, and that an annular distribution chamber for primary cleaning gas in the housing ends at the nozzle opening, which is an annular slot delimited by an annular rim of the housing and the guide body. 5. A filter unit according to claim 4, characterized in that the annular chamber tapers towards the annular slot, and that the included angle between an inner side wall of the distribution chamber and the outer side of the guide body is in the range of 2-45°. 6. A filter unit according to claim 4 or 5, characterized in that the guide body is carried by the housing, and preferably the guide body is permanently fixed to the housing. 7. A filter unit according to any one of claims 1- 6, characterized in that from the. nozzle opening the guide body has a first portion with increasing diameter in direction towards the outlet opening of the filter element. 8. A filter unit according to any one of claims 1- 7, characterized in that the guide body has an end portion with decreasing diameter in direction towards the outlet opening of the filter element. 9. A filter unit according to claims 7 and 8, characterized in that in between the first portion and the end portion the guide body has a second portion with substantially constant diameter. 10. A filter unit according to any one of claims 7-9, characterized in that the first portion and preferably also the end portion has/have a conical shape. 11. A filter unit according to any one of claims 1-10, characterized that the guide body has a longitudinal, through-going bore for feeding a part of the primary cleaning gas to the terminal end of the guide body. 12. A filter unit according to any one of the claims l-ll, characterized in that the unit comprises a plurality of filter elements, each having an associated filter cleaning nozzle which is connected to the source of pressurized primary cleaning gas via a feeding pipe that extends out of the clean-gas chamber to the control valve. 13. A filter unit according to claim 12, characterized in that the feeding pipes are mainly vertical. 14. A filter unit according to claim 12 or 13, characterized in that the individual feeding pipe is connected via a valve to a source of cleaning liquid. 15. A filter unit according to any one of the claims 1-14, characterized in that the filter element material in vicinity of the outlet opening is freely open to cleaning air from the nozzle. 16. A filter unit according to any one of the preceeding claims, characterized in that the pressure in the source of primary cleaning gas is settable or adjustable as well as the gas pulse time, interval and/or sequence.PATENT CLAIMS 1. A filter unit for filtering gas, comprising at least one filter element having an outlet opening and an associated filter cleaning nozzle, which nozzle is located in a clean-gas chamber at a distance from the outlet opening and is connected to a source of pressurized primary cleaning gas via a control valve, said nozzle having a nozzle opening for injection of primary cleaning gas and downstream thereof a guide surface for directing the primary cleaning gas and entrained secondary gas towards the outlet opening of the filter, characterized in that the guide surface is part of the outer side of a guide body that is located in the clean-gas chamber and is open to the secondary gas in the chamber. A filter unit according to claim 1, characterizedin that the nozzle opening is an annular slot which is delimited by an annular rim of a feeding pipe for primary cleaning gas and the guide body positioned in front of the feeding pipe. A filter unit according to claim 1 or 2, characterized in that the guide body portion facing towards the feeding pipe is curved, preferably hemispherical, with its central area positioned closest to the feeding pipe. A filter unit according to any one of claims 1-3, characterized in that on the side facing away from the outlet opening of the filter the guide body extends into a housing, and that an annular distribution chamber for primary cleaning gas in the housing ends at the nozzle opening, which is an annular slot delimited by an annular rim of the housing and the guide body. A filter unit according to claim 4, characterized in that the annular chamber tapers towards the annular slot, and that the included angle between an inner side wall of the distribution chamber and the outer side of the guide body is in the range of 2 -45 °. A filter unit according to claim 4 or 5, characterized in that the guide body is carried by the housing, and preferably the guide body is permanently fixed to the housing. A filter unit according to any one of claims 1- 6, characterized in that from the. nozzle opening the guide body has a first portion with increasing diameter in direction towards the outlet opening of the filter element. A filter unit according to any one of claims 1- 7, characterized in that the guide body has an end portion with decreasing diameter in direction towards the outlet opening of the filter element. 9. A filter unit according to claims 7 and 8, characterized in that in between the first portion and the end portion the guide body has a second portion with substantially constant diameter. 10. A filter unit according to any one of claims 7-9, characterized in that the first portion and preferably also the end portion has / have a conical shape. A filter unit according to any one of claims 1-10, characterized that the guide body has a longitudinal, through-going bore for feeding a part of the primary cleaning gas to the terminal end of the guide body. 12. A filter unit according to any one of the claims l-ll, characterized in that the unit comprises a plurality of filter elements, each having an associated filter cleaning nozzle which is connected to the source of pressurized primary cleaning gas via a feeding pipe that extends out of the clean-gas chamber to the control valve. A filter unit according to claim 12, characterized in that the feeding pipes are mainly vertical. A filter unit according to claim 12 or 13, characterized in that the individual feeding pipe is connected via a valve to a source of cleaning liquid. A filter unit according to any one of claims 1-14, characterized in that the filter element material in the vicinity of the outlet opening is freely open to cleaning air from the nozzle. 16. A filter unit according to any one of the preceeding claims, characterized in that the pressure in the source of primary cleaning gas is settable or adjustable as well as the gas pulse time, interval and / or sequence.