A rotary union, comprising: a housing having a cylindrical bore extending through a body of the housing, the cylindrical bore being substantially straight; a shaft rotatably mounted in the cylindrical bore, the shaft having a longitudinal axis extending along a centerline of the bore; a plurality of annular plates mounted on the shaft, the plurality of annular plates being spaced apart from one another at regular intervals along the longitudinal axis, each of the plurality of annular plates extending a radial distance from the shaft within the bore and having a radial fluid passage extending therethrough, the plurality of annular plates being rotatably and sealably engaged to rotate with the shaft; a plurality of spools disposed in alternating fashion between the plurality of annular plates, the plurality of spools sealably engaging the cylindrical bore and being rotatably engaged with the housing so as not to rotate with the shaft, each of the plurality of spools being axially disposed between two adjacent annular plates from the plurality of annular plates, each of the plurality of spools comprising: a radially extending wall having a fluid passage extending radially therethrough; and two seal rings slidably disposed along the longitudinal axis and biased in opposite directions so that the two seal rings slide against side faces of the two adjacent annular plates when the shaft is rotating; wherein the shaft, the plurality of annular plates and the plurality of spools are arranged to assemble in a stacked arrangement, the stacked arrangement being insertable as a whole into the cylindrical bore of the housing. a housing having a cylindrical bore extending through a body of the housing, the cylindrical bore being substantially straight; a shaft rotatably mounted in the cylindrical bore, the shaft having a longitudinal axis extending along a centerline of the bore; a plurality of annular plates mounted on the shaft, the plurality of annular plates being spaced apart from one another at regular intervals along the longitudinal axis, each of the plurality of annular plates extending a radial distance from the shaft within the bore and having a radial fluid passage extending therethrough, the plurality of annular plates being rotatably and sealably engaged to rotate with the shaft; a plurality of spools disposed in alternating fashion between the plurality of annular plates, the plurality of spools sealably engaging the cylindrical bore and being rotatably engaged with the housing so as not to rotate with the shaft, each of the plurality of spools being axially disposed between two adjacent annular plates from the plurality of annular plates, each of the plurality of spools comprising: a radially extending wall having a fluid passage extending radially therethrough; and two seal rings slidably disposed along the longitudinal axis and biased in opposite directions so that the two seal rings slide against side faces of the two adjacent annular plates when the shaft is rotating; wherein the shaft, the plurality of annular plates and the plurality of spools are arranged to assemble in a stacked arrangement, the stacked arrangement being insertable as a whole into the cylindrical bore of the housing. a radially extending wall having a fluid passage extending radially therethrough; and two seal rings slidably disposed along the longitudinal axis and biased in opposite directions so that the two seal rings slide against side faces of the two adjacent annular plates when the shaft is rotating; wherein the shaft, the plurality of annular plates and the plurality of spools are arranged to assemble in a stacked arrangement, the stacked arrangement being insertable as a whole into the cylindrical bore of the housing.