is inverted when installed in a fig1 system i e as illustrated in fig4 and 5 module 15 is generally rectangular in cross section throughout its length and has spaced apart horizontal plates 31 and 32 forming the top and bottom of the module and spaced apart partitions 33 extending between plates 31 and 32 and along the length thereof to form a section of the reverse air manifold 20 therebetween centrally of the module sleeves 34 at both ends of the unit 15 extend the reverse air manifold section the sleeves being provided with flanges 35 for connection to the next module spaced apart end walls 36 extend between plates 31 and 32 and outwardly from partition 33 and transversely of the module to form valve chambers 37 within the module one on each side of the reverse air manifold the outer sides of the valve chambers are closed by side walls 38 the latter having a port 39 therethrough and a flanged sleeve 40 for connection to the flanged sleeves 41 on bag houses 10 so that the module and bag house ports 39 and 26 can be brought into communication each valve chamber 37 has a reverse.