Historically, automotive air conditioning compressors of the shaft driven piston type have had a multi-piece, canister shaped outer shell.  The shell necessarily must have at least two basic halves or housings, so that shaft, pistons and piston drive means may be assembled inside before the two halves are joined.  Beyond the two basic housings, it is common to provide separate end caps, generally called heads, to close the ends of the cylindrical housings.  A good example may be seen in co-assigned U.S. Pat.  No.  5,219,273.  The heads have various refrigerant passages formed in them, which open to the outer ends of the cylinder bores across valve plates, which have a large diameter substantially equal to the inside diameter of the heads.  While the separate heads must be sealed, creating two more potential leak paths out of the compressor shell, it is convenient to sandwich the pistons valve plate between the housings and the heads as the heads are installed.  Another design approach integrates the heads to the two housings, as shown in U.S. Pat.  No.  5,100,306, which eliminates two seals and leak paths.  In order to be able to use the same kind of valve plate, however, it is necessary to make separate cylinder bore blocks that fit inside the housing halves.  The valve plates are then sandwiched between the now separate cylinder blocks and the integral heads at assembly. 
The ultimate integration of pans would be to combine housing, head and cylinder bore as one, thereby allowing only two basic pans to make up the outer shell of the compressor.  In that case, however, there is no easy access for assembling a valve plate between the outer ends of the cylinder bores and the integral heads.