If the physical arrangement will permit it, certain vapor lines should be sloped in order to prevent liquid from draining back to the evaporator outlets and to the suction of the compressors. Figure 9.6 shows several vapor lines and the recommended directions of their slopes. Figure 9.6a shows a section of a liquid recirculation system where the liquid/vapor line is inclined toward the low pressure receiver. This direction of slope aids in the return of liquid during normal operation and is also important should there be a power outage. In that case the compressors cease operating, interrupting the flow of vapor, but the liquid in the line continues to drain to the low-pressure receiver. The suction line of the compressor is sloped away from the compressor, but the discharge line, as in Fig. 9.6b, slopes away from the compressor toward the condenser.
The recommended magnitude of slope is often specified as 1:240, and while it may be easy to show slope on the design drawings, it is sometimes difficult in a long line to physically achieve the desired slope. Usually there is no problem for lines running above the roof, but sometimes within a building headroom may be sacrificed.
Figure 9.6 also shows the recommended orientation of the branches into and out of the vapor lines. The connections are made to the top of the vapor line, so that any liquid that inherently exists in a liquid recirculation system or
could be formed by condensation does not back up into the evaporators or accumulate at a compressor, particularly an idle one.
While branch connections should normally be to the top of the main, in the case of liquid lines, they can enter the main at the side or even at the bottom.