It would be possible to install a float or a capacitance probe directly in the vessel, but it is highly preferable to attach a float or capacitance chamber external to the vessel. One reason is that the level in the vessel may be subject to considerable agitation, whereas the level in the chamber will be much more tranquil. The float chamber requires two connections to the vessel, one above and one below the desired liquid level. The connections to the vessel should be made to the side of the chamber, rather than from the bottom. As Fig. 11.13a shows, in an ammonia system any oil in the vessel could collect in the line between the vessel and the chamber. Since oil has a different density than liquid ammonia, the float would settle at a level different than that in the vessel. As was mentioned in the previous section, the capacitance level sensor would be distorted by oil for an additional reason, which is that the dielectric constant of oil differs from that of liquid ammonia or the halocarbons.
If the liquid in the vessel is boiling or has compressor vapor bubbling through it to desuperheat the vapor, the density of the mixture in the vessel will be less than in the float chamber. The result, as illustrated in Fig. 11.13b, is that the liquid level in the vessel will be higher than in the float chamber.
Equalizer pipe lines should be a minimum of 30 mm (1–1/4 in) pipe for lowpressure vessels7. Chambers for low-pressure vessels should be a minimum of 75 mm (3 in) in diameter.