This section concentrates on the selection of the pipe size of suction risers in halocarbon systems that are designed for direct expansion (not liquid recirculation). When the physical arrangement requires vertical risers at the evaporator outlet, the dominant factor influencing the selection of the riser size is oil return. In halocarbon systems, the oil generally remains in solution in the refrigerant in the evaporator. This is in contrast to ammonia systems where the oil and ammonia separate. As the halocarbon refrigerant progressively vaporizes in flowing through the tubes of the evaporator, the oil concentration increases until the refrigerant leaves as small droplets of liquid with a high oil concentration. The objective is to return this oil through the suction line to the compressor, which can be accomplished by maintaining sufficiently high refrigerant vapor velocities in the riser.
The required vapor velocity to transport oil in a vertical riser is a function of the refrigerant, its density (thus the evaporating temperature and amount of superheat), the viscosity of oil, and the pipe diameter. As the evaporating temperature drops and with it the density of the vapor, a higher velocity must be provided. Higher vapor velocities are also needed as the diameter increases.
Figure 9.9 shows recommended design vapor velocities computed from the equation adapted from Reference 11:
The original research that developed the data shown in Fig. 9.9 was performed on CFCs and HCFCs with mineral oil. Equations 9.19 and 9.20 are assumed to also apply to new HFC refrigerants using synthetic oils, and these curves are also shown in Fig. 9.9.
Suction risers that are sized for full-capacity operation may be too large and unable to return oil at part load. The riser could be sized for the minimum flow rate, which of course, imposes a pressure drop penalty at higher velocities. Another approach is to use a double-suction riser, as illustrated in Fig. 9.10. When the refrigerant flow rate drops, oil will not be lifted into the suction line and collects in the oil trap. The oil blocks refrigerant flow in the right riser, diverting refrigerant to the left riser, which is sized to lift oil, even at the low flow rate. When the high flow rate resumes, the oil in the trap is carried up to the suction line. This quantity of oil may be excessive for the compressor to handle instantaneously, so an oil trap in the suction line is recommended.