The Refrigerant Oil Return Speed Influence on the Design of Vertical U-Tube Ground Heat Exchanger in the Direct Expansion Ground-Source Heat Pumps

In this paper, the oil return model of vertical U-tube ground heat exchanger in the direct expansion ground-source heat pumps was presented based on fluid mechanics principle. And the minimum refrigerant velocity was calculated according to nil oil volumetric flow rate and the relation between refrigerant velocity and interface velocity. The design of diameter and length for U-tube ground heat exchanger is also analyzed by considering three main factors: oil return, pressure drop and heat transfer. On the basis of analysis of these factors, the model to deal with the fuzzy relation was established. With the model established, economical matching of the refrigeration system can be obtained. And it also will help for the system design.