The merits of the direction of refrigerant feed have been a continuing subject of debate, but the fact is that both directions of feed are in successful operation.
The key requirement is proper design and application. Each type of feed has its inherent advantages.
• smaller refrigerant charge permits a smaller low-pressure receiver
• natural draining of the coil prior or during defrost
• continuous transport of oil out of the coil
• improved refrigerant-side heat-transfer coefficients for a given circulation ratio
• more uniform distribution of refrigerant through the various coil circuits.
When top feed is chosen, it may be advisable to design for a high circulation ratio, as Table 8.1 suggests.
Two of the many coil circuiting variations are shown in Fig. 8.10. Figure 8.10a shows horizontal headers with the tubes oriented in a vertical plane. This arrangement is often used for ice thermal storage units or in some cases for cooling air. Horizontal air flow should not be used because it would cause unequal loading of circuits. The circuiting in Fig. 8.10b has vertical headers with the planes of each tube circuit on an incline. With horizontal air flow, the lower circuits often draw a greater refrigerant flow rate than the upper circuits, so usually each circuit is equipped with an orifice (Sec. 6.9) having a smaller diameter in lower circuits than in the higher circuits.
The method of defrost also influences the direction of refrigerant feed. Hotgas defrost combines well with bottom feed where the hot gas is supplied at the top, so the flow of refrigerant during defrost is opposite to that for refrigeration. Air, water, or electric defrost methods are more compatible with top feed of refrigerant because prior to defrost the liquid supply valve closes and much of the liquid refrigerant simply drains out of the coil prior to the external application of heat. Because the orifices are always placed at the inlets of the coil circuits they are at the top of top-feed coils. Thus, when a top feed coil is defrosted with hot gas, this hot gas first encounters the orifices which may restrict the flow of vapor. This is another reason for top-feed coils to be more adaptable to defrosting from external heat.