Rifled tube: An effective answer in many applications
Before rifled tubing, the designer typically added more heat exchanger surface to raise the efficiency of an air conditioner. Now, because of the improved heat transfer characteristics of rifled tubing, designers can increase efficiency without expanding the size of air conditioning units.
Rifled tubing was developed to enhance the performance of evaporators and condensers in two-phase refrigerant flow applications. The improvement in heat transfer is due mainly to the 50% to 60% increase in internal surface area over plain tube, thereby reducing the liquid refrigerant film thickness.
Reduced film thickness results in an increased effective temperature difference between the tube wall and the refrigerant gas-liquid interface, and provides more heat transfer potential.
Also, in evaporator coils (depending on refrigerant velocity), the rifled tube helps promote annular flow, resulting in an increase in the amount of wetted surface area for evaporation. The tube-side heat transfer coefficient for rifled tubing is as much as two and one-half to three times that of plain tube for condensing and evaporating conditions.
The ridges in rifled tubing are designed to improve performance in both condensers and evaporators, but must have sufficient strength to minimize deformation when the tube is expanded into the fins.
The designs offered today somewhat compromise optimum performance to meet these criteria. Ridge height, apex angle, and helix angle all contribute to performance enhancement. Today, rifled tubing is available in 5/16-, 3/8-, and 1/2-in.-dia tube coils.
Using rifled tubing in an evaporator is more effective than in a condenser to improve capacity performance in refrigeration systems. Why? Because a 10% increase in evaporator coil performance will result in about a 3% increase in system performance, as compared to a 1.5% increase due to the same 10% increase in condenser coil performance.
Rifled tubing is also more effective under two-phase flow conditions. An evaporator coil has a higher percentage of its internal surface area in two-phase flow than does a condenser, which has desuperheating and subcooling sections.
Application economicsThe decision to use rifled tubing is usually determined by economics.
When rifled tube and raised-lance fincoils were introduced in 1983, substantial gains were made in material reduction for residential air conditioner evaporator coils. There were cases where designers were able to reduce the size of evaporators from three rows to two, with no loss in performance.
However, when comparing the economics of rifled vs. plain tubing, the additional cost of producing rifled tubing makes it difficult to justify without taking into account other factors.
The decision is somewhat more complex when considering the entire air-handling unit. If the combinations of coil cost and cabinet cost are reduced with rifled tubing, then it makes economic sense. Air conditioning evaporator coil applications seem almost automatic when it comes to the cost-effectiveness of rifled tubing.
Due to a lower overall effect on system performance, condensers are not as likely to justify rifled tubing. Medium- and low-temperature refrigeration applications require close analysis.
If the allocated space for the coil is fixed and plain tube coils cannot achieve the specified performance, rifled tubing may be an effective solution. This not only includes capacity ratings, but air pressure drop as well.
The ability to reduce the number of rows in a coil also has a positive effect in reducing air pressure drop.
Most rifled tube applications occur in refrigeration and air conditioning systems.
However, its use in single-phase fluid applications is growing. For example, rifled tube applications can be made for water coils, where the additional performance of rifled tubing is able to prevent adding more rows or fins per inch in a critical air pressure drop situation.
Rifled tube coils may not be the answer for every application, but they have certainly allowed many engineers and contractors to achieve cost, performance, and unit size objectives that would not be possible otherwise.
Rifled tubing should be considered when new air conditioning or refrigeration units are being developed, and certainly when trying to improve the performance of an existing model without incurring major retooling costs.
With the air conditioning and refrigeration industries sensitive to a new era of non-CFC refrigerants and synthetic compressor lubricants, there will be opportunities to develop new coil designs that optimize the performance characteristics of these chemicals.
Will other geometric configurations of rifled tubing provide better performance than those available today? Maybe. However, these opportunities for new designs must also be cost effective.