Special Topic: Tubes and Pipes
Figure 5: Effect of mass flowrate( G) on the heat transfer coefficient for a smooth tube( ST) and a hydrophobic( HYD) tube.
Figure 5 presents the variation of the heat transfer coefficient( HTC) with mass flow rate for the tubes studied here. The HTC’ s of the tubes studied perform about 10 % better at T sat
= 35 ° C than at T sat
= 45 ° C. The physical properties of refrigerant R32 perform better at a saturation temperature of 35 ° C than at 45 ° C. Specifically, at T sat
35 ° C, the liquid phase density of refrigerant R32 is higher than at 45 ° C, enabling the refrigerant to counteract the effects of gravity, particularly at high mass flow rates. This leads to an increased flow rate of the refrigerant within the tube, producing sufficient heat transfer and consequently enhancing the heat transfer coefficient. Moreover, the thermal conductivity of refrigerant R32 at a saturation temperature of 35 ° C exceeds the thermal conductivity at 45 ° C. This further enhances the heat transfer rate of the refrigerant.
Figure 6 shows the variation of pressure drop( DP) as a function of flow rate( G). This shows that there is no increase in pressure drop in the HYD tube.
To analyze the effect of hydrophobicity, the variation of the HTC with mass flow rate for ST tubes is compared to HYD tubes. Over the range of conditions considered here, the HTC of the HYD tube is approximately 20 % higher than that of the ST tube. The hydrophobic structure enhances the upper wall surface of the tube with a lower surface free energy, preventing the refrigerant from generating a liquid film on the upper wall surface. Due to the increase in contact angle, the upper wall surface of the tube forms large and small droplets, resulting in a droplet condensation flow pattern.
Conclusions
To assess the impact of the hydrophobic structure on tube-side condensation heat transfer, an experiment was performed that collected data over various scenarios( using refrigerant R32). Conditions included saturation temperatures being set at T sat
= 35 ° C and T sat
= 45 ° C, with mass flow rates of 100 kg m 2 s-1 and 150 kg m-2 s-1. Specific conclusions derived from this analysis are as follows:
( 1) Over the range of conditions considered here, the heat transfer coefficient of the HYD tube is approximately 20 % higher than that of the
ST tube. The hydrophobic structure enhances the upper wall surface of the tube with a lower surface free energy. This prevents the refrigerant from generating a liquid film on the upper wall surface.
( 2) The physical characteristics of refrigerant R32 exhibit superior performance at T sat
= 35 ° C when compared to the performance at
T sat
= 45 ° C. Consequently, both tubes demonstrate an enhancement of approximately 15 % at T sat
= 35 ° C when compared to T sat
= 45 ° C. Moreover, as the mass flow rate increases, the heat transfer coefficient of the HYD tube increases by approximately 20 %( when compared to a smooth tube). This enhancement is attributed to the hydrophobic structure, which promotes droplet condensation on the inner surface of the tube, increasing the droplet detachment rate and the heat transfer efficiency.
( 3) Performance of the HYD tube shows a clear trend as the vapor mass rises. This occurs when the refrigerant gas phase dominates over the liquid phase, preventing the formation of a liquid film from the large droplets produced by the hydrophobic structure. As a result, there is a significant increase at an average vapor mass of 0.5.
PRODUCTS
• GATES / GLOBES / CHECKS
PRESSURE CLASSES
• UP TO CLASS 2500( PN 420)
SIZES
• UP TO NPS 36( DN 900)
( 4) Future hydrophobic surfaces are being developed and tested; these will be the subject of another publication.
The hydrophobic tube offers a clear advantage over the smooth tube for various heat transfer applications and should be considered as a means to enhance heat transfer.
Reference:
PRODUCT QUALIFICATIONS
• API RP 591
• API 624 / ISO 15848 / TA LUFT
• ISO / PED / DIN / TSG / ATEX / CRN’ s
• ASME B16.34 SPECIAL CLASS- 100 % PRESSURE BOUNDARY NDE QUALIFICATION
1. Li W, Feng W, Liu, X, Li J, Cao B, Dou B, Zhang J, Kukulka D. J. Condensation heat transfer and pressure drop characteristics inside smooth and enhanced tubes with R410A and R32. Int J Heat Mass Tran 2023; 214: 124419. https:// doi. org / 10.1016 / j. ijheatmasstransfer. 2023.124419
About the Author
David Kukulka has been the Director of Engineering Development at Rigidized Metals Corporation for approximately 16 years. He is responsible for the development of the patented process to produce Enhanced Heat Transfer Surfaces for the Vipertex Division of Rigidized Metals. He is a registered Professional Engineer and also has a PhD in Mechanical Engineering with a specialty in Heat Transfer. He has more than 40 years of experience in heat exchanger design and enhanced heat transfer development.
YOUR TURN-KEY SOLUTION FOR DEMANDING PROJECT REQUIREMENTS
THIRD PARTY NDE EXAMINATION QUALITY SYSTEM
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BONNEY FORGE CAST STEEL VALVES ARE AVAILABLE
WITH CRYOGENIC CONFIGURATIONS FROM
OUR SPECIALTY VALVE LINE.
Figure 6: Variation of pressure drop( DP) with mass flow rate( G) for Smooth( ST) and Hydrophobic( HYD) tubes.
( 800) 345-7546 www. bonneyforge. com
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THIRD PARTY TESTED API 624 / ISO 15848 / TA LUFT
DESIGNED PER
• MSS SP-134
• BS 6364
• ASME B16.34
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