Michael B. Pate

Evaporation and condensation heat transfer and pressure drop in horizontal, 12. 7-mm microfin tubes with refrigerant 22

Using R-22 as the working fluid, a series of tests was performed to determine the evaporation and condensation performance of three tubes having many small, spiral inner fins. A smooth tube was also tested to establish a basis of comparison

An experimental comparison of evaporation and condensation heat transfer coefficients for HFC-134a and CFC-12

Experimental heat transfer coefficients are reported for HFC-134a and CFC-12 during in-tube single-phase flow, evaporation and condensation. These heat transfer coefficients were measured in a horizontal, smooth tube with an inner diameter of 8.0 mm and a length of 3.67 m. The refrigerant in the test-tube was heated or cooled by using water flowing through an annulus surrounding the tube. Evaporation tests were performed for a refrigerant temperature range of 5–15°C with inlet and exit qualities of 10 and 90%, respectively. For condensation tests, the refrigerant temperature ranged from 30 to 50°C, with et and exit qualities of 90 and 10%, respectively. The mass flux was varied from 125 to 400 kg m−2 s−1 for all tests. For similar mass fluxes, the evaporation and condensation heat transfer coefficients for HFC-134a were significantly higher than those of CFC-12. Specifically, HFC-134a showed a 35–45% increase over CFC-12 for evaporation and a 25–35% increase over CFC-12 for condensation.

A Generalized Performance Prediction Method for Adiabatic Capillary Tubes

Generalized dimensionless correlations were developed to predict refrigerant mass flow rate in an adiabatic capillary tube, for both subcooled and quality inlet conditions. Dimensionless parameters were created using the Buckingham Pi Theorem. These parameters were based on the effects of tube geometry, inlet conditions, and fluid properties. The correlations were based on experimental performance data with refrigerants R-134a, R-22, and R-410A that encompass an extensive range of operating conditions, including: condensing temperatures from 79 °F to 127°F (26°C to 53°C), capillary tube inner diameters from 0.026 to 0.090 in (0.66 to 2.29 mm), capillary tube lengths from 20 to 200 in (508 to 5080 mm), and inlet conditions from 30°F (16.7°C) sub-cooled to 35% quality. Corresponding to this range of refrigerants, capillary tube geometry, and operating conditions, the experimental mass flow rate range was between 3 and 375 lbm/h (1.4 to 170 kg/h). Independent assessments of both correlations were made by com...

Heat transfer and pressure drop during evaporation and condensation of R22 in horizontal micro-fin tubes☆

Abstract The heat transfer and pressure drop performance of three tubes with many small innerfins, called micro-fin tubes, having 9.5 mm outside diameter and 8.9 mm maximum inside diameter were obtained using R22 as the working fluid. The test apparatus had a straight, horizontal test section with a length of 3.67 m and was heated or cooled by water circulated in a surrounding annulus. Nominal evaporation conditions were 0 to 5°C (0.5 to 0.6 MPa) with inlet and outlet qualities of 10% and 90%, respectively. Condensation conditions were 39 to 42°C (1.5 to 1.6 MPa) with inlet and outlet qualities of 90% and 10%, respectively. Mass flux was varied from 150 to 500 kg m−2 s−1. The micro-fin tubes were generally found to have an enhancement of heat transfer ranging from 50 to 100% when compared to an equivalent smooth tube. The maximum increase in pressure drop was only 40%.

A Comparison of Augmentation Techniques During In-Tube Evaporation of R-113

An experimental study was conducted to determine the potential of three techniques for augmenting in-tube evaporation of refrigerants: high-fin tubes, microfin tubes, and twisted tape inserts. Five tubes with internal fins and one smooth tube with a twisted-tape insert were tested. Additionally, experiments were performed with two reference smooth tubes having diameters similar to the maximum inside diameters of the finned tubes. All experiments involved evaporating Refrigerant 113 (R-113) by direct electrical heating of the tube wall. Local evaporation heat transfer coefficients were measured as a function of quality for a range of mass fluxes and heat fluxes. Enhancement factors were calculated by forming ratios of the heat transfer coefficient for the augmented tube and a smooth tube of the same maximum inside diameter. Mass fluxes, pressure levels, and qualities were fixed when enhancement factors were calculated. For the internally finned tubes the enhancement factors varied from 1.1 to 2.8. An internally finned tube having helical spiral angles of 16 deg produced the largest enhancement of heat transfer. The tube with the twisted-tape insert typically had an enhancement factor of about 1.5. Pressure gradient ratios and enhancement performance ratios are also presented.

Heat exchangers for the air-conditioning and refrigeration industry; State-of-the-art design and technology

Abstract Recent developments in heat exchanger design for the air-conditioning and refrigeration industry are reviewed. Particular attention is placed on describing recent developments in heat transfer enhancement on the refrigerant side for evaporation and condensation applications. Both shell-side and tube-side refrigerant enhancement are covered. Data from a number of experimental studies are presented for commercially available surfaces.

Design Considerations for Air-Conditioning Evaporator and Condenser Coils

Major factors affecting heat transfer between the refrigerant and air in a plate finned-tube heat exchanger, otherwise known as an evaporator or condenser coil, are discussed. These factors are inside heat transfer coefficient, thermal contact resistance between the fin and the tube, and outside heat transfer coefficient. Correlations are presented to calculate the rate of heat transfer for each of these factors. Additional design considerations are presented and discussed for these three heat transfer mechanisms. In-tube augmentation, oil-refrigerant mixture effects, and frosting are also discussed, since they affect heat transfer in the coil. Tube circuiting procedures and standards available for rating coils are also presented.

A novel concept of passive shutdown heat removal in advanced nuclear reactors—Applications to PRISM and MHTGR

Abstract A novel concept of passive shutdown heat removal in advanced nuclear reactors is presented. In particular, its application to PRISM and MHTGR design has been discussed. The concept relies on shutdown heat removal from the reactor vessel by natural circulation of the containment building atmosphere (an inert gas) around the reactor vessel, and rejection of the heat from the gas to the outside environment through a system of heat-pipe heat exchangers located at the top of the containment structure. In comparison with the atmospheric air-cooling approach that is currently under consideration for PRISM and MHTGR, the attractiveness of the proposed heat-pipe emergency cooling system is that while it too is totally passive, it allows the reactor vessel to be housed in a conventional containment structure with an inert atmosphere which may be required for licensing the reactor in the U.S.A. Thus, the proposed system addresses the ongoing controversy over the containment issue for the PRISM and the MHTGR, while preserving the passive safety features of the shutdown heat removal technique. The proposed design concept may provide a viable alternative to the air-cooling approach that is being currently considered for the advanced reactors.

An acoustic performance analysis of AC-motor bathroom ventilation fans for a decade-long period, 2005–2015

The most common HVAC device in a residence, or in most other buildings for that matter, is the bathroom ventilation fan. Because these devices operate in close proximity to humans, the concept of loudness is particularly relevant to the acoustic performance of fans. In this regard, acoustic performance is an important component of indoor environmental quality, and the use of loudness as a method of achieving an acoustic rating is now widely accepted in codes and standards. Therefore, a long-term comparative study investigating changes in acoustic performances was performed for bathroom ventilation fans of the alternating current (AC) motor type, with a focus on fan loudness over an 11-year period from 2005 to 2015. Because of difference in performances and designs along with divisions created by testing standards and codes, the 1400 fans tested and analyzed were divided into low- and high-volume flow rate groups, namely below and above 42.5 L/s. In addition to specific conclusions reached from analyzing 11 years of fan performance measurements, major overall conclusions are that (1) the fan noise in terms of loudness has decreased, even to the point of being lower than typical human conversations in many cases, and (2) fan loudness has a weak relationship with volume flow rate and a more noticeable linear relationship with fan rotating speed. This study proves that acoustic performance improvements in bathroom ventilation fans have advanced acoustic-indoor environmental quality over the decade-long period of this study.

Effects of Operating Temperature on the Heat Transfer Characteristics of Photovoltaic Systems in the Upper Midwest

This paper presents the heat transfer characteristics of a stationary PV system and a dual-axis tracking PV system installed in the Upper Midwest, U.S. Because past solar research has focused on the warmer, sunnier Southwest, a need exists for solar research that focuses on this more-populated and colder Upper Midwest region. Meteorological and PV experimental data were collected and analyzed for the two systems over a one-year period. At solar irradiance levels larger than 120 W/m2, the array temperatures of the dual-axis tracking PV system were found to be lower than those of the stationary system by 1.8 °C, which is a strong evidence of the different heat transfer trends for both systems. The hourly averaged heat transfer coefficients for the experiment year were found to be 20.8 and 29.4 W/m2 °C for the stationary and tracking systems, respectively. The larger heat transfer coefficient of the dual-axis tracking system can be explained by the larger area per unit PV module exposed to the ambient compared to the stationary system. The experimental temperature coefficients for power at a solar irradiance level of 1000 W/m2 were −0.30% and −0.38%/ °C for the stationary and dual-axis tracking systems, respectively. These values are lower than the manufacturers specified value −0.5/ °C. Simulations suggest that annual conversion efficiencies could potentially be increased by approximately 4.3% and 4.6%, respectively, if they were operated at lower temperatures.