Conference Agenda

Natural refrigerants such as propane (R-290) and ammonia (R-717) often have superior refrigerant characteristics with the additional benefit of low global warming potentials (GWP). These benefits come with drawbacks of flammability and toxicity. R-290 is classified as an A3 refrigerant (flammable) and R-717 as B2L (toxic and slightly flammable). To use these refrigerants in heat pumps for space conditioning, the refrigerant must be isolated from the air stream, and this is typically done by a hydronic system. This work presents an alternative passive approach using a loop thermosiphon heat exchanger (LTHX) to replace the secondary loop heat exchanger. The primary advantage of the LTHX is that it is passive and can operate at higher evaporator temperatures in cooling mode and lower condensing temperatures in heating mode. This work first briefly discusses the design of a heat pipe heat exchanger (HPHX) which is the fundamental operating principle of the LTHX. The HPHX is comprised of multiple heat pipes where the size, quantity, and spacing are determined to meet the thermal load, while minimizing air side flow restriction. When considering manufacturability and operation of the HPHX there are significant challenges. These challenges lead to the design of an LTHX which alleviates these problems. The LTHX was prototyped and evaluated in heating and cooling mode over a range of conditions that are suitable for space conditioning applications. The evaluation showed that the LTHX could effectively transfer heat with efficiencies up to 98%. The LTHX saturated in heating mode at around 5,300 Btu/hr (1.6 kW, 0.4 RT) while cooling mode did not appear to saturate with a measured capacity of 6,400 Btu/hr (1.9 kW, 0.5 RT). Maximum temperature differences were on the order of 10 °F (5.6 °C) between return and supply air when the HPHX was performing most efficiently.

The fin-tube braze joint is critical for the high thermal performance of the brazed aluminum microchannel heat exchangers. Corrosion-related braze joint damage, e.g., loss of braze joints and decrease in braze joint fillet size, is expected to cause degradation of the thermal performance of the heat exchanger. This study provides a quantitative evaluation of the correlation between the thermal performance of brazed aluminum microchannel heat exchangers and braze joint integrity. A theoretical model is proposed to estimate the decrease in thermal performance according to the quantity of braze joint loss. The performance loss estimated by the model agrees well with the experimental results of heat exchangers brazed with different quantities of braze joints. A comparison of the performance of two brazed heat exchangers with different fin-tube joint sizes is experimentally performed. The results show a relatively small impact of the joint size on the thermal performance of the microchannel heat exchangers studied in this paper.

The hydrofluorocarbon (HFC) refrigerants used in the current refrigeration systems are facing a phase-down due to their higher greenhouse effect resulting in global warming, and thus HVAC&R industry has undergone a transition to low Global Warming Potential (GWP) refrigerants. Refrigerant mixtures are attractive alternatives since their composition can be tailored to comply with environmental regulations while preserving favorable thermophysical properties. However, the new low-GWP zeotropic mixture refrigerants have two or more components with different saturation temperatures at the same pressure level, known as temperature glide, which can cause the degradation of the overall heat transfer performance. The brazed plate heat exchangers (BPHX) provide excellent heat transfer performance due to a compact design and are used in several air-conditioning and refrigeration applications. In this study, flow boiling heat transfer and the associated pressure drop of the refrigerant mixture in a vertical BPHX were experimentally investigated. The single-phase water-to-water experiments were conducted in the tested heat exchanger with a counter-flow configuration. The flow boiling experiments charged with R-134a and R-454C were then performed in a pumped refrigerant loop to evaluate its thermal-hydraulic performance. Furthermore, parametric studies of various heat fluxes, mass fluxes, vapor qualities, and saturation temperatures were also conducted.

Globally, with the Kigali Amendment to the Montreal Protocol and other regulatory pressures, the HVACR industry is transitioning to lower Global Warming Potential (GWP) refrigerants. Historically, refrigerants were either single components, such as R-22 (GWP = 1820), or multiple component blends, with little temperature glide, such as R-410A (GWP = 2088) or R-404A (GWP = 3922).

To accelerate the transition away from high GWP refrigerants such as R-404A, thermodynamically comparable solutions will be required to minimize incumbent system design changes. Several new lower flammability (A2L) blends have been developed which include R-454A (GWP = 239), R-454C (GWP = 148), and R-455A (GWP = 146).

This paper will present the results of thermodynamic modeling analysis examining the impact of temperature glide of these different refrigerant blends on the sizing of heat exchangers. Data showing how rating a heat exchanger at its mid-point temperature condition vs its dew-point temperature will highlight the criticality of rating conditions in industry standards when making decisions in both low and medium temperature refrigeration applications. The data presented will include comparison of results from multiple thermodynamic design modeling software packages for both low and medium temperature refrigeration conditions. The modeling software packages used for data generation are ASPEN Plus®, and NIST Cycle_D-HX.

In recent years, environmental concerns accelerated the quest for alternative low-GWP working fluids to replace current HFCs and their blends. Simulation models are essential when designing and optimizing next-generation heat pump systems that employ low-GWP refrigerants for off-design performance. Experimental results indicate the necessity to optimize and redesign heat exchangers for zeotropic mixtures (e.g., R454C) to improve its performance when working as a replacement in a residential R410A heat pump. In this paper, a comprehensive system model has been used to evaluate the performance of a heat pump system. However, to enable multi-objective optimizations to explore the design space of heat exchangers, surrogate meta-models were used to reduce the computational time without compromising the accuracy of the predictions. The simulation results have been compared to the experimental results and validated within 7% error range. The developed simulation framework can detect several operational failure scenarios and accurately predict the off-design performance of the system when applying low-GWP refrigerants. Both structural (e.g., tube length and diameter) and circuitry parameters (e.g., tube, row, and circuit number) of finned tube refrigerant to air heat exchangers were optimized to provide highest seasonal performance (SEER2 and HSPF2), lowest refrigerant pressure drop and minimum usage of tube materials. The simulation results indicated that the optimized heat exchanger geometries can significantly improve the seasonal performance of the heat pump system when applying low-GWP zeotropic mixtures. The developed simulation model can function as a tool for determining optimal heat exchanger design for system performance without the need for extensive experiments.

Dual mode heat pumps are a crucial technology to minimize emissions from cooling and heating systems as we strive to achieve a decarbonized future. With the use of variable speed drives, these heat pumps have become more efficient. However, under extreme part load conditions, the control of these devices is challenging and current heat exchanger model capabilities cannot fully handle such phenomenon. This research presents comprehensive experimental analysis of an air-coil used in an R410A air-to-water heat pump. The coil has 2 rows of 36 tubes each. The refrigerant flow is distributed in 5 refrigerant circuits of 7 hairpins each (14 tubes per circuit), arranged one on top of the other, plus an additional hairpin located at the bottom of the coil that collects the refrigerant flow from the 5 circuits. In order to estimate the refrigerant temperature profiles in the coil circuitry, temperature sensors were attached on different locations of the tubes of the coil. Airflow maldistribution measurements were also conducted. Experiments were performed combining different compressor and fan speed values. The experimental temperature profiles suggest that the refrigerant distribution in each circuit is not equal and that some refrigerant accumulation and/or low refrigerant flow occurs in the lower refrigerant circuits. This is in part due to gravity, and the effect is more pronounced when operating the unit with low compressor speeds. The experimental data were used to validate existing air-to-refrigerant heat exchanger models and to identify future model development and validation initiatives. The predicted performance agreed with measured data within ±5%, but this deviation was significantly higher for low mass flux conditions, highlighting the need for improved modeling of gravity for accurate predictions of refrigerant mass flow maldistribution in headers.