Recovery Heat Exchanger Research Articles

Investigation on energy and mass distribution characteristics of granules in the composite heat exchanger

A composite heat exchanger for high-temperature granule waste heat recovery is proposed. Granular mass and energy distribution within the composite heat exchanger are simulated via the CFD-DEM coupling method. The effects of granular size, cooling gas velocity, and granular mass flow rate on granular distribution and the temperature distribution are analyzed. Results demonstrated that under the combined action of airflow impact force and gravity, the granule produces an uneven distribution in time and space. Larger granule tends to be positioned closer to the gas inlet horizontally. Along the direction from the gas inlet to the gas outlet, granules exhibit a trend of decreasing size, mass, and temperature. The influence of gas velocity predominantly affects the area coverage rate, whereas the effect of granular mass flow rate is minimal. The area coverage rate initially increases with rising gas velocity and subsequently decreases, peaking at 73.8% at 12 m/s. After the heat exchange process reached a stable state, the number of contacting granules accounted for only 0.37% of the total granules, making heat conduction between granules negligible. Convective heat transfer dominates, accounting for approximately 86.52% of the total heat exchange, while radiation heat transfer accounts for about 13.48%.

Influence of Control Strategy on Heat Recovery Efficiencyin a Single-Duct Periodic Ventilation Device

The subject of the research was a single-duct, decentralised periodic ventilation unit, using accumulative heat exchanger for heat recovery (also called single-core fixed-bed regenerator). It can achieve high efficiency of heat recovery but is vulnerable to pressure differences between the interior of the building and the outside. To counter this, two control strategies were proposed: adjustment of the fan speed based on an air flow sensor and adjustment of the working cycle length based on temperature sensors. The strategies were tested experimentally in actual working conditions. Due to the use of cheap and simple sensors, it was possible to retain the low price of the device. Both control strategies proved to be successful in equalising the amount of supplied and removed air in a single cycle. Moreover, the heat recovery efficiency increased by more than 10% compared to the default working mode.

Comparative analysis of inlet and outlet cavity designs on the thermal-hydraulic performance of compact heat exchangers for combustion engine exhaust heat recovery

ABSTRACT Improving the Internal Combustion Engines (ICEs) overall thermal efficiency is crucial to reduce fuel consumption and emissions. To achieve this goal, waste heat recovery from exhaust gases remains the most promising strategy, which involves the utilization of compact heat exchangers. This study evaluates the performance of novel Monolithic Integrated Thermal Exchange Matrixes (MITEMs) designs for recovering exhaust heat from a 70 hp diesel engine, focusing on the impact of inlet/outlet cavities. Computational Fluid Dynamics simulations were employed to analyze the heat transfer rate, pressure drop, and overall thermal-hydraulic performance of different MITEM designs. The Rectangular Channels with Large Cavities (RCLC) design exhibited superior performance, enhancing the heat transfer rate by 14.7% compared to the baseline design while limiting pressure drop increase to 9.2%. The RCLC design increased local Nusselt numbers and heat transfer coefficients by up to 60%, achieving peak velocities of 98 m/s within cross-sectional planes. These findings provide valuable insights for optimizing exhaust heat recovery systems in diesel engines.

Experimental study on heat storage characteristics of an air-sand moving bed heat exchanger

The air-sand moving bed heat exchanger, designed for storing fluctuating medium-temperature gas waste heat, offers significant potential for energy conservation and carbon reduction in industrial settings. However, its operational characteristics are insufficiently explored. This study investigates the heat storage process involving direct contact and cross-flow of air and quartz sand within a rectangular heat exchanger chamber. Comprehensive analyses were conducted to explore flow resistance characteristics, temperature response, and heat storage performance. Results indicate that pressure drop increases with higher air velocity and temperature, and lower air pressure, particle diameter. The relative discrepancy between fitted pressure drops and experimental data is below 10%. The primary heat transfer zone exists during particle descent. As the air-particle mass flow ratio increases, the descent rate of this zone along the airflow direction slows. Additionally, higher air temperatures elevate the optimal air-particle mass flow ratio. With inlet temperatures for air and particles set at 350 °C and 30 °C respectively, and a mass flow ratio of 0.81, an exergy efficiency of 58.93% and a power density of 600.64 kW·m−3 are achieved. These findings offer crucial insights for air-sand moving bed heat exchangers for medium-temperature gas waste heat recovery.

Multi-objective optimization and life cycle assessment of mass-integrated combined heat and power system

As the effects of climate change become more widely recognized, technical innovation and green energy will promote the growth of combined heat and power systems. This study proposes a novel mass-integrated combined heat and power system and conducts energy analysis, exergy analysis, and techno-economic analysis for the system. The optimization strategy integrated polynomial regression and non-dominated sorting genetic algorithm III is established, with system thermal efficiency, exergy efficiency, and return on investment (ROI) as objective functions. The system's environmental consequences are then assessed throughout its life cycle using life cycle assessment (LCA) under ideal operating circumstances. The findings reveal that the waste heat recovery heat exchanger has the highest exergy destruction in the system, with a value of 674.61 kW. Furthermore, the intermediate heat exchanger 1 and intermediate heat exchanger 2 (IHX2) with lesser exergy efficiency have an exergy destruction of less than 32.00 kW. The IHX2 is the most expensive equipment in the system, costing $90,931.19, but it also provides the most potential for system improvement. The multi-objective optimization findings suggest that the thermal efficiency, exergy efficiency, and ROI for the system are 0.17, 0.97, and 0.30, respectively. The LCA demonstrates that the system has a negligible influence on global warming and ozone generation with corresponding LCA findings of less than 4.20. The construction and operation phases of the investigation system have the most significant environmental consequences. The correlation between the raw materials necessary for the construction of the equipment in the system and the reaction temperatures, conditions, and waste composition during the preparation of the working fluids is large, so it is necessary to take corresponding environmental protection measures during production and processing to reduce the system's environmental emissions from the source and promote ecologically sustainable development.

Proposal and comparison of two heat recovery measures for the coal-based Allam cycle: Double expansion and lower turbine backpressure

Improving the thermodynamic efficiency of the coal-fired semi-closed supercritical CO2 cycle (coal-based Allam cycle) can be achieved by implementing self-heat recuperative coal drying and recompression with more syngas heat integrated with the efficient power cycle. To prevent the heat exchanger from overtemperature during syngas heat recovery, two measures are considered: double expansion and lower turbine backpressure. For the double expansion, a portion of the recycle CO2 is fed into an expander and the exhaust is mixed with the flue gas in the main recuperator to raise the recycle flow rate. For the lower turbine backpressure, the turbine backpressure is decreased to reduce exhaust flue gas temperature. Thermodynamic analysis indicated that self-heat recuperative coal drying and recompression could enhance the net efficiency by 4.28 % points compared to the basic cycle, but the heat exchanger for raw syngas heat recovery faces overtemperature problem with the cold stream exceeding 777 °C. Both the measures proposed above can effectively reduce the temperature to 730–760 °C with slightly decreased efficiencies of 46.93 %∼47.68 % for double expansion and 47.12 %∼47.81 % for lower turbine backpressure. Furthermore, economic assessment shows that compared to the basic cycle, the cost of electricity decreases by 2.49 % and 5.04 % for the schemes with double expansion and lower turbine backpressure, respectively. Therefore, the proposed design with lower turbine backpressure demonstrates advantages in both thermodynamic and economic performance.

Thermal and electrical performance analysis of a lightweight micro CHP system for recreational vehicles

For comfortable use of a Recreational Vehicle (RV), there is a need to supply electrical energy to recharge the batteries and supply the installed equipment, as well as thermal energy, for hot water and ambient heating. A conventional solution for supplying electrical energy is the installation of photovoltaic modules combined with connection to an electrical grid, when possible. Water heating is generally done using a passage gas heater or hybrid gas/electric storage heater. Imported vehicles have gas heating and many other vehicles have diesel heaters. To develop an alternative to currently available energy supply methods, this work proposes the creation of a cogeneration system for RVs. From a literature review, a research gap was identified in relation to micro CHP devices for RVs. In the development of this work, a micro CHP prototype was created, with 980 W of maximum electrical power, consisting of a single-cylinder internal combustion engine, an automotive alternator and heat exchangers for heat recovery. The prototype was enclosed and instrumented, allowing to evaluate the thermal power obtained in water flowing through exhaust gases, lubricating oil and cooling air heat recovery systems, in addition to measuring the electrical power of the alternator. Operational tests were carried out with 46 different test conditions, combining variations in the engine speed (N) and in electrical load. The prototype electrical efficiency reached 10.18 % ±0.2 % at 3008 rpm with 83 % load. The maximum electrical power was 0.746 kW±0.015 kW at 3637 rpm and 100 % load. The maximum thermal energy recovery rate was 3.267 kW±0.039 kW. Utilization factor reached 65.57 % ±0.33 % at 3208 rpm with 63 % load. Compared to traditional means of providing electrical power and thermal power for RVs, the prototype proved to be more advantageous when the demand is for electrical power and space heating. However, for conditions in which the demand is for electrical power and water heating, or just electrical power, the current solutions available are still more efficient.

Fast cooling miniature Joule-Thomson cryocooler with improvement of energy recovery and thermal mass distribution

Open-cycle conical miniature J-T cryocoolers are favored for applications in the infrared detection field, due to their fast cooling and compact lightweight features. In this type of cryocooler, the regenerative heat exchanger for energy recovery plays a vital role in both its miniaturization and fast cool-down, because its geometry greatly determines the distribution of its heat transfer area and thermal mass. To examine and optimize the effects of the geometry on its fast-cooling performance, a validated transient analysis model, being capable of simulating the entire operating process of an open-cycle miniature J-T cryocooler, was applied to analyze and compared the transient heat transfer and flow characteristics, and then the cooling performance under different operating conditions for one cylindrical and three different cone-angle J-T cryocoolers in this study. Researches show that in terms of geometry, it is advisable to increase the heat transfer area near the cold end of heat exchanger while ensuring a relatively small thermal mass for the entire cryocooler. By analysis, a novel structure of heat exchanger with reduced volume and mass was proposed to enhance the fast-cooling performance and miniaturize cryocooler. Studied results indicate that the performance of this novel cryocooler is improved in terms of reducing the cool-down time by 28% and the volume by 50%. Additionally, this optimized structure exhibits excellent fast-cooling performance under broader operating conditions.

Waste heat recovery of blast furnace slag considering resource utilization: Localized cooling enhancement in moving bed heat exchanger

When utilizing moving bed heat exchanger for waste heat recovery from blast furnace slag, it's essential to enhance the cooling rate in the high-temperature section to meet the requirements for resource utilization. In this study, we introduced moving bed heat exchanger with airflow-assisted cooling, building upon the shell-and-tube design. This innovation enhanced the cooling rate of the section above the glass transition temperature in the heat exchanger, surpassing the critical cooling rate, through the introduction of cooling airflow in the high-temperature section. Experimental studies were conducted to investigate the impact of airflow Reynolds number, granular Peclet number, and the dimensionless height of the section with cooling airflow on Nusselt number, recovery efficiency, local cooling rate, and pressure drop. Multi-objective optimization of these parameters were performed using hierarchical analysis, leading to the determination of the optimal operational parameters for airflow-assisted cooling moving bed heat exchanger, resulting in Pes=10.18, Reg=2969, and Hap/H=1/9. The Nusselt number, recovery efficiency and pressure drop corresponding to the optimal parameters were 3.593, 79 % and 407.57 pa, respectively.

Exergetic, economic and exergy-based sustainability analysis of a power generation system with CO2 capture and methanol production

This study proposes a new biomass-based Combined Heat and Power (CHP)/renewable methanol production through captured carbon dioxide. A liquid organic hydrogen carrier (LOHC) stores and releases hydrogen. Literature review shows that LOHC has been integrated into such a system for the first time. The exergetic, exergoeconomic, and economic performance are researched. On average, results show that the exergetic efficiency of the system is 6.6 %. Depletion ratio (DR) is the depletion rate of the exergy source given to the system, which is desired to have low values. The sustainability index (SI) is the DR's reverse, representing the required exergy source per exergy destruction (exergy depletion). High values of this index indicate less exergy source consumed in the system. Exergetic ecological index (EECI) compares the useful products or depleted exergy per consumed exergy source destruction. The negative value of EECI means that the depletion of the exergy source is more significant than the product exergy and vice versa. The exergy-based sustainability indices, including DR, SI, and EECI, are found as 0.934, 1.071, and −0.868, respectively. Exergoeconomic analysis reveals that the exergy destruction rate at the system's capital cost (REX) value is 0.087×102. Heat Recovery Steam Generator (HRSG), gasifier, and Heat Exchanger 1 (HE1) are obtained as 5.153×10−2 (kW/€), 1.707×10−2 (kW/€), 3.094×10−2 (kW/€) respectively. The payback period and the REX are 14.56 years and 0.087×10−2 (kW/€), respectively; it seems long for an engineering application, but these technologies are relatively new, and their costs might decrease soon. Exergy destruction or lost power per capita, which means depleted investment, costs much less than 1. Although the thermodynamic values of the system seem low because irreversibilities occurred in chemical reactions, CO2 saved and produced methanol and cannot be ignored in terms of its environmental and economic benefits, considering CO2 taxes and the methanol economy.

Synthesis of heat-integrated water networks considering detailed heat exchanger design

This paper introduces a mathematical programming approach for synthesising heat-integrated water networks (HIWNs) with a detailed heat exchanger design. A common assumption of constant heat capacities of water streams is relaxed. The specific heat capacity of water is now considered as a nonlinear function of temperature to avoid errors in calculating energy balances. In addition, most previous works have assumed constant individual heat transfer coefficients for water streams and utilities when estimating heat transfer areas. This assumption can lead to significant over- or under-estimation of heat transfer areas. In this research, individual and overall heat transfer coefficients for heat exchangers are calculated based on the detailed design of plate heat exchangers for heat recovery. The objective function of the proposed mixed integer nonlinear programming (MINLP) model is to minimise the total annualised cost (TAC). The TAC includes operating costs for freshwater and utilities, pumping costs for overcoming pressure drops in heat exchangers, and investment costs for heat exchangers. Three examples are solved in this work to verify the proposed model.

Efficient simultaneous optimization approach for the synthesis of large-scale heat-integrated water networks

This study develops a generalized superstructure and a compact mixed-integer nonlinear programming (MINLP) model for achieving efficient energy and water utilization in designing integrated process water systems with large sizes. Utilizing non-isothermal mixing reduces the need for many heat exchangers for indirect heat recovery. Hence, an exchanger-centric modeling strategy for heat integration is embraced over a hot–cold streams matching-based approach. A novel two-step approach is proposed to address the nonconvex and nonlinear challenges and ensure the efficiency of getting high-quality solutions. Initially, a targeting model incorporating linear heat integration is adopted to reduce the superstructure and partially initialize the design model. Subsequently, in the synthesizing stage, a two-level global search procedure is devised, integrating adaptive exchanger adjustment with multi-start local optimization. The robustness and efficiency of the proposed approach are confirmed through the resolution of four large-scale problems within one hour, yielding improved results compared to the best-known existing solutions.

Energy, exergy, thermoeconomic analysis of a novel multi-generation system based on geothermal, kalina, double effect absorption chiller, and LNG regasification

The current work proposes a novel CCHP system based on geothermal energy, which can generate hot water, chilled water, and power. Kalina is considered for power generation, double-effect absorption chiller cycle for cooling production, and LNG regasification cycle for thermal integration of absorption chiller and electric power generation. The hot water is generated using the heat recovery exchanger. Energy, exergy, economic, and environmental assessments are made on the proposed system to see how it can improve energy and exergy efficiency compared to the single state. Also, an environmental assessment is carried out to analyze the reduction of CO2 emission rate compared to other studies that used biomass and fossil energy sources. The findings were confirmed considering the outcomes of the previous works, and the proposed system showed remarkable performance in terms of exergy and energy efficiency, which were 85.22 % and 52.79 %, respectively. Also, the highest irreversibility exists in the LNG Regasification sector, with a ratio of 81 %. The economic estimation determined that the two parameters, total cost rate and cost per exergy, are 186.74 $/h and 8.3 $/GJ, respectively. Therefore, the innovation of the work lies in the heart of heating, cooling, and power products compared to the previous works.

The characteristics of heat exchanger for local utilization of wastewater heat under different operating conditions

RELEVANCE. The authors research the local utilization of wastewater heat (in close proximity to the place of their formation) based on a heat exchanger. To select rational characteristics of the heat exchanger and correctly assess the potential energy effect, it is necessary to take into account the influence of the operating conditions of the device (duration of individual use of the shower, mass flow of heated and heating water, temperature of heated water at the inlet to the heat exchanger, temperature of the flow at the moment the device is turned on).THE PURPOSE. The purpose of the work is to research the dependence of the efficiency of wastewater heat utilization on the operating conditions of the heat exchanger and to identify the parameters that have the greatest impact on the effect of energy-saving measures.METHODS. Based on a verified mathematical model of the thermal operation of a recovery heat exchanger, the temperature distribution inside the flows of the heated and heating water in time is calculated (from the moment of switching on until reaching a stationary operating mode). Based on the data obtained (the temperature of the heated water at the outlet of the heat exchanger at each point in time), the absolute and relative heat savings are determined under various operating conditions.RESULTS. The influence of the non-stationary phase of the heat exchanger operation on its energy efficiency is considered. For a specific heat exchanger configuration, the time required for the device to reach a steady state of thermal operation is determined. It was revealed that the greatest influence on the relative and absolute savings of thermal energy is exerted by such operating conditions as mass flow and temperature of heated water at the entrance to the heat exchanger. The temperature of the flow at the initial moment of time has the least influence on the energy effect.CONCLUSION. The operating conditions that have the greatest impact on the effect of local wastewater heat recovery are determined. The need to take these conditions into account when designing a heat exchanger and choosing its optimal parameters has been confirmed.

Heat recovery of decentralised façade units: A case study

Indoor air quality affects the health of the premises' users and the efficiency of their work. Sealing the building envelope to reduce the demand for thermal energy reduces the number of air changes. One of the solutions that improve indoor air conditions is decentralised façade units to alternate air supply and exhaust. An important issue is the implementation of heat recovery in this type of solution, the aim of which is to reduce the energy consumption of ventilation and air conditioning systems. There is no description of research in the literature on the use of various methods of heat accumulation in façade units. The available descriptions are limited to aluminium, ceramics, and PCM. The novelty presented in this study is the use of gravel and gypsum as materials for storage in heat recovery exchangers.The objective of the article is to identify important factors influencing the efficiency of heat recovery, the parameters of air flowing through decentralised façade units, and the internal air temperature.The heat recovery exchanger located in a decentralised façade unit for alternating air supply and exhaust was analysed. Two exchanger filling configurations were tested: gravel and gypsum. Three variants of cycle length settings were analysed: 2 min, 6 min, and 10 min. Airflow, temperature before and after the heat recovery exchanger, and temperature inside and outside the room were measured.Research showed that the type of material that fills the heat recovery exchanger is more important than the duration of the cycle or airflow. The heat recovery efficiencies achieved for gypsum are 72 – 87 %, and for gravel 92 %. The material used and the setting do not affect the temperature values before the heat exchanger. The setting has no significant impact on the temperature after the heat recovery exchanger and on the indoor air temperature. In turn, the type of material significantly affects the values of these parameters.

Design and Analysis of Pancake with Different Pitch Coil Heat Exchanger for Low-Grade Heat Recovery

Design and Analysis of Pancake with Different Pitch Coil Heat Exchanger for Low-Grade Heat Recovery

IMPROVING THE PLANTS EFFICIENCY OF THERMAL INCINERATION HOUSEHOLD WASTE BY RECOVERING WASTE HEAT

The work is devoted to research on the creation of recovery exchanger for waste heat exhaust gases of household waste incineration plants. The purpose of the work is to develop a technical solution for the heat recovery exchanger of waste incineration plants (WIP) and determine its thermal efficiency indicators. The main objectives of the study were to analysis of the modern experience of using the WIP and establish requirements for the creation of a exhaust gas heat recovery exchanger, develop a new technical solution for the heat recovery exchanger, and determine the change patterns in its main thermal indicators in different operating modes of the WIP. The known methods of thermal calculation of heat exchangers and the results of previous studies on the development and implementation of heat recovery equipment operating on dusty gases were used. The results of work on the creation of a new technical solution for an air-heating heat recovery exchanger with the ability to clean working surfaces from dust deposits are presented. The heat exchange surface of the heat recovery exchanger is composed of steel panels formed by tubes with membranes. The tubes applied have circular flow turbulizators on their internal surfaces. Turbulizators provide heat transfer intensification by 1.4 to 1.8 times with a moderate increase in aerodynamic resistance compared to other methods of heat transfer intensification. The regularities of changes in the main indicators of the heat recovery unit in different operating modes during the year in the practical range of changes in its input parameters were established. The research results show that, depending on the initial temperatures of gases and air, the excess air ratio in exhaust gases and the dust level of the working surface, the heat recovery exchanger provides heating capacity of 72-263 kW; cooling of exhaust gases to a temperature of 107-245 °C; heating of air to 96-220 °C. It was also established that the deposition of dust on the heat exchange surface of the heat recovery unit under the considered conditions leads to a decrease in the heating air temperature by 1.3-1.4 times and an increase in the final exhaust gas temperature by 1.1-1.2 times. At the same time, the heating capacity of the heat recovery exchanger is reduced by half. To increase heat recovery efficiency, it is necessary to periodically clean the working surfaces of the heat recovery unit with compressed air.

Compact heat pipe heat exchanger for waste heat recovery within a low-temperature range

Heat pipe heat exchangers (HPHX) are attracting attention for being capable of effective heat exchange through phase change. Much waste heat is generated at low-temperature sources, but there has been a lack of research on this. Most previous studies focus on large HPHXs over 1 m in length. In the present study, a compact HPHX (185 × 70 × 140 mm3) with nine heat pipes was fabricated, and the potential for waste heat recovery (WHR) was investigated at low heat sources. Also, the thermal performance of the HPHX was compared with and without the fin of the heat pipe. Experiments were conducted by changing the hot-side gas temperatures (50, 75, 100, and 125 °C) and mass flow rates (0.03, 0.04, and 0.05 kg/s). The heat transfer rate and pressure loss had a linear relationship with the hot-side temperature and mass flow rate. Despite a low temperature 50 °C, more than 100 W of heat can be recovered using a compact HPHX. At least 1.8 times improved thermal performance was obtained in HPHX in which finned heat pipes were installed. These results are expected to serve as indicators for various applications that generate low-temperature waste heat, such as clothes dryers and kitchen hoods.

Fluid-thermal characteristics of solar-exhaust gas powered dryer: An experimental validation

A new method of supplementing heat energy in a solar-exhaust gas greenhouse dryer has been experimentally evaluated in this paper. An internal combustion engine using diesel produced exhaust gas which was channeled to a hybrid recuperative heat exchanger (HRHE) for energy recovery. Fluid and thermal characteristics of the dryer were reported for three modes of drying: solar mode (SM), solar-exhaust gas mode (SEGM), and exhaust gas mode (EGM). Consequently, the dryer room air temperature were found as: 14.82–58.46 °C, 34.49–61.97 °C and 25.75–30.77 °C respectively. Moisture evaporated as a result of temperature variations were computed as: 0–20.8 g, 0–17.79 g and 0–22.33 g respectively. Fluid characteristics of exhaust gas included: average density of 0.7218 kg/m3, volumetric flow rates from 4.2×10−3 – 1.67×10−2 m3/s, maximum residence time of 418 s in HRHE, mass flow rates from 11.32 to 45.07 kg/h, velocity in connectors ranging from 2.14 to 8.52 m/s, velocity in tubes from 0.035 to 0.14 m/s, Reynolds number ranging from 2681 to 10674 in connectors and 344–1368 in tubes, Nusselt number ranging from 14 to 43 in connectors and constant at 3.66 in tubes. Available energy in exhaust gas was found in the range of 2082.32–16002.5 kJ/h, corresponding to temperatures of 197.19–359.82 °C as a result of engine speeds varied from 750 to 2500 rpm. Kinetic energy in exhaust gas increased with increased velocity for both tubes and connectors to a maximum of 39.79 kJ/h in connectors and 1.289×10−2 kJ/h in tubes. The significance of this study is in promotion of faster product drying in SEGM and slower drying for delicate products requiring low heat energy supply in EGM.

Frost accumulation in the non-hygroscopic rotary heat exchanger for heat recovery

In this paper, a process of frost accumulation inside the non-hygroscopic rotary heat exchanger depending on operating conditions is investigated. The study was based on the results of the numerical simulations conducted in the original computer program. The analysis considers both the threshold conditions of frost accumulation and the conditions of frost layer growth. For this purpose, three indicators related to frost accumulation were distinguished: a frost accumulation limit temperature, a surface of the matrix covered with the accumulated frost layer, a maximum thickness of the accumulated frost layer. The analyzes assessed the influence of operating parameters on these indicators, focusing on the return air relative humidity. It was revealed that the highest values of this parameter cause the greater thickness of the frost layer. However, the largest ‘frost’ accumulation area was created in the range of average return air relative humidity values – in the analyzed cases it was RH2i=38.5 %. The paper also includes a detailed analysis of the threshold temperature of frost accumulation. These limits of outdoor air temperature were determined for a wide range of operating parameters’ values: both airstream relative humidity and nominal temperature effectiveness. Additionally, eight probable configurations of active heat and mass transfer areas within rotary heat exchanger channels were revealed, which are related to the initiation of frost accumulation. Consequently, the locations of the initial ‘frost’ accumulation area were predicted. It was also found that the greatest impact on the frost accumulation threshold temperature has the return air relative humidity RH2i, although this influence is not unequivocal. The most unfavorable conditions regarding nominal temperature effectiveness in terms of frost accumulation are also unclear. In some cases, it was even demonstrated that an increase in the effectiveness can eliminate the ‘frost’ accumulation area.