Coated Heat Exchanger Research Articles

Robust anti-icing double-layer superamphiphobic composite coatings for heat exchangers

The formation and accumulation of ice on the heat exchangers of air conditioners significantly reduce the performance issues, as well as the stability and heating efficiency. Superhydrophobic anti-icing coatings passively achieve multifunctional anti-icing properties by minimizing water droplet contact and promoting Cassie ice formation. However, long-term performance in frigid environments remains challenge for these coatings due to limitations in anti-icing durability and mechanical properties. Herein, a double-layer polymer-SiC/F-SiO2 superamphiphobic composite coating is developed. The bottom layer comprises a polymer PAI and SiC composite, and the top layer consists of F-SiO2. The composite coating demonstrates superior performance in simultaneous frost prevention, low ice adhesion, easy frost removal, exceptional mechanical strength, and long-lasting anti-icing durability. Our developed double-layer coating exhibits low ice adhesion strength down to 9.2 kPa, remarkable mechanical resilience against scratching and flushing, and delayed frost properties. These superior anti-icing properties, manifested by both lower adhesion strength and improved frost repellency, lead to a doubling of frosting time and the easier removal of ice on coated heat exchangers compared to traditional units. The development of this novel superamphiphobic composite coating provides a practical approach to creating durable anti-icing materials, leading to significant improvements in air conditioner performance.

Experimental and numerical studies on moisture adsorption/desorption characteristics across the circular fin tube desiccant coated heat exchanger

To improve the energy transfer abilities, enhance the quality of air, and curtail the growth of microorganisms, the desiccant coated heat exchangers are a capable substitute compared to conventional heat exchangers due to their capability of decoupling the sensible and latent loads and utilizing low-grade thermal energy. The concept of desiccant coated heat exchangers (DCHE) can be employed in various heat exchange/transport systems like heat pumps, atmospheric water harvesters, adsorption chillers, and air conditioners. Thus, in this research work, a circular fin tube desiccant coated heat exchanger (DCCHE) has been proposed and designed and a dynamic numerical model is established for simulating the combined heat and mass transport occurring across the adsorption/desorption process of a DCCHE employing finite element approach. The moisture and temperature distribution and flow of fluid are considered in the transient numerical model covering several domains such as water flow, air flow, and the coated desiccant. The desiccant material utilized in the current analysis is silica gel. The validation study showed good agreement, and the maximum error between the experimental result and simulated model for the humidity ratio of air and temperature of outlet air were observed to be ± 8 % and ± 7 %, respectively. A parametric analysis is carried out to assess the effect of varying the different input parameters on DCCHE’s performance. The field emission scanning electron microscopy results concluded that a uniform coating is formed on the circular fin surface. The experimental examination on the sorption kinetics of the silica gel showed that the moisture uptake capability was 0.34 g/g. Moreover, the proposed circular fin tube desiccant coated heat exchanger has been compared with the conventional plate type fin tube DCHE to assess its performance in terms of thermal coefficient of performance and specific dehumidification energy.

Novel MOF-303/G coated wire-finned heat exchanger for dehumidification applications–Experimental investigation

Adsorption dehumidification outperforms other technologies regarding moisture removal rate, lower energy consumption, and environmental friendliness. The selection of desiccant material, the heat exchanger used, and the method of regeneration significantly impact the performance of desiccant dehumidifiers in terms of water vapour removal rate and energy consumption. Metal-organic framework materials are a new class of microporous materials with high water adsorption characteristics compared to conventional desiccants like silica gel. In this work, a novel MOF-coated electrically heated wire finned heat exchanger was developed, and its performance was experimentally investigated. Wire-finned tubes coated with MOF-303/G outperformed other wire-finned tubes coated with Aluminium fumarate, MIL-100(Fe), MIL-100(Fe)/G, MOF-801 and MOF-801/G, achieving water uptake of 22 %. Also, the addition of graphene nanoplatelets during the synthesis of MOF-303 resulted in a significant enhancement of its thermal diffusivity to reach 0.184–0.23 mm2/s at temperatures ranging between 25 °C and 115 °C. The Coefficient of Performance (COP) for the developed multi-tube MOF-303/G coated heat exchanger was experimentally tested and compared to other desiccant-coated heat exchangers. Results showed that COP values ranging from 0.6264 to 1.02 to 1.2 at the regeneration temperatures of 45 °C, 60 °C and 70 °C, respectively, were achieved, which are higher than those reported in the literature.

Experimental analysis and investigation of desiccant coated heat exchanger applications involving condensation and sorption mechanisms

This study focuses on the application of desiccant coated heat exchangers (DCHEs) in conditions where condensation occurs below the dew point temperature. Previous experimental studies on DCHEs and DCHPs have predominantly examined the sorption mechanism above the dew point, leaving a gap regarding material selection and system design in the presence of condensation. To address this, a guide is introduced that combines the understanding of condensation and sorption processes for desiccant applications. The guide is applied to evaluate the suitability of a superabsorbent-LiCl (SAP-LiCl) composite desiccant for desiccant coated heat pump (DCHP) applications. The stability of the desiccant in the presence of condensed water droplets is validated. Additionally, an experimental investigation explores the relationship between sorption and condensation mechanisms, identifying the transitional coolant temperature at which dehumidification shifts from condensation-dominant to sorption-dominant. By incrementally adjusting the coolant temperature in 5 °C steps, the transitional temperature of the SAP-LiCl based DCHE is determined to be approximately 15 °C under the target operation scenario, rendering its suitability for additional below-dew-point applications. The study reveals optimal operational parameters for DCHE, demonstrating a potential energy saving of 20 %. Based on these findings, a DCHP prototype is constructed to assess the feasibility of DCHE in real-world applications. The transient behaviors of this prototype are analyzed during each operating cycle, yielding a coefficient of performance of 4.97 and a cooling capacity of 1.5 kW under baseline operating conditions.

Experimental study on wavy fin desiccant coated heat exchanger for hybrid air conditioning systems

The desiccant-coated heat exchanger (DCHE) is an energy-efficient alternative dehumidification equipment for humidity control in air conditioning. It exhibits a higher dehumidification performance than other solid desiccant systems and can simultaneously handle both latent and sensible loads. Taking the clue of research outcomes from the alternative heat exchangers to enhance the performance, a wavy fin DCHE is designed, fabricated and tested. Objectives of the study are to (i) analyse the dynamic performance of DCHE, (ii) compare the heat and mass transfer characteristics of plain and wavy fin-type DCHEs, and (iii) examine the influence of different operating parameters on the performance. The experimental investigation shows that the dehumidification capacity and thermal coefficient of performance of the proposed wavy fin DCHE are 0.34 g/s and 1.041, respectively, which are 75% and 60% higher than the conventional plain fin type. In addition, the developed DCHE has an average cooling capacity of 0.85 kW, which is 42% higher. Besides, the total quantity of moisture adsorbed also increased by 35%. Furthermore, the sensitivity analysis of the parametric study identified air velocity, cold water temperature, and inlet humidity ratio as the most influential operating parameters on the performance of wavy fin DCHE.

Solar regenerated carbon-based composite desiccant coated heat exchangers for air dehumidification

Desiccant dehumidification can realize decoupling of latent and sensible heat loads in dehumidified air conditioning, thereby reducing energy consumption. This paper proposes a solar regenerated carbon-based composite desiccant coated heat exchangers (DCHE) for air dehumidification. Carbon-based composite desiccant is electrostatically sprayed onto the surface of the heat exchanger to form a dehumidification coating. An experimental system for evaluating the dehumidification and regeneration performance of DCHE is established. The Taguchi method is employed to evaluate the impact of operating parameters such as air velocity, water flow rate, and regeneration temperature on the dehumidification and regeneration efficiency of DCHE. The result shows that air velocity is the primary factor impacting the dehumidification performance of DCHE, with regeneration temperature and cooling water flow rate following in importance. With the faster air velocity and the lower cooling water flow rate, DCHE shows the better dehumidification performance. Moreover, excessively both high and low regeneration temperatures can result in a degradation in the dehumidification performance of DCHE. Under optimal operating conditions, the dehumidification rate (EDE) and thermal coefficient of performance (COPth) of DCHE can reach 7.5 kg/h and 2.3, respectively. The moisture content of the air reduces from 19.6 g/kg to 8.0 g/kg.

Performance study of a heat pump fresh air unit based on desiccant coated heat exchangers under different operation strategies

Fresh air system is getting more and more important nowadays. The heat pump fresh air system based on the desiccant coated heat exchanger (DCHE) that can handle the latent and sensible load of fresh air efficiently has a large energy saving potential. To improve the performance of the heat pump fresh air system based on DCHE, in this paper, a control system based on PLC is established and the operation strategies of the system are investigated. An operation strategy including automatic control of the opening of the electronic expansion valve based on the suction superheat of the compressor, control of the switchover period based on the relative humidity of supply air, and control of compressor frequency based on evaporation and condensation temperature is established. By using the optimal operation strategies, the dehumidification rate and COP of the system can reach 4.42 kg/h and 6.2 in summer, compared with the unoptimized strategy it increased 8% and 25%, respectively. The two performance indices can reach to 3.04 kg/h and 6.98, with an increase of 89% and 78%.

Theoretical study on the dehumidification behaviors of dual-desiccants coated cross-flow heat exchanger with staged adsorption-desorption process

The conventional desiccant coated heat exchangers (DCHEs) suffer from the inefficient utilization of regenerative energy and the inadequate dehumidification capacity of a single desiccant. In this study, a concept of dual-desiccants cross-flow DCHE with staged adsorption-desorption process was proposed to strengthen dehumidification capacity and energy efficiency simultaneously. In order to guide the construction of proposed DCHE, the steady-state dehumidification capacity of single-desiccant (specifically, silica gel, FAM Z01, FAM Z05 and EMM-8) cross-flow DCHE was investigated by thermodynamic analysis. The dual-desiccants cross-flow DCHE was thus constructed, and its numerical model was established to investigate the dehumidification characteristics. Theoretical analysis revealed that single-desiccant cross-flow DCHEs exhibit different optimal relative humidity ranges for dehumidification. The minimum regeneration temperature driving dehumidification cycle increases as the lower supply air humidity is required. Numerical results demonstrated that the staged adsorption process ensures a large dehumidification capacity across a wide humidity range, including low humidity conditions. Meanwhile, the staged desorption process facilitates the cascade utilization of energy and the direct recovery of regenerative waste heat. For dual-desiccants cross-flow DCHE utilizing silica gel and EMM-8, the moisture removal capacity and dehumidification coefficient of performance can reach 0.012 kg/kg and 0.41, respectively.

Alginate Biopolymeric Coated Heat Exchanger for Atmospheric Water Harvesting

Alginate Biopolymeric Coated Heat Exchanger for Atmospheric Water Harvesting

Experimental investigation of a thermo-responsive composite coated heat exchanger for ultra-low grade heat utilization

There is a growing interest in desiccant coated heat exchanger (DCHE)-based systems due to characteristics of isothermal dehumidification and compact structure. To enhance the dehumidification efficiency and reduce the regeneration temperature of DCHE-based systems, a thermo-responsive composite desiccant coated heat exchanger (PSLCHE) was developed. This study focuses on investigating the influence of operating parameters (such as air temperature, relative humidity, regeneration temperature and adsorption/desorption time ratio) on the dynamic performance, average dehumidification capacity (Davg) and thermal coefficient of performance (COPth) of the system. Under the air condition of 30 °C & 60% RH, the PSLCHE demonstrates remarkable performance with the Davg of 7.3 g·kg−1 and COPth of 0.93 at 40 °C. This study provides a route to the design of highly efficient air conditioning systems using thermo-responsive composite for ultra-low grade thermal energy at 40 °C.

Performance evaluation of wavy fin desiccant coated heat exchanger for hot and humid climatic conditions

The desiccant coated heat exchanger (DCHE) is a novel type of solid desiccant equipment that can simultaneously handle both latent and sensible loads. The systems based on DCHE are considered as an energy efficient alternative to dehumidification in conventional air conditioning. Hence, the present study proposes a novel wavy fin based DCHE which can perform better than plain fin configuration. The main objectives of this paper are (i) to numerically evaluate the heat and mass transfer characteristics of wavy fin DCHE and compare them with the plain fin type, (ii) to propose a novel method of estimating the effective cycle time for DCHE operation, and (iii) to identify the critical geometric and operating parameters influencing the performance of DCHE. The investigation reveals that the dehumidification capacity of the proposed wavy fin DCHE is 61% higher than that of the plain fin DCHE. Further, the dehumidification increases by 53% when the DCHE is operated on the proposed effective cycle time. In this work (i) a high performance DCHE based on wavy fin configuration is proposed, (ii) a methodology for determining and operating the DCHE on an effective cycle time is presented and (iii) an overview to choose a suitable geometric design for wavy fin DCHE in air conditioning applications is provided.

Numerical study on the heat and moisture transfer characteristics of FAM Z01 coated heat exchanger with different direction configuration of air flows

The internal cooling fluid is used to remove adsorption heat and improve dehumidification performance in desiccant coated heat exchangers (DCHEs), and its direction configuration not only has a significant impact on the heat and moisture transfer characteristics, but also sometimes determines the complexity of theoretical analysis. In this study, a simplified two-dimensional numerical model was developed to simulate the heat and moisture transfer within an air-cooling type FAM Z01 DCHE. The proposed model considers the local heat and moisture transfer mechanism and the effect of direction configuration of air flows. A parametric analysis is carried out for cross flow, counter current flow and cocurrent flow DCHEs. Results shows that the numerical model can receive a highly-reliable accuracy. The dehumidification performance indices of three DCHEs exhibit complex variation patterns under various parameters. The counter current flow DCHE demonstrates a better heat and moisture transfer performance compared to cross flow and cocurrent flow DCHEs. It can be found that the maximum relative deviation of dehumidification coefficient of performance DCOP between the cross flow DCHE and the other two DCHE is just 14.1%, indicating a feasible way for simplifying numerical modeling by transforming cross flow DCHE into counter current or cocurrent flow DCHE.

Enhanced moisture sorption through regulated MIL-101(Cr) synthesis and its integration onto heat exchangers

Through regulated and less toxic MIL-101(Cr) synthesis and superabsorbent polymeric binders, this work reports MIL-101(Cr)-coated heat exchangers with twice the water uptake of the silica gel-coated control towards energy-efficient cooling.

Performance evaluation of MIL-101(Cr) based desiccant-coated heat exchangers for efficient dehumidification

Metal-organic frameworks (MOFs) with ultrahigh water uptake and moderate regeneration temperature are ideal sorbents for desiccant-coated heat exchangers (DCHEs). Herein, MIL-101(Cr) is synthesized through the hydrothermal method and the product shows an exceptional adsorption capacity of 1.17 g/g at 90 % RH and steep water uptake at middle RH (P/P0 = 0.35–0.45). Then MIL-101(Cr) coated heat exchangers (MCHE) are fabricated by the spraying method and a testing platform is established to investigate the dehumidification performance. Parameter analysis revealed that a medium switchover period (9–12 min), low-temperature heat sources (48–60 °C), and lower cooling water temperature facilitate the overall performance. Experiments under two different heat exchanger layouts are conducted, showing MCHEs in the parallel mode present better heat/mass transfer performance and MCHEs in the series mode tend to acquire higher heat recovery efficiency (η). Hot and humid climates are favorable in both modes while low-RH climates require lower inner cooling fluid temperature. Under five typical inlet air conditions, the dehumidification capacity of the parallel mode and the series mode ranges from 0.26 to 0.43 kg/h and 0.24–0.40 kg/h, and the η ranges from 0.30 to 0.47, 0.32–0.53, respectively.

Performance evaluation of a high-efficient hybrid adsorption refrigeration system for ultralow-grade heat utilization

50–80 °C heat sources are abundant in renewable energy areas and industrial processes. However, they are difficult to be effectively used due to their ultralow grade. In order to solve this problem, this paper presents a high-efficient hybrid adsorption refrigeration system combing with desiccant coated heat exchangers. In this study, the performance of the hybrid system is experimentally investigated and parametric effects are fully discussed. Results indicate that the hybrid system operates efficiently across a wide heat source range of 50–80 °C. A high-temperature hot water facilitates the cooling production of the hybrid system, while a low-temperature hot water is more beneficial to the COP promotion. Also, a lower cooling water temperature, higher air temperature, higher relative humidity and longer switch time can enhance system performance. The optimal cooling capacity is quantified as 4579 W when powered by 80 °C hot water and the COP reaches its maximum of 0.673 under a heat source temperature of 50 °C. Comparison results show that compared with present systems, the hybrid system has greater adaptability as well as higher efficiency in utilizing 50–80 °C ultralow-grade heat sources, and can satisfy the simultaneous demand for cooling and dehumidification, which offers a promising way to high-efficient energy utilization and sustainable development.

Investigation of zirconium (Zr) coated heat exchanger surface for the enhancement of heat transfer and retardation of mineral fouling

BackgroundFouling is a major problem in many industrial heat exchangers as in these devices water has been used for heat exchanging purposes. With the commencement of foulants deposition on heat exchanger surface the performance of heat exchangers is deteriorated due to lower thermal conductivity of the deposits formed on the heat exchanger surface. This study investigated the heat transfer and mitigation of CaCO3 fouling by applying zirconium coating layer on heat exchanger surface. MethodsZirconium was chosen as a coating material because of its surface adhesion and one of the best corrosion resistance materials. The Physical Vapor Deposition approach was implemented to develop a coating layer on heat exchanger surface. Heat transfer was investigated at numerous temperatures and flow rates to examine the impact of coating on heat transfer. Fouling experiments were conducted to examine the CaCO3 accumulation on the uncoated SS316L and Zirconium coated surfaces. After the fouling experiments, the coupon was disassembled from the test segment for the investigation of surface texture, after fouling effect and the morphology of deposits was assessed by using Field Emission Scanning Electron Microscopy (FESEM). Significant findingsResults reveal that the Zirconium coated surface declined the fouling deposition and improved the heat transfer.

Zeolite-coated fin–coil heat exchanger for CO2 recovery from simulated dry flue gas via low-temperature-heat-driven TSA

Adsorbent-coated heat exchangers are expected to achieve not only high adsorption and desorption rates based on the rapid heat transfer in their adsorbent layers but also a low pressure drop owing to their structured adsorbent layers. In this study, a CaA-zeolite-coated adsorber was experimentally investigated to improve the CO2 capture performance of low-temperature-heat-driven temperature swing adsorption (TSA) from dry flue gas, a major source of greenhouse gas emissions. At a feed CO2 concentration of approximately 10 vol%, a flow rate of 1 L-STP/min, and a temperature swing range of 20 °C–80 °C, the performance of the coated adsorbent was first compared with that of a heat exchanger of a similar volume packed with CaA zeolite pellets. Due to the enhancement in heat transfer of the coated adsorbent, almost the same CO2 concentration and a higher CO2 recovery ratio as those of the packed adsorbent were obtained at an appropriate cycle time despite that the amount of coated adsorbent was only one-third that of the packed adsorbent. Therefore, the accelerated heat transfer in the adsorbent layer significantly affected the increased mass transfer rate in or around the coated adsorbent. Next, the effect of the space velocity (SV) was investigated using a coated heat exchanger that had a three-times-longer length in the gas flow direction and approximately the same thickness of the adsorbent coating layer as the above coated heat exchanger. Although a separate discussion on the larger adsorber size is necessary, the CO2 recovery concentration was over 50% and the recovery ratio exceeded 80% as the SV was reduced by one-third. Furthermore, the fin pitch of the heat exchanger was doubled and the thickness of the adsorbent coating was doubled while maintaining the same volume of the heat exchanger and the same amount of adsorbent coating. This was done to reduce the heat capacity of the heat exchanger, which is a factor in heat loss in TSA, by reducing the number of fins used. The doubled adsorbent coating layer thickness and halved contact area with the gas flow resulted in a decrease in CO2 capture. Thus, the effects of an increased adsorbent layer thickness and a reduced contact area should be determined to further increase the effectiveness of the adsorbent-coated heat exchangers.

Experimental study on the performance of a novel superabsorbent polymer and activated carbon composite coated heat exchangers

Desiccant coated heat exchangers have attracted great attention for their ability to handle both sensible and latent heat loads. However, widely used salt-embedded composite silica gels risk salt solution leakage. To alleviate this problem, a novel superabsorbent polymer (sodium polyacrylate) and activated carbon composite coated heat exchanger (C-DCHE) was developed and tested. Effects of air-side and water-side parameters on the performance of C-DCHE in terms of average dehumidification capacity (Davg) and thermal coefficient of performance (COPth) were investigated and compared to an activated carbon coated heat exchanger (A-DCHE). With the outdoor air of 38 °C&60% RH, Davg and COPth of C-DCHE can be up to 6.1 g kg−1 and 3.6, an improvement of 43% and 26% over A-DCHE, respectively. Besides, the performance of C-DCHE was compared with conventional and salt-embedded silica gels coated heat exchangers in recent publications. C-DCHE showed superior air supply comfort owing to a lower average outlet air humidity ratio and larger COPth under most conditions. Especially, at low regeneration temperatures of 40∼50 °C, its COPth can be 50%–220% higher. This paper provides a promising direction for salt-free composites in DCHE-based systems.

Performance assessment of heat exchanger coated with encapsulated ultra-high salt content desiccant

Desiccant coated heat exchanger (DCHE) provides an effective way to decouple the sensible and latent heat loads and improves the energy efficiency by raising the evaporation temperature of air conditioning system. In this paper, a new encapsulation of porous membrane is adopted to fabricate the ultra-high salt content composite desiccant, AFLi30@P, whose salt content can reach over 85w %. Its adsorption capacity can be as high as 3.8 g g−1 at 27 °C and 70 % humidity. And then, DCHE based on AFLi30@P is developed. Experiments on both desiccant and DCHE levels are conducted to investigate the effects of temperature and humidity conditions on the dehumidification performance. The results reveal that, different from air temperature and dehumidification temperature, air humidity and regeneration temperature has positive effects on the DCHE dehumidification performance. Comparison between vertical and horizontal placements presents that horizontal placement of AFLi30@P on DCHE can avoid the effect of gravity to realize more uniform adsorption and heat transfer capacity. Dehumidification capacity of AFLi30@P DCHE can reach 1.08 g g−1 under the conditions of 27 °C air temperature, 70 % air humidity, 30 °C dehumidification temperature and 60 °C regeneration temperature.

Performance evaluation of PVA/PEO/LiCl composite as coated heat exchangers desiccants

Composite desiccants have led to a growing interest because of their eco-friendliness, sustainability, and excellent dehumidification properties. In this paper, we present a method for constructing a dehumidification composite of PVA/PEO/LiCl that is eco-friendly and recyclable. The results show that PVA/PEO/LiCl has better dehumidification performance than SG, SG/LiCl, PVA/PEO, and PVA/PEO/CaCl2 as coated heat exchangers desiccants. The addition of LiCl caused the internal hydrophilic polymeric hygroscopic carriers to work in concert with the membrane porosity. This resulted in a considerable increase in the membrane's hygroscopic rate and capacity, as well as improved mechanical qualities. As the concentration of LiCl was increased, the surface sealing of the PVA/PEO polymer improved dramatically, and the shape of the polymer-formed film changed from loose to firmly packed. The structural shift toward densification boosted the strength of the co-blended dehumidified films. With a PVA/PEO/LiCl ratio of 1:1:2.5 and the coating thickness of 10–15 µm, the greatest dehumidification effectiveness is achieved. During five subsequent sorption and desorption procedures, the sorption of PVA/PEO/LiCl remained between 70 and 75%, demonstrating its outstanding reusability and cycling stability.