Refrigerant Charge Research Articles

Seasonal performance comparison of R-410A and R-454B in a variable-speed air-cooled scroll chiller

In recent years, there has been increasing concern about the impact of air conditioning and refrigeration on global warming. This is particularly related to the emissions of refrigerants with high global warming potentials (GWP), such as R-410A, which is used in air conditioning and chiller systems. There has been a concerted effort within the HVAC industry to find lower GWP refrigerants to replace R-410A in HVAC systems. In this paper, a 10.5 kW (3 TR) air-cooled variable-speed scroll chiller has been utilized to conduct an experimental comparison of R-410A (GWP of 2088) and its low GWP A2L alternative R-454B (GWP of 466) according to AHRI 551/591 testing condition at rating and part load condition with optimized charge. The compressor speed and suction superheat were matched for both refrigerants at all the test conditions. R-454B shows 98 % capacity and 102 % efficiency compared to R-410A at rating conditions of 35 °C outdoors and water return temperature of 12 °C. The IPLV of the R-454B chiller was just 1 % higher than R-410A. The discharge temperature of R-454B and compressor isentropic efficiency is 8 % higher and almost like R-410A, respectively. The optimized charge of R-454B was 5 % lower refrigerant charge compared to R-410A. The LCCP analysis for major Indian cities over a 15-year operational span demonstrates a notable reduction, ranging from 6.6 % to 7.3 %, in overall R-454B emissions compared to R-410A. The study demonstrates that R-454B is a drop-in replacement to R-410A designs and reduces the direct GHG emission from the chiller by 76 %.

Quantitative detection of refrigerant charge faults in multi-unit air conditioning systems based on machine learning algorithms

Refrigerant charging discrepancies constitute the predominant malfunctions in air conditioning systems. Achieving the optimal charging level is crucial for system performance, underscoring the importance of precise refrigerant level prediction. This study introduces an algorithm designed for the quantitative detection of refrigerant charging errors by integrating the Markov Transition Field (MTF), Convolutional Neural Networks (CNN), and Multi-head Self-Attention (MSA) mechanisms. A high-precision enthalpy difference chamber was employed to establish a Variable Refrigerant Flow (VRF) refrigerant charging test bench. This setup facilitated the analysis of system parameter sensitivity to charging faults and aided in the creation of a training dataset for the algorithm. Comparative analysis was conducted against Support Vector Machines (SVM), Random Forests (RF), CNN with Self-Attention (AT), and MTF-CNN-MSA. The findings reveal that our method adeptly captures temporal dependencies and dynamic shifts in time series as visual representations, offering novel insights for discerning fault patterns within such data. Notably, the maximum pressure variations at high-pressure and low-pressure points were 0.25 MPa and 0.07 MPa, respectively, with temperature shifts of 12 °C and 3.5 °C at the high and low-temperature points. The high-pressure and high-temperature points are particularly sensitive to changes in refrigerant charging, and parameters from these sections were utilized to construct the dataset. The CNN-MSA algorithm demonstrates consistent performance across various fault types, effectively delineating fault characteristics. The accuracies achieved by SVM, RF, CNN-AT, and MTF-CNN-MSA were 84.38 %, 73.75 %, 88.13 %, and 93.75 %, respectively. In comparison, the CNN-MSA algorithm was able to more accurately detect refrigerant charge faults at different levels.

Visualization Study on Oil Return Characteristics of Vapor Compression Heat Pump System

Vapor compression heat pump technology is a widely utilized method for energy conversion. Lubricating oil plays a crucial role in the heat pump system cycle by effectively reducing wear on the compressor’s moving parts and preventing refrigerant leakage. However, it can also create an oil film in the heat exchange equipment, which increases thermal resistance and diminishes heat transfer efficiency. This study utilizes a vapor compression heat pump system test bench to investigate factors influencing the system’s oil circulation rate, the two-phase flow patterns of refrigerant and lubricating oil, and the impact of oil circulation on system performance. The findings reveal that as the compressor speed increases, the oil circulation rate initially decreases before increasing again. Additionally, a decrease in the evaporator’s heat load leads to a reduction in oil circulation at high temperatures, while it increases at low temperatures. Furthermore, increasing the opening of the electronic expansion valve results in a gradual decrease in the oil circulation rate, whereas an increase in the refrigerant charge correlates with a rise in the oil circulation rate. The oil return flow pattern can primarily be categorized into three states: slow oil return, oil film flow, and high-speed oil return. These patterns are closely related to the degree of superheat, with lower superheat levels intensifying oil return.

The role of miscible PAG lubricants in ammonia refrigeration systems reduction and compactness

Today’s society is going through an expanding use of natural refrigerants, earlier present only in big industrial facilities. This has made necessary the charge reduction and compactness improvement of the equipment, to fit the safety measures and standards requirements. The refrigerant charge and the complexity of an ammonia refrigeration system are mainly because of flooded evaporators use and the need for an efficient oil separation system, since conventional lubricants and ammonia are immiscible. The use of ammonia miscible lubricants can address the oil separation problem. A Poly-Alkylene-Glycol (PAG) can be designed and formulated to be miscible with ammonia, and the plant operated like a conventional halocarbon refrigerant one. The circulating lubricant will be recovered through the compressor suction port of the compressor. This will help in the operation of Direct Expansion (DX) evaporators, reducing to a testimonial size the oil separation circuit. This article is a translation of the article by Muñoz-Alonso M, Dixon L, Seeton CJ, Karnaz J. The role of miscible PAG lubricants in ammonia refrigeration systems reduction and compactness. In: Proceedings of the 9th IIR Conference on the Ammonia and CO2 Refrigeration Technologies. Ohrid: IIF/IIR, 2021. DOI: 10.18462/iir.nh3-co2.2021.0018 Published with the permission of the copyright holder.

Enhancing Semiconductor Chiller Performance: Investigating the Performance Characteristics of Ultra-Low-Temperature Chillers Applying a Liquid Receiver

This study investigates the implementation of a cryogenic chiller utilizing a mixed-refrigerant cascade refrigeration cycle (MRCRC). In this setup, R-404A is employed in the high-temperature circuit (HTC), while a mixture of refrigerants is utilized in the low-temperature circuit (LTC). Unlike a conventional MRCRC that operates without a receiver to maintain the composition ratio, this research explores the impact of receiver installation on system performance. Experiments were conducted with and without a receiver to assess performance improvements and device behavior. With a fixed refrigerant charge of 4 kg, the suction and discharge pressures of the LTC compressor remained low and stable after the receiver’s installation. The addition of a receiver significantly reduced the cooling time, with further reductions observed as the refrigerant charge increased. The system achieved evaporative heat capacities of 0.59, 1.76, and 2 kW for refrigerant charges of 4, 7, and 9 kg, respectively. Notably, at the maximum refrigerant charge of 11 kg, the evaporative heat capacity peaked at 3.3 kW. These findings indicate that incorporating a receiver is crucial for enhancing the cooling performance of cryogenic coolers using mixed refrigerants and stabilizing device operation. This contrasts with previous studies that omitted receivers due to concerns over potential alterations in the composition ratio of the mixed refrigerant.

Experimental study of the performance of a water-to-water heat pump equipped with a liquid receiver with different charge levels

This paper presents an experimental investigation of the effects of a decreasing refrigerant charge on the performance and operating conditions of an R513A water-to-water heat pump equipped with a large liquid receiver, an electronic expansion valve, and a vapor accumulator at the compressor inlet. The mass of the refrigerant inside the machine is progressively extracted to simulate refrigerant leakages occurring during the normal operation or in case of failures. The results show that the subcooling is the parameter mostly affected by a refrigerant charge variation since it rapidly collapses to 0 K with the charge reduction. Furthermore, it is possible to identify three different zones in which almost all the properties analyzed (COP, heating capacity, operating pressures and expansion valve opening) exhibit peculiar trends with the charge decrease: subcooling sensitivity zone, constant parameters zone, and compressor failure risk zone. The extension of these zones is determined by the size of the liquid receiver which is installed in the system and, for the heat pump under consideration, is between 100% and 95% of the initial charge for the first zone, between 95% and 40% for the second zone, and below 40% for the third zone.

Design and experimental characterization of a propane-based reversible dual source/sink heat pump

The current paper presents the design and energy performance analysis of a propane-based reversible Dual Source/Sink Heat Pump (DSHP). DSHPs offer an alternative to conventional water to water and air to water heat pumps, leveraging the strengths of both technologies in an efficient manner. The developed prototype incorporates an innovative Dual Source/Sink Heat eXchanger (DSHX), enabling the unit operating in various modes, including space heating, space cooling, and domestic hot water production using brine, air or both simultaneously as a source/sink. The DSHX serves as as both a condenser or an evaporator, directly rejecting or absorbing heat from air and/or brine. By eliminating secondary loops and defrost cycles, the DSHX minimizes energy losses. The main novelty of this work lies in the DSHX that integrates external units typically duplicated in DSHPs into a single component, eliminating the need for split refrigerant flow rates, thus avoiding maldistribution, refrigerant charge increase and draining valves. A steady state experimental campaign was conducted in a climatic chamber to characterize the DSHP prototype and validate the DSHX performance models. Heating capacity up to 11.2 kW and COP values up to 4.7 were achieved at nominal compressor speed by supplying hot water at 35 °C with an ambient temperature of 7 °C. Similarly, when producing cold water at 7 °C, cooling capacity and EER reached 9.8 kW and 3.6, respectively, at nominal compressor speed using air as heat sink at 35 °C. The effects of various operating parameters on the overall coefficient of performance and heat duty in both heating and cooling modes, considering air or brine as heat source/sink are analyzed in detail. Results demonstrate enhancements of approximately 15 % in capacity and efficiency compared to earlier work. Moreover, four deterministic models were created in order to predict the behaviour of the DSHX and validated against experimental results, reaching deviation values below 15 %.

Experiments on the performance of low temperature air source heat pump based on parallel connection of evaporators

An air source heat pump experimental bench with evaporators connected in parallel at low ambient temperatures is established to investigate the key parameters and operating performance after this retrofit. Experimental tests are conducted on the novel heat pump unit to study the optimal refrigerant charge, heat production capacity and COP at different ambient temperatures, to observe the frost condition in the operation of the unit, and to optimise the control logic. Results show that the optimum refrigerant charge amount after retrofit is 26 kg. Further experiments show that under the condition of −12 °C, heat production capacity of the retrofitted unit is increased by 5.39%, the COP is increased by 4.13%, and the evaporating temperature is increased by 1.50 °C; under the condition of 2 °C, the heat production capacity of the retrofitted unit is increased by 5.72%, the heating COP is increased by 2.51%, and the evaporating temperature is increased by 2.93 °C. At the same operating time, the modified unit has less frost on the evaporator surface. The optimum suction superheat after the modification is 1 °C, and it is at this point that the unit has the highest heating COP, which is 7% higher than at 0 °C suction superheat. The heat production and COP are less affected by the supplement gas superheat. Through the modification of the parallel evaporator, the overall heating performance and frost prevention ability of the unit are greatly improved.

REFRIGERANT CHARGING UNIT FOR THE RESIDENTIAL AIR CONDITIONERS: AN EXPERIMENT

In the present work, an automatic R410A refrigerant charger for residential air conditioners is fabricated and tested. The charger operates on the throttling principle and uses the suction pressure of the compressor to estimate the refrigerant charge level. This helps to reduce the risk of compressor damage and ensures the correct composition ratio of R410A refrigerant when charged into the machine. The charging process is controlled by the LabVIEW platform, which provides adequate control and visualization of the charging process. The developed charger meets expectations in solving the technical problems encountered when charging R410A refrigerant for residential air conditioners. It is compact, portable and can be directly controlled through the LabVIEW interface, allowing real-time visualization of the charging process. The present work is expected to make a significant practical contribution, serving as a useful reference for the future manufacturing of compact portable equipment in the residential air conditioning field.

Simulation of diffusion of combustible refrigerants R1234yf and R290 leakage in automotive air conditioning

Electric vehicles that utilize a heat pump system have a refrigerant charge increase of at least 400 g compared to traditional fuel vehicle air conditioning systems. If combustible refrigerants are used, the risk of combustion increases when the refrigerant leaks and spreads to the passenger compartment. This paper dynamically monitored the volume concentrations of combustible refrigerants R1234yf and R290 as they leaked and entered the passenger compartment accompanied by air supply by numerical simulation. The results indicated that refrigerants are more prone to accumulate in the rear row than in the front. After a leak, the average volume concentration of R1234yf at the four air outlets was 1.58 %, and 3.36 % for R290. at the breathing points of four passengers, the average volume concentration was 0.99 % for R1234yf and 2.39 % for R290. Near the feet of the passengers, the average volume concentration was 0.95 % for R1234yf and 2.27 % for R290. The highest volume concentration of R1234yf in the passenger compartment was below its LFL, whereas all monitoring points for R290 exceeded its LFL. Compared to experimental data, the difference in maximum refrigerant volume concentration was approximately 1.2 %.

Refrigerant charge estimation method based on data-physic hybrid-driven model for the fault diagnosis of transcritical CO2 heat pump system

The transcritical CO2 heat pump system is a promising and excellent heating scheme. However, the operating pressure of the transcritical CO2 heat pump system is high, and refrigerant leakage is a frequent problem. It is worth noting that the operating principle of CO2 heat pump systems is unique. Thus, conventional refrigerant charge estimation methods for subcritical systems cannot be directly applied to transcritical systems. In this study, a novel refrigerant charge estimation method for the transcritical CO2 heat pump system is proposed. Based on the heat and mass transfer characteristics of the transcritical system, the grey box model constrained by physical laws is constructed. Based on the machine learning algorithm, the correction model of the grey box model is built. The hybrid model combines the advantages of the high generalization ability of the physic-driven model and the nonlinear fitting ability of the data-driven model. The results show that the novel estimation method can accurately predict the refrigerant charge, enabling fault diagnosis of refrigerant leakage. The data-driven correction model can improve the prediction accuracy of the grey box model. The Mean Relative Error of the hybrid-driven model can be controlled within 5 %.

Refrigerant charge influence on the performance of a transcritical CO2 system with flash-tank for low-temperature refrigeration

While the use of CO2 as a natural refrigerant become widespread, there is a scarcity of research on refrigerant charge effects in transcritical CO2 refrigeration applications, especially for systems with flash-tank and two-stage compressors. In this work, an experimental investigation of the effect of the Normalized Refrigerant Charge (NRC) on the parameters and performance of a Flash-Tank Vapor Injection (FTVI) CO2 refrigeration system is carried out. Within the explored NRC range of −0.02 to 0.247, an optimal NRC of 0.18 was identified that yields a maximum COP regardless of operating conditions. Results suggest that typical operating parameters such as temperature and pressure are not being affected by the NRC because of the charge buffering function of the flash-tank. However, the liquid–vapor separation capacity of the flash-tank causes a reduction in the refrigeration capacity when the system is not charged properly. This refrigeration capacity detriment related to undercharged conditions becomes more severe under high ambient temperatures, with a maximum COP penalty of 20 % when comparing the lowest charge versus the optimal charge performances at a gas cooler outlet temperature of 40 °C.

Experimental investigation on performance of heat pump air circulation evaporating separation system for saline wastewater treatment

There is a gradual increase in the discharge of saline wastewater which is harmful to the environment and human health. A novel heat pump air circulation evaporating separation (HP-ACES) system for saline wastewater treatment is proposed and experimentally investigated. The results show that the system evaporation efficiency (SEE) initially decreases before increasing with the expansion valve opening at different refrigerant charges. However, the SER increases by 0.67 kg/h at 1.21 kg refrigerant charge while they descend and then rise at 1.53 and 1.86 kg refrigerant charges. Noted that the effects of expansion valve opening on the SER and SEE are lessened with refrigerant charge. Moreover, there exist the optimal values of total air flow and air flow proportion of packing concentration tower at which the SER and SEE peak, and both of the solution concentration and cooling water flow negatively affect the SER and SEE. Besides, compared with similar evaporating separation systems, the HP-ACES system has higher energy efficiency for treating high saline wastewater and the SEE ranges from 0.86 to 1.64 kg/kWh (5–25 % CaCl2 solution). However, the SEE is low, and it is necessary to reveal the cause and propose the optimization schemes in the future.

Experimental study on system characteristics of vapor compression refrigeration system using small-scale, gas-bearing, oil-free centrifugal compressor

Small-scale refrigeration centrifugal compressors hold the potential for superior efficiency compared to currently used positive displacement compressors. However, traditionally, centrifugal compressors have been employed in large refrigeration systems with cooling capacity of several hundred kilowatts or more. Conversely, the application of small-scale centrifugal compressors in smaller refrigeration systems, with cooling capacity in the tens of kilowatts, has largely remained theoretical, lacking practical experimental studies. For a better understanding of the operational characteristics of such systems, this study constructed a 45 kW R245fa refrigeration system test bench based on the small-scale oil-free centrifugal compressor utilizing gas foil bearings. The experimental investigation analyzed the impact of refrigerant charge amount, compressor speed, and coolant flow rate on system performance. Results indicate that the system achieves its peak cooling capacity of 49.73 kW with a system COP of 3.64 at 65,000 rpm. Moreover, at 50,000 rpm, the system demonstrates its highest system COP of 4.23 with a cooling capacity of 26.07 kW. In comparison to traditional positive displacement compressor refrigeration systems, it exhibits higher energy efficiency. Additional simulation studies were conducted to evaluate the effects of substituting R1233zd(E) for R245fa on the system performance and to assess the feasibility of such a substitution.

Experimental study on Joule-Thomson refrigeration system with R1150/R290/R601a for ultra-low temperature medical freezer

Since the global outbreak of the COVID-19, the requirement for ultra-low temperature medical freezers has increased rapidly. Joule-Thomson refrigeration systems, known for their simple structure and low cost, are an excellent choice for ultra-low temperature medical freezers. To further improve the performance of Joule-Thomson system for large-capacity ultra-low temperature freezer, this study conducted a series of experiments on the performance of a ternary hydrocarbon mixture R1150/R290/R601a. The impacts of refrigerant charge, capillary tube size, compressor speed, and environmental temperature on the steady-state and dynamic performance were investigated in detail. The results showed that under a fixed system configuration and charge, the freezer can achieve an ultra-low temperature of −82.2 °C and a COP of 0.256 at an environmental temperature of 25 °C, while maintaining a low compressor pressure ratio not exceeding 12. Furthermore, the matching relationship between capillary size and charge was emphatically studied, and it was found that the optimal refrigerant charge increased as the inside diameter of the capillary tube decreased or its length increased. The inside diameter had a more significant effect on system performance than the length. The suitable charge was 225–275 g for capillary tubes with an inside diameter of 1.4 mm, and increased to 400–450 g for an inside diameter of 0.7 mm. In addition, the experiments with various compressor speeds indicated that, before the cabinet temperature dropped to −40 °C, the variation of compressor speed had little effect on the pull-down rate. However, the higher the compressor speed, the lower the cabinet temperature that can be achieved. When the speed was 3500 rpm, the system achieved its lowest daily energy consumption, which was 8.42 kW·h/24 h.

A METHODOLOGY TO COMPARE REFRIGERANTS BASED ON THEIR THERMOPHYSICAL PROPERTIES

To compare different refrigerants in a fair way has been always a matter of discusión. There are some properties like Isoentropic COP, volumetric capacities, pressure ratio that can be compared easily and supply some relevant information. However, how they are translated into real systems is not so direct. For instance, isentropic COP is obtained under the assumption that all the refrigerants are able to work in the same condensing and evaporating temperatures, but these variables depend on the temperature lift of the secondary fluid and in real systems, they will not be the same. The size of the heat exchangers and, hence, the cost depend on heat transfer coefficients and pressure drop, and extrapolating this data in a straightforward way is not easy; even the required refrigerant charge is not easy to determine. In this work, the authors propose a methodology to compare different refrigerants based on the external operating conditions, the approach compares the system performance not only in terms of efficiency parameters but also in term of operation map, size and cost of the system and refrigerant charge. To do that, a basic modelling approach has been developed based on quite extended correlations for heat transfer coefficient, pressure drop and void fraction in the heat exchangers. The developed methodology does not pretend to be an exact design guide but a guide that researchers and students can use to estimate qualitatively the differences among refrigerants

Effects of simultaneous soft faults in a reversible and variable-speed air-to-water heat pump

The widespread adoption of air-to-water heat pumps is driven by their high heating efficiency, with some systems offering reversibility for cooling applications. Despite their benefits, these systems are susceptible to soft faults such as undercharge, overcharge, and outdoor unit airflow restrictions (condenser fouling in cooling mode or evaporator fouling in heating mode). Detecting and addressing these faults before they escalate into hard faults is critical. While previous experimental studies have focused on residential heat pumps with imposed faults, most were conducted on air-to-air systems, primarily in cooling mode and with fixed-speed compressors. This study addresses the gap by investigating a reversible air-to-water heat pump employing R290 and a variable-speed compressor. The experimental work consists of imposing faults of undercharge, overcharge, condenser and evaporator fouling, individually and in combination. The investigation analysis considers the system’s behavior under these faults in cooling and heating modes and how the control strategy (subcooling or superheat) and a compressor suction accumulator affect it. In cooling mode, the combination of 85% undercharge and 50% condenser fouling worsens the EER up to 39% if the superheat is controlled and up to 10% if the subcooling is controlled. In heating mode with subcooling control, 85% undercharge combined with a 50% evaporator fouling can worsen the COP by 32%. However, there are cases where those same fault levels only impact the COP by 4% due to the charge requirements of the condition and that the excess charge is stored in the compressor suction accumulator. As compressor speed can significantly affect the refrigerant charge requirements of the system, this work shows its potential for fault detection, particularly for undercharge. These findings can provide a basis for developing Fault Detection and Diagnosis methodologies for reversible air-to-water heat pumps with variable-speed compressors.

Performance of liquid-line magnets and CNT nano-lubricant in a refrigerator with varying mass charge of R600a refrigerant

Performance of liquid-line magnets and CNT nano-lubricant in a refrigerator with varying mass charge of R600a refrigerant

Thermoeconomic assessment of the air-cooled tropical data center through energy-saving strategies

Rapid developments in the electronic information industry drive the increased energy usage and carbon emission of data center buildings, prompting the focus on the energy efficiency and environmental sustainability. Expanded operation envelopes of tropical data centers is assessed to analyze the potential for the building energy savings and carbon emission reduction through collaborative analysis of operation modes (OMs), supply air temperature (SAT), and outdoor air temperature (OAT). The OMs of compression vary with the setpoints of SAT, in which the average exergy efficiency of compressors at alternate operation mode is 6.8 % and 8.0 % lower than that of double and single compression operations. As SAT rises from 20 °C to 32 °C, the system exergoeconomic factor increases from 5.4 % to 8.0 %, and the average carbon cost decreases by 36.5 %. Additionally, with just an 8.5 % increase in exergy cost (i.e., Case 8) at OAT rising from 30 to 34 °C, the high SAT and low refrigerant charges provide considerable exergy cost advantages versus resisting the OAT fluctuations. Dynamic operation strategies are also proposed and compared to cope with the impacts of tropical environments. Compared to the 26 °C SAT baseline, the average energy savings are 9.1–14.7 %, indicating the ability to fully utilize outdoor and indoor conditions.

Effect of oil concentration, viscosity, and surface tension on tube temperature and heat transfer coefficients of R1234ze(E)/oil spray evaporation on an enhanced tube bundle

Shell-and-tube evaporators are widely used in desalination, refrigeration, and petrochemical industries. The heat transfer process in spray-type evaporators combines the droplets impingement at the top rows of the bundles and falling film evaporation for the bottom rows, with a transitional region in the center of the bundle. Spray evaporators are a promising alternative to flooded-type shell and tube heat exchangers because they run with significantly lower refrigerant charge amounts.This study presents new data for the spray evaporation of low-GWP refrigerant/oil mixtures on an enhanced tube bundle. The experiments involved R134a (HFC) and R1234ze(E) (HFO) paired with synthetic Polyolester (POE68 and POE220) lubricants. The tests utilized a boiling enhanced tube (re-entrant cavity tube) and were conducted at saturation temperatures of −6.7 °C, 2.2 °C and 10 °C (20°F, 36°F and 50°F), with heat flux varying from 3 to 35 kW/m2 and oil circulation ratio (OCR) ranging from 0.4 % to 2 %. With the addition of oil, the bundle heat transfer coefficient (HTC) of refrigerant/oil mixtures decreased by 20–50 % at an OCR of 1.2 % and a heat flux of 13.5 kW/m2, across the tested saturation temperatures. The average wall superheat across the bundle exhibited at least a 48 % increase for all the refrigerant/oil mixtures tested at an OCR of 1.2 %. The viscosity of the refrigerant/oil mixture showed a limited influence on heat transfer compared to surface tension, mainly because the oil concentrations were quite small. Oil promoted a partial dry-out of the tubes, especially at the bottom of the bundle. Foaming was observed and increased with heat flux and oil concentration. Foam sheets on the tube bundle created an additional barrier for the liquid refrigerant to reach the tube heat transfer surfaces and thus decreased the overall bundle heat transfer coefficient.