Evaporator Coil Research Articles

Potential use of evaporator coils for water extraction in hot and humid areas

Limits of water production from atmospheric water vapor are investigated for cases where fresh hot humid air is cooled over evaporator coils of refrigeration machines and then directed to open localized areas. This application may be considered as a source of limited amounts of potable water at a free cost since the water is a by-product of the climate conditioning process. For high air velocities, water yield was found to diminish due to insufficient evaporator capacity. On the other hand frosting effect was found to bind the vapor condensation process due to coolant starvation. A working chart was developed for quick prediction of water yield for any combination of atmospheric air temperature in the range (25 < T a < 40°C) and relative humidity in the range 30% < ϕ < 100%. The water yield due to air dehumidification on finned evaporator coils was calculated using a developed model that relates the surface efficiency to the enthalpy change of cooled moist air. The surface was treated as an extended finned coil with mean efficiency for wet and dry conditions. For typical hot humid weather (Jeddah, Saudi Arabia, 21°23'N and 39°E), the daily variation of water yield showed to follow the relative humidity pattern with minimum during midday hours. The maximum water yield for August 1st was predicted as 17.6 kg/m 2 day for an air speed of 2.25 m/s. On the basis of actual climatic data, the monthly estimated average water yield during August and February were 509 and 401 kg/m 2, respectively.

A Numerical Study of a Wavy Fin and Tube CO2 Evaporator Coil

Carbon dioxide is among the promising natural refrigerant alternatives to HCFCs and HFCs for refrigeration. In the perspective of this development, a numerical model for dry evaporator coil design and simulation, based on correlations for carbon dioxide, is presented, along with typical cooling coil simulations. The model uses the NIST database for refrigerant properties and provides adequate flexibility for local parameter calculations across the coil. The heat transfer and pressure drop data used to validate this model originate both from a dedicated test bench built in our laboratories and from other sources. These data were predicted satisfactorily over the operating range corresponding to refrigeration applications. Apart from the air side pressure drop, which is predicted with a maximum uncertainty of 25%, a comparison between experiments and calculation are within 1°C for air and CO2 outlet temperatures and within 13.5% for capacity and CO2 pressure drop. Simulations at low and moderate temperatures were performed on coil configurations typically used in supermarket applications. Key parameter distributions, including temperatures, pressures, and relative humidity, were tracked inside and outside the tube coil. Tube relative positions in the coil largely influenced phase repartition and overall operation. The resulting air temperature distribution gradients were also affected.

Prediction of the energy consumption of household refrigerators and freezers via steady-state simulation

A simplified model to assess the energy performance of vapor compression ‘on–off’ controlled refrigerators is presented herein. The model consists of first-principles algebraic equations adjusted with experimental information obtained from the refrigeration system under study. The experimental work consisted of controlling and measuring the system and component operating conditions in order to gather key information for the development and validation of the model. The methodology showed similar accuracy to that using more sophisticated dynamic simulation codes, but with lower computational costs. When compared to experimental data, the model predicted AHAM energy consumption tests within a ±5% deviation band. A sensitivity analysis considering the number of tube rows in the condenser coil, the number of fins in the evaporator coil and the compressor stroke is also reported. The refrigeration system under study was a top-mount ‘Combi’ 600-l refrigerator, ‘on–off’ controlled by the temperature of the fresh-food compartment.

Evaporator Air-Side Fouling: Effect on Performance of Room Air Conditioners and Impact on Indoor Air Quality

This study investigated experimentally the effects of real fouling material at the air side of the evaporator's heat exchanger coil on the performance of room air-conditioner units as well as the impact of some fouling material compositions on indoor air quality. At the end of the experiments, examination of the biological contents of the fouling material on the coil was performed; it was found that the coil frontal face had heavy colonies of Aspergillus fungi of different species. This result is important to fill in the knowledge gaps between biological deposition, colonization, and occupant problems of staying in an environment with a fouled evaporator coil. The results of the effect of such fouling materials on the unit performance show that the unit coefficient of performance at the standard air face velocity of 1.53 m/s for a clean evaporator coil was 2.82. It decreased to 1.89 (67%) after injection of 100 g of fouling materials, dropped to 1.79 (63.4%) after injection of 200 g of fouling materials, and fell to 1.23 (43.6%) after injection of 300 g of fouling materials. However, these results demonstrate that the predominant effect of fouling is to cause significant degradation in a room air conditioner's coefficient of performance.

시설원예 습도환경 제어를 위한 냉각식 제습기 개발

During the winter season in Korea, the relative humidity of greenhouse at night often exceeds 90% because air temperature inside the greenhouse is usually controlled using a heater with all of windows closed to minimize heat loss, thereby requiring the use of a dehumidifier that can maintain optimum humidity levels of <TEX>$70{\sim}80%$</TEX> to provide a good growth condition of crops. Also, such a high humid condition can cause the development of a pest, such as insects, fungi or diseases. However, the use of most conventional dehumidifiers for low temperature dehumidification is limited because their performance is degraded due to frost accumulation on the evaporator coil. This study was carried out to develop a refrigeratory-based dehumidifier suitable for low temperature dehumidification in greenhouse cultivation. The developed dehumidifier consists of a condenser and an evaporator installed separately so that relative and absolute humidity levels can be reduced when air passed through the condenser and evaporator, respectively. The prototype dehumidifier showed a dehumidification capacity of <TEX>$5{\sim}7kg/h$</TEX> when air with a temperature of <TEX>$15{\sim}25^{\circ}C$</TEX> and a relative humidity of <TEX>$70{\sim}95%$</TEX> came into the dehumidifier. Under the condition that either temperature or relative humidity was fixed, the amount of condensed water was proportional to the levels of both temperature and relative humidity.

Defrost cycle performance for a circular shape evaporator air source heat pump

Air source heat pumps have numerous advantages in many applications over other heating equipment with regard to energy efficiency. However, there are two main problems with air source heat pumps: (1) heating capacity decreases when the outdoor air temperature becomes lower and (2) when there is frost formation on the outdoor heat exchanger surfaces in humid climates. This paper will examine the defrost cycle for a residential heat pump with circular shaped evaporator coil in more detail paying special attention to the high humidity conditions encountered in maritime climates. The investigation was to optimise the efficiency of an air source heat pump operating under a range of conditions that would include defrost. Performance optimisation was achieved through a series of experiments carried out to the EN14511 test standard from which it was possible to note the best defrost initiation condition, defrost operating time and intervals between defrosts that most benefited the performance of the heat pump.

Experimental investigation of the performance of industrial evaporator coils operating under frosting conditions

This paper describes a field experimental investigation of the effects of frost formation on the performance of a low-temperature large-scale evaporator coil used in industrial refrigeration systems. A series of experiments were conducted to determine the in situ coil cooling capacity of the evaporator over time as frost builds on its surfaces. Field-measured quantities include inlet and outlet air temperatures, inlet and outlet air relative humidity, and air volume flow rate. These measurements provide a baseline set of experimental data that can be used to validate numerical models of industrial evaporators operating under frosting conditions.

Comparison of parallel- and counter-flow circuiting in an industrial evaporator under frosting conditions

This paper describes a theoretical model of a large-scale, ammonia-fed evaporator coil used in an industrial refrigeration system and operating under low temperature air and refrigerant conditions that are typically encountered in refrigerated storage spaces. The model is used to simulate the performance of counter-flow and parallel-flow circuited evaporator coil designs under frosting conditions. The counter-flow frost model is validated using in situ data obtained from a field-installed evaporator coil. The performance of an evaporator in a parallel-flow circuit arrangement is simulated and compared to counter-flow circuiting. The effects of coil circuiting are evaluated in terms of the frost distribution across the evaporator coil and the associated reduction in cooling capacity during operation.

The impact of fouling on the performance of filter–evaporator combinations

This paper presents results of experiments performed on different combinations of five types of filters of varying efficiencies (MERV4, 6, 8, 11, and 14) and four types of evaporator coils with depths and fin geometries under clean and fouled conditions. The fouled conditions were obtained after injection of 600 g (1612 g/m 2 of coil face area) of dust upstream of the filter–coil combination, which was meant to simulate a year of operation in the field. The air-side pressure drops of the coils and filters and air-side effective heat transfer coefficients of the coils were determined from the measurements under the clean and fouled conditions. Depending upon the filter and coil, the coil pressure drops increased in the range of 6–30% for an air velocity of 2.54 m/s. The impact was significantly greater for tests performed without an upstream filter (the coil pressure drops increased from 43% to 200%). The largest relative effect of fouling on pressure drop occurs for coils with fewer rows, primarily due to higher fin densities. The impact of fouling on air-side effective heat transfer coefficients was found to be relatively small, which ranged from −14% to 4%. In some cases, heat transfer was actually enhanced due to additional turbulence caused by the presence of dust. However, with large dust deposits, heat transfer is degraded because the dust also acts as insulation and creates an uneven air velocity.

Application of Exergoeconomic Techniques to the Optimization of a Refrigeration Evaporator Coil With Continuous Fins

An optimization for the geometrical parameters of continuous fins on an array of tubes of a refrigeration evaporator is developed in this paper using the exergy method. The method is based on exergy, economic analysis, and optimization theory. As there are humid air and refrigerant single- and two-phase streams involved in the heat transfer process, then there are irreversibilities or exergy destruction, due to pressure losses İΔP, due to temperature difference İΔT and due to specific humidity gradient İΔω. These principal components of total irreversibility are not independent, and their relative contribution to total irreversibility of a cross-flow refrigeration evaporator is investigated. A change in geometry was obtained by varying the evaporator tube diameter for a selected evaporator capacity, and hence the evaporator tube length and total heat transfer area are calculated for a fixed evaporator face length. In this way, the effect of changes in the geometry on the total number of exergy destruction units of the heat exchange process is investigated. The optimum balance between the three components of irreversibility (İΔP,İΔT, and İΔω) is also determined, thereby giving the optimum solution for the heat exchanger area. The total cost function, which provides a measure of the contribution of the evaporator to the total cost of the refrigeration system, is expressed on the basis of annual capital and electrical energy costs. The total cost function is minimized with respect to the total heat transfer area and the total number of exergy destruction units (NI). The relationship between the operational variables, heat transfer area, refrigerant and air irreversibilities, and the total annual cost for this type of evaporator are developed, presented, and discussed. The pressure, temperature, and specific humidity irreversibilities are found to be 30.34%, 33.78%, and 35.88%, respectively, of the total irreversibility, which is 8.5% of the evaporator capacity.

Advances in control of frost on evaporator coils with an applied electric field

This paper introduces an innovative technique on use of an applied electric field for control of frost over evaporator coils with fin density and geometric configuration of interest to freezer/refrigerator applications. The technique discussed in this paper, referred to as the “dielectric barrier discharge” (DBD) method, may be particularly suitable for application in evaporator coils with high fin density. Experiments conducted with a small-scale laboratory test-module, as well as a full-scale supermarket evaporator are presented. The DBD technique is based on generating localized non-resistive heating within fins of an evaporator coil via application of a high-voltage, alternating current through electrodes. Our experiments demonstrate that the defrosting time using DBD is substantially shorter than conventional techniques, while the energy consumption associated with the process is less than one half of the corresponding energy of the electrical resistance heating methods. Basic operational principles of the technique, its advantages and limitations when compared to conventional electrical defrosting techniques are discussed and presented for the first time in this paper.

Aerosol Generation by Blower Motors as a Bias in Assessing Aerosol Penetration into Cabin Filtration Systems

In cabin filtration systems, blower motors pressurize a vehicle cabin with clean filtered air and recirculate air through an air-conditioning evaporator coil and a heater core. The exposure reduction offered by these cabins is evaluated by optical particle counters that measure size-dependent aerosol concentration inside and outside the cabin. The ratio of the inside-to-outside concentration is termed penetration. Blower motors use stationary carbon brushes to transmit an electrical current through a rotating armature that abrades the carbon brushes. This creates airborne dust that may affect experimental evaluations of aerosol penetration. To evaluate the magnitude of these dust emissions, blower motors were placed in a test chamber and operated at 12 and 13.5 volts DC. A vacuum cleaner drew 76 m 3 /hour (45 cfm) of air through HEPA filters, the test chamber, and through a 5 cm diameter pipe by a vacuum cleaner. An optical particle counter drew air through an isokinetic sampling probe and measured the size-dependent particle concentrations from 0.3 to 15 μm. The concentration of blower motor aerosol was between 2 × 10 5 and 1.8 × 10 6 particles/m 3 . Aerosol penetration into three stationary vehicles, two pesticide application vehicles and one tractor, were measured at two conditions: low concentration (outside in the winter) and high concentration (inside repair shops and burning incense sticks used as a supplemental aerosol source). For particles smaller than 1 μm, the in-cabin concentrations can be explained by the blower motor emissions. For particles larger than 1 μm, other aerosol sources, such as resuspended dirt, are present. Aerosol generated by the operation of the blower motor and by other sources can bias the exposure reduction measured by optical particle counters.

MEPR versus EEPR valves in open supermarket refrigerated display cabinets

This paper presents the comparative experimental field performance of mechanical evaporator pressure regulating valves (MEPR) and electronic evaporator pressure regulating valves (EEPR) under the identical operating conditions of supermarket open multi-deck refrigerated display cabinets. The main goal of the supermarket refrigeration system design is to keep the displayed product at the required constant temperature, while minimising the cooling load to increase the overall energy efficiency of the system. Field tests have shown that the electronic evaporator pressure valve has a significant effect on improving the cabinet temperature and reducing the rate of frost formation on the evaporator coils with subsequent improvements in the air curtain strength.

VISUAL DOCUMENTATION OF AIR FLOW PATTERNS IN A CONTROLLED ATMOSPHERE STORAGE

Rapid reduction of fruit temperature has long been recognized as a key parameter in extending post harvest fruit quality. Research documenting times required to cool apples and pears at various locations in two 1000 bin controlled atmosphere storage rooms provided evidence of non-uniform air circulation patterns (Hellickson, et al. 1996, 1997). Video documentation of neutrally buoyant helium filled soap bubble movement in one of the same rooms verified the non-uniform air circulation patterns. Bin spacing throughout the room and bin placement beneath the evaporator coil directly influenced air distribution uniformity and quantities that did not circulate through the cooling unit. Improved bin placement within storerooms may result in shorter cooling times, reduced energy consumption and improved fruit quality after extended storage periods.

Energy recovery effectiveness in an enthalpy wheel used in air conditioning systems

SYNOPSIS Maintaining good Indoor Air Quality inside air-conditioned spaces demands more fresh air for ventilation. This in turn increases the heat load on the evaporator coil. For minimising this additional load, a heat recovery unit called an enthalpy wheel is introduced to exchange sensible and latent heat between the fresh air and the return exhaust air. This paper aims to present a simulation of the flow and heat transfer characteristics of the enthalpy wheel. A typical case of hot and humid ambient conditions is considered for analysis. As the wheel is under quasi steady state, transient flow patterns are included to predict the performance of the enthalpy wheel. The amount of energy transferred, during the counter current flows of fresh and return air, is deduced from the heat and mass transfer analysis. It has been found that the effectiveness of heat exchange is nearly 77%. This analysis would be very useful in optimising the design parameters for increasing the heat transferred by the enthalpy wheel.

Pressure drop of HFC refrigerants inside evaporator and condenser coils as determined by CFD

Pressure drop of HFC refrigerants inside evaporator and condenser coils as determined by CFD

Electric defrosting analysis of a finned-tube evaporator coil using the enthalpy method

When moist air comes into contact with a surface maintained at a subfreezing temperature, which is also below the dew-point temperature of water vapour in air, frost will form. Although this frost formation process is associated with a temporary increase in the heat transfer rate in the initial stages of deposition, the overall thermal performance over an extended period of time is degraded due to the insulating effect of the frost and also due to blockage of the coil and the accompanying reduction in air flow. One method of defrosting employs electric heat to melt the frost of the coil. This paper describes how the enthalpy method can be used to calculate the defrosting time and the temperature distribution in the frost layer during the defrosting process of a finned-tube evaporator coil. While the analysis presented applies mainly to electric defrosting, the method can be easily modified to handle different defrosting methods and processes.

Numerical and experimental studies of refrigerant circuitry of evaporator coils

This study attempts to analyze the performances of evaporator coils with complex refrigerant circuitry using a distributed simulation model, which has three elements: branch, tube and control volume. The governing equations for a control volume are presented in the paper together with the computer simulation procedure for branches, tubes and control volumes of a coil. The model predictions on four test coils are validated with experimental data collected under different airflow conditions using R134a as a refrigerant. Using this model, the heat transfer and fluid flow characteristics of the coils are studied. The study shows that while the thermal resistance of refrigerant side is comparable to that of airside, the coil comprehensive performance can be improved by changing the refrigerant mass velocity along the flow path. Compared with a common coil, using a complex refrigerant circuitry arrangement where the refrigerant circuits are properly branched or joined may reduce the heat transfer area by around 5% in coil design. A guideline is proposed for the refrigerant circuitry arrangements to improve the coil performance.

Energy optimisation studies in a dairy industry ice bank tank

SYNOPSIS In the dairy industry, ice bank tanks are used for chilling water to 1°C. The chilled water is then used for cooling milk from its arrival temperature of between 5°C to 12°C down to a temperature of 4°C and also for cooling during the pasteurisation process. An ammonia based vapour compression system is used to chill the water. The load on the plant is not constant throughout the day since the arriving milk temperature and quantity are not fixed. Hence, during part load conditions, ice builds up considerably in the ice bank tank. This results in reduced heat transfer rates and other flow problems. However, the ice build-up to a certain level is beneficial, as it helps to meet sudden demands from the load side. Beyond this level, though, problems occur as more ice is accumulated. In the present work, the load from the chiller, pasteuriser and thermal resistance due to ice build up on the evaporator coil is calculated for a week. This has been undertaken to determine the additional energy spent in storing cold in the form of ice blocks. An ice flake thickness of 50 mm is considered to be the optimum. Further build-up can be controlled by operating the compressor for the time needed according to the load. This paper brings out a method to prevent the ice layer thickness exceeding 50 mm by running the compressor according to load fluctuation, thereby achieving a power saving.

Pressure drop of HFC refrigerants inside evaporator and condenser coils as determined by CFD

This paper presents pressure-drop data for inside evaporator and condenser coils of refrigeration and air conditioning systems using hydrofluorocarbon (HFC) refrigerants and blends. Computational fluid dynamics (CFD) was used to simulate the two-phase flows of different refrigerants through two types of coil common to the evaporator and condenser components found in a typical vapour-compression system. A comparison of single-phase and two-phase flows is made with respect to pressure drop across the evaporator, firstly at constant temperature for single-phase saturated vapour, and secondly at increasing temperature with varying components such as density and viscosity for two-phase vapour. Pressure losses of HFC refrigerants and blends were compared with data for conventional refrigerants and data provided by other researchers.