Heat Pump Dehumidifier Research Articles

A practical solution for multivariable control of temperature and humidity in greenhouses

This paper presents a simple and effective control solution to regulate temperature and humidity inside a greenhouse, which are the main climatic variables affecting crop growth and health. A multivariable control strategy with inverted decoupling is proposed in two schemes. The nighttime control scheme uses heating and dehumidification, and the daytime control scheme uses natural ventilation, dehumidification, and humidification. A system identification methodology is used to obtain low-order linear models from data measured in a greenhouse. This simplifies the design of proportional–integral (PI) controllers and allows the use of well-known control techniques. For instance, anti-windup is used to deal with the saturation of the actuators and for a bumpless transfer mechanism to switch between the nighttime and the daytime control schemes. Setpoint weighting is used for the humidity controllers to attenuate undesired effects on the temperature control loop when the humidity setpoint is changed. Furthermore, split range control is used for the first time to control humidity by combining a heat-pump dehumidifier and a fogging system. The control solution presented in this work allows the integral absolute error to be reduced, on average, by 88.5% for the interactions affecting the temperature control loop, and by 85.6% for the humidity control loop, compared to decentralized PI control. The control schemes have been tested in simulation with real disturbance data from a Mediterranean greenhouse and in preliminary form in a real facility to conclude that the proposed multivariable control strategy could be effective in decoupling the temperature and the humidity control loops.

Efficient management of a dehumidifier in a greenhouse under warm weather conditions

The reduction of humidity inside a greenhouse is a key factor to ensure the quality of the crop and avoid the incidence of cryptogamic diseases. The method commonly used is a combination of heating and natural ventilation, although it is very deficient from the energy and environmental points of view. In cold and humid climates the use of a heat pump dehumidifier (HPD) can reduce energy consumption. To evaluate this technology in a Mediterranean climate, an HPD was installed in a greenhouse of the Cajamar Experimental Station in Almería. The environmental humidity inside the greenhouse decreased when using the HPD. The efficiency of the HPD was related to the value of the condensation temperature of the water vapor and to the temperature and humidity of the air inside the greenhouse. In addition, it was found that the energy efficiency of the system varied according to climate. For efficient management of HPD it is essential to know the influence of climate on energy efficiency. The flow of condensed water vapor and the power consumed presented differing behaviors. Thus, the production of liquid water correlated linearly with the condensation temperature, increasing the slope of the linear relationship with the temperature of the air inside the greenhouse. However, the power consumption was correlated linearly with the air temperature. The management of the system is efficient when the air temperature is above 15.0ºC and the relative humidity is less than 90%.

Comparison of different drying techniques for shiitake mushroom (Lentinus edodes): Changes in volatile compounds, taste properties, and texture qualities

The impact of heat pump dehumidifier drying (HPD) on the sensory qualities of shiitake mushroom was comprehensively evaluated in comparison with hot air drying (HAD) and vacuum freeze drying (VFD). The data showed that HPD substantially improved the characteristic flavors (volatile sulfide levels and equivalent umami concentration) of dried mushrooms by partially inhibiting enzymatic and Maillard reactions. The rehydration, shrinkage, and microstructural characteristics of HPD mushrooms were intermediate between VFD and HAD mushrooms. Results of sensory quality analysis indicated that HPD mushrooms outperformed HAD and VFD mushrooms in terms of overall quality. Additionally, HPD (0.85 kWh kg−1) had significantly (P < 0.05) lower energy consumption than HAD (2.65 kWh kg−1) and VFD (6.67 kWh kg−1). It can be concluded that HPD is a commercially attractive low-energy technique for the production of high sensory caliber dried shiitake mushrooms.

Performance Investigation of a Constant Temperature Heat Pump Dryer

Abstract Heat pump drying systems (HPDS) are systems composed of a heat pump (HP) that carries out a drying process which is energy efficient and environmentally friendly. However, in the HPDS, HP technology is used in the drying process, which makes it a very complex system coupling a hot air cycle and compressed refrigeration cycle. The control strategy of the drying process is important when developing heat pump dryers (HPDs) with high efficiency and reliability. An auxiliary condenser can be used to release excess heat from the system and maintain a constant temperature drying process, while equipping an evaporator bypass is a simple method for controlling the heat pump dehumidification rate. In this study, a novel enclosed drying system that incorporates a commercial 2.65 kW HP dehumidifier with an evaporator bypass was designed and constructed. The drying performance, such as the drying time and drying degree, and the HP performance, such as the coefficient of performance of an enclosed system with an air bypass, were experimentally investigated. According to the results of the investigation, increasing the circulating air bypass ratio can greatly reduce the drying time and improve the drying degree and the drying performance of the drying system. However, the air bypass reduces the performance of the HP, especially at a higher air bypass ratio, which should be taken into account in research and design of HPDS in the future. Besides, an attempt has been made to investigate the feasibility of using this novel HPD to preheat the drying room before drying process and refrigeration storage in drying room after drying process. Contribution of this research can diversify the function of the HPD.

An improvement of rose flowers drying process recovering volatile compounds by heat pump systems

Low-temperature drying is becoming recognized as an optimal type of hot drying technology for agricultural and sideline products. This study presents an experimental study on drying roses with a heat pump dehumidifier. The rose drying process took 40 hours across three drying stages, where the hot air temperatures were 45°C, 50°C and 55°C respectively. The dried roses were brightly colored and good quality, also, rose water was obtained during the drying process when the waste heat was recovered. The major components of the rose water were rose ether, nerol, citronellol, acetic acid, phenylethyl alcohol, methyleugenol, and so on, which are typical compounds of rose essential oil. The results show that a heat pump is one of best choices for drying rose flowers, and that rose water can also be effectively recovered during the drying process.

Review on low‐temperature heat pump drying applications in food industry: Cooling with dehumidification drying method

AbstractMoisture content of food material is a key factor influencing the quality of storage and extending the quality of a food product. Manufacturers develop new technologies to process sensitive food materials to supply new products with improved properties and high quality. The process in which the air is cooled sensibly while the moisture is removed from food is called cooling with dehumidification process. The system fundamentally acts as a heat pump, which pumps the heat from the dehumidified air to a different air stream in another area, with the aid of a refrigerant gas to carry the heat. This article reviews the potential of low‐temperature heat pump dehumidifier drying used in the food industry. Moreover, this describes the principle of cooling with dehumidification (CWD) process, the importance of studying psychrometric charts to understand CWD process, measures used in identify energy efficiency, comparison of this method over common dryers, applications, advantages and limitations of the CWD drying method.Practical applicationsMultiple measures have been taken to increase the drying efficiency of convection drying, especially by the application of cooling with dehumidified techniques. This drying technique has comparably higher energy efficiency, better and consistent product quality and the ability to control drying temperature and humidity over other conventional methods. Low‐temperature heat pump dryers are used increasingly applications in the food industry for the drying of grains, fruits, vegetables, herbs, spices, fish, meat, pet foods, and other heat‐sensitive food products in several countries.

Design and Development of a Heat Pump Based Moisture Control System for Industrial Drying of Medicinal Plants

Moisture control is one of the most common preservation methods used for enhancing the shelf life of agricultural produce. When processing or storing of fresh produce, a special attention has to be given in maintaining low temperature of the drying air. A heat pump based dehumidifier, which comprised of an evaporator, condenser, compressor, expansion device, a control unit, main frame and a covering chamber, was designed and fabricated for moisture control of agricultural produce. The air with high relative humidity coming from the evaporator was passed through the condenser unit and reheated up to 30 °C. The control unit was designed using Arduino controller to maintain preferable relative humidity in the storage room with minimum working cycle. Duty cycle of the system was 20 minutes ‘switch on’ and 5 minutes ‘switch off’ time. The performance of the heat pump based dehumidifier system was evaluated by studying the temperature and relative humidity of empty store room and with stored Gotukola (Centella asiatica). The relative humidity of the empty store room was found to be reduced from 50% to 16% within 6 hours of operation of heat pump dehumidifier. It was also possible to reduce the moisture content of fresh Gotukola from 88% to 12% within 27 hours. During this period the air temperature of the storeroom varied from 25°C to 44 °C. The specific moisture extraction rate of the heat pump dehumidifier system was 0.49 kg/kWh. The designed and developed heat pump based moisture control system performed satisfactorily and could be effectively used in moisture control of medicinal plants.

Evaluation of a dehumidifier in a mild weather greenhouse

The high humidity that surrounds crop plants, aggravated in mild weather greenhouses by high transpiration and lack of ventilation, encourages plant diseases and physiological disorders. A heat pump dehumidifier (HPD) has been installed in a mild weather greenhouse. Its development and the characteristics of the dehumidification process have been studied. The results indicate that the HPD reduced the risk of crop damage caused by humidity. Its effectiveness was related to the temperature value and the air humidity inside the greenhouse. HPD operation was not suitable under dry weather conditions. Conversely, when humidity was excessive, the HPD did not eliminate the risk of moisture damage. The appropiate conditions for using an HPD to eliminate humidity damage risk are relative humidity between 84% and 88%, and air temperature higher than 15.0 °C. Under these conditions, the condensed steam production value was 14.2 kg h−1, the specific moisture extraction rate was 2.3 kg (kW h−1), and the coefficient of performance was 2.5.

Influence of heat pump drying on tomato flavor

ABSTRACTTomato flavor from a heat pump dehumidifier dryer was characterized and compared to fresh and freeze-dried tomato samples for the retention of fresh flavor compounds. The monitored quality parameters were nonvolatile, volatile, and odor intensity. There were no differences in nonvolatile profiles; however, significant changes in key odor-impact volatiles were observed. Commercial spray-dried tomato was also evaluated to examine thermally induced volatile changes. The volatile and sensory profiles of heat pump dried tomato were comparable to freeze-dried tomato, with good retention of fresh aroma. In contrast, loss of compounds contributing to fresh green aroma ((E)-2-hexenal, 1-penten-3-one, 1-hexanol) and presence of heat-induced compounds (dimethyl sulfide, furfural, pyrrole derivatives) were detected in the spray-dried tomato.

Heat-pump dehumidifier as an efficient device to prevent condensation in horticultural greenhouses

In the context of increasing energy costs, alternative methods to the energy consuming venting-heating method must be considered for greenhouse dehumidification. In this paper the performance of a heat pump used as a dehumidifier is investigated. Contrary to the classical control aiming at maintaining the greenhouse air at a relative humidity set point, the considered device is designed as a preventive tool to avoid condensation on the crop and limit the energy consumption. The experimental set up was run during winter inside a 2350 m2 plastic greenhouse in the West of France for a set temperature of 16 C. During the experiment, no condensation occurred on the plants with a mean condensation rate of 12 W m 2 and a mean electrical power of 7.62 kW, for an overall efficiency of 4.9. Moreover, the energy retrieved by vapour condensation was given back to the greenhouse as sensible heat, contributing to the total heating of the greenhouse. While dehumidifying the greenhouse air, the device reduces, or may even rule out the gas consumption. The total energy consumption of the heat pump during the season was compared to simulated values for venting-heating dehumidification, with or without an exchanger. The heat pump dehumidifier was shown to be 6 to 8.5 times less energy consuming than the former and 3-8 than the latter, depending on the exterior climate. Using the energy cost of several significant countries, a preliminary operative cost study was conducted and showed that the heat pump can be competitive as a dehumidification alternative.

Mathematical, Fuzzy Logic and Artificial Neural Network Modeling Techniques to Predict Drying Kinetics of Onion

In this study, the influence of drying temperature (40, 50, 60C) and airflow velocity (2 and 3 m/s) on drying onion was evaluated by a custom designed fluidized bed dryer equipped with a heat pump dehumidifier. A comparative study was performed among nonlinear regression techniques, fuzzy logic and artificial neural networks to estimate the dynamic drying behavior of onion. Among nine mathematical models, approximation of diffusion with R2 = 0.9999 and root mean square error = 0.004157 showed the best fit with experimental data. Fuzzy logic tool in MATLAB with Mamdani model in the form of If–Then rules along with triangular membership function used for simulation, interpolation and obtaining a theoretical increase in experimental moisture ratios were used. Feedforward–backpropagation neural system with application of Levenberg–Marquardt training algorithm, hyperbolic tangent sigmoid transfer function, training cycle of 1,000 epoch and 2-5-1 topology was determined as the best neural model in terms of statistical indices. Practical Applications Forecasting kinetics of food drying by application of precise mathematical and dynamic modeling techniques as well as preparing particular patterns for describing such drying behaviors of diverse food products seem unavoidable for processing factories to optimize the quality of their dried food products and reduce operational costs. For this purpose, in this study, we applied different temperatures and airflow velocities to dry onion by a custom designed fluidized bed dryer equipped with a heat pump dehumidifier, predicting its drying behavior by regression, fuzzy logic and artificial neural network techniques and comparing accuracy of those models. Also, this paper compares prediction patterns of drying onion by fluidized bed drying with those of other food products dried by various drying methods. The results of this study will be helpful for all researchers and producers who want to know more about the nuances of impacts of different fluidized bed drying variables on the patterns of heat and mass transfer in fruits and vegetables.

The Characteristic of Heat Pump Dehumidifier Drier in the Drying of Red Chili (Capsium annum L)

Preservation of agricultural products have a very important to avoid the dropped of selling price when the harvest season arrives. Agricultural products are perishable because of high water content approximately 80 % which if the moisture content above 10% encourages proliferation of spoilage microorganisms in the material and cause the short shelf life. One way that has been done to preserve agricultural products such as red chili is dried using direct sunlight. Weakness of the sun drying can not reduce the water content to below 10% so spoilage microorganisms can live and breed, cause the short self life. Drying using Heat Pump Humidification Technology (HPD) is one solution to overcome the disadvantages of the direct sun drying. HPD can reduce the moisture content up to or below 10% will increase the self life and make easier to crush and to produce a chili powder. The purpose of this study to determine the appropriate mathematical model of the drying phenomena. The constant of the model equations are determined by Sum Square Error (SSE). Before being dried chilies dipped in hot water to remove the wax. PHD is used as an air dehumidifier that circulate in the dryer. Humidity difference causes the displacement of water from the chilies to dry air and lower the moisture content of chili. Moisture content of chili every 5 minutes is analyzed using gravimetric method. The results showed that the texture of the chili is very fragile with a moisture content of less than 10% and in the best conditions, the moisture content of chili is 6:35 wt%. From the research found that the appropriate model is Lewis model and the constant of the models obtained (k = 0.0056). Due to the moisture content of the product below 10 wt%, PHD can be considered as an agricultural product dryer.

Analysis of indoor environmental conditions and heat pump energy supply systems in indoor swimming pools

For indoor swimming pools, a lot of energy is needed to control the indoor temperature, relative humidity and pool water temperature. Meanwhile, the indoor air contains a high specific enthalpy due to water evaporation. A new heat pump dehumidifier is studied to reduce energy consumption. The most significant feature of this system is that it can not only recover the latent heat from indoor moist air, but also absorb heat from outdoor air to heat the indoor air and pool water. First, indoor environmental conditions, including space parameters and pool temperature, are analyzed based on human thermal comfort and energy saving. Subsequently, the models of heat and moisture gain are built. After that, the construction and operating modes of the heat pump dehumidifier are described, and the system model is established based on polynomial equations model. In a case study, an indoor swimming pool with a heat pump dehumidifier in Shanghai is studied. When outdoor air specific enthalpy is higher than 18.6 kJ/kg, the requirement of pool water heating can be met only by the heat pump dehumidifier, thus, auxiliary pool heater will not to be put into use. At last, economic analysis between the heat pump dehumidifier and conventional dehumidifier is conducted.

Minimum Fluidization Velocity of Food Materials: Effect of Moisture and Shape

Changes in fluidization behaviour of three geometrical shaped food particulates, with changes in moisture content during drying, were investigated using a fluidized bed dryer. The three food particulates were cylindrical (beans), parallelepiped (potato) and spherical (green peas). Fluidization behavior was characterised for cylindrical shape particles with three length diameter-ratios of 1:1, 2:1 and 3:1, parallelepiped particles with three aspect ratios of 1:1, 2:1 and 3:1 and spherical particles. All drying experiments were conducted at 50oC and 15% RH using a heat pump dehumidifier system. Fluidization experiments were undertaken for the bed heights of 100, 80, 60 and 40mm and at 10 moisture content levels.Data was analysed using SAS, and an empirical relationship of the form Umf = A + B e-Cm was developed for the change of minimum fluidization velocity with moisture content during drying for cylindrical particulates for the L:D ratio of 1:1, and spherical behaviour was best fitted to the linear model of Umf = A + Bm. Due to irregularities in shape, the minimum fluidisation velocity of parallelepiped particulates (potato) could not be fitted to any empirical model. The experimentally determined minimum fluidisation velocities were compared with predicted minimum fluidisation velocities using a generalised equation.

Density Variation of Different Shaped Food Particulates in Fluid Bed Drying: Empirical Models

Three particular geometrical shapes, parallelepiped, cylindrical and spherical, were selected from potatoes (aspect ratio = 1:1, 2:1, 3:1), cut beans (length:diameter = 1:1, 2:1, 3:1) and peas respectively. The density variation of food particulates was studied in a batch fluidised bed dryer connected to a heat pump dehumidifier system. Apparent density and bulk density were evaluated with non-dimensional moisture at three different drying temperatures of 30, 40 and 50 oC. Relative humidity of hot air was kept at 15% in all drying temperatures. Several empirical relationships were developed for the determination of changes in densities with the moisture content. Simple mathematical models were obtained to relate apparent density and bulk density with moisture content.

The Drying Constant and its Effect on the Shrinkage Constant of Different-Shaped Food Particulates

Experiments were undertaken to study the relationship between the drying constant and the shrinkage constant of food products during drying. Three particular geometrical shapes of parallelepiped, cylindrical and spheres were selected from potatoes (aspect ratio = 1:1, 2:1, 3:1), cut beans (length: diameter = 1:1, 2:1, 3:1) and peas, respectively. The volumetric shrinkage behaviour and drying behaviour of food particulates were studied in a batch fluidised bed dryer connected to a heat pump dehumidifier system at three different drying temperatures of 30, 40 and 50 °C. Shrinkage constant was evaluated with non-dimensional moisture and compared with the drying constant for all materials at these three temperatures to understand the effects of the drying constant on the shrinkage constant for each material under investigated drying conditions. Simple mathematical models were obtained for the relationship between the volumetric shrinkage constant and the drying constant.

Optimization of processing parameters of horse mackerel ( Trachurus japonicus) dried in a heat pump dehumidifier using response surface methodology

Response surface methodology (RSM) was employed to optimize the drying conditions of horse mackerel dried in a heat pump dehumidifier (HPD). The independent variables were drying air temperature (20–30 °C), drying air velocity (1.5–2.5 m/s) and sodium chloride (NaCl) content in the osmotic solution (5.0–15.0%), respectively. The coefficient of determination, R 2 values for specific moisture evaporation ratio (SMER), drying rate (DR), total colour difference (Δ E) and total volatile base nitrogen (TVBN) were greater than 0.89. The combined effects of these variables on SMER, DR, Δ E and TVBN were investigated. The results showed that SMER, Δ E and TVBN varied curvilinearly with increasing of temperature, velocity and NaCl content in the osmotic solution and DR increased linearly. Based on response surface and desirability functions, the optimum conditions for horse mackerel drying by HPD were: drying air temperature 30 °C, drying air velocity 1.5 m/s and NaCl content in the osmotic solution 9.9%.

Simulation model for complex refrigeration systems based on two-phase fluid network – Part II: Model application

A simulation model of complex refrigeration system based on two-phase fluid network is developed in part I of the paper [Shao, S.Q., Shi, W.X., Li, X.T., Yan, Q.S., 2008. Simulation model for complex heat pump systems based on two-phase fluid network – part I: model development. International Journal of Refrigeration 31 (3), 490–499.]. One same fluid network is used in the model to describe the refrigeration system in different operating modes with the method of the factual and fictitious branches. When there are many heat exchangers with concurrent cooling and heating, the traditional models are not able to define the flowing directions. The developed model, however, whether the initial flowing direction of the connecting branches is right or not, is able to predict the flow and heat transfer. Three typical complex refrigeration systems such as the multi-unit inverter air conditioner, heat pump with domestic hot water and multi-unit heat pump dehumidifier are simulated with the developed model as demonstrations on how to use it. The model can evaluate the influences of one or more parameters on the system performance, which can be used for the optimization of the system by all-condition performance analysis. It is shown that the two-phase fluid network model is effective and convenient for the simulation of different complex refrigeration system, especially for performance analysis, system evaluation, and optimization based on the performance prediction.

Drying characteristics of horse mackerel ( Trachurus japonicus) dried in a heat pump dehumidifier

In this study, a heat pump dehumidifier (HPD) was designed and constructed to investigate the drying characteristics of horse mackerel experimentally. Effects of surface load, drying air temperature, sodium chloride (NaCl) content in the osmotic solution, tempering, evaporator bypass air ratio (BAR) and air velocity on the drying characteristics of horse mackerel have been investigated. Experimental results showed that a high surface load of 8 kg/m 2 resulted in lower drying rate (DR), and surface load of 6 kg/m 2 was preferred by considering specific moisture evaporation rate (SMER) and total colour difference (Δ E). DR increased with increase in the drying air temperature. In view of SMER and Δ E, temperature at 20–30 °C was optimum air temperature for drying horse mackerel by HPD. However, NaCl content in the osmotic solution and tempering played a minor role during drying horse mackerel by HPD. BAR of 0.6–0.8 obtained the maximal SMER. The optimum air velocity for drying horse mackerel by HPD was 2.0–3.0 m/s. Furthermore, HPD provided an alternative way to process intermediate moisture fillets.

Fluidization Characteristics of Moist Food Particles

Changes in fluidization behaviour of green peas particulates with change in moisture content during drying were investigated using a fluidized bed dryer. All drying experiments were conducted at 50 + 2 0C and 13 + 2 % RH using a heat pump dehumidifier system. Fluidization experiments were undertaken for the bedheights of 100, 80, 60 and 40 mm and at 10 moisture content levels.Fluidization behaviour was best fitted to the linear model of Umf = A + B m. A generalized model was also formulated using the height variation. Also generalized equation and Ergun equation was used to compare minimum fluidization velocity.