Latent Heat Load Research Articles

An experimental study of structured packing dehumidifier/regenerator operating with liquid desiccant

The present work presents an experimental investigation on the performance of the structured packing cross flow desiccant dehumidification system (DDS). This system is referred as DDS; its heart is the dehumidifier/regenerator. It is used to meet a latent heat load by air dehumidification. Calcium chloride (CaCl 2) solution is used as the working desiccant material in this system. The structured packing has a density (specific surface area) of 390 m 2/m 3, corrugation angle of 60° and void fraction of 0.88. The effect of relevant parameters such as air flow rate, desiccant solution flow rate, desiccant solution temperature and concentration and packing thickness on the performance of the system is studied. The performance of the system is evaluated using the mass transfer coefficient, moisture removal rate (MMR), effectiveness and the coefficient of performance (COP). The remarkable increase of mass transfer coefficient and MRR for both deh/reg is observed by increasing both air and solution flow rates. Eventually, the payback period (PP) of the DDS is 11 months with annual running cost savings (Δ C RC) of about 31.24% compared with vapor compression system (VCS) dehumidification. The overall environmental impacts of DDS are nearly 0.63 of VCS. This may emphasize the need of incorporating a desiccant system along with air conditioning applications.

INFLUENCE ON VENTILATION MODE ON DISCOMFORT INDEX AND SURFACE CONDENSATION

Ventilation is one of the most important techniques for cooling. However, the influence of humid must be taken into account when is ventilated under the hot and humid climate. In this paper, we discuss the passive cooling effect by the proposed ventilation mode which take account of humidity. The proposed ventilation mode are as follows: a ventilation mode where a plenty of natural ventilation operates when being outside air temperature during the period: 18°C ≤ AT ≤ 27°C (AT: ambient temperature) throughout the year. Next, then the passive cooling effect is discussed based on the results of a series of parametric simulation studies. The cooling effect is verified in four ways: (1) hourly discomfort index fluctuation without air-conditioning, (2) hourly discomfort index fluctuation with air-conditioning, (3) monthly latent heat load with air-conditioning throughout the year, and (4) possibility of surface condensation by ventilation.The results are obtained as following:(1) Discomfort index large drop by ventilation mode in the nighttime, (2) ventilation mode reduce latent heat load. Furthermore, large thermal mass reduce latent heat load, and (3) large thermal mass reduces the fear of surface condensation.

Prediction of heat exchanger performance by the thermal network method (Improvement of an evaporator's performance and restraining air flow bypass by adjusting the refrigerant‐flow path pattern)

AbstractThis paper presents the influence of latent heat load by changing the relative humidity of air flow distribution in the super‐heated gas region in an evaporator and the effect of the refrigerant flow path pattern. To estimate the influence, we applied the thermal network method that can consider refrigerant quality distribution in the heat exchanger. If the super‐heated gas region spreads to the upper wind row, humid air goes through the heat exchanger without drying, called “air flow bypass.” To suppress this phenomenon and simultaneously obtain the best performance, we chose an optimum path pattern on the basis of this method. As a result, the prediction shows that performance of the evaporator is recovered 5% for a simple 2‐row 2‐path heat exchanger when 40% of the unbalance of air flow rate occurs. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20302

DEVELOPMENT OF DEHUMIDIFICATION FILTER USING NATURAL MESOPOROUS MATERIAL

A new dehumidification filter using natural mesoporous material, Wakkanai siliceous shale, was developed for effective treatment of the latent heat load. The dehumidification ability of the developed filter was evaluated. It was demonstrated that this filter performed as an air-to-air heat exchanger under high rotating speed. The total heat exchange efficiency was 80% at 22 rpm. This filter removed 3.3 g/kgDA of water vapor at 1 rpm when the regeneration temperature was 40°C. Moreover the rate of energy reduction to remove outdoor air load was estimated to be 56% by a combination of this filter and a sensible heat exchanger.

Performance analysis of a no-frost hybrid air conditioning system with integrated liquid desiccant dehumidification

This paper reports a performance analysis for a hybrid air conditioning system. In this system, the sensible heat load is primarily treated by a vapor compression heat hump; the latent heat load is treated by a liquid dehumidification system that uses a lithium chloride solution as a desiccant. In addition, by decreasing the humidity ratio of air flowing through the outdoor heat exchanger of the vapor compression heat pump, frosting can be retarded. The overall system performance was evaluated by a cycle simulation conducted both in summer and winter modes. Compared to a traditional air conditioning system, the hybrid air conditioning system improves the coefficient of performance (COP) by approximately 20% and 100% in summer and winter, respectively.

C122 潜熱・顕熱分離空調を利用したタスク空調に関する研究(空調システムの新しい展開-潜熱・顕熱分離空調II)

The air-conditioning system that separated the latent heat load and the sensible heat load has attracted a great deal of attention for energy-saving system. We will design the air-handling units of outdoor air (OHU) for latent heat system. OHU can use Task/ambient air conditioning (TAG) system. TAG system are being investigated and are effective air conditioning methods for offices. In this study, subjective experiments with a TAG system were conducted to investigate thermal comfort and productivity by field measurements.

Latent and sensible heat transfer in air‐cooling applications

PurposeJoint descriptions of both heat and mass transfer and thermodynamic aspects of air‐cooling applications cannot be easily found in the literature. Numerical analyses are a notable exception since suitable physical models and realistic boundary conditions are a prerequisite of accurate simulations. Thus, it is believed that the experience gained with numerical simulations might be of some help also to designers of air‐conditioning and drying systems. This paper seeks to address this issue.Design/methodology/approachIn the text, the physical implications of governing equations and boundary conditions utilized in numerical simulations are extensively discussed. Particular attention is paid to the thermodynamically consistent definition of latent and sensible heat loads, and to the correct formulation of the heat and mass transfer analogy.FindingsComparisons of analytical and numerical results concerning forced flows of humid air over a cooled plate validate the assumptions made in numerical simulations, both for air‐conditioning applications (almost always characterized by low rates of mass convection) and drying applications (almost always characterized by high rates of mass convection).Originality/valueFinally, with reference to the cold plate problem investigated here, the effects of the suction flow induced by condensation on the Nusselt number are quantified.

Entrainment of refrigerated air curtains down a wall

Refrigerated air curtains are used in open supermarket display cases as a barrier between the warm ambient air and the cold refrigerated air. Entrainment of ambient air into the curtain by shear layer mixing contributes to both the sensible and the latent heat load on the display case. To better understand the fluid dynamics which govern entrainment, velocity and temperature measurements of the curtains were made in a refrigerated display case, which was modified to allow a more fundamental flow. In particular, a vertical solid wall was installed to approximately represent a fully-stocked configuration. As such, negatively-buoyant wall jets (with high inflow turbulence) in the Reynolds number range of 4200–8000 and in the Richardson number range of 0.13–0.58 were examined. To define the air curtain vortex structures, flow visualization of the curtain interface was employed. The results of which showed that the entrainment of the ambient air was found to be governed by a variety of eddy engulfing structures. Particle Image Velocimetry was used to examine the velocity profiles of the air curtains in a non-intrusive manner, the measurements of which indicated negatively-buoyant acceleration following the jet exhaust, followed by a more linear curtain growth characteristic of isothermal wall jets. In addition, thermocouples were used to obtain the net increase in temperature of the curtain due to entrainment, where it was found that the dimensionless thermal energy loss decreased with decreasing Reynolds number.

Water conservation through energy management

The objective of this paper is the simultaneous management of two important industrial resources – energy and water. A class of industrial processes where heat transfer is accompanied by mass transfer (water evaporation and condensation) are concerned, and special attention is given to them where the transfer of heat is a limiting stage. These processes are considered not in isolation but in integration with all related processes and subsystems. More specifically, the set of processes and subsystems include cooling systems, systems of contact economizers for flue gas energy recovery, scrubbers, direct steam heaters, driers, related systems of coolers and the heat exchanger networks. The proposed preliminary design procedure is based on concepts and tools related to Pinch technology especially modified for consideration of simultaneous heat and mass transfer. The manipulated parameters include process' latent and sensible heat loads and systems' structural changes with efficiency criteria allowing sacrifice of quantity for quality of performance. Two specific procedures allow targeting of maximum flue gas energy recovery and minimum temperature of cooling water providing guidance to structural design and system operation. The application of the proposed methodology is designed to achieve water conservation through minimization of water evaporation losses. Its significance is demonstrated through three popular industrial applications (a power station, a brewery and a tissue factory).

Reducing Effect of Latent Heat Load by Fresh Air Heat Load Reduction System Using Underground Double Floor Space

This paper presents a feasibility study of a fresh air load reduction system by using an underground double floor space. The fresh air is introduced into the double slab space and passes through the opening bored into the footing beam. The air is cooled by the heat exchange with the inside surface of the double slab space in summer, and heated in winter. Then the heat is transferred to the adjacent soil through foundation slab, wall and to the room just above the mat through floor slab. This system not only reduces sensible heat load of the fresh air by heat exchange with earth but also reduces latent heat load of the fresh air by ad/de-sorption of underground double slab concrete. In this paper, we used a model for evaluation of fresh air latent heat load reduction by hygroscopic of air to earth exchange system taking into account coupled heat and moisture transfer of underground double floor space. In conclusion it shows the validity of the proposed method for a design tool and the quantitative effect of the system.

휜-튜브형 액체건조제 제습기의 높이에 따른 성능특성에 관한 실험적 연구

Several desiccant cooling systems have been developed in terms of cost and performance. In this study a fin-tube exchanger has been used for liquid desiccant dehumidification system. This dehumidifier has been designed to study the absorption characteristic of the aqueous triethylene glycol(TEG) solution which has the flow range from 20 to 50 LPM. The dehumidifier performance characteristics of working factor variables such as inlet solution flow rate, air flow rate, solution concentration and brine temperature have been analyzed. This dehumidifier has the ability to provide running while saving the latent heat load of total energy. The result of this experiment can provide useful data for hybrid air conditioning system.

액체 건조제 제습을 위한 핀튜브형 재생기의 성능인자 영향 연구

Liquid desiccant dehumidification system can be used effectively to save energy consumed in air conditioning as an alternative compared with conventional air conditioning systems by reducing latent heat load. The dehumidifier and the regenerator form the heart of this system. The latent part of the cooling load is handled using liquid desiccant. In this study, the experimental regenerator has been designed to study the regeneration characteristics of the aqueous triethylene glycol(TEG) system. Thebr/ performance factors of the regenerator with finned tube heat exchanger were evaluated by a series ofbr/ experimental runs. The regeneration process is highly dependent on the liquid desiccant conditions, such as, temperature, concentration and flow rate. In addition, the effects of the inlet air temperature, humidity and flow rate were discussed. Data obtained are useful for design guidance and performance analysis of a regenerator, particularly for a liquid desiccant cooling system.br/

Use of desiccant dehumidification to improve energy utilization in air-conditioning systems in Beirut

Humidity and indoor moist surrounding affect air cleanliness and protects harmful microorganisms when relative humidity is above 70%. In humid climates, the humidity issues are a major contributor to energy inefficiency in HVAC devices. The use of liquid desiccant dehumidification systems of supply air is a viable alternative to reduce the latent heat load on the HVAC system and improve efficiency. Thermal energy, at a temperature as low as 40–50°C, required for the operation of a liquid desiccant hybrid air conditioner can be efficiently obtained using a flat-plate solar collector. In this work a model of a solar-operated liquid desiccant system (using calcium Chloride) for air dehumidification is developed. The system utilizes packed beds of counter flow between an air stream and a solution of liquid desiccant for air dehumidification and solution regeneration. The desiccant system model is integrated with a solar heat source for performance evaluation at a wide range of recorded ambient conditions for Beirut city. Standard mass and energy balances are performed on the various components of the system and a computer simulation program is developed for the integrated system analysis. The desiccant system of the current study replaces a 3 TR (10.56 kW) vapour compression unit for a typical house as low latent load application, and is part of a hybrid desiccant–vapour compression system for a high latent load application, namely a small restaurant with an estimated cooling load of 11.39 TR (40 kW), including reheat. The relevant parameters of the desiccant system are optimized at peak load, and it is found out that there is an important energy saving if the ratio of the air flow rate in the regenerator to that in the dehumidifier is about 0.3 to 0.4. The COP of the desiccant unit is 0.41 for the house, and 0.45 for the restaurant. The size of the vapor compression unit of the restaurant is reduced to 8 TR when supplemented by a desiccant system. The performance is studied of the desiccant system integrated with a solar collector system and an auxiliary natural gas heater to heat the regenerator. The transient simulation of the solar desiccant system is performed for the entire cooling season. The solar fraction for the house is equal to 0.25, 0.47, and 0.68 for a collector area of 28.72, 57.44, and 86.16 m2, respectively. The solar fraction for the restaurant is 0.19, 0.38, and 0.54, for the same collector areas. The life cycle savings for the house run solely on desiccant system were positive only if natural gas is available at a cheap price. For the restaurant, the economic benefit of the desiccant system is positive, because the need for reheat in the vapor compression system is eliminated. For a gas price of 0.5638 $/kg, the payback period for the restaurant turned out to be immediate if the energy is supplied solely by natural gas, and 11 years if an 86.16 m2 solar collector is implemented to reduce the fuel consumption. Copyright © 2003 John Wiley & Sons, Ltd.

Dehumidification and Simultaneous Removal of Selected Pollutants from Indoor Air by a Desiccant Wheel Using a 1M Type Desiccant

Solid-desiccant dehumidifiers are increasingly becoming an integral part of desiccant based air-conditioning systems because of their effective handling of latent heat loads compared to conventional vapor compression units. In these units, either a silica gel or a molecular sieve is used for dehumidification of air. Both of them have the capability to co-adsorb various chemical pollutants during dehumidification of air. However, the shape of the isotherm for water vapor on these materials is not favorable for desiccant cooling applications. A mixture (IM desiccant) containing a silica gel, a molecular sieve, and a hydrophobic molecular sieve that was coated on an aluminum foil was studied for its capability for simultaneous removal of moisture and some selected pollutants from air. Experimental data were obtained in a fixed bed adsorber that simulated the operation of a rotary desiccant wheel. Air to be dehumidified and cleaned and the hot regeneration air were cycled in a specific time interval through this bed. The shape of the water isotherm on IM desiccant was found to be in between that of silica gel and molecular sieve 13×, but its uptake capacity was significantly lower than that of either silica gel or molecular sieve. A flow rate of about 100 L/min that provided a face velocity of about 132 cm/s was used in the adsorption step. The flow rate during regeneration was about 50 L/min. The temperature of the inlet air was about 23°C and its relative humidity was varied between 20 percent and 80 percent. The concentrations of pollutants were as follows; carbon dioxide: 1050 and 2300 ppm; toluene: 32 ppm; 1,1,1-trichloroethane: 172 ppm, and formaldehyde: 0.35 ppm. A complete breakthrough of all the pollutants was observed during an adsorption cycle. However, a removal efficiency greater than 50 percent for these pollutants was observed if the adsorption cycle time was about 1 minute.

WATER VAPOR ADSORPTION-DESORPTION CHARACTERISTICS OF A MIXED ADSORBENT

Air conditioning systems that are incorporating desiccant based dehumidifiers are finding increased applications over traditional vapor compression refrigeration systems due to their efficient handling of latent heat loads. Generally, silica gel, molecular sieves, or activated alumina coaled on a rotating honeycomb type desiccant matrix is used as a dehumidifier in these systems. However, shapes of the isotherm for water vapor adsorption on these materials are not favorable for optimal performance of desiccant cooling systems. Theoretical studies have shown that a material having a type I isotherm for water vapor, whose shape lies between that on molecular sieve 13X and silica gel would lower installation and operating costs of desiccant based air-conditioning systems. An adsorbent was prepared by mixing a silica gel, molecular sieve 13X, and a hydrophobic molecular sieve. The mixed adsorbent was mixed with a binder and was further coated on both sides of a 0.0014“ thick aluminum foil. Both plain and corrugated foils were used in preparing a desiccant matrix. A plain sheet was inserted in between two corrugated sheets and then they were rolled into a cylinder. The shape of water adsorption isotherm and equilibrium water adsorption capacity were obtained for this desiccant matrix. The shape of the water isotherm in the temperature range of 288-308 K was found to be more favorable than that on silica gel or molecular sieve 13X. However, the total water adsorption capacity of the new mixed adsorbent was significantly lower than these materials. The water adsorption data on the mixed adsorbent could be correlated according to Polyani's potential theory

Modeling transport processes in heat and mass releasing porous media

A computer algorithm was developed which estimates the latent and sensible heat loads due to the heating or cooling of heat and moisture releasing porous media. The algorithm also predicts the moisture loss and temperature distribution which occurs during the heating or cooling process. This paper discusses the modeling methodology utilized in the current computer algorithm and describes the development of the heat and mass transfer models. The results of the computer algorithm are compared to experimental data taken from the literature.

New ideas for energy utilisation in combined heat and power with cooling: II. Applications

Chemical dehumidification can be beneficial in a HVAC plant, both in terms of capital and operating costs. The sorption system can be integrated into a traditional plant, usually satisfying the latent heat load, but it can satisfy even the whole load with particular cycles which also use evaporative cooling. A number of applications are examined here, both integrated and self-sufficient systems, driven by natural gas or waste heat, working with liquid or solid sorption. Different lay-outs, performance and savings suggest new ideas for better energy utilisation in HVAC combined heat and power plants.

Energy conservation through hybrid air-conditioning cycles: computer modeling studies

SYNOPSIS In this paper the authors have presented a review, analysis and computer modelling of desiccant based hybrid air-conditioning (A/C) cycles for typical hot and humid conditions. Three hybrid cycles based on solid desiccant dehumidifier (viz. ventilation/condenser, recirculation/condenser and ventilation/heat exchanger cycles) have been evaluated and compared in performance with a conventional vapour compression A/C system. Computer modelling and analysis is based on dehumidifier performance data and the air psychrometric properties data correlations. Typical performance results in parametric variations for less and more humid conditions prevailing in Indian cities are presented. It is found that there is considerable scope for energy savings and improvement in the COP with hybrid A/C over the conventional vapour compresion cycle and desiccant cooling cycle alone for high latent heat load conditions.

Bulk Refrigeration of Fruits and Vegetables Part II: Computer Algorithm for Heat Loads and Moisture Loss

A computer algorithm was developed that estimates the latent and sensible heat loads due to the bulk refrigeration of fruits and vegetables. The algorithm also predicts the commodity moisture loss and temperature distribution which occurs during refrigeration. Part I focused upon the thermophysical properties of commodities and the flowfield parameters which govern the heat and mass transfer from fresh fruits and vegetables. This paper, Part II, discusses the modeling methodology utilized in the current computer algorithm and describes the development of the heat and mass transfer models. Part II also compares the results of the computer algorithm to experimental data taken from the literature, and, describes a parametric study which was performed with the algorithm. In addition, this paper also reviews existing numerical models for determining the heat and mass transfer in bulk loads of fruits and vegetables.

Bulk Refrigeration of Fruits and Vegetables Part I: Theoretical Considerations of Heat and Mass Transfer

A computer algorithm was developed that estimates the latent and sensible heat loads due to the bulk refrigeration of fruits and vegetables. The algorithm also predicts the commodity moisture loss and temperature distribution which occurs during refrigeration. This algorithm includes the combined phenomena of transpiration, respiration, air flow, and convective heat and mass transfer. The development and performance of the computer algorithm are presented in two parts. This paper, Part I, discusses commodity thermophysical properties and flowfield parameters which govern the heat and mass transfer from fruits and vegetables. Commodity thermophysical properties include transpiration and respiration, while flowfield parameters include psychrometric properties and convective heat and mass transfer coefficients. Part I describes the modeling treatment of these properties and parameters. The second paper, Part II, discusses the heat and mass transfer models, compares algorithm results to experimental data, and...