Showing papers on "Heat exchanger published in 2005"

Latent Heat Storage Materials and Systems: A Review

Abstract: The use of a latent heat storage system using Phase Change Materials (PCM) is an effective way of storing thermal energy (solar energy, off-peak electricity, industrial waste heat) and has the advantages of high storage density and the isothermal nature of the storage process. It has been demonstrated that, for the development of a latent heat storage system, choice of the PCM plays an important role in addition to heat transfer mechanism. The information on the latent heat storage materials and systems is enormous and published widely in the literatures. In this paper, we make an effort to gather the information from the previous works on PCMs and latent heat storage systems. This review will help to find a suitable PCM for various purposes a suitable heat exchanger with ways to enhance the heat transfer, and it will also help to provide a variety of designs to store the heat using PCMs for different applications, i.e. space heating & cooling, solar cooking, greenhouses, solar water heating and waste heat recovery systems. Measurement techniques of thermophysical properties, studies on thermal cycles for long term stability, corrosion of the PCMs and enhancement of heat transfer in PCM are discussed. New PCM innovations are also included for the awareness of new applications. This paper contains a list of about 250 PCMs and more than 250 references.

Experimental and numerical investigations of the behaviour of a heat exchanger pile

Abstract: Note: Sols Reference LMS-CHAPTER-2005-001 Record created on 2006-11-09, modified on 2017-11-27

Air-cooled heat exchangers and cooling towers

Abstract: The heat released in the condenser must be rejected to the environment. In regions lacking large bodies of water this rejection must be to the atmosphere. The requirement for a small temperature difference between the exhaust steam and the atmosphere, while rejecting large quantities of heat to air having a low heat capacity compared with water, poses a challenging technological problem which has resulted in a number of different engineering solutions.

Estimates of heat stress relief needs for Holstein dairy cows.

Abstract: Estimates of environmental heat stress are required for heat stress relief measures in cattle. Heat stress is commonly assessed by the temperature-humidity index (THI), the sum of dry and wet bulb temperatures. The THI does not include an interaction between temperature and humidity, although evaporative heat loss increases with rising air temperature. Coat, air velocity, and radiation effects also are not accounted for in the THI. The Holstein dairy cow is the primary target of heat stress relief, followed by feedlot cattle. Heat stress may be estimated for a variety of conditions by thermal balance models. The models consist of animal-specific data (BW, metabolic heat production, tissue and coat insulation, skin water loss, coat depth, and minimal and maximal tidal volumes) and of general heat exchange equations. A thermal balance simulation model was modified to adapt it for Holstein cows by using Holstein data for the animal characteristics in the model, and was validated by comparing its outputs to experimental data. Model outputs include radiant, convective, skin evaporative, respiratory heat loss and rate of change of body temperature. Effects of milk production (35 and 45 kg/d), hair coat depth (3 and 6 mm), air temperature (20 to 45 degrees C), air velocity (0.2 to 2.0 m/s), air humidity (0.8 to 3.9 kPa), and exposed body surface (100, 75, and 50%) on thermal balance outputs were examined. Environmental conditions at which respiratory heat loss attained approximately 50% of its maximal value were defined as thresholds for intermediate heat stress. Air velocity increased and humidity significantly decreased threshold temperatures, particularly at higher coat depth. The effect of air velocity was amplified at high humidity. Increasing milk production from 35 to 45 kg/d decreased threshold temperature by 5 degrees C. In the lying cow, the lower air velocity in the proximity of body surface and the smaller exposed surface markedly decrease threshold temperature. The large variation in thresholds due to environmental and animal factors justifies the use of thermal balance-based indices for estimating heat stress. Such an approach may make possible estimates of threshold temperatures at which heat stress relief is required for widely different cattle types and environmental situations.

High Flux Heat Removal with Microchannels—A Roadmap of Challenges and Opportunities

Abstract: Heat fluxes in IC chips and other electronics equipment have reached the current limits of air-cooling technology. Some of the applications require heat fluxes well beyond the limit of 100 W/cm 2 , thus demanding advanced cooling solutions. Liquid cooling technology has also received attention as the advances in single-phase liquid cooling in microchannels have shown considerable promise. The extension of compact heat exchanger technology to microscale applications offers many new possibilities. The liquid cooling technology is expected to reach heat dissipation rates as high as 10 MW/m 2 (1 kW/cm 2 ) with enhanced microchannels and a junction-to-air temperature difference of 50°C. The challenges facing flow boiling systems are also evaluated. This paper reviews the fundamental technological developments in liquid cooling as well as flow boiling and presents possible solutions in integrating the cooling system with a building's HVAC unit in a large server-type application. The opportunities and challenges...

Study of a novel silica gel-water adsorption chiller. Part I. Design and performance prediction

Abstract: A novel silica gel–water adsorption chiller is designed and its performance is predicted in this work. This adsorption chiller includes three vacuum chambers: two adsorption/desorption (or evaporation/condensation) vacuum chambers and one heat pipe working vacuum chamber as the evaporator. One adsorber, one condenser and one evaporator are housed in the same chamber to constitute an adsorption/desorption unit. The evaporators of two adsorption/desorption units are combined together by a heat-pipe heat exchanger to make continuous refrigerating capacity. In this chiller, a vacuum valve is installed between the two adsorption/desorption vacuum chambers to increase its performance especially when the chiller is driven by a low temperature heat source. The operating reliability of the chiller rises greatly because of using fewer valves. Furthermore, the performance of the chiller is predicted. The simulated results show that the refrigerating capacity is more than 10 kW under a typical working condition with hot water temperature of 85 °C, the cooling water temperature of 31 °C and the chilled water inlet temperature of 15 °C. The COP exceeds 0.5 even under a heat source temperature of 65 °C.

The optimum high pressure for CO2 transcritical refrigeration systems with internal heat exchangers

Abstract: The system performance of a CO2 refrigeration system is greatly affected by the compressor discharge pressure. An internal heat exchanger (IHX) with high effectiveness is an important factor to achieve high system performance. The expression traditionally used to describe the heat exchange effectiveness is not suitable for CO2 systems. As a result a practical effectiveness expression for IHX, based on enthalpy difference, has been derived and is reported in this paper. Detailed analysis on the relationship between the optimum high pressure Pk,opt and other systematic parameters was performed. Evaporating temperature has little influence on Pk,opt; and IHX can minimize the sensitivity of the system to the refrigerant quality x at the evaporator outlet. Based on simulation data, a correlation of Pk,opt was developed that predicts the simulation values with a deviation of less than 3.6% in the whole range and 0.94% when the evaporating temperature t1=5.3 °C. The results reported in this paper can be used in optimum control and performance evaluation of the whole system.

Apparatus and method for facilitating cooling of an electronics rack employing a heat exchange assembly mounted to an outlet door cover of the electronics rack

Abstract: An apparatus is provided for facilitating cooling of an electronics rack. The apparatus includes a heat exchange assembly mounted to an outlet door cover hingedly affixed to an air outlet side of the rack. The heat exchange assembly includes a support frame, an air-to-liquid heat exchanger, and first and second perforated planar surfaces covering first and second main sides, respectively, of the air-to-liquid heat exchanger. The heat exchanger is supported by the support frame and includes inlet and outlet plenums disposed adjacent to the edge of the outlet door cover hingedly mounted to the rack. Each plenum is in fluid communication with a respective connect coupling, and the heat exchanger further includes multiple horizontally-oriented heat exchange tube sections each having serpentine cooling channel with an inlet and an outlet coupled to the inlet plenum and outlet plenum, respectively. Fins extend from the heat exchange tube sections.

Zero emissions closed rankine cycle power system

Abstract: A fuel combustion power generation system is provided operating as a closed loop Rankine cycle and with zero atmospheric emissions. The fuel is combusted with oxygen in a combustor to generate high temperature products of combustion. The products of combustion are routed to a first side of a heat exchanger. A second side of the heat exchanger has a working fluid of the closed loop Rankine cycle passed therethrough to boil the working fluid into a gas. The working fluid is then expanded, condensed back to a liquid and pumped back to high pressure for return to the heat exchanger the products of combustion enter a condenser, where gases are collected and liquids recirculated or released. The products of combustion can be expanded upstream of the heat exchanger. The fuel can be a gaseous fuel or a solid or liquid fuel, such as coal or biomass, with gasification before combustion.

Carbon dioxide as refrigerant for tap water heat pumps: A comparison with the traditional solution

Abstract: Increased concern about the environmental impact of the refrigeration technology is leading toward design solutions aimed at improving the energy efficiency of the related applications, using eco-friendly refrigerants, i.e. ozone-friendly and with the least possible global warming potential (GWP). In this respect, carbon dioxide (ASHRAE R744) is seen today as one of the most promising refrigerants and is raising great interest in industrial and scientific fields. In the present work, the plant options are investigated, which are related to the design of air/water heat pumps for tap water using CO2. A comparison is made, in terms of energy efficiency, between a system working with CO2 and a similar one working with HFC R134a; such a comparison is carried out by means of a simulation model of a refrigerating machine/heat pump, characterized by a detailed representation of the heat exchangers, based on their subdivision into elementary volumes. Results show that carbon dioxide is an interesting substitute for synthetic fluids, if the design of the system is focused to take advantage of its properties. q 2005 Elsevier Ltd and IIR. All rights reserved.

Air-cooled heat generating device airflow control system

Abstract: A system for controlling airflow around an air-cooled heat generating device includes a cooling fluid line and heat exchangers positioned along the cooling fluid line to cool airflow around components in the heat generating device. The heat exchangers are positioned adjacent to the heat generating device such that the heat exchangers are directly in the path of at least one of airflow supplied into the components and airflow exhausted from the components. The system also includes valves positioned along the cooling fluid line to control cooling fluid flow through respective heat exchangers. The system further includes a secondary heat exchanger configured to absorb heat from the cooling fluid and a controller configured to control the valves to thereby control the cooling fluid flow through the heat exchangers.

Two-stage transcritical carbon dioxide cycle optimisation: A theoretical and experimental analysis

Abstract: The aim of this paper is to investigate, both experimentally and theoretically, the potential of improving the cycle efficiency through two stage compression with intermediate cooling, at operating conditions typical of air conditioning. The experimental set-up consists of two closed loop air circuits acting as heat sink and heat source for gas-cooler and evaporator, respectively. The tested refrigerating circuit includes two tube-and-fin heat exchangers as gas-cooler and evaporator, a back-pressure valve as throttling device and a double-stage semi-hermetic compound, two-piston, reciprocating compressor equipped with oil separator and intercooler. A full set of thermocouples, pressure transducers and flow-meters allows measurement of all the main parameters of the CO 2 cycle, enabling to perform heat balance both air and refrigerant side. Tests were run at fixed evaporation pressure, evaporator outlet superheating and gas-cooler outlet temperature, varying the gas-cooler outlet pressure in the range 8–11 MPa. The optimal gas-cooler pressure for this application as well as the effect of the intercooler efficiency on the cycle performance were investigated. A FORTRAN code for the simulation of an improved two-stage cycle was validated against the experimental results; a theoretical analysis performed with this code is proposed for optimisation and energy performance evaluation of such a cycle.

Pressure Effects on Flowing Mildly-Cracked n-Decane

Abstract: It is anticipated that traditional methods of cooling that employ the sensible heat transfer provided by fuels will not be sufficient to meet the cooling requirements of future high-performance aircraft. One potential solution is the use of endothermic fuels, which absorb heat through chemical reactions. However, few studies have analyzed the effects of pressure on a chemically reacting, flowing fuel. An experiment is described that studies the effects of pressure on flowing, mildly cracked, supercritical n-decane. The experimental results are studied with the aid of a unique two-dimensional computational fluid dynamics model that simulates the formation of cracked products from experimentally derived proportional distributions. This model is used to study the effect of pressure on the flow properties of the fuel. The experiments indicate that increasing pressure enhances bimolecular pyrolysis reactions, relative to unimolecular reactions. Increasing pressure also increases the overall conversion rate of supercritical n-decane flowing through a reactor. This is primarily because pressure increases the density, which increases the residence time of n-decane flowing through the reactor.

Thermodynamic analysis of a ground-source heat pump system for district heating

Abstract: This study deals with the thermodynamic analysis of ground-source heat pump (GSHP) systems for district heating. The mass, energy, entropy and exergy balance relations are derived and applied to a GSHP system with a U-bend ground heat exchanger. The performance characteristics of this GSHP system are evaluated in terms of energetic and exergetic aspects. Based on the measurements conducted on 7 January 2004, the heat extraction rate from the soil is found to be, on average, 61.4 W m−1 of bore depth, while the required borehole length in meter per kW of heating capacity is obtained as 11.71. The entering water temperature to the unit is measured to be 15.3°C. The heating coefficient of performance of the heat pump (COPHP) is about 2.85, while that for the whole system is obtained to be 7.4% lower than COPHP. The exergy efficiency values for the heat pump unit and whole system are found to be 66.8 and 66.6%, respectively. Copyright © 2005 John Wiley & Sons, Ltd.

Performance assessment of HC-290 as a drop-in substitute to HCFC-22 in a window air conditioner.

Abstract: As per the Montreal Protocol, CFCs and HCFCs are being phased out. HCFC-22 is used in window air conditioners. This paper presents the experimental performance study of a window air conditioner with propane (HC-290), a natural refrigerant, as a drop-in substitute to HCFC-22. Experimental results showed that HC-290 had 6.6% lower cooling capacity for the lower operating conditions and 9.7% lower for the higher operating conditions with respect to HCFC-22. The coefficient of performance for HC-290 was 7.9% higher for the lower operating conditions and 2.8% higher for the higher operating conditions. The energy consumption of the unit with HC-290 was lower in the range 12.4–13.5% than HCFC-22. The discharge pressures for HC-290 were lower in the range 13.7–18.2% than HCFC-22. For HC-290, the pressure drop was lower than HCFC-22 in both heat exchangers. This paper also presents simulation results for the heat exchangers of an HCFC-22 window air conditioner with HC-290 as a drop-in substitute. The simulation has been carried out using EVAP-COND, a heat exchanger model developed by NIST [National Institute of Standards and Technology. EVAP-COND: simulation models for finned-tube heat exchangers, Maryland, USA (2003). http://www2.bfrl.nist.gov/software/evap-cond/ [18] ]. The simulated evaporator capacities are within ±4% of the experimentally measured cooling capacities for both refrigerants. Simulation results for HC-290 and HCFC-22 are compared. The exit temperatures of HC-290 are lower by 0.3–1.2 °C in the condenser and are higher by 2.1–2.4 °C in the evaporator than HCFC-22. Evaporating pressures of HC-290 are lower by 2.1–3.3% as compared to HCFC-22. The pressure drops of HC-290 are lower in both the evaporator and the condenser as compared to HCFC-22. The outlet temperatures of air for HCFC-22 and HC-290 in both heat exchangers are nearly the same.

Vapor compression heat pumps with refrigerant mixtures

Abstract: Refrigerant mixtures are widely considered as the next-­generation environment-­friendly refrigerants because they allow to tune refrigerant properties for the best compromise between conflicting goals: lower global warming potential (GWP), zero ozone depletion potential (ODP), low or no flammability and toxicity. Nevertheless, refrigerant mixtures can behave quite differently during evaporation and condensation than pure refrigerants. This paper first discusses key differences between pure and mixed refrigerants, presents a case study on the impact of mixtures on heat exchanger design and quantifies the potential efficiency losses/gains using a large temperature glide mixtures. The paper further details the benefit of mixtures on capacity control.

A gas turbine engine frame with an integral fluid reservoir and air/fluid heat exchanger

Abstract: Disclosed is a structural frame (38) for a gas turbine (10) engine comprising an integral fluid reservoir (60) and air/fluid heat exchanger (68;168;268). A central hub (56) includes a reservoir (60) for storing a fluid and an outer rim (62) circumscribes the hub (56). A heat exchanger (68;168;268) is fluidly coupled to the reservoir (60) and is in simultaneous communication with the fluid and an air stream.

Heat and mass exchanger

Abstract: A mass and heat exchanger includes at least one first substrate with a surface for supporting a continuous flow of a liquid thereon that either absorbs, desorbs, evaporates or condenses one or more gaseous species from or to a surrounding gas; and at least one second substrate operatively associated with the first substrate The second substrate includes a surface for supporting the continuous flow of the liquid thereon and is adapted to carry a heat exchange fluid therethrough, wherein heat transfer occurs between the liquid and the heat exchange fluid

Influence of Surface Properties and Characteristics on Fouling in Plate Heat Exchangers

Abstract: The scope of the present work was to investigate the influence of surface properties of corrugated heat exchanger plates on calcium sulpate crystallization fouling from aqueous salt solutions. The applied surface treatment technologies for the heat exchanger plates are ion beam implantation, ion sputtering, carbo-nitriding & oxidizing, and Ni-P-PTFE coating. Ten pairs of Alfa Laval M3 heat exchanger plates were treated with these techniques and used for this study. Other process parameters such as temperature, solution composition, flow velocity, and the corrugation inclination angle of the plates were kept constant in order to allow for a comparison between the different surface treatments. To evaluate the influence of surface roughness on CaSO4 crystallization fouling, one pair of plates was electropolished. Surface energy and surface roughness measurements were carried out to characterize the treated surfaces.