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00493 A practical algorithm for diagnosing control loop problems
13
Space heating and cooling
a light pipe with a 95% specular reflectance. The infra-red radiation transmitted through the material into a passive stack was found to enhance the natural ventilation flow by up to 14%. The effect is greater in summer than in winter, which is highly desirable as there is often a lack of driving force for natural stack ventilation in summer. Impact of scale build-up models on energy OOlOO489 utilization in heat exchangers Duffuaa, S. 0. and Budair, M. 0. Inf. J. Energy Rex, 1999, 23, (I I), 9991005. Presented in this paper is an energy utilization index. It has the potential to be used during investigations of the impact of scale build-up in heat exchange equipment. The investigation is carried out using a variety of scale build-up models. The models employed in this paper are the linear, logarithmic, and asymptotic. It has been demonstrated by a realistic example that the type of scale build-up model assumed greatly affects energy utilization and maintenance scheduling for heat exchange equipment. Numerical study of the natural convection of 00100490 Boussinesq equations in parallelepipedal cavities with isothermal walls and heated from two sides: influence of walls conditions Chmaissem, W. and Daguenet, M. Energy Convers. Manage., 1999, 40, (IO). 1041-1056. A new calculation code using a two-dimensional finite-element method, valid in permanent and laminar flow, is investigated. Elements of symmetry existing in boundary conditions imposed on velocities, as well as on temperatures, are considered, then elements of symmetry existing only in boundary conditions imposed on velocities and, finally, boundary conditions containing no symmetry. The flow is two-cellular so that the Rayleigh number remains inferior to a value in the order of IO”. Beyond this value, secondary cells can appear, following the geometry of the enclosure. Optimum channel geometry for solar air heaters of 00100491 conventional design and constant flow operation Hegazy, A. A. Energy Corwers. Manage., 1999, 40, (7). 757-774. This study presents a remarkably simple criterion for determining the channel optimum depth-to-length ratio (D/L),,, which effectively maximizes the useful energy from collectors that are designed to heat a fixed mass rate of air flow. The engineering accuracy of the analytically derived criterion has been corroborated over a wide range of ambient variables, design parameters and operating conditions. The results demonstrate clearly the decisive role of the (D/L) ratio in enhancing heater performance. A poor choice of this ratio can significantly affect the thermal conversion efficiency, especially for lower specific mass rates. In contrast, decreasing the (D/L) ratio heyond the optimal value leads to a slight enhancement in heater efficiency. Of significance is the finding that, by choosing (D/L),,,, the air temperature rise is also maximized. For variable flow operation, as the optimal. however, the ratio of D/L = 2.5 x lo-” is recommended Moreover, the reported results and discussions have provided valuable conceptual insights regarding generalization of the performance curves of conventional solar air heaters. It is found that the analytically determined channel depth parameter, in conjunction with the air specific mass rate, are quite successful in correlating the performance in general terms. Performance of film absorbers under the effect of a 00/00492 contaminant non-absorbable gas Sahir. H. M. and Eames, 1. W. Applied Energy, 1999, 63, (4), 255-267. This paper presents a method of calculating the effects of the presence of non-absorbable gases on the performance of the absorber of vapour absorption refrigeration systems. The paper, which uses data for lithium bromide/water system with air as a contaminant, shows the detrimental effects of such gases on the overall effectiveness of the absorber. It is shown that either introducing turbulence into the vapour/gas mixture in the absorber or choosing a larger absorber can offset the reduction in performance, due to the presence of a non-absorbable gas. Calculations of the reduction in coefficient of performance, due to hydrogen build-up in the absorber of a lithium bromide/ water system. are also presented. A practical algorithm for diagnosing control loop 00100493 problems Salshury T. I. Energy & Buildiqs:s, 1999, 29, (3) 217-227. This paper describes an algorithm for detecting and diagnosing poor performance in feedback control loops. The performance of the control loop is assessed based on criteria derived from parameters that can he estimated from superficial knowledge of the controlled process. The algorithm can distinguish between three types of control problems: oscillatory response, sluggish response and plant failure. The method is particularly applicable to heating, ventilating and air-conditioning (HVAC) systems where linear (PI) control law is normally used despite the existence of often quite non-linear operating characteristics. In these systems, the control performance varies as the plant is exercised across its operating range. In practice, periodical re-tuning of HVAC control loops during the yearly load cycle may he necessary in order to obtain consistent control performance. The algorithm presented in this paper can act as the basis for determining when re-tuning is necessary in addition to detecting plant failure. Potential advantages of detecting control problems in HVAC systems are energy savings, reduced maintenance costs and improved
52
Fuel and Energy Abstracts
January 2990
thermal comfort conditions. The paper describes the techniques comprising the algorithm and presents results from tests carried out on a simulated airhandling unit (AHU). 00/00494 Roof components models simplification via statistical linearisation and model reduction techniques Palomo Del Barrio, E. Energy & Buildirtgs, 1999, 29, (3) 259-281. This paper addresses the simplification of detailed models for predicting the thermal hehaviour and the cooling performance of huildings roof components designed on the basis of evaporative and radiative cooling principles. Detailed models usually consist of a large set of non-linear ordinary differential equations whose integration requires a lot of time compared with the one needed for thermal analysis of conventional buildings. For natural roof cooling techniques to be taken into account in the building design process, simplifying such models is hence required. This paper presents a rigorous and systematic procedure for simplifying detailed roof models: in a first phase, statistical linearization techniques are applied for going from an original model including N non-linear ordinary differential equations to one equivalent linear model formed by N linear ordinary differential equations; in a second one, model reduction techniques are applied for replacing the equivalent linear model before by a set of n < N linear ordinary differential equations. The proposed methodology has been applied to a detailed roof planted model. 00/00495 Some aspects concerning modelling the flow and heat transfer in horizontal mantle heat exchangers in solar water heaters Rosengarten. G. et rrl. Inr. J. Energy Rus.. 1999. 23. (I IO). lOO7-1016. An experimental and numerical investigation has been conducted to study the heat transfer process in horizontal mantle heat exchangers used in solar water heaters. A rectangular cavity has heen used as a simplified geometry. With the aid of particle image velocimetry (PIV) the flow field in the centre-plane of the rectangular cavity has been visualized. Three-dimensional flow simulations were performed using a commercial CFD package. The impinging jet formed by the inlet flow directed towards the opposite wall was found to produce localized turbulence in the cavity, with an inlet Reynolds number as low as 360. This turbulence was found to effect the flow field and heat transfer in the cavity when the inlet Reynolds number was above 1200. It has heen demonstrated that with the boundary conditions used in this study, majority of the heat transferred was in the bottom half of the cavity. In conclusion, this is not the ideal situation for optimization of solar water heating systems. 00100498 Steady-state heat transfer in an insulated, reinforced concrete wall: theory, numerical simulations, and experiments Jones, G. F. and Jones, R.W. Energy & Building.s, 1999, 29, (3), 293-306. In this paper, the steady-state heat transfer in an in siru constructed, insulated, reinforced concrete wall is examined. The wall consists of two reinforced concrete slabs that sandwich an insulation layer. The two halves of the wall are joined by a series of steel wires to provide the needed structural integrity but unavoidably act as thermal shunts that increase the heat transfer rate. Nonetheless, the thermal performance of the wall surpasses that for an uninsulated wall because of the presence of the insulation. A comparison is made of the heat transfer rates predicted by a three-dimensional, numerical model with laboratory test data. The results from these data, and from numerous sensitivity studies performed with the model, show that the steady-state heat transfer in the wall may he approximated by the isothermal planes model. A more accurate estimate is obtained by a weighted average of the isothermal planes result and a simple parallel path model in which the isothermal-planes result is weighted by the factor 0.85. An appropriate value of the weighting factor, specific to a particular wall panel configuration, may he obtained using a correlation and graphical results that are presented in the paper. These quantitative results for the steady-state panel conductance are expected to be useful in exploring manufacturing strategies that would improve the thermal resistance of the panel, in designing energy-conserving buildings that employ such panels, and in establishing accurate energy standards and energy code compliance methods. 00/00497 Thermal transients in buildings: development and validation of a numerical model Nannei, E. and Schenone. C. Emqy & Buikdir~g.s. 1999. 20. (3). ZOY-215. The present paper describes the development of a simplified numerical model able to simulate thermal and hygrometrical transients in huildings and its validation by means of a climatic room. The numerical code is based on energy and mass balance equations and on the heat transfer equation for time-dependent conduction. The program calculates the indoor air temperature and the temperature of the inner surface of the walls according to the geometrical characteristics and thermophysical properties of the walls, the transients of outdoor temperature, the air renewals and the supplied heat flow. Validation of the code was carried out hy means of a test room immersed in an artificial, controlled climate. to achieve definite and repeatable boundary conditions. Comparison between the experimental data and the numerical results for thermal unsteady state conditions shows a good agreement and hears witness to the reliability of the simulation program.
Space heating and cooling
a light pipe with a 95% specular reflectance. The infra-red radiation transmitted through the material into a passive stack was found to enhance the natural ventilation flow by up to 14%. The effect is greater in summer than in winter, which is highly desirable as there is often a lack of driving force for natural stack ventilation in summer. Impact of scale build-up models on energy OOlOO489 utilization in heat exchangers Duffuaa, S. 0. and Budair, M. 0. Inf. J. Energy Rex, 1999, 23, (I I), 9991005. Presented in this paper is an energy utilization index. It has the potential to be used during investigations of the impact of scale build-up in heat exchange equipment. The investigation is carried out using a variety of scale build-up models. The models employed in this paper are the linear, logarithmic, and asymptotic. It has been demonstrated by a realistic example that the type of scale build-up model assumed greatly affects energy utilization and maintenance scheduling for heat exchange equipment. Numerical study of the natural convection of 00100490 Boussinesq equations in parallelepipedal cavities with isothermal walls and heated from two sides: influence of walls conditions Chmaissem, W. and Daguenet, M. Energy Convers. Manage., 1999, 40, (IO). 1041-1056. A new calculation code using a two-dimensional finite-element method, valid in permanent and laminar flow, is investigated. Elements of symmetry existing in boundary conditions imposed on velocities, as well as on temperatures, are considered, then elements of symmetry existing only in boundary conditions imposed on velocities and, finally, boundary conditions containing no symmetry. The flow is two-cellular so that the Rayleigh number remains inferior to a value in the order of IO”. Beyond this value, secondary cells can appear, following the geometry of the enclosure. Optimum channel geometry for solar air heaters of 00100491 conventional design and constant flow operation Hegazy, A. A. Energy Corwers. Manage., 1999, 40, (7). 757-774. This study presents a remarkably simple criterion for determining the channel optimum depth-to-length ratio (D/L),,, which effectively maximizes the useful energy from collectors that are designed to heat a fixed mass rate of air flow. The engineering accuracy of the analytically derived criterion has been corroborated over a wide range of ambient variables, design parameters and operating conditions. The results demonstrate clearly the decisive role of the (D/L) ratio in enhancing heater performance. A poor choice of this ratio can significantly affect the thermal conversion efficiency, especially for lower specific mass rates. In contrast, decreasing the (D/L) ratio heyond the optimal value leads to a slight enhancement in heater efficiency. Of significance is the finding that, by choosing (D/L),,,, the air temperature rise is also maximized. For variable flow operation, as the optimal. however, the ratio of D/L = 2.5 x lo-” is recommended Moreover, the reported results and discussions have provided valuable conceptual insights regarding generalization of the performance curves of conventional solar air heaters. It is found that the analytically determined channel depth parameter, in conjunction with the air specific mass rate, are quite successful in correlating the performance in general terms. Performance of film absorbers under the effect of a 00/00492 contaminant non-absorbable gas Sahir. H. M. and Eames, 1. W. Applied Energy, 1999, 63, (4), 255-267. This paper presents a method of calculating the effects of the presence of non-absorbable gases on the performance of the absorber of vapour absorption refrigeration systems. The paper, which uses data for lithium bromide/water system with air as a contaminant, shows the detrimental effects of such gases on the overall effectiveness of the absorber. It is shown that either introducing turbulence into the vapour/gas mixture in the absorber or choosing a larger absorber can offset the reduction in performance, due to the presence of a non-absorbable gas. Calculations of the reduction in coefficient of performance, due to hydrogen build-up in the absorber of a lithium bromide/ water system. are also presented. A practical algorithm for diagnosing control loop 00100493 problems Salshury T. I. Energy & Buildiqs:s, 1999, 29, (3) 217-227. This paper describes an algorithm for detecting and diagnosing poor performance in feedback control loops. The performance of the control loop is assessed based on criteria derived from parameters that can he estimated from superficial knowledge of the controlled process. The algorithm can distinguish between three types of control problems: oscillatory response, sluggish response and plant failure. The method is particularly applicable to heating, ventilating and air-conditioning (HVAC) systems where linear (PI) control law is normally used despite the existence of often quite non-linear operating characteristics. In these systems, the control performance varies as the plant is exercised across its operating range. In practice, periodical re-tuning of HVAC control loops during the yearly load cycle may he necessary in order to obtain consistent control performance. The algorithm presented in this paper can act as the basis for determining when re-tuning is necessary in addition to detecting plant failure. Potential advantages of detecting control problems in HVAC systems are energy savings, reduced maintenance costs and improved
52
Fuel and Energy Abstracts
January 2990
thermal comfort conditions. The paper describes the techniques comprising the algorithm and presents results from tests carried out on a simulated airhandling unit (AHU). 00/00494 Roof components models simplification via statistical linearisation and model reduction techniques Palomo Del Barrio, E. Energy & Buildirtgs, 1999, 29, (3) 259-281. This paper addresses the simplification of detailed models for predicting the thermal hehaviour and the cooling performance of huildings roof components designed on the basis of evaporative and radiative cooling principles. Detailed models usually consist of a large set of non-linear ordinary differential equations whose integration requires a lot of time compared with the one needed for thermal analysis of conventional buildings. For natural roof cooling techniques to be taken into account in the building design process, simplifying such models is hence required. This paper presents a rigorous and systematic procedure for simplifying detailed roof models: in a first phase, statistical linearization techniques are applied for going from an original model including N non-linear ordinary differential equations to one equivalent linear model formed by N linear ordinary differential equations; in a second one, model reduction techniques are applied for replacing the equivalent linear model before by a set of n < N linear ordinary differential equations. The proposed methodology has been applied to a detailed roof planted model. 00/00495 Some aspects concerning modelling the flow and heat transfer in horizontal mantle heat exchangers in solar water heaters Rosengarten. G. et rrl. Inr. J. Energy Rus.. 1999. 23. (I IO). lOO7-1016. An experimental and numerical investigation has been conducted to study the heat transfer process in horizontal mantle heat exchangers used in solar water heaters. A rectangular cavity has heen used as a simplified geometry. With the aid of particle image velocimetry (PIV) the flow field in the centre-plane of the rectangular cavity has been visualized. Three-dimensional flow simulations were performed using a commercial CFD package. The impinging jet formed by the inlet flow directed towards the opposite wall was found to produce localized turbulence in the cavity, with an inlet Reynolds number as low as 360. This turbulence was found to effect the flow field and heat transfer in the cavity when the inlet Reynolds number was above 1200. It has heen demonstrated that with the boundary conditions used in this study, majority of the heat transferred was in the bottom half of the cavity. In conclusion, this is not the ideal situation for optimization of solar water heating systems. 00100498 Steady-state heat transfer in an insulated, reinforced concrete wall: theory, numerical simulations, and experiments Jones, G. F. and Jones, R.W. Energy & Building.s, 1999, 29, (3), 293-306. In this paper, the steady-state heat transfer in an in siru constructed, insulated, reinforced concrete wall is examined. The wall consists of two reinforced concrete slabs that sandwich an insulation layer. The two halves of the wall are joined by a series of steel wires to provide the needed structural integrity but unavoidably act as thermal shunts that increase the heat transfer rate. Nonetheless, the thermal performance of the wall surpasses that for an uninsulated wall because of the presence of the insulation. A comparison is made of the heat transfer rates predicted by a three-dimensional, numerical model with laboratory test data. The results from these data, and from numerous sensitivity studies performed with the model, show that the steady-state heat transfer in the wall may he approximated by the isothermal planes model. A more accurate estimate is obtained by a weighted average of the isothermal planes result and a simple parallel path model in which the isothermal-planes result is weighted by the factor 0.85. An appropriate value of the weighting factor, specific to a particular wall panel configuration, may he obtained using a correlation and graphical results that are presented in the paper. These quantitative results for the steady-state panel conductance are expected to be useful in exploring manufacturing strategies that would improve the thermal resistance of the panel, in designing energy-conserving buildings that employ such panels, and in establishing accurate energy standards and energy code compliance methods. 00/00497 Thermal transients in buildings: development and validation of a numerical model Nannei, E. and Schenone. C. Emqy & Buikdir~g.s. 1999. 20. (3). ZOY-215. The present paper describes the development of a simplified numerical model able to simulate thermal and hygrometrical transients in huildings and its validation by means of a climatic room. The numerical code is based on energy and mass balance equations and on the heat transfer equation for time-dependent conduction. The program calculates the indoor air temperature and the temperature of the inner surface of the walls according to the geometrical characteristics and thermophysical properties of the walls, the transients of outdoor temperature, the air renewals and the supplied heat flow. Validation of the code was carried out hy means of a test room immersed in an artificial, controlled climate. to achieve definite and repeatable boundary conditions. Comparison between the experimental data and the numerical results for thermal unsteady state conditions shows a good agreement and hears witness to the reliability of the simulation program.