Minimum Airflow Research Articles

Experimental Study on the Combustion System Optimization in the Case of a 36 kW Condensing Boiler

The Energy-related Products Directive that came into force in all EU countries on September 25, 2015 is focusing attention on the condensing boilers more than ever before since they currently represent the most advanced and environmental friendly heating technology based on fossil fuels. In this context, an experimental study referring to the combustion system of a condensing system of 36 kW nominal heat output - defined in central heating mode and condensing regime - was developed. The analyzed condensing system is of premixed type, consisting of an air blower, a gas valve, a fuel gas-air mixer and a cylindrical multihole burner. Beside nominal heat output, another important requirement is a wide heat output range, for flexibility relative to the load. Consequently, the minimum heat output of the boiler should be as low as possible. This problem of maximum and minimum heat output of the boiler is, in fact, a question of maximum and minimum air flow that enters in the combustion system, which is decided by minimum aria of nozzle of the fuel gas-air mixer. In order to establish the optimum solution for the analyzed combustion system, four fuel gas-air mixers, Siemens type, had been experimented. The paper presents the results of this study.

Time-averaged ventilation for optimized control of variable-air-volume systems

Typical variable air volume (VAV) terminals spend the majority of time at their minimum airflow setpoints. These are often higher than the minimum ventilation requirements defined by code, resulting in excess energy use and a risk of over-cooling the spaces. We developed and tested a time-averaged ventilation (TAV) control strategy in an institutional building on the UC Berkeley campus to address this issue. Whenever a zone does not require cooling, TAV alternates the VAV damper between partially open and fully closed so that the average airflow matches a predefined ventilation setpoint. Compared to the existing, base case scenario using single-max VAV logic, this strategy reduced the mean zone airflow fraction from 0.44 to 0.27 during the intervention period. The corresponding reductions in average heating, cooling, and fan power were 41%, 23%, and 15% respectively. In addition to being programmed directly in a native control system, TAV may be applied via sMAP as a low-cost retrofit strategy in any building that has a BACnet network and direct digital control (DDC) to each VAV terminal.

Development and Evaluation of Applicable Optimal Terminal Box Control Algorithms for Energy Management Control Systems

Common energy management control systems (EMCS) in building HVAC systems could be made much more energy efficient without sacrificing comfort. Most researchers have focused on implementing optimal control algorithms in Variable Air Volume (VAV) systems with EMC functions. Previous studies have paid little attention to using terminal box EMC functions integrated with main AHU (Air Handling Unit) systems. Terminal boxes with EMCS may cause occupant discomfort and waste excessive energy if they do not have the proper operation control functions. The objective of this study is to evaluate the impact of energy consumption and estimate building energy savings on the optimal minimum air flow of single duct VAV terminal boxes and develop applicable optimal terminal box control algorithms for EMCS. This paper presents a dynamic model of a VAV terminal box with hydronic reheat, develops optimal terminal box control algorithms and applies the developed EMCS algorithms to an actual building. The results of this study show that optimal terminal box control algorithms can stably maintain the set room air temperature and reduce energy consumption for varying heating loads compared to conventional control algorithms.

Air diffusion performance index (ADPI) of overhead-air-distribution at low cooling loads

The air diffusion performance index (ADPI) concept and its implementation in the diffuser selection guideline (ASHRAE Handbook-Fundamentals, 2009) provide designers a simple method to design overhead-air-distribution systems. However, the current guideline considers only five diffuser types and the range of sensible cooling loads used to develop the guideline (65W/m2–250W/m2) is high for current buildings. Applying this outdated guideline may cause draft discomfort and/or unnecessary fan-energy waste. This paper presents new experimental results to update the ADPI-based guideline to include 15 diffuser types at low cooling loads of 25W/m2 and 50W/m2. Compared to the high loads in the current guideline, the ADPI values generated in this work are greater due to the lower loads used. Most ceiling-mounted diffusers produce higher ADPI at a large range of airflow than do sidewall and other diffuser types. ADPI for ceiling diffusers has the least variation with changes in loads or airflow rates. The results show that the minimum airflow set-point for a variable air volume system varies with diffuser type. For ceiling diffusers other than round and perforated diffusers, the minimum set-point can be reduced to lower than 30% of the peak flow rate without significantly reducing ADPI.

Immediate Effects of the Vocal Function Exercises Semi-Occluded Mouth Posture on Glottal Airflow Parameters: A Preliminary Study

The study aimed to quantify immediate alterations in the airflow glottogram between the Vocal Function Exercises semi-occluded mouth posture (/ol/ with lip buzz) and the sustained vowel /o/ in individuals with normal voices, and to determine if noted changes were in agreement with established semi-occluded vocal tract aerodynamic theory. Based on semi-occluded vocal tract aerodynamic theory, we hypothesized the following immediate changes in the flow glottogram during the /ol-buzz/ condition: a greater open quotient, a greater skewing quotient, a greater maximum flow declination rate, increased average airflow, decreased peak airflow, and increased minimum airflow. A cohort of eight men with normal voices produced the sustained vowel /o/ and the Vocal Function Exercises semi-occluded mouth posture (/ol-buzz/). Flow glottograms for both conditions were obtained from the inverse-filtered oral airflow signal via a circumferentially vented pneumotachograph mask. Data revealed that open quotient and minimum airflow rates increased significantly between conditions. All other measures trended in the directions predicted by aerodynamic theory, but were not statistically significant. The Vocal Function Exercises semi-occluded mouth posture appeared to provide an effective vocal tract semi-occlusion that immediately altered the flow glottogram in accordance with predictions made by computer-modeled aerodynamic theory.

Two-dimensional assessment of the nasal valve area cannot predict minimum cross-sectional area or airflow resistance.

Clinicians who manage nasal obstruction often comment on the shape and size of the nasal valve (NV) area. However, correlation of the symptoms of obstruction, nasal airflow dynamics, and the endoscopic appearance of the anatomic cross-sectional area of the NV is poorly understood. Endoscopic imaging and calculation of the NV area is investigated as a tool for either clinical or research use. To describe and evaluate a two-dimensional measurement of the minimum cross-sectional area (MCA) of the NV by using endoscopic imaging. A cross-sectional study of patients with symptoms of nasal obstruction who were undergoing nasal assessment was performed. The NV was measured with digital imaging taken from the endoscopy. Adobe Photoshop was used to calculate the digital MCA of the NV based on pixel count and a reference marker placed in the image field. Airway parameters were assessed by using a nasal obstruction visual analog scale, nasal airway resistance via rhinomanometry, and acoustic rhinometry derived MCA (acoustic MCA). Correlation of the digital MCA and airway parameters was made and interobserver correlation of the MCA measures was calculated. Thirty-three nasal airways were assessed: mean (standard deviation) digital MCA (0.28 ± 0.13 cm(2)) and mean (standard deviation) acoustic MCA (0.51 ± 0.15 cm(2)). Correlation of the digital MCA with visual analog scale was poor (Pearson r = 0.10, p = 0.56). Similar finding between digital and acoustic MCA was poor (Pearson r = 0.50, p = 0.56, respectively) despite a moderately strong interobserver correlation for the digital MCA (Pearson r = 0.79, p < 0.001). Qualitative endoscopic assessment of the NV may help clinicians predict NV dysfunction but simple two-dimensional measures seemed to be of limited value in accurately assessing the three-dimensional NV quantitatively.

Minimum Outdoor Air Control and Building Pressurization with Lack of Airflow and Pressure Sensors in Air-Handling Units

AbstractIdeally, minimum outdoor air and building static pressures are controlled through feedback controllers with the use of airflow and pressure sensors in air-handling units (AHUs). Unfortunately, because these sensors are not always installed in AHUs, it is common to maintain the minimum outdoor airflow by fixing the AHU control dampers at minimum positions and maintain the building static pressure by tracking the return fan speed with the supply fan speed. The purpose of this paper is to evaluate the system performance of these alternative controls. The outdoor airflow and building static pressure of an AHU is simulated using a nonlinear network solution. The simulation results show that the fixed damper position method can approximately maintain a constant outdoor airflow ratio with an appropriate damper connection if the building static pressure is perfectly controlled. The return fan speed linear tracking method along with the fixed damper position method shows performance in maintaining the mini...

Analysis of the Influence of the Permeability of the Envelope in the Performance of Different Types of Mechanical Ventilation Systems

The requirement of a minimum air flow rates in dwellings is new in the Spanish regulation. The value of the minimum air flow rate for each room of the dwelling depends on the type of room, its occupation or its dimensions. To achieve these air flow rates, in the houses built in the Basque Country, mechanical ventilation systems are installed, usually: simple flow with continuous extraction, hygrorregulated simple flow or double flow with heat recovery ventilation systems.The permeability of the envelope is a very important parameter, that must be cared for the correct performance of the ventilation system installed. This performance must be measured in terms of energy consumption, but also in terms of indoor air quality, because the objective of the ventilation system is to ensure an adequate indoor air quality.It has been analyzed the quality of indoor air and the energy consumption of a dwelling for the three systems of ventilation, and for three levels of permeability. The analysis of the indoor air quality has been done checking 15 contaminants. As conclusion, we can say that in the case of the two simple flow ventilation systems, the permeability affect mainly on the indoor air quality, but in the case of the double flow with heat recovery ventilation systems on the energy consumption.

Demand-controlled ventilation for multiple-zone HVAC systems—Part 2: CO2-based dynamic reset with zone primary airflow minimum set-point reset (RP-1547)

In Part 1 of this article, a CO2-based dynamic reset was proposed and evaluated as an energy-saving demand-controlled ventilation strategy by reducing the outdoor airflow rate when the occupancy is under design occupancy. Further energy-saving potential still exists when the zone primary airflow rate is modulated or reset instead of only resetting the outdoor airflow. Two new control strategies are presented: a CO2-based dynamic reset, now combined with a zone primary airflow set-point reset in Part 2. The system level of these control strategies is the same as CO2-based dynamic reset in Part 1. The additional control features are added at the zone level. The zone primary airflow rate minimum set-point will first be reset to increase system ventilation efficiency, which will lead to a reduced system outdoor airflow rate. With this proposed dynamic reset with zone primary airflow set-point reset, the design minimum zone primary airflow rate can be designed lower than the regular set-point used in variable air volume terminal box minimum set-points. This new setting reduces the energy consumption for the system supply fan as well as energy consumption for terminal reheating. Both energy simulation and airflow simulation were conducted in different locations of a building. In this article, the results show that the proposed strategies of CO2-based dynamic reset with zone primary airflow set-point reset consume less energy when compared to CO2-based dynamic reset presented in Part 1 and the baseline case “without demand-controlled ventilation.” The average annual system outdoor airflow rates for the two options of the proposed demand-controlled ventilation are 44.1% and 45%, respectively, less than the outdoor airflow rate for the case without demand-controlled ventilation. Therefore, the proposed demand-controlled ventilation control strategies are suggested to consider for CO2-based demand-controlled ventilation for multiple-zone single duct VAV systems with terminal reheat.

Effects of diffuser airflow minima on occupant comfort, air mixing, and building energy use (RP-1515)

There is great energy-saving potential in reducing variable air volume box minimum airflow set-points to about 10% of maximum. Typical savings are on the order of 10%–30% of total HVAC energy, remarkable for an inexpensive controls set-point change that properly maintains outside air ventilation. However, there has long been concern whether comfort and room air mixing are maintained under low flows through diffusers, and this concern has prompted variable air volume minima to be typically set at 20%–50% of maximum. RP 1515 evaluated occupants’ thermal comfort and air quality satisfaction in operating buildings under both conventional and reduced minimum variable air volume flow set-points, and measured the air diffusion performance index and air change effectiveness for typical diffuser types in the laboratory. The hypotheses were that lowered flow operation would not significantly reduce comfort or air quality and that HVAC energy savings would be substantial. The hypotheses were almost entirely confirmed for both warm and cool seasons. But beyond this, the reduction of excess airflow during low-load periods caused occupants’ cold discomfort in the warm season to be halved, a surprising improvement. It appears that today's widespread overcooling of buildings can be corrected without risk of discomfort by lowering conventional variable air volume minimum flow set-points.

IAQ and energy impacts of ventilation strategies and building envelope airtightness in a big box retail building

Heating, ventilating, and air conditioning (HVAC) systems in buildings are designed to provide thermally comfortable conditions and to maintain acceptable indoor air quality (IAQ). At the same time, the operating costs of HVAC systems are often a large percentage of the total energy consumption of buildings, which constitutes 40% of the primary energy consumed in the U.S. As efforts are pursued to reduce building energy use, which may involve reductions in outdoor air ventilation rates, it is important to consider the impacts of these measures on IAQ. To better understand the IAQ and energy trade-offs of various ventilation approaches, simulations were performed using the multizone airflow model, CONTAM, for a big box retail store in two climates. Annual energy simulations were also performed in EnergyPlus. Simulations included ventilating at the minimum outdoor airflow rate prescribed by the ASHRAE Standard 62.1–2013 Ventilation Rate Procedure during occupied hours and at a lower rate for 24 h a day. Additional cases included increasing the base outdoor air intake rates and changing the building envelope airtightness. Annual airflow and contaminant (carbon dioxide, formaldehyde, ozone, particulate matter, and a generic volatile organic compound) simulations were performed using emission rates from previous studies of retail buildings. This study demonstrated that ventilating at a lower rate for 24 h a day saved energy compared with ventilating at the higher rate prescribed in ASHRAE 62.1 and that the simulated indoor contaminant concentrations did not exceed common benchmarks or health guidelines.

Type III home sleep testing versus pulse oximetry: is the respiratory disturbance index better than the oxygen desaturation index to predict the apnoea-hypopnoea index measured during laboratory polysomnography?

ObjectivesIn its guidelines on the use of portable monitors to diagnose obstructive sleep apnoea, the American Academy of Sleep Medicine endorses home polygraphy with type III devices recording at a...

A Study on the Control Method Without Stratification of Single Duct VAV Terminal Units

The objective of this study was to propose a stratification control method for the minimum air flow of a variable air volume (VAV) terminal unit. The minimum air flow rate of the VAV terminal unit is the key factor affecting thermal comfort, indoor air quality (IAQ), stratification and energy consumption depending on the operating mode of the VAV system. Therefore, selecting the proper minimum air flow is very important. In this study, a building simulation was conducted to evaluate the indoor thermal comfort, stratification, IAQ and energy consumption according to the fixed minimum air flow rate of the VAV terminal units. In addition, an algorithm was proposed considering the stratification and variable minimum air flow rate depending on the indoor load. The results of the fixed minimum air flow rate simulation showed that the minimum air flow rate considering stratification satisfies all the conditions of indoor thermal comfort, IAQ and without stratification. The proposed method was found to be more effective than the existing control method. The energy comparison with the existing control method showed that the proposed method reduces the energy consumption.

Extending air temperature setpoints: Simulated energy savings and design considerations for new and retrofit buildings

The thermostat setpoint range (deadband) in office buildings impacts both occupant thermal comfort and energy consumption. Zones operating within the deadband require no heating or cooling, and the terminal unit airflow volume rate may be reduced to its design minimum. Wider deadbands allow energy savings as well as lower total airflows through the terminal. The extent of such savings has not been systematically quantified. Reference models representing standard HVAC and building design practice were used to simulate the impact of thermostat setpoint ranges on annual HVAC energy consumption. Heating and cooling setpoints were varied parametrically in seven ASHRAE climate zones and in six distinct medium-sized office buildings, each representing either a new building design or a building controls retrofit. The minimum airflow volume rates through the VAV terminal units were also varied to represent both standard and best practices. The simulations are compared to empirical data from monitored buildings. Without reducing satisfaction levels, by increasing the cooling setpoint of 22.2 °C (72 °F) to 25 °C (77 °F), an average of 29% of cooling energy and 27% total HVAC energy savings are achieved. Reducing the heating setpoint of 21.1 °C (70 °F) to 20 °C (68 °F) saves an average of 34% of terminal heating energy. Further widened temperature bands achieved with fans or personal controls can result in HVAC savings in the range of 32%–73% depending on the climate. It is demonstrated that in order to fully realize energy savings from widening thermostat temperature setpoints, today's typical VAV minimum volume flow rates should be reduced.

Acoustic and Aerodynamic Measures of the Voice During Pregnancy

Known influences of sex hormones on the voice would suggest pregnancy hormones could have an effect, yet studies using acoustic measures have not indicated changes. Additionally, no examination of the voice before the third trimester has been reported. Effect of pregnancy on the voice is relatively unexplored yet could be quite relevant to female speakers and singers. It is possible that spectral and aerodynamic measures would be more sensitive to tissue-level changes caused by pregnancy hormones. In this first longitudinal study of a 32-year-old woman's pregnancy, weekly voice samples were analyzed for acoustic (fundamental frequency, perturbation ratios of shimmer and jitter, Harmonic-to-Noise Ratio, spectral measures, and maximum phonation time) and aerodynamic (average airflow, peak flow, AC/DC ratio, open quotient, and speed quotient) parameters. All measures appeared generally stable during weeks 11-39 of pregnancy compared with 21 weeks postpartum. Slight decrease in minimum airflow and open speed quotient may reflect suspected vocal fold tissue changes. It is recommended that future studies monitor and test correlations among hormone levels, visualanalyses of vocal fold mucosa, aerodynamic function, and glottal efficiency.

Demand control ventilation: Influence of terminal box minimum airflow setting on system energy use

Abstract Demand control ventilation is one of the most energy efficient ways to achieve the optimum indoor air quality (IAQ). The interest in it has led to a lot of research and publications particularly for single duct VAV system with terminal reheat since the update of the ASHRAE standard 62 in 2004. However, the VAV terminal box (TBX) control algorithms have been given less attention and the influence of the VAV TBX airflow setting on the system energy use has not been quantified. Thus, this study investigated how the VAV TBX minimum airflow setting influences system energy use as well as building IAQ. First, the DOE developed benchmark building model and EnergyPlus software were used to simulate the building annual hourly thermal load. Then, the system control, energy use and IAQ models were developed for the selected building. Two VAV TBX control methods, the single maximum and dual maximum control methods, were investigated in further detail using two different minimum airflow settings from ASHRAE Standard 90.1-2010. The results indicated that when the TBX minimum airflow is reset from 30% to 20%, a reheat energy savings as great as 40% was achieved and an annual system energy savings up to 30% was reached.

Optimised hydrodynamic parameters for the design of photobioreactors using computational fluid dynamics and experimental validation

A numerical simulation using computational fluid dynamics (CFD) was utilised to investigate the flow hydrodynamics of cylindrical bubble column type photobioreactors (PBRs) with a 30 l culture medium. To establish the reliability of the simulation study, the CFD model was validated using particle image velocimetry (PIV) computed data under various air flow rates. There were 32 simulation cases for the study comprising two PBR designs, four air flow rates and four nozzle size diameters. Hydrodynamic analyses such as % volume of dead zones, average circulation time and turbulence intensity inside the simulated PBRs were evaluated. Results have shown that the most appropriate PBR for microalgae cultivation was a design with internal baffle and an extended cone-shaped bottom section. In addition, the recommended nozzle diameter was found to be 10 mm and a minimum air flow rate of 0.10 vvm. To eliminate dead zones inside the PBR, the flow rate can be slightly increased but should not exceed 0.15 vvm. Practical evaluation through laboratory experiments has further confirmed the results of the study where the biomass concentration of Chlorella vulgaris from the proposed PBR was significantly higher compared to the standard PBR design. Based on the numerical investigation and practical evaluation, the improved PBR can be seen to be more effective in culturing microalgae particularly for larger scale mass production.

Evaluation of the Impact of Minimum Airflow on the Energy Consumption of Single Duct VAV Terminal Boxes

Terminal boxes are critical components of variable air volume (VAV) systems. The minimum airflow of terminal boxes is a key factor for comfort, indoor air quality (IAQ) and energy cost. If the minimum airflow is higher than needed, terminal boxes will have significantly more simultaneous heating and cooling, and air-handling units (AHUs) will consume more fan power. Buildings will have IAQ problems if the minimum airflow is less than required. A dynamic building simulation and field experiment were carried out to evaluate the impact of minimum airflow on the energy consumption of single duct VAV terminal boxes. A comparison of the simulation data with the existing constant setpoint revealed 20% annual energy savings in the improved minimum airflow setpoint. The experiment showed that the room air temperature can be maintained stably and the vertical difference in the room air temperature is lower than the comfort value when the improved minimum airflow setpoint is applied. Measurements of the CO2 levels also revealed no indoor air quality problems.

Stability and accuracy of variable air volume box control at low flows. Part 1: Laboratory test setup and variable air volume sensor test

Variable air volume systems with direct digital controllers have been widely adopted in the HVAC system of commercial, industrial, and large residential buildings because they provide better energy efficiency and occupant comfort. Normally, a variable air volume terminal unit defines a minimum airflow rate to satisfy the space ventilation requirement and/or the proper operation of a terminal heating coil, if so equipped. However, it has been found that variable air volume terminal units often fail to perform as expected at the minimum airflow range (below 500 fpm [2.5 m/s]). Under such a flow range, the embedded airflow sensor becomes inaccurate, and the designed minimum airflow rate is less than the minimum controllable airflow rate. This results in a series of problems, including lack of ventilation, uneven airflow control, reduced damper and operator life, and energy waste. Through designed laboratory and field tests, this study (ASRHAE Research Project RP-1353) aims to identify the major factors that cause inaccuracy and instability issues in variable air volume terminal units and the relationship between the major factors and performance of the airflow sensor, controller, and terminal unit system. Laboratory tests performed in this study included a variable air volume sensor test, controller test, and system test. Four variable air volume boxes from three manufacturers and four controllers from four manufacturers were tested systematically. Two identical test beds with high accuracy (±0.5%) reference airflow meters were designed and constructed in the test facility. The size of the reference airflow measuring stations was carefully selected to provide maximum airflow measuring accuracy and maximum available system pressure drop. This article describes the laboratory test setup and summarizes the variable air volume sensor test results. A companion article summarizes the controller test, system test, and field test results. From the variable air volume sensor test, three factors, namely, inlet conditions, low variable air volume damper positions, and low airflow rates, are identified as strongly impacting variable air volume terminal unit performance.

Expert-Based-Computer Aided Design and Component Selection for Dust Collection Systems

Environmental regulations regarding industrial pollution are getting tougher to keep high air quality. The design of proper system to meet specific requirements relays on selection of different filtration components. Components are available in a wide range of designs to meet various requirements of air filtration level, quantity, and characteristics of the contaminants to be removed. An expert system package for the selection of dust collectors' components is developed. The developed package utilizes database as well as knowledge base to aid the selection process based on practice and past experience. Industrial case studies are demonstrated to test the developed package and compare its results against the current systems design. The developed package in one case study estimated a minimum air flow rate of 60000 m 3 /hr, which is 30% higher than the existing value and a static pressure drop due to ducts of about 1900 Pa which is 30 % lower than the current value leading to improved air quality. In another case study the package results indicated the need to minimize the value of air flow to be 2300 m 3 /hr, which is 50% lower than the current value and increase the external static pressure to 1700 Pa which is 70% higher than the existing design value leading to better filtration efficiency.