Movement and control of evaporating droplets released from an open surface tank in the push–pull ventilation system

Push–pull ventilation systems are effective local ventilation methods to control airborne contaminants generated in industrial buildings, among which droplets are typical. In this paper, the numerical simulations of water droplets released from an open surface tank into the push–pull flow field are carried out and the effects of ambient relative humidity and the pull-flow velocity on the ventilation system performance are discussed based on the droplet evaporation and movement. It was found that the movement and evaporation of droplets were closely related to the push–pull flow mechanism and the droplet initial diameter. When the control effect was good due to the presence of air closure in the flow field (pull-flow velocity ranging from 1.5 m/s to 3.0 m/s), droplets were unlikely to move away from the closure and the evaporation of droplets smaller than 40 μm was obvious. Whereas when the control effect was poor (pull-flow velocity equaling 1.0 m/s), large droplets still moved around the tank surface but small droplets were subject to dispersing, and in such a case droplets smaller than 60 μm evaporated obviously. Moreover, the effect of ambient relative humidity (ranging from 0 to 80%) on controlling droplets was rather limited and no more than ±6%. In addition, the system could save airflow rate and energy consumption by reducing the pull-flow velocity which was excessive originally in ventilation design. Finally, the paper put forward a new index to evaluate the control effect from another standpoint based on whether the droplets did harm to the environment.     

An energy impact assessment of ventilation for indoor airborne bacteria exposure risk in air-conditioned offices

Treatment of fresh air in ventilation systems for air-conditioned offices consumes a significant amount of energy and affects the indoor air quality (IAQ). In this study, energy impact on the ventilation systems was examined against certain IAQ objectives for indoor airborne bacteria exposure risk in air-conditioned offices of Hong Kong. The relationship between thermal energy consumptions and indoor airborne bacteria exposure levels based on regional surveys was investigated. The thermal energy consumptions of ventilation systems operating for carbon dioxide (CO₂) exposure concentrations between 800 and 1200 ppmv for typical office buildings and the corresponding failure probability against some target bacteria exposure levels were evaluated. The results showed that, for a reference indoor environment at a CO₂ exposure concentration of 1000 ppmv, the predicted average thermal energy saving of ventilation system for a unit increment of the expected risk of unsatisfactory IAQ of 1% was 55 MJ m⁻² yr⁻¹ and for a unit decrement of 1%, the predicted additional thermal energy consumption was 58 MJ m⁻² yr⁻¹ respectively. This study would be a useful source of reference in evaluation of the energy performance of ventilation strategies in air-conditioned offices at a quantified exposure risk of airborne bacteria.     

通风空调对室内空气品质的影响

加强通风空调系统对室内空气品质的正面作用,除了加大通风量外,近年来提高送风量的品质措施越来越多,本文强调的要深层次认识通风空调自身污染对室内空气品质的负面影响。针对目前空调系统的设计,提出了消除通风空调系统对室内空气品质负面影响、有效发挥其正面作用的一些措施。     

Evaluation of distributed environmental control systems for improving IAQ and reducing energy consumption in office buildings

Conventional heating, ventilation, and air conditioning (HVAC) systems are incapable of providing control over individual environments or adjusting fresh air supply based on the dynamic occupancy of individual rooms in an office building. This paper introduces the concept of distributed environmental control systems (DECS) and shows that improvement in indoor air quality (IAQ) and energy efficiency can be achieved by providing required amounts of fresh air directly to the individual office spaces through distributed demand controlled ventilation (DDCV). In DDCV, fresh air is provided to each micro-environment (room or cubicle) based on input from distributed sensors (CO₂, VOC, occupancy, etc.) or intelligent scheduling techniques to provide acceptable IAQ for each occupant, rather than for groups or populations of occupants. In order to study DECS, a numerical model was developed that incorporates some of the best available models for studying building energy consumption, indoor air flow, contaminant transport and HVAC system performance. The developed model was applied to a DECS in a model office building equipped with a DDCV system. By implementing DECS/DDCV and intelligent scheduling techniques it is possible to achieve an improvement in IAQ along with a reduction in annual energy consumption compared to conventional ventilation systems.     

Multiple-zone ventilation and temperature control of a single-duct VAV system using model predictive strategy

Conventional ventilation control schemes for VAV systems cannot achieve acceptable indoor air quality (IAQ). Ventilation control at the zone level is one of key factors affecting IAQ and also impacts thermal control. This paper proposes a multi-input–multi-output (MIMO) controller to control temperature and ventilation of multiple zones in a building with a model predictive control (MPC) strategy. The controller follows the ventilation rate procedure of ASHRAE Standard 62.1 and meets its ventilation requirements. Simulation-based experiments under four types of typical weather conditions are conducted to evaluate the controller's performance. The experimental results demonstrate that the controller is capable of maintaining ventilation air requirements and temperature of multiple zones.     

Ventilation rates in schools

Research shows that poor indoor air quality (IAQ) in school buildings can cause a reduction in the students’ performance assessed by short-term computer-based tests; whereas good air quality in classrooms can enhance children's concentration and also teachers’ productivity. Investigation of air quality in classrooms helps us to characterise pollutant levels and implement corrective measures. Outdoor pollution, ventilation equipment, furnishings, and human activities affect IAQ. In school classrooms, the occupancy density is high (1.8–2.4 m²/person) compared to offices (10 m²/person). Ventilation systems expend energy and there is a trend to save energy by reducing ventilation rates. We need to establish the minimum acceptable level of fresh air required for the health of the occupants. This paper describes a project, which will aim to investigate the effect of IAQ and ventilation rates on pupils’ performance and health using psychological tests. The aim is to recommend suitable ventilation rates for classrooms and examine the suitability of the air quality guidelines for classrooms. The air quality, ventilation rates and pupils’ performance in classrooms will be evaluated in parallel measurements. In addition, Visual Analogue Scales will be used to assess subjective perception of the classroom environment and SBS symptoms. Pupil performance will be measured with Computerised Assessment Tests (CAT), and Pen and Paper Performance Tasks while physical parameters of the classroom environment will be recorded using an advanced data logging system. A total number of 20 primary schools in the Reading area are expected to participate in the present investigation, and the pupils participating in this study will be within the age group of 9–11 years. On completion of the project, based on the overall data recommendations for suitable ventilation rates for schools will be formulated.     

Energy impact assessment for the reduction of carbon dioxide and formaldehyde exposure risk in air-conditioned offices

Treatment of fresh air in ventilation systems for air-conditioning consumes a considerable amount of energy and affects the indoor air quality (IAQ). In this study, energy impact on ventilation systems was examined against certain IAQ objectives for indoor formaldehyde exposure risk in air-conditioned offices of Hong Kong. Thermal energy consumptions for ventilation systems and indoor formaldehyde exposure concentrations based on some regional surveys of typical offices in Hong Kong were reviewed. The thermal energy consumptions of ventilation systems operating for CO₂ exposure concentrations between 800 ppmv and 1200 ppmv for typical office buildings and the corresponding formaldehyde exposure risks were evaluated. The results showed that, for a reference indoor environment at a CO₂ exposure concentration of 1000 ppmv, the average thermal energy saving of ventilation system for a unit increment of the acceptable formaldehyde exposure limit of 1 h (loss of life expectancy of 0.0417 day) was 280 MJ m⁻² yr⁻¹; and for a unit decrement of the exposure limit of 1 h, an additional average thermal energy consumption of 480 MJ m⁻² yr⁻¹ was expected. This study would be a useful source of reference in evaluation of the energy performance of ventilation strategies in air-conditioned offices at a quantified exposure risk of formaldehyde.     

Ventilation and energy performance of partitioned indoor spaces under mixing and displacement ventilation

Large variation in indoor air quality (IAQ) and thermal comfort can occur in partitioned office spaces due to heterogeneous air mixing. However, few published studies examined IAQ, thermal comfort, and energy performance of partitioned occupied spaces, which are commonly found in today’s buildings. The objective of this study is to evaluate indoor environmental quality and air conditioning performance of a partitioned room under two typical ventilation modes: (1) mixing ventilation and (2) displacement ventilation. For a total of six representative air-conditioning scenarios, three-dimensional computational fluid dynamics (CFD) simulations are performed to examine temperature distribution, ventilation effectiveness, energy consumption, and local thermal comfort for two partitioned spaces. Simulation results indicate that temperature distribution in a partitioned room is a strong function of ventilation strategy (mixing vs. displacement), but marginally affected by diffuser arrangements. Local age-of-air (air freshness) significantly varies with both diffuser arrangement and ventilation strategy. Regarding energy consumption, displacement ventilation can achieve an indoor set-point temperature in the partitioned spaces about two times faster than mixing ventilation. Under mixing ventilation, the time to achieve a set-point temperature was notably reduced when each partitioned space is served by its own diffuser. For the same supply airflow rate, displacement ventilation can generate local draft risk at ankle level, while mixing ventilation may result in a draft sensation in wider areas around an occupant. Overall, the results suggest that mixing ventilation system can save energy if each partitioned zone is served by its own diffuser such as a multi-split air conditioning. However, when multiple partitioned zones are served by only one diffuser, displacement ventilation is more energy-efficient and can achieve higher ventilation effectiveness than mixing ventilation.     

个性化通风系统的热舒适性分析

个性化通风系统可以改善空气质量、改善人体热舒适性。介绍了个性化通风改善热舒适性方面的研究成果,指出可以利用局部热感觉对整体热感觉的影响、通过个性化送风手段来满足人的不同热舒适要求。     

Modeling the resiliency of energy‐efficient retrofits in low‐income multifamily housing

Residential energy efficiency and ventilation retrofits (eg, building weatherization, local exhaust ventilation, HVAC filtration) can influence indoor air quality (IAQ) and occupant health, but these measures’ impact varies by occupant activity. In this study, we used the multizone airflow and IAQ analysis program CONTAM to simulate the impacts of energy retrofits on indoor concentrations of PM_2.5 and NO₂ in a low‐income multifamily housing complex in Boston, Massachusetts (USA). We evaluated the differential impact of residential activities, such as low‐ and high‐emission cooking, cigarette smoking, and window opening, on IAQ across two seasons. We found that a comprehensive package of energy and ventilation retrofits was resilient to a range of occupant activities, while less holistic approaches without ventilation improvements led to increases in indoor PM_2.5 or NO₂ for some populations. In general, homes with simulated concentration increases included those with heavy cooking and no local exhaust ventilation, and smoking homes without HVAC filtration. Our analytical framework can be used to identify energy‐efficient home interventions with indoor retrofit resiliency (ie, those that provide IAQ benefits regardless of occupant activity), as well as less resilient retrofits that can be coupled with behavioral interventions (eg, smoking cessation) to provide cost‐effective, widespread benefits.     

Simulation of ventilation and fire in the underground facilities

A revised fire outbreak and evacuation simulation model, MFIRE, is presented in the paper. The model provides information for setting up an emergency ventilation scheme, establishing safety procedures and minimizing damage in underground network systems. MFIRE simulates the interdependence between the ventilation system and its pertinent fans and structures, and the changes in ambient conditions and the heat source. A laboratory based fire simulation was conducted in a small physical tunnel network to verify the MFIRE. The rates of air flow and temperature distribution in each tunnel are compared with the simulated results obtained by MFIRE. Regarding air flow, the experimental rates correlate with the simulated results very well. Because of the reduced physical scale of the laboratory model, the simulated temperature distributions do not quite correlate with the laboratory data. MFIRE is employed to simulate a hypothetical fire outbreak in the Taipei Mass Rapid Transit System. The simulation is designed to investigate the direction and rate of air flow, temperature distribution and emergency ventilation responses. The results have confirmed that the proposed “push–pull” ventilation model can exhaust the high temperature air and smoke out of the underground facilities efficiently once the fire breaks out.     

A combined system of chilled ceiling,displacement ventilation and desiccant dehumidification

Different types of heating, ventilation, and air-conditioning (HVAC) systems consume different amounts of energy yet they deliver similar levels of acceptable indoor air quality (IAQ) and thermal comfort. It is desirable to provide buildings with an optimal HVAC system to create the best IAQ and thermal comfort with minimum energy consumption. In this paper, a combined system of chilled ceiling, displacement ventilation and desiccant dehumidification is designed and applied for space conditioning in a hot and humid climate. IAQ, thermal comfort, and energy saving potential of the combined system are estimated using a mathematical model of the system described in this paper. To confirm the feasibility of the combined system in a hot and humid climate, like China, and to evaluate the system performance, the mathematical model simulates an office building in Beijing and estimates IAQ, thermal comfort and energy consumption. We conclude that in comparison with a conventional all-air system the combined system saves 8.2% of total primary energy consumption in addition to achieving better IAQ and thermal comfort. Chilled ceiling, displacement ventilation and desiccant dehumidification respond consistently to cooling source demand and complement each other on indoor comfort and air quality. It is feasible to combine the three technologies for space conditioning of office building in a hot and humid climate.     

Sick building syndrome—A case study in a multistory centrally air-conditioned building in the Delhi City

Recently, airtight envelope system has become popular in the design of office buildings to reduce heating and cooling loads. Maintaining allowable indoor air quality (IAQ) for such airtight buildings totally depends on mechanical ventilation systems. Subsequently, poor operation of the ventilation system in such office buildings causes ineffective removal of polluted indoor air, and displays a sign of “sick building syndrome” (SBS). User's perception is an important parameter for evaluating IAQ. A questionnaire study was carried out to investigate the prevalence of the SBS at a multistory centrally air-conditioned Airport Authority of India (AAI) building in the New Delhi city. Quantification of the perceptions of the users regarding IAQ was done by converting their responses to a SBS score. The quantified answers were then subjected to statistical analysis. Qualitative analysis of the questionnaire was carried out to evaluate relationships between SBS score and carbon dioxide (CO₂) and other parameters related to building and work environment. Quantitative analysis of IAQ was also conducted by monitoring indoor concentrations of four pollutants, namely, nitrogen dioxide (NO₂), sulphur dioxide (SO₂), suspended particulate matter (SPM) and carbon monoxide (CO). Concentrations of pollutants were complying with IAQ standards as given by ASHRAE and WHO. The SBS was higher on the third floor as compared to other floors and the control tower. The main symptoms prevailing were headache (51%), lethargy (50%), and dryness in body mucous (33%). The third floor and the control tower were affected by infiltration, mainly from entrance doors. A direct relation between the average SBS score and CO₂ concentration was found, i.e., the average SBS score increased with CO₂ concentration and vice versa, clearly signifying the usefulness of SBS score in IAQ.     

Energy conservative building air conditioning system controlled and optimized using fuzzy-genetic algorithm

In this work, the combined effect of the energy conservative variable refrigerant volume (VRV) system and the variable air volume (VAV) system was experimentally investigated using genetic fuzzy optimization method that yielded better thermal comfort, indoor air quality (IAQ) requirements without compromising on the energy savings potential. The proposed system was tested using the demand controlled ventilation (DCV) combined with the economizer cycle ventilation (ECV) techniques and examined for a year-round building air conditioning (A/C) application. The supply air temperature (SAT) set points were varied under three distinct strategies and the optimal solutions obtained for the fuzzy systems designed resulted in an enhanced energy conservative potential. The test results of the proposed system were compared with the conventional fan coil A/C system. Based on the three strategies of the supply air temperature, the proposed system yielded an improved per day energy savings potential of 54% in summer and 61% in winter design conditions. Furthermore, for the strategies considered the proposed system achieved an annual energy conservative potential of 36% and exhibited more possible ways to achieve thermal comfort, IAQ and energy conservation.     

An experimental method for contaminant dispersal characterization in large industrial buildings for indoor air quality (IAQ) applications

Spatial contaminant distribution in large semi-open building areas such as sports arenas, warehouses, atriums, malls and other industrial halls, is critical to estimating exposure, health risks and building energy performance. It is considered a challenge to measure experimentally how effectively the ventilation system removes or dilutes air pollutants in a large open space, taking into consideration the usual non uniformity of the air flow created by local heat and contamination sources, geometrical obstructions and the air distribution system. An experimental method is presented in this paper which can be used for both the experimental evaluation of ventilation effectiveness as well for CFD modeling validation in large open space applications. The developed method which is based on a passive perfluorocarbon tracer gas (PFT) system has been successfully used for the experimental evaluation of ventilation effectiveness in an ice skating arena in the Boston area and for the validation of a developed CFD model for the analysis of the IAQ in ice rink facilities. The method is very easy to use, fast and fairly inexpensive and it does not interfere with human and other activities within the building environment.     

The effect of source motion on contaminant distribution in the cleanrooms

In the recent decades, cleanrooms have found growing applications in broad range of industries such as pharmacy and microelectronics. Concerns about negative effects of the contaminant exposure on the human health and product quality motivate many researchers towards understanding of the airflow and contaminant distribution though these environments. With an improvement in computational capacity of the computers, computational fluid dynamics (CFD) technique has become a powerful tool to study the engineering problems including indoor air quality (IAQ). In this research, indoor airflow in a full-scale cleanroom is investigated numerically using Eulerian-Eulerian approach. To evaluate the ventilation system effectiveness, a new index, called final efficiency, is introduced which takes all aspects of the problem into account. The results show that the contaminant source motion and its path have a great influence on the contaminant dispersion through the room. Based on the results, the contaminant distribution indexes, e.g. final efficiency and spreading radius, are improved when the source motion path is in the dominant direction of the ventilation airflow. Consequently, the efficiency of an air distribution system which provides a directional airflow pattern shows the least source path dependency. This study and its results may be useful to gain better understanding of the source motion effects on the indoor air quality (IAQ) and to design more effective ventilation systems.     

Prototype development of the rooftop turbine ventilator powered by hybrid wind and photovoltaic energy

In order to respond the suggestions made in the previous works, such as (1) improving the design of the rooftop ventilator commonly used in Taiwan, (2) making the “push–pull” airflow model in the ventilation duct effective for the bathroom ventilation and (3) combining the energy demands of the ventilator with renewable energy to reduce energy consumption, this study develops a prototype of the rooftop turbine ventilator powered by hybrid wind and photovoltaic energy. A low-speed wind tunnel experiment is performed to investigate the prototype's ventilation performance. The experimental results indicate installing an inner fan at low outdoor wind speed (0 and 5 m/s) increases the ventilation rate. The ventilation rate was not improved by installing an inner fan at a high outdoor wind speed. A rated rotation speed close to 1500 rpm is highly recommended when installing the inner fan. This study also introduces the general application modes of the proposed ventilator, and their electricity specifications.     

Comparing mixing and displacement ventilation in classrooms: pupils' perception and health

Several studies have found that indoor air quality (IAQ) in schools is often poor and may affect the health of the pupils. Building ventilation is a means to reduce pollutants indoors, but different designs should be evaluated for their effectiveness in different environments. In a field experiment performed at four classrooms in one school building, air was supplied either in the mixing or in the displacement mode, and we collected information on exposures, pupils' perception of IAQ and climate, and health symptoms and performed clinical examinations. The room temperature, relative humidity, concentration of CO?, and cat allergen were measured at the breathing height and were similar during each ventilation mode. The children perceived IAQ were similar in the two ventilation regimes, and there were few differences in symptom reports or clinical parameters. However, the pupils reported more eye symptoms during displacement ventilation. PRACTICAL IMPLICATIONS: Both mixing and displacement ventilation may be appropriate in school classrooms as long as the overall design, ventilation rates, and maintenance of systems are satisfactory.     

Regulatory standards related to building energy conservation and indoor-air-quality during rapid urbanization in China

The number of airtight buildings equipped with air-conditioning units along with levels of energy consumption from residential and commercial buildings has both increased markedly in China since 1990 due to rapid economic growth and urbanization. During this same period, home refurbishment/decoration/remodeling activities in newly constructed or existing apartments have become very popular and brought attention to a wide range of health concerns. This paper reviews building energy-saving and indoor-air-quality (IAQ) related standards in China. In summary, the two systems of building energy-saving and IAQ-related standards have been already established separately, although Chinese IAQ standards contain some indices related to building ventilation and energy (e.g. fresh air volume, relative humidity, and temperature). Building energy-saving systems are applicable to buildings existing in a wide range of climatic conditions. Formaldehyde was selected as a pollution index in “Chinese Evaluation Handbook of Ecological Residence Technology” (promulgated in 2001) for buildings mainly contaminated with harmful compounds emitted from interior decorating materials. As part of its IAQ control strategy, China promulgated a series of IAQ-related standards and compulsory national standards for limits of harmful substances contained in interior decorative materials (LHSCIDM), which placed strong emphasis on source control. When enacting the IAQ-related standards, China adopted some of the standards used in developed countries and related international standards for reference.     

Numerical analysis for evaluating the “Exposure Reduction Effectiveness” of room air cleaners

An “Exposure Reduction Effectiveness” index is proposed to quantify the performance of room air cleaners based on their capability of reducing occupants' exposure to pollutants of interest. The index has advantages over existing index such as CADR. It is applicable to both room average and local breathing zones, and can be used to compare air cleaning with the other two IAQ strategies: ventilation and source reduction. A computational fluid dynamics model is developed and used to assess the effectiveness of a room air cleaner for VOCs and particles of different sizes. The simulation results show: (1) species transport model and drift-flux Eulerian model can be used to predict concentration distributions of gaseous pollutants and particulate matters when there's an air cleaner operating and CFD simulation is an efficient tool to analyze personal ventilation; (2) the Exposure Reduction Effectiveness (ERE) index is effective for describing the removal effectiveness of room air cleaners in local zones as well as the room average.