Constant Air Volume System Research Articles

Prediction model for vertical distribution of oil particles under thermal plume and displacement ventilation in a machining workshop

Oil particle concentrations exhibit an inhomogeneous distribution in large-space machining workshops with a displacement ventilation system. The inhomogeneous distribution can lead to variations in the required supply air volume used to control the indoor particle concentration at different heights. To rapidly calculate the particle concentration distribution in the vertical direction, this study established a zonal particle concentration prediction model based on the mass and energy balance by considering the interaction between the heat source thermal plume, wall thermal plume, and supply airflow. An experiment was conducted in a scaled chamber to validate the proposed prediction model. The result showed that the relative error between the prediction model and experiment ranged from −17% to 10%. A case study was performed to demonstrate the applicability of the prediction model to the control of a displacement ventilation system. Using the prediction model, the control system can maintain the particle concentration in the work area below 0.5 mg/m³with variable particle source emission rates and reduce the supply air volume by at least 24% and 55% compared with the variable and constant air volume systems, respectively, neither of which can address the inhomogeneous distribution of oil particle concentration. The prediction model can also be used for the design of displacement ventilation systems when the inhomogeneous particle concentration distribution is considerable.

Subzone division optimization with probability analysis-based K-means clustering for coupled control of non-uniform thermal environments and individual thermal preferences

Collective air distribution is widely used for thermally comfortable indoor environments but is limited to unified thermal comfort requirements. Advanced collective air distribution has the potential to generate non-uniform thermal environments for individual thermal preferences. This study proposes a subzone division optimization method for nonuniform thermal environments to improve thermal preference satisfaction. The proposed method divided non-uniform thermal environments into subzones using probability analysis-based K-means clustering, minimizing and maximizing the thermal environment differences within and among the subzones, respectively. The number of subzones was determined to maximize thermal preference satisfaction with the coupled control between the nonuniform environments and individual thermal preferences while limiting the percentage of thermal discomfort. The results based on experiments of advanced collective air distribution, i.e., stratum ventilation, showed that thermal preference satisfaction and the percentage of thermal discomfort increased with an increasing number of subzones. Compared with the conventional method (i.e., uncoupled control without subzone division optimization) and recent method (i.e., coupled control without subzone division optimization), the proposed method improved the thermal preference satisfaction by 34.7 % and 11.7 %, respectively, for four subzones. Moreover, with the proposed method, increasing the system control flexibility increased thermal preference satisfaction. Compared with the variable air volume system, the constant air volume system and the system with variable supply airflow rate and temperature improved thermal preference satisfaction by 5.8 % and 22.5 %, respectively. The proposed method contributes to extending the capability of collective air distribution for individual thermal preferences.

Optimizing demand-controlled ventilation with thermal comfort and CO2 concentrations using long short-term memory and genetic algorithm

Heating, ventilation, and air conditioning systems are the largest consumers of energy in commercial and residential sectors. Minimizing their energy consumption without compromising indoor air quality and thermal comfort would result in environmental and financial benefits. However, most buildings still utilize constant air volume systems with on/off control to meet the thermal loads. Previous studies show that CO2-based demand-controlled ventilation is the most widely used strategy to determine the optimal level of supply air volume. Conventional CO2 mass balanced models, however, do not take the thermal comfort level into account. This paper aims to develop and implement a practical and efficient optimization framework to help office building designers and operators enhance thermal comfort and indoor air quality while minimizing energy costs for ventilation using demand-controlled ventilation. The objective function was constructed to consider thermal comfort indices, indoor air quality, and ventilation power demand. Long Short-Term Memory networks in conjunction with a generic algorithm optimization were utilized to generate the search spaces and solve the objective function. The proposed optimization strategy achieved near-optimal results within approximately 1 h of computation time. The findings indicate a reduction of the average ventilation power by 5.6% compared to the current on/off control approach and a slight increase of 0.25% in ventilation power when compared against the minimum ventilation rate recommended by ASHRAE. Additionally, the optimized approach leads to a saving of 26.9 kg per day of greenhouse gas emissions in terms of carbon dioxide equivalent.

BIM-Based Automated Multi-Air Distribution Layout Generation for Office Buildings: A Case Study

Although HVAC (Heating Ventilation, and Air Conditioning) layout design has transitioned from 2D drawings to 3D BIM models, the traditional manual process of sizing and placing terminal units is still a time-consuming task, which can lead to inappropriate airflow and wastage of resources due to human error. The purpose of this paper is to develop a highly robust method for sizing and placing HVAC terminal units in rooms, while also systematically avoiding obstacles based on the locations of air terminal units and FCUs (Fan Coil Units) to which they belong. The method employs a “scan” approach to solve the problem of insufficient robustness caused by the traditional grid division method. Rule-based iterative algorithms are utilized for the sizing of terminals, airflow verification, and obstacle avoidance to generate a complete set of FCU and CAV (Constant Air Volume) system terminal layouts within a building. The method was tested for the automated design of HVAC terminal layouts in six different buildings and successfully completed the task within seconds, demonstrating the method’s immediacy and robustness. Moreover, airflow organization tests showed that the terminal layouts generated by the algorithm had a 95% pass rate, indicating the effectiveness of the method.

Comparative Numerical Energy Analysis of Decentralized Ventilation Adapting to Local Norway Climates

The present work evaluates the performance of different decentralized ventilation control strategies in the local Norway climate. Evaluation is performed using a monthly time-step energy analysis and the IDA-ICE simulation tool for a comparative primary energy analysis on the strategy combinations. Primary energy comparison is conducted with respect to a centralized constant air volume system for comparative analysis. The evaluation results show that the representative decentralized ventilation (DV) system has the most significant energy performance. The lower heat recovery efficiency significantly impacts on the ventilation energy of DV system in the cold climate, and the low specific fan power can efficiently be used for zone cooling in summer.

Feasibility study for the use of variable air volume systems for office buildings

The main factor contributing to greenhouse gas emissions is the building up of the surrounding area. Studies have shown that buildings globally consume 30-40 % of energy use and release 40-50 % of global carbon dioxide emissions. Among all systems in houses, heating, ventilation and air conditioning (HVAC) systems are by far the most energy intensive. They consume approximately 50 % of the total energy demand of buildings. However, the systems are some of the most important systems in today's buildings. The number of these systems that are being installed has increased dramatically over the past few years. This is mainly due to the increasing demands for thermal energy, comfort and climate change. This paper presents a feasibility and ecology study between two ventilation or air-conditioning systems: constant air volume (CAV) and variable air volume (VAV). One of the purposes of this work is to determine the energy costs for each of the systems. An air conditioning system that saves operating costs usually requires a large initial investment. In this case, engineers must decide whether it is worth paying the additional upfront costs for a system that has lower operating costs. Despite the low attractiveness from the point of view of the investor, the VAV systems reduce the amount of greenhouse gas emissions and the amount of energy resources for servicing the commercial sector. Such system have less metal consumption. Thus, the cost of metal processing is also reduced. The results of this study can contribute to the future selection of ventilation systems, as well as contribute to the design and improvement of the systems under study. Energy saving is one of the main reasons why VAV systems are very popular today for the design of ventilation and air conditioning systems for office buildings and in many industries abroad. With these systems, the volume of transported air is reduced as soon as the operating load falls below the maximum projected load. The calculation of emissions of harmful substances into the environment was made while ensuring the operation of CAV and VAV systems.

Static pressure set-point reset smoothing scheme based-on improved iterative learning control and variable trajectory for air-conditioning system

Variable static pressure is the main strategy for air volume adjustment in variable air volume air-conditioning system and can offer huge energy savings compared with constant air volume systems. When parameter set-points reset with load, dynamic processes are excited. Some processes take a long time to reach a new stable state with large overshoots. Improved iterative learning control (ILC) algorithm is studied to ameliorate transition processes of the static pressure reset control loop. A mathematical model of the static pressure control loop is established, and a new desired trajectory is given to improve the system tracking performance. Simulation and experimental studies are performed on traditional Proportion Integration Differentiation (PID) control, Proportion Differentiation (PD) type ILC, fuzzy gain PD type ILC and variable speed-integral PID type ILC. Results show that the system has the smallest overshoot and the shortest adjustment time with the fuzzy gain PD type ILC. Meanwhile the system can be betterly stabilized near the set-point with smallest evaluation index and the desired trajectory can be tracked with better dynamic performance. Studies show that the dynamic process is smoothed by improved ILC and variable trajectory. It is of great significance to enhance the whole system stability.

Simulations of a novel demand-controlled room-based ventilation system for renovated apartments

The study simulated and assessed a novel control algorithm for an innovative room-based ventilation system for renovated apartments. The novel system is a manifold of fans that connects to an air-handling unit to control the supply of airflow to each room in an apartment. The exhaust side has a 3-way damper to control the division of extract airflows. The simulation required demand-control of ventilation airflows in each room. This included CO2- and temperature-based control of supply to dry rooms and humidity- and temperature-based control of exhaust from wet rooms. The object-oriented software IDA-ICE includes a graphical interface for assembling the controls, which enabled custom simulations. The controls efficiently maintained sufficient air quality in each room and ensured balance of supply and exhaust. The room-based demand-controlled ventilation system achieved 74% savings in fan energy consumption relative to the reference constant air-volume system. The simulations indicated the need for less-resisting overflow vents in doorways to prevent infiltration heat loss when supplying bedrooms with greater airflow. Infiltration heat losses increased by 18% with closed doors despite the use of acoustic vents to assist overflow. Future measurements will aim to validate the demand-control algorithm and the performance of the novel system in real apartments.

Static balance duct design method and temperature dependent variable air volume systems

In hot and humid tropical countries, the air-conditioning system has a high energy usage in commercial buildings. This study introduces a static balance duct design method for a temperature dependent variable air volume system. Ducts are looped together, which cause the air to self-balance and minimize the pressure loss across the duct network. The static balance duct network can easily identify the critical zone of highest pressure loss. This zone appears at the longest duct length from the air source and is always consistent for different load conditions. The air supplied to this critical zone is controlled by the zone temperature sensor, which directs feedback to the variable frequency drive fan of the air-handling unit. The rest of the variable air volume terminal units are zone temperature controlled. The duct looping design mitigates the problems in pressure variations and complements the temperature dependent control strategy. An open office case study is carried out to demonstrate the merit of the static balance duct design method. An actual installation was completed in a shopping mall. The recorded temperature readings verified that the static balance duct network and temperature dependent variable air volume air-conditioning system is operational and functional. Approximately 27% energy savings is achieved compared to the constant air volume system. Practical application: The static balance duct design method is a combination of equal friction duct design method and a loop duct network. This static balance duct network can identify the critical zone of highest pressure loss and this zone is always consistent. In this temperature-dependent variable air volume (TDVAV), the variable air volume (VAV) terminal unit at the critical zone is eliminated. The zone temperature sensor at the critical zone provides direct feedback to the variable frequency drive fan. All other terminal units are temperature dependent. The static balance TDVAV system enhances energy savings, improves the performance of VAV system and reduces the faults for VAV systems.

Model-based space temperature cascade control for constant air volume air-conditioning system

Constant air volume (CAV) air-conditioning system is still widely used for cooling/heating buildings because of the requirement of air changes in many occasions and its simple system design as well as low primary cost. The space temperature of the CAV air-conditioning system is usually controlled by using conventional proportional-integral (PI) control algorithm, which tracks the space temperature by regulating the opening of the water valve directly. However, the control performance of the space temperature is usually unsatisfactory because of the significant thermal inertia of the building mass and the large volume of the indoor air. This paper proposes a model-based cascade control method for the space temperature control of the CAV system aiming to improve the control robustness and accuracy. A supply air temperature prediction model is developed to predict the system demand supply air temperature based on the real-time load condition and space temperature. The prediction is used as the set-point, and the space temperature is well controlled by regulating the water valve opening. The proposed control method was validated in a simulation CAV system as well as on an experimental platform. The validation results show that the proposed model-based cascade control method achieves better space temperature control performance than the conventional PI control. This control method may well track the space temperature directly both in the virtual CAV system and the real experimental system.

New Approach for Refined Efficiency Estimation of Air Exchange Organization

In the modern conditions, energy efficiency is one of the most important world problems. One of the important factors influencing the overall energy efficiency of buildings is air distribution in rooms. Literature review shows different options of efficiency estimation of air exchange with significant limitations. Some of them have non-obvious physical meaning. Certain of them do not take into account possible room zoning. Consideration of turbulence intensity in the efficiency estimation was not found. In this work, we propose an approach to estimate the air exchange efficiency in different kind of rooms. It is a relation of minimum room demands and inlet air potentials. Additional definitions are introduced for the parameters and demands estimation. The mechanical energy of the air is used for the estimation, which includes the energy of averaged motion and turbulent pulsation. Special approach is offered for turbulence intensity computation for energy calculations. The example of efficiency estimation of air exchange in a museum room with constant air volume system of air conditioning is solved.

Demand controlled ventilation (DCV) in school and office buildings: Lessons learnt from case studies

Demand controlled ventilation (DCV) refers to a ventilation system with air flow rates that are controlled based on a measurement of an indoor air quality (IAQ) and/or on a thermal comfort parameter. DCV operates at reduced air flow rates during a large amount of the operation time and thus consumes less energy for fan operation and heating/cooling the supply air. The aim of the present research is to assess the IAQ, ventilation efficiency, and the operation and energy efficiency of real operating DCV systems in moderate climates.Measurements are carried out for at least two weeks in autumn and winter 2015-2016. The following parameters were monitored: CO2 concentrations and air temperatures at different positions in the room and at the extract air grill, position of the variable air volume (VAV) boxes, supply and extract air flow rates and the occupancy of the room. Four case studies with large and varying occupancy rate and with different use and ventilation systems are selected. Two classrooms and three landscaped offices were evaluated.The results show that a DCV is interesting in rooms with a large and varying occupancy rate such as lecture rooms and landscaped offices. A good IAQ is measured in all cases studied even at reduced air flow rates. The effect of the reduced air flow on the ventilation efficiency is negligible. The VAV boxes react well to predefined set points for CO2 concentration. During the measurement period, the reduction for fan energy ranges from 25 to 55% and ventilation heat losses 25–32% compared to a constant air volume system (CAV) with a design airflow rate of 29 m3/(h.pers), i.e., IDA3 in EN 13779. However, commissioning of the DCV is necessary to obtain and maintain these performances.

Influence of asynchronous demand behavior on overcooling in multiple zone AC systems

The cooling demands of different zones in an air conditioning (AC) system are different (with large discrepancies) even at the same moment, and the degree of variance changes with time. This asynchronous behavior of demand greatly influences the overcooling degree in multiple zone AC systems. This paper describes the asynchronous demand quantitatively and reveals its relationship with the overcooling degree in a multiple zone AC system. The Lorenz curve and Gini index are introduced in this study and used to describe the demand characteristics. Two typical multiple zone AC systems, namely, constant air volume (CAV) and variable air volume (VAV) systems, are considered as examples, and their overcooling degree under different demand profiles are analyzed. Under different asynchronous demands, the overcooling degree in the CAV system changes from 1 to 3.5, while that in the VAV system changes from 1 to 1.5. In this paper, the influence of the regulation ability of the AC system on energy consumption is also discussed. This paper presents a new perspective to study the demand pattern and explores the method to reduce the overcooling phenomenon in multiple zone AC systems.

Dynamic modelling of solar storage system: a case study of leisure centre

Purpose – The purpose of this paper is to present the advantages of a solar store system with transpired solar air collector (TSC) in North Wales, UK. The collectors are designed as a proposal to meet the target of the solar air storage and heating project to improve the efficiency of solar collectors in the UK. Design/methodology/approach – IES software simulation is used to examine the potential of a solar store system in Deeside Leisure Centre compared to the traditional constant air volume (CAV) system and CAV system with heat pump. The design parameters and configurations are determined on the basis of the monitoring results gained from recent experiments. Findings – The result demonstrates good agreement between simulations and monitoring results and the solar store system demonstrates considerably lower energy consumption compared to the traditional CAV system with and without heat pump. Originality/value – The usage of TSC is proven to be useful in improving COP of the heat pumps and reducing overall energy consumption in a leisure center. The framework proposed in this study could also be applied to different building types in order to highlight their advantages.

Determination of dust and microorganism accumulation in different designs of AHU system in Shaanxi History Museum

Serial environmental measurements were conducted in the Shaanxi History Museum with two different air handling unit (AHU) designs to obtain a distribution of airborne and dust-borne microorganisms, and to determine the effects of occupant density, temperature and relative humidity (RH) on airborne or dust-borne microbial counts. Samples were collected from five segments of three AHU systems during 9:00–10:00 and 15:00–16:00. The results indicated that a high density of occupants caused a serious accumulation of airborne bacteria and fungi regardless of whether the system was a constant air volume (CAV) or variable air volume (VAV) system. In both of the aforementioned systems, the same downward trend in airborne bacterial counts was observed, where the highest counts were found at the return air segment, followed by the filter and supply air segments. Nevertheless, airborne fungal counts showed opposing trends in CAV and VAV systems. Our results also demonstrated that dust loading in the VAV system was in the range of 1.1–13.3 g/m2 which meets the Chinese standard, while dust-borne bacterial counts (4.6 × 104–36.9 × 104 CFU/gdust) and fungal counts (5.3 × 104–1129 × 104 CFU/gdust) far exceeded relevant threshold value. Furthermore, dust loading was more closely correlated with dust-borne bacteria levels than dust-borne fungi levels. In addition, airborne or dust-borne fungal counts were higher than the bacterial counts in the same sampling locations in the two systems. The predominant fungal genera that were consistently identified in different segments of both CAV and VAV systems were Penicillium, Aspergillus and Cladosporium. Moreover, RH exerted a greater influence on microbial growth than temperature.

The Air Conditioning System of a Large Public Building Energy Consumption Simulation Research

The large building air conditoning system with different modes were made , which include the constant air volume system, variable air volume system, the temperature and humidity independent control system. To calculate energy saving rate, the energy consumption simulated by DeST energy simulation software.

Reconciling Indoor Air Quality and Energy Efficiency in Air-Conditioned Classrooms: Case Studies for Portugal and Macao

Growing awareness on the subject of classroom's indoor air quality and concerns regarding the probable link between indoor air quality, student's attendance and academic performance, support recent legislation limiting classroom's indoor pollutant concentrations. A consequence of this legislation is the generalized shift towards air-conditioned classrooms and increased budgets for schools energy running costs. According to the scientific literature, demand control ventilation systems are suited for classrooms, ensuring indoor air quality and thermal comfort with significant energy savings. Based on detailed building energy simulation this paper compares a demand control ventilation system and a (traditional) constant air volume system considering a typical classroom located in two distinct climatic regions: Portugal and Macao. Based on hourly results of classroom's indoor CO 2 concentrations and indoor air temperatures, and on annual air-conditioning energy consumptions, advantages of the demand control ventilation system are discussed.

Decoupling dehumidification and cooling for energy saving and desirable space air conditions in hot and humid Hong Kong

The combined use of dedicated outdoor air ventilation (DV) and dry cooling (DC) air-conditioning system to decouple sensible and latent cooling for desirable space air conditions, better indoor air quality, and energy efficiency is proposed for hot and humid climates like Hong Kong. In this study, the performance and energy saving potential of DCDV system in comparison to conventional systems (constant air volume (CAV) system with and without reheat) for air conditioning of a typical office building in Hong Kong are evaluated. Through hour-by-hour simulations, using actual equipment performance data and realistic building and system characteristics, the cooling load profile, resultant indoor air conditions, condensation at the DC coil, and energy consumptions are calculated and analyzed. The results indicate that with the use of DCDV system, the desirable indoor conditions could be achieved and the annual energy use could be reduced by 54% over CAV system with reheat. The condensate-free characteristic at the DC coil to reduce risk of catching disease could also be realized.

A Dynamic Simulation of Indoor Polydispersion of Particles in All-Air Air-Conditioned Environment with Dynamical Particle Sources

This paper reports an investigation of the impact of indoor dynamic particle source on the operating strategies of all-air air-conditioning systems using a state-space model. In the case of constant air volume system, the simulation results indicated that if the designed air supply rate was properly determined, the indoor particle concentrations would satisfy the specified requirements of indoor air quality standards. For a variable air volume system, if the strength of indoor particle source is, in a relative sense, large compared to the cooling load, the particle concentrations could markedly increase and would probably bring about indoor air quality problem. The partial load ratio, a dimensionless ratio of the partial source ratio to the partial cooling/heating load ratio, has been suggested as an indicator to guide the design and operation of the air supply rate of variable air volume system. This operating strategy builds on the basis of the identification of indoor sources and takes the relative importance of cooling load and particle sources into the evaluation. The computational results showed that the ventilation performance of variable air volume system was improved by applying the partial load ratio to the operation of variable air volume system.

Effect of neutral temperature on energy saving of centralized air-conditioning systems in subtropical Hong Kong

Higher room temperature can still let the occupants have a neutral thermal sensation if higher air speed is provided. With a suitable scheme of neutral temperature and comfort air speed, reduction of energy consumption of the central chiller plant may surpass the additional energy requirement of the air side equipment, then both energy saving and thermal comfort can be achieved for the entire air-conditioning system. To evaluate this, the energy consumptions of a centralized air-conditioning system using the common air side alternatives were studied for the subtropical Hong Kong. The alternatives are variable air volume (VAV) system, constant air volume (CAV) system and fan coil (FC) system. Each of them was associated to a central chiller plant to serve a high-rise office building. The studying range of the room air temperature was from 23 °C to 30 °C. It is found that the VAV and FC systems can provide both thermal comfort and energy saving for higher room temperature, but CAV system is not feasible when the room air temperature is above 27 °C. If the indoor air speed threshold is considered, the neutral temperature can be brought up to 26.5 °C, and the energy saving potentials of VAV and FC systems would be 12.9% and 9.3% respectively.