| dc.description.abstract | Executive summary
This technical report is intended as a preliminary study on airflow measurements as they are basic for the evaluation of the efficiency of air handling units (AHUs) in ventilation systems. The report targets readers intending to measure the field performance of the AHU. Its goal is to study solutions for airflow measurement of ducted air in ventilation systems.
The original purpose of this project was to measure the field efficiency of rotary heat recovery in buildings. For this purpose, measurements of the temperatures, airflows and relative humidity in all the ducts very close to the air handling unit were needed to calculate the field efficiency. The initial goal was to measure in nine different buildings and give some inputs regarding the discrepancy between rated and measured efficiency in buildings of different sizes and used for different purposes. However, it was soon unveiled the lack of suitable equipment to measure airflows in the ducted air for AHUs. For our purpose, it was necessary to have equipment that could measure accurately and that could be moved among the different buildings. It also was unveiled the lack of existing standards for measurements in the field of airflows in a reliable way. Thus, the purpose of this project was shifted to an overview and investigation of the existing knowledge and equipment for field airflow measurements, and these are the results presented in this report.
Heating, ventilation and air-conditioning (HVAC) play an essential role in achieving the desired indoor climate. The energy used by the HVAC in the building sector accounts for a large proportion of the total energy use in most countries around the world. Although extensive studies on energy conservation measures and innovations for the HVAC systems have been performed and implemented, the field performance of these measures in practice tends to be easily overlooked. The real performance can be considerably reduced from improper installation, system fault and lack of commissioning and in-field verification. IEA Annex 34 has shown that energy savings of 20-30 % can be reached by re-commissioning building HVAC system (Jagpal 2006). Monitoring airflow rates in ducted air in a ventilation system can indicate problems in the installation and operation of an AHU. The field performance of the AHU could imply whether the building energy use is in accordance with the design intent or is detrimentally affected.
However, it was not an easy task to perform airflow measurements in operating AHU. Most of the measurement recommendations of standards are defined for lab conditions where the air velocity profile in the duct is fully developed. In practice, it is difficult to follow the standards’ requirements in most cases. Operating systems have bends or devices that disturb the flow, thus it is difficult to find stretches of ducts long enough or to get access to the straight parts of the ducts close to the AHU. Consequently, the air velocity profile at the measuring points is usually not fully developed which increases the measurement complexity and reduces the accuracy. Especially measuring the airflow rate close enough to the heat exchanger (to avoid branching of flows) was found to be a problem due to turbulence and distorted velocity profiles.
During this project, scientific literature, researchers, equipment suppliers and contractors around the world have been contacted in the search for suitable solutions. However, none of the sources had perfect solutions for field airflow measurements. This report provides an overview of relevant studies and standards concerning laboratory measurements in a building-configuration-manner of rotary heat exchangers. Additionally, it focuses on testing currently available methods and devices that could be used for reliable measurement of airflow rates. Airflow measurement techniques investigated in this project are summarized herein as follows:
1. Pressure differential technique
The typical airflow rate measuring device employs a pressure differential method (e.g. Pitot tube, Venturis or orifice plate) to determine the airflow. This method is relatively accurate, but it is fragile, expensive and introduces an additional pressure penalty, and their sensitivity
becomes low for low airflow rates (Yu, Li et al. 2011). Additionally, this method is mostly used for laboratory test instead of field test due to the extra pressure loss and the strong intervention to the practical systems. In this project, the orifice plate method to determine the airflow rate is regarded as the reference value considering its high accuracy and reliability.
2. Velocity traversal technique
Another method is to use velocity traversal which needs to place one or more velocity sensors to various designated positions in the duct to obtain the area-averaged velocity. The flow regime, the duct diameter and velocity profile in the duct influence the number of measuring points and the measuring accuracy. The turbulence caused by the bends, system devices such as fans, filters, coils and limited straight ductwork in the AHU may lead to inaccurate velocity results and thus a larger number of required measuring points, which also means more labour work. Moreover, the traverse method cannot capture the airflow variations. The measured airflows are assumed constant within the time period of measuring velocities in the different points across the cross-section of the duct. The sensor probe will interfere with the velocity profile which in turn decreases the measurement accuracy.
3. Ultrasonic airflow measurement technique
The third method is using ultrasonic airflow measurement device, which is considered as a superior alternative to the pressure differential and velocity traverse methods in this project. The measurement accuracy of the ultrasonic device is proven almost equally good as pressure differential device (e.g. orifice plate) without extra pressure loss and interference on velocity profile. The ultrasonic method can monitor constant and time-dependent airflows. This ultrasonic measurement device is able to measure a wide range of air velocities without degrading the sensitivity due to its linear response (time difference between the propagation of the ultrasonic waves) to flow velocity change (Olmos 2004). This especially benefits the low flow velocity measurement. The difficulty of connecting pre-fabricated ultrasonic flow measurement module with existing ductwork in the practical system may impede its application in existing AHU.
4. Tracer gas technique
Tracer gas method is widely used in ventilation to determine the airflow rates and air distributions. The difference between the tracer gas and aforementioned three methods is that the tracer gas method directly determines the airflow rate while the other methods measure air velocity firstly and calculate the airflow rate by interoperating with cross-section area. The tracer gas technique in the AHU also enables to reveal leakages and airflow recirculation in rotary heat wheels. However, the poor mixing of the tracer with the airflow in the AHU causes wrong airflow rates, which almost always occurs in the practical system. In addition, this method demands proper preparation and specific knowledge for interpretation of the tracer concentration. It is not intuitive to obtain the airflow and difficult to perform compared to other techniques. It has a larger cost that the previous measurement techniques.
The products and measuring techniques presented in this report are suitable for measuring airflow rates in ventilation ducts, provided they are installed according to their requirements. Based on this study, we have not found an ideal portable equipment available for measuring airflow rates in AHU as they all require minimum straight duct lengths to achieve fully developed flow profiles or good mixing between tracer gas and air. Sufficient lengths of straight ducts are normally not available in practice. When the real efficiency of the heat or energy recovery in the AHU is to be measured, the accuracy and reliability of the measurement are expected to be lower than in the laboratory measurements. The measuring equipment must be chosen based on the characteristics of each ventilation system, especially ductwork diameter and configuration, available space to perform the measurements and minimum straight duct lengths before and after disturbances. This study did not find one single device that could be universally used for all different systems.
Further investigation on this issue and new development for airflow measurement devices are encouraged, so that the real performance of rotary heat exchangers can be easily verified and continuously monitored when installed in buildings. The ideal field measuring equipment should be able to accurately measure airflow rates, temperatures and relative humidity. The measurements should be carried out automatically (requiring little manual labour) and the results should be logged and easily exported to a computer for analysis. The device should also be of minimal disturbance for the airflow, as disturbances will induce pressure drop and increase energy use for the fans. | nb_NO |
