CPD PROGRAMME
Professional development
The CIBSE Journal CPD Programme
Members of the Chartered Institution of Building Services Engineers (CIBSE) and other professional bodies are required to maintain their professional competence throughout their careers.
Continuing professional development (CPD) means the systematic maintenance, improvement and broadening of your knowledge and skills, and is therefore a long-term commitment to enhancing your competence. CPD is a requirement of both CIBSE and the Register of the Engineering Council (UK).
CIBSE Journal is pleased to offer this module in its CPD programme. The programme is free and can be used by any reader. This module will help you to meet CIBSE’s requirement for CPD. It will equally assist members of other institutions, who should record CPD activities in accordance with their institution’s guidance.
Simply study the module and complete the questionnaire on the final page, following the instructions for its submission. Modules will be available online at
www.cibsejournal.com/cpd while the information they contain remains current.
You can also complete the questionnaire online, and receive your results by return email.
This module considers the methods and practicalities of measuring airflow Continuous airflow measurement in ventilation and air conditioning systems
When air conditioning and ventilation systems are designed, there are often assumptions made that the airflow rate delivered through the ductwork system will match the design intent. However, this is not likely to be the case, leading to inefficient or ineffectual ventilation. This CPD article will consider the methods and practicalities of measuring airflow, specifically concentrating on applications of continuous monitoring.
The opportunity for continuous airflow measurement It is only in cases where there are specific – and possibly special – needs to monitor the air flow rate continuously that there would normally be permanent air volume flow rate monitoring devices. This would often be where a particular air volume flow rate or room air pressure (normally as a means of maintaining positive movement of air and contaminants) must be assured to meet varying laboratory ventilation or process needs. However, the challenges of supplying sufficient outdoor air proportions are not restricted to laboratory or process use. Variable air volume (VAV) systems are notoriously demanding to design and operate to ensure optimum outdoor air fraction, as individual zones vary their demands on the total supply air flow rate. Actively monitoring volume flow rates in the outdoor air inlet duct together
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Air Handling Unit
Damper Room Damper Fan Damper Figure 1: Simplified recirculation ‘economiser’ system
with zone flow rates provides information for the control system to properly modulate the outdoor and recirculation air fractions. Such control may be driven by the needs of extract systems where variable flow fume hoods are being used. In this case, there is not only a requirement to maintain appropriate total supply flow rates (and resulting room pressures), but also the most energy efficient fresh air proportions in the variable volume supply air to the room where the fume hoods
are located (see CIBSE Journal CPD, October 2013, for further details of variable volume fume hoods). Alternatively, and more generally, accurate
airflow measurement can provide essential real time information for the commonly used systems such as the recirculation air system shown in Figure 1. The system, known as an economiser
system, will only properly economise if appropriate volumes of outside air are mixed
with the recirculated air – the proportions being set by the indoor and outdoor temperatures (or by comparing enthalpies or possibly moisture contents) and the required amount of fresh air. Without a direct measure of the incoming air volume flow rate, it may take some time – in some cases, many years of operation – before, for example, a poorly performing damper or a faulty actuator is discovered. This could result in long-term inefficient operation. This may
manifest itself as in the simulated data in Figure 2 for an economiser system. The actual flow rate through the outdoor
air intake can be adversely affected by such common phenomena as varying outside outdoor wind velocities; stack induced flow within the building altering the building’s pressure profiles; obstructed air intake and discharge louvers; frosted air filters; malfunctioning dampers/actuators; and faulty control loops. Beyond the day-to-day operational needs
when the building has a change of use, a variation in occupancy, altered indoor environmental quality demands or undergoes a refurbishment, real time air flow rate measurement (through both main ducts and individual sub-networks and branches) can enable the building operator to obtain information to inform the recommissioning of the system to satisfy new operational conditions.
February 2014 CIBSE Journal 65
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