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 fi nal 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.
Airfl ow pressure drop in HVAC ductwork
Fans consume a great deal of energy, and so their effi ciency is crucial to preventing waste. The airfl ow through ducted systems is key to maintaining maximum effi ciency. This CPD takes a detailed look at fl ow and pressure requirements for such systems
It is somewhat odd that despite the universal adoption of fans in industrial, commercial (and increasingly domestic) systems, the underlying concepts that determine the size, selection and effi ciency are still uncertain to many. It is thought that fans consume more than 20% of the electricity in buildings, and so are excellent candidates for optimisation when seeking opportunities to reduce the carbon footprint and the operating cost in the built environment. This CPD will consider the fl ow and pressure requirements to allow air fl ow through ducted systems. A future CPD will apply this to consider the pressure profi le through the whole system and the appropriate selection of fans.
The ‘total’ story When examining the air fl ow through a duct, it is convenient to consider the pressures in the fl owing air in terms of duct static, velocity and total pressure. The development of these concepts comes from a standard relationship, the Bernoulli Equation, which applies the conservation of energy to incompressible fl owing fl uids. The equation (for a ‘frictionless’ system) is:
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Potential Energy + Pressure Energy + Kinetic Energy = Constant (or Total Energy) The potential energy relates to the elevation of the fl uid (for example, its height above a datum such as ground fl oor level – think of the energy required to carry a barrel of water up several fl ights of stairs); pressure energy is due to the force of the fl uid all around it (air at the bottom of a cold lift shaft will have a pressure energy related to the force imposed by the weight of the column of air above it); and kinetic energy relates to the movement of the fl uid (to the square of the fl uid velocity). As the sum of the three is constant
in a closed system such as a duct or pipe (ignoring friction and assuming
Figure 1: Air moving through gradually expanding duct
Lower velocity
High velocity energy
Low velocity energy
incompressible fl uids) it means that if one of the values changes, then one or both of the others must compensate to keep the sum of the three constant. So, for example if a round duct very gradually expands (as in Figure 1) the velocity goes down as the area of the duct increases. The potential energy stays the same (as the centreline of the duct is still at the same elevation) and hence the pressure energy must increase to compensate, for the loss in velocity energy. Air is, of course, compressible but, at the pressures experienced in HVAC ductwork, it is assumed that the air will not compress and, if the temperature does not vary, the density of the air (kg/cu m) will remain
August 2011 CIBSE Journal
41
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