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.
Designing ducted air system pressure drops for low carbon operation
The design ‘pressure drop’ in ducted air and piped water systems not only affects the sizing calculations, system noise and potentially controllability but also the installation cost and lifetime carbon footprint of a system
In the August CPD, the concept of static, velocity and potential pressures combining to give total pressure was described. Total pressure will always reduce as a fluid passes along a conduit (pipe/duct) – this is normally calculated as a drop in static pressure. Potential pressure will be related to the change in height as the conduit runs through the building and, although significant for liquid systems (with high densities), it is less so with air systems. The speed of the flowing fluid will directly affect the velocity pressure (in a square law relationship), and the static pressure will in turn compensate for any rise or fall in velocity pressure (the latter frequently referred to as ‘static regain’). This CPD will consider the application
of these concepts to examine how ducted air system designs might be adjusted to reduce their energy consumption. CIBSE Guide Bi
Galvanised mild steel rectangular duct 3 metres
1.5 metres
Outdoor O
Intake mesh
60% free area ζ = 1.47
A B Fan Air volume flowrate 0.7 m3 C D
Heater ΔPs
E 2 metres F = 25Pa (from manufacturer) /s (from design requirements)
Duct pressure drop 1 Pa/m (from design requirements) Duct size 350mm x 350mm (Using duct sizing data from CIBSE Guide C4) Duct air velocity 5.7m/s (From duct sizing data from CIBSE Guide C4) ζ factors from CIBSE Guide C4
Figure 1: Simple ventilation system asserts that some 8% of
electrical consumption in air conditioned and mechanically ventilated buildings is likely to be due to the motors powering the fans. This absolute fan energy use may well have reduced for new buildings in the intervening years since publication
www.cibsejournal.com
Atmospheric pressure
of Guide B, as the requirements of the Building Regulations to moderate building cooling loads have strengthened and the maximum allowable power consumed by fan systems have become more stringent. Nonetheless, contribution to energy use and the resulting carbon footprint will be significant. Taking a very simplified air handling
system, as in Figure 1, pressure drop can be established using the principles
O A Intake B Fan C
outlined in the August CPD. (More detailed commentary and rationale is given in the appendices of CIBSE Guide B, Section 3, as well as in the excellent examples of the CIBSE Guide to HVAC Building Services Calculationsii
D Heater
to determine the pressure drop through the rectangular galvanised steel ductwork and the individual fittings may be obtained from CIBSE Guide Ciii
Static pressure Total pressure
E
Velocity pressure F
Fundamentals, Chapter 21iv Bend
or ASHRAE .
G
H R Diffuser
October 2011 CIBSE Journal 63 .) The data
G H
Room R
Bend ζ = 0.25
1 metre
Fan total pressure
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