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.
Going transcritical with CO2
This module looks at the growing use of carbon dioxide in transcritical refrigeration applications
Carbon dioxide (R–744) is regaining its place in the refrigeration world, having almost disappeared from widespread use in building services in the early days of air conditioning. Driven by environmental concerns, legislation1
is requiring
increased adoption of ‘alternative’ refrigerants, of which CO2
is one.
However, the development of transcritical CO2
systems also provides opportunities
for using this largely benign, naturally occurring chemical to power air-sourced heat pumps that can effectively produce temperatures suitable for domestic hot water.
The rise and fall of CO2 The resurgence of CO2
is due to its
environmental credibility as a ‘natural refrigerant’, but it is also non-flammable and has thermodynamic properties that give smaller volumetric refrigerant flowrates compared to the well- established halocarbon systems. It also has the benefit of being categorised as non-toxic (although it is an asphyxiant gas). CO2
as a gas makes up more than
390 ppm (0.039% by volume) of the earth’s atmosphere – one of the more significant gases. The ease by which it can
www.cibsejournal.com
The many personas of CO2 CO2
is well known both to the general public and engineers. Atmospheric CO2
be produced led to its early application in refrigeration in the mid 1800s, and it was subsequently applied widely in food refrigeration. The use of CO2
refrigeration in sea
transport enabled the liberalisation of the world fresh food market, and it reigned until the middle of the 1940s, when newly invented synthetic halocarbons became popular owing to their higher efficiencies and lower operating pressures. These displaced CO2
so that by 1960 CO2 marine applications2
as the favoured refrigerant, was rarely used in .
Its use continued in ‘cascade systems’ for industrial and process applications, where it provides low temperature refrigeration ‘cascaded’ with a higher temperature cycle (employing another refrigerant) to provide final heat rejection. However, over the last 40 years, the environmental consequences of chlorofluorocarbons (CFCs) and, subsequently, hydrochlorofluorocarbon (HCFCs) and, then, the realisation of the global warming penalty of hydrofluorocarbons (HFCs) has turned the focus back to natural occurring refrigerants such as CO2
, ammonia, and the hydrocarbons. Figure 1: The phases of carbon dioxide
is closely associated with climate change – providing about 60% of the enhanced greenhouse effect – where relatively small changes in global concentrations are linked with rises in global temperatures. The global warming potential (GWP) of CO2
is relatively low compared with
synthetic refrigerants such as R134a, which has a GWP 1,300 times that of CO2
, or even the recently developed
‘environmentally friendly’ refrigerants such as HFO–1234yf, with a GWP of 4. By comparison, naturally produced methane has a GWP of 25. CO2
is produced by the combustion of coal or hydrocarbons, through respiration, December 2012 CIBSE Journal 53
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