CPD PROGRAMME 20 MPa
18 MPa 15 MPa 13 MPa
10 MPa 9 MPa 8 MPa
7 MPa 6 MPa
5 MPa
4 MPa 3.5 MPa
3 MPa
2.5 MPa 2 MPa
5 qevap Evaporator 6 1 0 °C 3 Throttle -10 °C 4 q 6 20 °C 10 °C 1 wcomp Compressor
30 °C 40 °C 4
3 Space heater 3a 100 °C 120 °C 140 °C 160 °C 180 °C
50 °C 60 °C 70 °C 80 °C 90 °C qspace
Hot water tank
q hot w 2
the gas cooler and the cold discharge from the evaporator. An example application is shown in fi gure 4 for a heat pump feeding domestic hot water and space heating. In the example, the evaporator is operating at 0°C. To ensure good performance, the heating load – in this case, the space heating – should be designed to work on a wide temperature drop, allowing the gas cooler to work most effectively and provide the greatest opportunity for the evaporator to absorb heat from the outdoor heat source. Systems are available using hermetic and semi- hermetic multistage CO2
compressors 150 200
250 300 350 400 450 Enthalpy (kJ/kg)
Figure 4: Pressure enthalpy diagram for example CO2 Urieli:
www.ohio.edu/mechanical/thermo/)
the refrigerant at constant pressure, until it becomes a superheated gas at the intake of the compressor. The compressor increases the pressure, also adding heat to the refrigerant, and consuming power to drive the motor. The hot, high-pressure, superheated gas enters the condenser, where heat is rejected (or, in a heat pump, passed to the heat transfer medium) and the gas reverts back to a liquid. The warm, high-pressure liquid (often subcooled) passes through a pressure-reducing device (an expansion device) at constant enthalpy. The whole cycle is below the critical point, so is known as a ‘subcritical’ process. The expansion device is designed primarily to ensure that superheated gas enters the compressor by sensing the condition of the low-pressure refrigerant leaving the evaporator to control the fl ow. For a transcritical CO2
cycle, the process
does not look greatly different; however, the heat rejection occurs above the critical point and the temperature reduces while the CO2
remains a gas, and there is no
condensation. Both the absolute operating pressures and the range of pressures is far higher than for halocarbon systems. By the system – and specifi cally the ‘gas cooler’ – rejecting heat above the critical temperature, it allows the CO2
system
to operate so that it can provide heat at an exergy that is useful for heating domestic hot water (while the evaporator is operating at low temperatures suitable
www.cibsejournal.com 500 550 600 transcritical heat pump (based on work created by Israel
for cooling, or for drawing heat from low- temperature outdoor air in a heat pump application). At its simplest, the pressure reduction
is undertaken using a high-pressure expansion valve (HPEV) that controls the fl ow of refrigerant based on the pressure in the heat rejection (gas cooler) part of the cycle. In principle, the HPEV is normally held closed by a spring that works in opposition to the pressure in the gas cooler. There are enhancements that have been applied to the expansion process to improve the seasonal performance such as using two stages of expansion, with an intermediate liquid receiver with a direct vapour feed into the end of the evaporator process. A liquid receiver is often variously used (sometimes in association with an additional expansion valve to prevent damaging liquid CO2 entering the compressor) at the start or end of the evaporator to provide a buffer that allows the metering of the refrigerant to maintain optimum design pressures in the gas cooler. The effi ciency of the system is closely
related to optimising the lift provided by the compressor and the temperature at the outlet of the gas cooler – commercially available systems use tuned control algorithms to ensure seasonal performance is maximised. A heat exchanger is often used to exchange heat between the relatively hot discharge from
Further reading: For an introduction to the basic refrigeration process, see Refrigeration – inside the box, CIBSE Journal CPD March 2009.
There is an excellent presentation
at
www.phi–usa//Papers/CO2– presentation–LAM–2003-06.pdf that looks at the comparative attributes of CO2 CO2
– a refrigerant from the past with
prospects of being one of the main refrigerants in the future, by Nekså, P., provides both an historical perspective and description of key components (freely available – fi nd via a web search engine). For a historical perspective, see Pearson,
A., Carbon dioxide—new uses for an old refrigerant, International Journal of Refrigeration, Volume 28, Issue 8, December 2005. For safe commissioning and operation systems, the pre-eminent reference
of CO2
is the Institute of Refrigeration’s (IoR) Safety Code Of Practice for Refrigerating Systems Utilising Carbon Dioxide Refrigerant.
References
1
EC.europa.eu/clima/policies/f–gas/index_en.htm – accessed 11 November 2012.
2 Forbes Pearson, S., Report on Lloyds Register of Shipping, 1970.
3 Impact of CO2 on human decision-making and
productivity, Satish, U., et al., Sunny Upstate Medical University, CARTI Presentations, April 2010.
4
www.ohio.edu/mechanical/thermo/Applied/ Chapt.7_11/
Chapter9.html – accessed 11 November 2012.
.
to match the range of loads suitable for building services engineering. For larger systems, such as supermarkets, CO2 transcritical systems are now frequently used to provide high temperature loads in conjunction with lower temperature, subcritical CO2
systems. © Tim Dwyer, 2012.
December 2012 CIBSE Journal
55
Pressure
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