CPD Programme
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typically use modern fluorinated refrigerants (such as the HFCs R407C and R410A) that, although less damaging than the previously used CFCs, still have a signicant global warming potential (GWP). Ammonia, a naturally forming chemical, offers no harmful effects to the ozone layer. The ammonia water working fluid has zero ozone depletion potential (ODP) and zero GWP. Apart from the intrinsic environmental benefits, a low GWP also provides a credit when assessing BREEAM (Building Research Establishment Environmental Assessment Methodology) ratings for the building; and similarly the lack of CFC will ensure that the Energy and Atmosphere prerequisite for the LEED (Leadership in Energy & Environmental Design) rating system’s Fundamental Refrigerant Management will be satisfied, [1]. Ammonia refrigerant is toxic and
flammable. However, the commercially available units are designed for outdoor installation. The heat pumps contain a fully sealed ammonia circuit, with no need for any contact with the fluid during operation or servicing. The risk of hazardous ammonia escape is therefore significantly reduced. Considering the circuit shown in Figure
1 with reference to the bottom right of the diagram, the condensed refrigerant is expanded and enters the evaporator at a relatively low temperature and pressure. Heat is thus drawn into the system from the surrounding air by the ‘evaporator’ as the ammonia evaporates (the evaporator is shown diagrammatically wrapping around the other components – this can be seen more clearly in the sectional diagram of a production unit in Figure 2). Heat is also added into the system by
the gas burner as it heats the refrigerant and absorbent solution in the ‘generator’ – this should ideally be configured to allow condensing of the combustion gases to maximise performance. This heat is being used to ‘boil out’ the ammonia from solution, so that the now higher temperature and higher pressure ammonia vapour can pass into the top of the ‘condenser’ (that is shown combined with the systems ‘absorber’) and exchange heat to the return water from the heating system (shown in this example at 40C) so that the ammonia condenses and returns via the expansion device back to the low pressure evaporator (to receive more heat from the surrounding air). At the same time, the absorbent (water)
from the generator, after the refrigerant is boiled out, continues to another heat
68 CIBSE Journal October 2010
Defrosting valve 50˚C
40˚C Solution pump
Weak
solution
Strong solution
Refrigerant vapour
exchanger called the ‘absorber’, which is at low pressure. It is here that the ammonia vapour and water join up again generating further heat as they combine. The heat is extracted in the condenser/absorber section to improve system efficiency. The now cool low-pressure mixture is pumped back to the generator, using the solution pump to complete the process. The water (from the building heating
circuit or coil of an indirect cylinder) is raised in temperature, which in this case is by 10K. The water used as an absorbent is quite separate from that circulating in the heating system. (For a more complete description of the absorption refrigeration system, see the CIBSE Journal CPD article from November 2009 – available online at
www.cibsejournal.com).
Applications Gas-absorption heat pumps can be utilised in schemes that would traditionally use
Refrigerating fluid
Figure 1: Internal thermodynamic cycle of a gas-absorption heat pump
boilers for space heating applications. As the heat pump is typically designed for outdoor installations, the plant room size could be reduced, thereby offering space efficiency as well as fuel efficiency. An example of a 41kW heat pump is shown in Figure 3. As with most types of heat pumps, the optimum performance available from gas- absorption heat pumps is achieved when supplying low temperature loads such as underfloor heating. Here water outlet temperatures at, for example, 35C to 45C are more appropriate for heat pumps compared with traditional radiator circuits requiring temperatures around 80C. Gas-absorption heat pumps are also
capable of producing domestic hot water with temperatures in excess of 65C via an indirect cylinder, in a similar manner to conventional commercial boilers. However, the efficiency would be significantly reduced to reach required storage temperatures for the potable (or domestic) hot water to prevent risks from legionella bacteria. (See section 5 of CIBSE TM13 [2] and HSE ACoP L8 [3] for guidance on averting the risk of Legionnaire’s disease) For larger load applications it is possible to connect multiple gas-absorption heat pumps to ‘cascade’ through the use of a digital controller. In the case of some products this can allow the linking of many units to provide total installed capacity of several megawatts. Using appropriate controls, it is possible
Figure 2 : Internal view of a gas-absorption heat pump
to design and install a system where a combination of gas absorption heat pumps and commercial boilers are used i.e. bivalent systems. Here the gas absorption heat pump capacity may be selected to match the base thermal load of the building, as a technology to contribute to achieving a low carbon footprint.
www.cibsejournal.com
Hot water
Burner Generator
Absorber/Regenerator
Condenser/Absorber Pipe in pipe
Evaporator
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