CPD PROGRAMME


elements in the refrigeration process. In a three-pipe system, as shown in Figure


1, one pipe would be the liquid line (as connected to the outlet of a condenser); one a high-pressure vapour line (effectively at the pressure of the compressor discharge); and one a low-pressure vapour line (effectively operating at the pressure at the inlet to the compressor). When cooling is required, a control unit


would open the unit’s coil to the liquid line (through the expansion valve) and provide the outlet into the low-pressure vapour line, so acting as an evaporator. For heating, the coil would be opened to the high-pressure vapour line and the outlet to the liquid line, so acting as a condenser. In both cases, the amount of cooling or heating would be controlled by the EEV to produce the required degree of superheat, or sub-cooling, respectively. Where there is an appropriate balance of


heating and cooling loads across a building (the ‘sweet spot’ typically being where the cooling amounts to 40%1


of the total


building load), manufacturers show that there is potential to operate with an effective coefficient of performance (COP) in excess of seven (as shown in Figure 2). This COP would not prevail throughout the whole season’s operation, being dependent on both the external conditions and the load balance. In a heat recovery VRF system, there are opportunities to capture heat not required for space heating that may otherwise be rejected, and use it productively, for example, in a thermal store or to generate domestic hot water. Similarly, there is potential to use the heat pump facility of the VRF to supply heat for hot water generation. The thermodynamics of the vapour compression refrigeration cycle will mean that when using a refrigerant, such as the


commonly used R410A, the maximum temperature that can be efficiently generated is about 50°C. This is usable for underfloor heating applications and potentially some ‘low temperature’ heat emitters, as well as preheating for domestic water. But as the difference between source (for example, the outdoor or room air) and the required target temperature (for example, the hot water store) increases, so does the compressor pressure ratio, and this will reduce the capacity of the system, alongside a falling system COP.


Hot water generation A number of methods have been identified to lift the upper temperatures of the cycle specifically for building services applications. A method being offered by several VRF manufacturers is to provide a cascade arrangement, where a second refrigeration cycle (using R134a refrigerant) uses the heat from a condenser in the VRF system to provide heat for the R134a evaporator. This cascaded system (Figure 3) can provide a useful solution where the demand for heat (for example in hot water storage) can absorb the variations in the space heating demands from the building, so more fully utilising the heat generated to produce the cooling required by the remainder of the installation.


System considerations As with any air source heat pump, the outside coils are likely to accumulate frost when the system is in heating mode (since the outside coil is acting as an evaporator at low temperature). This may be cleared via a number of automatic mechanisms, depending on the number and layout of external coils – though any defrost cycle will affect the system performance (and ‘derate’


the installed capacity). However, the effect of the cycle is unlikely to be noticeable in the room units. When there are multiple external units, such defrost cycles can be effectively sequenced while still maintaining heating to the internal units. One of the developments that enable VRF


is the control mechanism to maintain the lubricating oils in the compressor. Recovery of oil entrained in the refrigerant is principally achieved by capturing the oil before it enters the main distribution pipework. However, there will always be some carryover of lubrication oil into refrigerant flow, and this is recovered through a automatic dedicated cycle (lasting a few minutes), where the electronic expansion valves in individual units are automatically manipulated (they are all linked via the system’s control network) and the compressor operates to pump the oil from the system back to the outside unit. There is likely to be a significant network


of insulated copper refrigerant piping to serve a VRF-based system, including many brazed joints and some flared connections. The design for the refrigerant network must comply with the requirements of safety standards (such as BS EN 378 Refrigerating systems and heat pumps – Safety and environmental requirements and ASHRAE Standard 15 – Safety Standard for Refrigeration Systems). Similarly, the installation must be installed and operated to comply with the current standards, most notably the ‘F-Gas’ regulations.2 The cost of installing a VRF system is similar


to that of a three- or four-pipe water-based fan-coil system.3


Particularly where there are


opportunities for heat recovery, which may include using excess space heat for low and higher temperature hot water production, this electrically-powered system can be both straightforward and flexible to install and operate, and provide competitive seasonal efficiencies and operating costs. © Tim Dwyer, 2013.


Further reading: ASHRAE Handbook – HVAC Systems and Equipment (2012), chapter 18 has one of the most comprehensive introductions to VRF, together with examples of application (and provided some source material for this article).


References


1 LG UK – Heating with Air Conditioners to reduce carbon emissions, Powerpoint presentation, 2013.


2 (In Britain) The Fluorinated Greenhouse Gases Regulations 2009, The Ozone-Depleting Substances (Qualifications) Regulations 2009.


Figure 3: Cascaded refrigeration process to provide hot water heating (Source: LG)


3 ASHRAE Handbook 2012 – HVAV Systems and Equipment, page 18.3


www.cibsejournal.com April 2013 CIBSE Journal


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