8 7 6
5 4 0
Cooling-based operation
Max.COP(7.1)
Heating-based operation
0 20 40 60 80 100 Operation ratio (%)
Outdoor temperature: 7˚C DB/6˚C WB Indoor temperature: 20˚C DB/15˚C WB
Figure 2: Potential operating COP at ideal conditions can reach more than 7 (Source: LG)
accommodate changes in the sum of the demands from the multiple connected room units. These are limited to providing either cooling, or – in the case of a reversible ‘heat pump’ system – heating across all the units at any one time. The reach of a traditional multi-split system would typically be limited to less than 100 metres of total pipe run (so, for example, if there were four internal units they could each be 12.5m metres away from the external unit).
Evolution Over the last 15 years, variable flow systems have evolved from simple multi-splits so that, through more sophisticated control mechanisms, they can extend to supply dozens of indoor units connected with several hundred metres of pipework to one or more outdoor units using variable flow (controlled by the demands at the room unit). They can also supply terminal units spread over 100m vertically from the outdoor unit. Rather than use separate gas and liquid pipe runs to each and every unit, these systems use a single pair (or trio) of refrigeration pipes that can serve many units (and sub groups), and are known as VRF systems. Each outdoor unit would typically provide
up to about 200kW cooling capacity and the distributed indoor units up to 35kW each unit. The outdoor unit will commonly have multiple compressors, and at least one of these would be a variable capacity device. The compressors would normally be rotary or scroll compressors that inherently have little vibration and are controllable through modulating speed or digitally altering the geometry. The appearance of the indoor units is
similar to that of a traditional (water based) fan coil unit and they are similarly flexible with their positioning in the space. As with any coil that can operate below the dew point temperature of the room air, they should
48 CIBSE Journal April 2013
always be fitted with appropriate condensation drains. The room units will contain a fan, a refrigerant coil and controls providing both autonomous and integrated control operation. The room unit’s temperature will be set by the condition of the refrigerant running through the coil in the unit, in response to room – or zone – temperature sensors. The specific controls will be determined by
the manufacturer’s particular arrangements, but they would typically enable temperature control, fan speed control and, if included, dampers to allow fresh air supply. However, the headline operational benefit of VRF is through its integrated operation with both the other terminal units and the outdoor compressor unit. All will be connected through a common control system, which may itself be linked in to a central building management system and protocols such as BACNet and LONworks. There are number of configurations offered
that include single-mode systems (offering either heating or cooling concurrently to all the indoor units, which may be simply referred to as ‘heat pump’ VRF systems) and dual-mode systems employing two or three refrigerant pipes sequentially, serving units that individually can concurrently provide heating or cooling (these are frequently known as ‘heat recovery’ VRF systems). Operationally, single-mode VRF systems
will be similar to traditional multi-splits (aside from their ability to service more extensive systems). They can take advantage of the diversity of the timing of loads (for example, the temporal variation in peak cooling required for east- and west-facing office zones) to reduce the main system size (just as VAV does in ducted air systems). While operating in heating mode, all the internal coils will be acting as condensers, and the heating will be controlled by the electronic ‘expansion’ valve (sometimes referred to as a ‘linear expansion valve’) metering the flow. This will maintain
an appropriate level of sub cooling (and so coil temperature) in the refrigerant as it leaves the room coil to return via the common return pipe to the external condensing unit. The electronic expansion valve in the external unit will, in turn, act to maintain an appropriate degree of superheat at the compressor intake, while the flow of refrigerant is modulated to meet the flow demands of all the distributed units. Conversely, when operating as cooling
devices the internal units would function as evaporators, and the degree of cooling provided will be controlled through the local expansion valve maintaining appropriate superheat (and so coil temperature) at the exit of the local coil. Where there are concurrent loads for
heating and cooling in different zones, dual- mode (heat recovery) systems allow heat to be moved around the building between areas that require heat and those that require cooling. Heat would effectively be moved from areas that require cooling to those that require heating without the need for the refrigerant to pass through the condenser/evaporator coil in the external unit. The external compressor will still operate (to provide the compression for the cycle), and will adjust capacity to satisfy the total cooling being delivered by the sum of the individual units. The room units contain the same coils and
fans as for single-mode systems. However, by applying appropriate controls and some sophisticated piping arrangements, they allow concurrent heating and cooling within separate room units. Each of the indoor units is individually controlled, maintaining a required refrigerant flow to meet the load requirements by the use of electronic expansion valves (EEV), and the sum of all the flows is matched by modulation of the compressor output. The EEV also allows each individual unit to be isolated when an area is unoccupied or no conditioning is required. In two-pipe heat recovery systems, a
control unit local to a number of internal units provides a redirection of refrigerant between internal units so that some can act as evaporators providing cooling while, concurrently, others may act as condensers providing heating. The external unit will provide the compression to power the refrigeration cycle and, in some circumstances, if the cooling load of individual units matches the heating load of other units (plus the heat of compression from the external unit), the external coil will be bypassed and so the internal unit coils will be providing the complete evaporation and condensation
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Coefficient of performance (COP)
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