15 December 2021
|
How the ratio of energy consumed to energy produced can reduce operating costs for VRF systems
Harriet Evans, MHI sales director at Beijer Ref UK & Ireland, explores some of the key points to consider when designing a VRF system.
Variable Refrigerant Flow (VRF) air conditioning is a system that delivers high performance in cooling and heating for all commercial applications. It regulates its capacity to meet demand and often covers larger design loads than splits or multi splits.
MHI’s KXZ range of VRF includes both heat pump and heat recovery models, but what do they offer and how do you make the right choice for each project?
There is one clear operational difference between the two. A heat pump can operate in a single mode only, meaning that all indoor units are in cooling or heating at any given time. A heat recovery system, meanwhile, allows each indoor unit to be in either cooling or heating mode, irrespective of what the rest of the system is doing.
Making the right choice
When designing a system, the type of areas being served is the main consideration. Open plan office space, for example, would normally be considered one zone, regardless of how many indoor units are involved. For this reason, a heat pump system could be the right choice as all the units would be required to be in the same mode.
When specifying for a number of different areas, such as a collection of small offices or meeting rooms, or a collection of open plan spaces, then ideally each area should be able to regulate its own temperature and cool or heat as required. This is where a heat recovery system would come into its own.
This additional flexibility is the main reason why heat recovery systems are chosen, alongside the ability to reduce carbon footprint, meet energy-saving standards and cut building operation costs. That must be balanced initially against higher initial capital costs because of more components and the installation of three main refrigerant pipes around the building from the outdoor unit, rather than two with a heat pump system.
System efficiency
Air conditioning takes energy from one area and rejects it elsewhere. Normally this would see heat energy from a room rejected outside, using electricity to power the system’s compressor and the associated fans which circulate air.
Efficiency, whether EER in cooling or COP in heating, is a ratio of energy consumed to energy transferred / rejected / introduced: 1 kW of energy consumed to 1 kW of capacity produced, equals an EER or COP of 1; 1kW consumed to 4 kW produced gives a ratio of 4. The higher the number, the more efficient the equipment is.
When energy is rejected to ambient, for example in cooling with a heat pump, that energy is lost from the system, meaning efficiency is calculated at that point and cannot be changed. But if the energy can be reused, system efficiency can be improved.
A heat pump system has two main refrigerant pipes, a gas line and a liquid line, running from the outdoor unit to each indoor unit. A heat recovery system has three main pipes; a suction line, hot gas line and a liquid line. This allows both cooling and heating to operate simultaneously and reuse the energy recovered elsewhere.
VRF offering simultaneous operation of both cooling and heating
Operating half of the illustrated system in cooling would generate 50 kW capacity from 12.5 kW of electricity, meaning 50 kW of energy is rejected to ambient and an EER of 4. By reusing that energy and rejecting it into rooms requiring heating, another 50 kW capacity would be generated in the form of heat energy, taking the total to 100 kW, but still only needing the system to power half of that. No energy is lost from the system and so efficiency, in effect, doubles.
The amount of energy recovery taking place is fluid depending on the ratio of cooling to heating operation. The best efficiency ratings are usually when the system is operating in a 60/40 split and it is possible to achieve ratings of 9.0 with KXZ.
Go with the flow
Flow Selectors are required on a 3-pipe heat recovery system to divert the refrigerant flow if the associated indoor unit is in cooling or heating. They are available in single and 4 port versions, and are selected by the capacity and number of indoor units.
Both the low and high pressure gas lines connect to the box from the main run with the liquid line connecting straight to the indoor unit. On the 4 branch model, the liquid line is a small header which splits four directions. Because the liquid line bypasses the PFD box, it means that there are less joints and so less chance of refrigerant leaks, as well as less time spent on installation.
The flow selector determines what mode the indoor unit is operating in. In cooling it opens up the low pressure gas line to the unit and in heating it opens up the high pressure gas. This determines if the flow is suction from the indoor unit or hot gas into the unit.
Normally, a single unit would be connected to one branch of the flow control box. However, connecting more than one unit to a branch creates a small 2-pipe system, ideal for common areas and awkward shapes where airflow needs to be split and optimised.
KXZ VRF
Beijer Ref offers MHI’s KXZ range of VRF, including both heat pump and heat recovery models , mini and maxi chassis and a capacity range covering 12 to 168 kw from a single system.
Micro KXZ covers mini VRF, with single fan models of 12, 14 and 15.5 kW, which are available in single and 3 phase, as well as the 3 phase twin fan which covers 22, 28, and 33.5 kW.
Maxi VRF covers both heat pump and heat recovery options. The modular heat pump KXZE1 option is available from 28 to 56 kW from a single module, with up to 168 kW achievable from 3 unit system. High coefficient of performance (COP) variations are available from the KXZ-XE1 range, covers 22.4 to 100 kW and sees COP increased from 3.6 to 4.5.
Heat recovery systems complete the line-up, again with high COP variations, capable of meeting loads from 22.4 to 168 kW.
All products are approved by Eurovent, the independent testing body which supports operating and sound level data.
