34 Energy Efficient Air Conditioning Real efficiency gets it right
Efficiency is just a matter of getting everything right, says Colin Goode, VRF pre-sales engineer at FG Eurofred.
WITHIN MONTHS OF manufacture two identical cars can be returning vastly different fuel consumption figures, depending on how they have been driven, maintained and cared for. It is also the case that car manufacturers’ published fuel consumption figures are rarely achieved in everyday driving conditions – the difference between lab testing and real life. The same is true of air conditioning systems.
While choosing the right system is, of course, vitally important, it is only one component of the overall efficiency of air conditioning.
Selection of system type
There is plenty of information available from manufacturers to help contractors and specifiers choose the right equipment. However, there are a number of things to consider that may not be immediately apparent.
Firstly, do you opt for a multiple split system or a single VRF? For smaller applications multiple one to one split systems are normally cheaper to purchase and install than a single heat recovery VRF system. However, the VRF system is normally more efficient by way of its superior part load performance and energy recovery capability. This offsets the additional cost of the VRF system and has lower running cost over the life of the system.
Capacity
Even before getting as far as looking through the brochures, it is extremely important to calculate the duty of the system you will need. Too big a system will increase the initial cost and will never reach full capacity; too small a system will see the units running flat out constantly and never reaching their target temperature.
Finally, consider whether some form of heat recovery – either through heat- recovery ventilation units, a three-pipe VRF or both – would improve the overall efficiency of the system. This would apply particularly where a building needs heating and cooling at the same time, such as where one side is in full sun and the other is always in shade.
Installation
The siting of a unit can often be a compromise between efficiency, aesthetics and practicality. For very good reasons, cassettes, for example, are often positioned too close to walls, pillars, light fittings and other obstructions, which can cause
ACR News February 2014
recirculation of cooled air directly into the unit’s intake, causing it to shut down prematurely.
Ducted indoor units using open ceiling void return-air often suffer from erratic room temperature control caused by the fluctuating temperatures in the void space acting on the return-air sensor in the unit. The solution is to control the indoor unit from a room-mounted sensor, or to add a return air duct directly from the room to the rear of the unit.
Often, there are very few places that outdoor units can be located, which is why they often end up clustered together on back walls, alleyways, light wells or even inside ventilated plant rooms. While being in a shaded position is good, being too close together can cause heat from one being discharged into its neighbour. Also, because of restricted space, they can end up next to a kitchen extractor, heating flue or some other heat source, which will inevitably significantly reduce efficiency when in cooling mode.
Maintenance
Obviously, the better maintained a system is the more efficient it will be, so it is important to ensure that units are positioned for ease of access for service and maintenance. Maintenance should be undertaken at adequate frequencies for the building in question; typically twice a year for most commercial office buildings. Other buildings or environments may require more frequent maintenance visits to maintain clean filters, check condensate drains and keep outdoor unit heat exchanger coils free from debris. In addition to the normal maintenance procedures most systems will need a leak test to comply with F-Gas regulations. A further consideration is the pipework, which should be kept as short as possible to prevent capacity drop, and the number of joints kept to a minimum.
Ensuring that the pipework is correctly insulated is also important as not only can poor insulation affect the efficiency, it can also lead to condensation building up on the pipes to the point where water damage can occur to the fabric of the building.
Controls
Modern controls offer a lot of flexibility with multiple on/off times, programmable set back temperatures, different temperature settings for individual areas, set point limits and various password levels to prevent the ‘thermostat jockeys’ from setting unusually high or low temperatures. Some of the new room controllers also
have touch screens and most systems can also interface with building management systems if required.
To try to remove the human element even further, systems can be linked to passive infrared sensors, which can switch off a room unit if no people are detected inside a particular space. Additionlly, by installing magnetic sensors that can detect when windows are opened, individual indoor units can be switched off.
Commissioning
There are a number of key checks to make during commissioning, including strength/leak testing, ensuring all moisture has been removed and that the overall system refrigerant charge is correct; the system’s efficiency will be significantly reduced if it is under or over charged. Controls must be set up and time schedules entered in each of the room or central controllers. Indoor units must be checked and everything run-tested. However, all too often the system commissioning gets rushed and is not fully completed or documented because the installation has overrun and the project handover deadline must be met. This impacts on the system operation, overall efficiency/operating costs and end user training.
Both the handover instructions to the people who are actually going to use the system and the operation and maintenance manuals, for those who will have to maintain and operate the units in the future, have to be clear and understandable.
Finally you can get all these previous stages absolutely right, but if the manufacturer’s service and maintenance instructions and frequencies for filters, coils and grilles are not followed, it can all fall apart. There is no secret about getting an air conditioning system running efficiently; it is just a matter of getting everything right.
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