How to select the right system?m

هشام فكري محمد

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4 سبتمبر 2008
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How to select the right system

In split systems an outdoor condensing unit serves one or more (up to four) indoor units. Simple split systems are used only for cooling the indoor environment. Heat pump splits may be ‘reversed’ to provide indoor cooling or heating as required.

With VRF (Variable Refrigerant Flow) systems an outdoor unit serves up to 40 or more indoor units. In simple (2 pipe) heat pump systems all the indoor units are controlled to be in either cooling or heating mode simultaneously. More complex heat recovery systems (3 pipe) allow indoor units to function individually (or in zones) to serve the local requirement for cooling (eg on the sunny side) or heating (eg on the North side) with minimum energy consumption by the system.

The choice between VRF and split systems is no longer a simple one for specifiers, but help is at hand, says Mitsubishi Heavy Industries’ General Manager, David Lettis.

When specifying an air conditioning system, the decision used to be a fairly easy one to make - splits were for small installations, VRF systems were for larger operations, and chillers/AHUs were the only solution for the largest commercial projects. At either end of the spectrum, that still holds true in many cases. However, it is in the middle zone – medium-sized systems - where system selection becomes a more complex process, as VRF becomes ever more attractive in both smaller and larger projects.



Until recently there was one simple fact in selecting the air conditioning system for small to medium sized projects - splits were cheaper to buy. VRF systems, whilst potentially more energy efficient in operation and energy recovery, were more expensive to install. Splits were simpler and cheaper, and in many applications a series of split systems gave greater design flexibility than earlier generation VRF systems.

In more recent years the balance has changed. The relative cost of VRF systems has come down as a result of production efficiencies, and increasing skills and knowledge among specialist installation contractors. Most premium brand split systems now include inverter technology (to optimise energy consumed by compressor operation) which has existed in VRF specifications since their inception in the 80s.

Initial costs for modern energy efficient split systems for any given installation might be a third or more less than the capital costs of an equivalent VRF system, but in some applications, multiples of split system might be more expensive to install. High performance split systems and VRF systems qualify for Enhanced Capital Allowances (ECA), giving certain tax advantages to the user.

The latest generation of VRF solutions, including the new KX series from Mitsubishi Heavy Industries, offers increased design flexibility, reduced installation costs, and a range of compact lower output systems. It means that VRF can now be the most cost effective choice for some smaller installations. Add to that the increase in connectable capacity (the diversity maximum had been increased to allow up to 21kW of connectivity to one 14kW outdoor unit) and the KX6 VRF Compact looks like the smart choice for a whole range of smaller installations.



Cost is obviously a major driver, but it is not the only criterion. Local authorities are becoming more restrictive when it comes to the visual impact of outdoor units, particularly those in a roof-top location, and are demanding reductions in both the number and size (especially height) of outdoor units. A larger sized split system installation, with, perhaps, thirty outdoor units, could well run into planning difficulties, making a VRF system a viable alternative.

In the past, very large VRF outdoor units could still fall foul of the planners. The latest generation of VRF systems, however, have radically reduced dimensions. MHI’s KX6 outdoor unit, for example, is only 845mm high, making it a much more flexible and attractive proposition than in the past.

The choice for specifiers, then, is far from clear. However, there is an answer. Talk to the technical team at MHIE or our distributors who have a wealth of experience and a long track record of providing the optimum solution.
 

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