Keith Carpenter, southern regional sales manager of Emerson Network Power, discusses the increasing importance of the 'free cooling' chiller concept in IT and commercial building applications

Today’s focus on energy efficient design of both buildings and equipment, shows that the ‘Free Cooling’ Chiller has never been more relevant to all sectors of the construction and mechanical services industries. ‘Free Cooling Chillers’, whilst not a new concept, provides significant direct financial benefits to the end user. 
The benefits offered, over site-specific bespoke designs, are that being fully packaged, all necessary components and control systems are internal, greatly reducing design and installation costs. Furthermore, the inherent results of 20 years or more experience of brand leaders in the design and operation of ‘free cooling’ systems, aids the smooth running and satisfactory completion of projects.

Free Cooling Defined
‘Free Cooling Chiller’ specifically refers to package air-cooled chillers, having an integral free cooling circuit with a self-contained control system. The term ‘free cooling’ encapsulates how ambient air, temperature permitting, is used to cool the chilled water supply, utilising compressors, evaporators, condensers, etc., as in a conventional air cooled chiller, but with the addition of an integral dry cooler circuit, thereby reducing power consumption and DX mechanical cooling. The dry cooler circuit is built into the same mechanical assembly that supports the condenser coils and fans and, where capacity is approximately 300 kW and above, the coil area is generally maximised by sophisticated “W” formation coil arrangements. The condenser fans also serve the dry cooler circuits and, using intelligent control routines, can control the compressor head pressure whilst maximising the free cooling operation.
                
Possible Savings
The savings available from a ‘Free Cooling’ Chiller will depend upon the application. Traditionally they have been used in data centres and office complexes highly populated with IT, where the load is relatively constant.  In such applications, studies show that a ‘Free Cooling’ chiller will typically reduce energy consumption costs by about 35%.
The recent significant increase in the use of ‘free cooling’ chillers within commercial properties to meet higher expectations regarding workplace air-conditioning, together with the green policies of many companies and the inexorable drive towards energy efficient buildings, shows this to be a growth sector.     In a typical office environment the seasonal diversity of the cooling load is generally considered to be about 45%, equating to a saving per annum of approximately 50%.

Fit For Purpose
When designing a free cooling system, remember the higher the design chilled water temperatures, the higher the energy savings. The maximum benefit can be obtained when the design incorporates a floating chilled water set point, optimizing the chilled water temperature to cooling load requirements.
A water glycol mix must be utilized to prevent both the risk of freezing and the free cooling coils splitting. Typically a 20% ethylene glycol mix is suitable throughout the UK. Glycol need only be added to the secondary water circuits feeding the chillers if the installation features primary and secondary circuits with heat exchangers in between.
The selection of a ‘free cooling’ chiller, requires the chiller to be as efficient as is possible as standard. Generally speaking, this requires the use of peak efficiency Twin Rota Screw or Fully Hermetic Scroll compressors.  Chillers of good design with high-energy efficiency will include a sickle blade fan arrangement, the fan system controlled via inverter drives or featuring an EC (DC voltage) fan motor with in-built speed control.
Manufacturers also offer ‘Free Cooling’ Chillers’ with the options of onboard pump sets, pressurization units, heat recovery systems, buffer vessels and a wide range of options, which reduces site construction time.
Sound emissions are also critical, with the key design element being the chiller fan section. In most 'free cooling' chillers of good design coil areas are maximized such that airflow rates and fan power requirements are near identical to conventional chillers. Most ‘free cooling’ chillers are supplied with levels of acoustic packages, for instance, 40 db (A) @ 10 meters not being uncommon.
   
More To Come
Most manufacturers chiller ranges reflect the gains in efficiency and performance made by employing the latest compressor, condenser coil, evaporator and fan design technology. The place in the market of the ‘Free Cooling’ chiller is becoming more extensive.  The traditional market place, serving data centres and IT suites is well established and secure while future growth in this sector is expected as a direct impact of increased IT loadings.