CHILLERS
cooling capacity, integrated Free Cooling and input and output temperatures of 18°C and 12°C respectively. The electricity costs were estimated at 15 Euro cents per kilowatt hour. Under these conditions, the annual operating costs for the pure glycol system with only one refrigerant circuit were 33,000 Euros cheaper. This takes account of the fact that a water-glycol mixture requires more pump power and that the losses in capacity caused by heat transfer must be offset by increased electricity consumption on the fans in the precision air-conditioning unit. So why is it that the glycol-free system ultimately costs far more to operate? The key factor is the compact integrated brazed plate heat exchanger between the inside and outside circuits. Firstly, the heat transfer losses that occur in this part of the system increase compressor running time. Secondly, the pressure drop that occurs in the flow through the brazed plate heat exchanger pipes increases the pump power requirement significantly in both the interior and Free Cooling circuits. This additional energy consumption means that at least part of the efficiency benefits of Free Cooling is cancelled out. In contrast, the pure glycol system can exploit the benefits of
Free Cooling without compromise and need not compensate for any heat transfer losses on the heat exchanger.
This means that on closer analysis, the argument that glycol-free systems are cheaper to run is equally untenable – at least under conventional pre-existing site conditions such as those in our example. But it can make
financial sense to use glycol-free systems at sites where Free Cooling is unfeasible and therefore where permanent compressor operation is required. Although even then, there is still the disadvantage that each additional component increases the statistical probability of a system failure. So whatever the case, system separation entails other risk factors in the data centre.
Beehive Coils
www.acr-news.com
August 2016 45
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