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CHILLERS


Believing


is one thing – knowing is another


Natascha Meyer, product manager for chillers at Stulz GmbH looks at the debate around glycol-free chilled- water systems with integrated free cooling.


W


hen it comes to CW (chilled-water) systems for data centre cooling, many air conditioning specialists believe that it makes operational and economic sense to do away with water-glycol refrigerant in the data centre interior. But more detailed analysis suggests that this theory is the exception, not the rule. Whether pure water should be used for data centre cooling chillers, dispensing with glycol altogether, is an issue of ongoing debate. The basis for this discussion is the fact that using glycol comes with a range of disadvantages: ■ heat transfer is not as eff ective with a water-glycol mixture.


■ glycol is costlier than water. ■ larger pumps are required to circulate a water-glycol mixture than for pure water, increasing not only the scale of the project but electricity consumption as well.


Proponents of glycol-free data centre interiors argue that using pure water would reduce investment and operating costs and improve cooling capacity. In their view, glycol should only be used where anti-frost requirements render it indispensable: in the pipeline systems leading to the chiller outside the data centre.


But closer examination of the facts quickly shows that there are loopholes in this argument because it omits to say that system separation is a prerequisite for dispensing with


44 August 2016


glycol. Instead of a single chilled water circuit between the interior air conditioning units and the chiller on the roof of the building, a glycol-free data centre interior means splitting the system into two chilled water circuits: an interior circuit with pure water and an outside circuit for Free Cooling, which is still fi lled with a water-glycol mixture.


This system separation takes the heat load from the water circuit and transfers it to a brazed plate heat exchanger in the water-glycol circuit which then conveys the heat from the interior of the building to the outside chiller equipped with a Free Cooling system. Obviously, this uses less glycol than a CW system with only one refrigerant circuit. But it requires additional components: alongside the heat exchanger it necessitates a pump, as well as frost-repelling pipeline heaters for the pure water circuit, and a number of additional components such as special piping and wiring work. So dispensing with glycol doesn’t just save money: it also creates extra work. Ultimately there is so much additional eff ort involved that it negates the savings made by removing glycol from the equation. So in the fi nal analysis, the supposed reduction in investment costs is untenable. But what about the theory that glycol-free systems are cheaper to operate? A system comparison using the example of a continuously operational data centre in Hamburg provides some basic information on this. Operating costs were calculated for an air-cooled chiller with a 700kW


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