For cooling in data centers, centralized multi-zone systems with water-cooled machines (chillers) are most commonly installed. They are more efficient than freon air conditioners because the heat transfer fluid circulating between the external and internal units does not turn into a gaseous state, and the compressor-condenser unit of the chiller only starts operating when the temperature rises to a certain level. One of the most critical questions when designing a chiller system is: what coolant is best to use? It can either be water or an aqueous solution of polyhydric alcohols – propylene glycol or ethylene glycol. Let’s explore the advantages and disadvantages of each option.
Physics and Chemistry
From the perspective of physical properties (specific heat capacity, density, kinematic viscosity), water is considered the optimal coolant. Moreover, it can be safely discharged onto the ground or into the sewage system. Unfortunately, in our latitudes, water is only used indoors since it freezes at 0 °C. The density of the coolant decreases while its volume increases during freezing. This process occurs unevenly, and it cannot be compensated for by using an expansion tank. Regions of freezing become insulated, static pressure on the walls of the pipes increases, leading to breakage. Aqueous solutions of polyhydric alcohols lack these disadvantages. They freeze at much lower temperatures without forming localized pockets. Their density decreases much less during crystallization compared to the transformation of water into ice, meaning their volume does not increase significantly – even frozen glycol solutions do not damage pipes.
Customers often choose propylene glycol because it is non-toxic. Essentially, it is an approved food additive E1520, used in baking and other food products as a moisture-retaining agent. It is also applied in cosmetics and many other industries. If the system is filled with an aqueous solution of propylene glycol, no special precautions are needed: the customer will only require an additional tank to compensate for leaks. Working with ethylene glycol is more complicated – this substance is classified as moderately toxic (Class 3 hazard). Its maximum allowable concentration in the air is 5 mg/m3, but due to its low volatility at normal temperatures, one can become poisoned by the vapors of this polyol only by breathing them for an extended period.
The situation is worst with wastewater: water and propylene glycol do not require disposal, but the concentration of ethylene glycol in common water use facilities should not exceed 1 mg/l. Because of this, data center owners will have to budget for special drainage systems, isolated containers, and/or a dilution system for the drained coolant with water: simply discharging it into the sewage system is not permissible. The volumes of water required for dilution are hundreds of times greater than the volumes of coolant, and spilling it on the ground or floor is strongly discouraged – this toxic polyol must be washed away with large amounts of water. Nonetheless, the use of ethylene glycol in modern air conditioning systems is also quite safe when all necessary precautions are observed. data centers this is also quite safe when all necessary precautions are observed.
Economics
Water can be considered almost free compared to the cost of heat carriers based on polyatomic alcohols. An aqueous solution of propylene glycol for the chiller-fan coil system is quite expensive—around 80 rubles per liter. Considering the need for periodic replacement of the heat carrier, this adds up to significant amounts. The price of an aqueous solution of ethylene glycol is almost half that, but one must also account for disposal costs, which are relatively small. There are nuances related to viscosity and thermal conductivity: the heat carrier based on propylene glycol requires a higher pressure generated by the circulation pump. Overall, the operating costs of a system with ethylene glycol are significantly lower, which is why this option is often chosen despite the slight toxicity of the heat carrier. Another way to reduce costs is to use a dual-circuit system with a heat exchanger, where ordinary water circulates in heated indoor spaces while a non-freezing glycol solution carries heat outside. The efficiency of such a system is somewhat lower, but the amount of the expensive heat carrier is significantly reduced.
Summary
In fact, all the listed cooling system options (except for purely water-based ones which are not feasible in our latitudes) are valid. The choice depends on the total cost of ownership, which must be calculated for each specific case during the design stage. One thing to avoid at all costs is changing the concept when the project is almost ready. Moreover, changing the heat carrier is out of the question once the engineering systems of the future data center are being installed. Such indecision and dilemmas can lead to significant expenses, so the choice should be made once and for all.
Source: habr.com
