Chiller data center cooling: which coolant to choose?

For air conditioning, data centers most often install centralized multi-zone systems with water-cooling machines (chillers). They are more effective than freon air conditioners, because the coolant circulating between the external and internal units does not go into a gaseous state, and the compressor-condenser unit of the chiller is switched on only when the temperature rises to a certain level. One of the most fundamental questions in the design of a chiller system: which coolant is better to use? It can be water or an aqueous solution of polyhydric alcohols - propylene glycol or ethylene glycol. Let's try to understand the advantages and disadvantages of each option.

Physics and chemistry

From the point of view of physical properties (heat capacity, density, kinematic viscosity), water is considered the optimal coolant. In addition, it can be easily drained onto the ground or into the sewer. Unfortunately, in our latitudes, water is used only indoors, since it freezes at 0 ° C. In this case, the density of the coolant decreases, and the volume occupied by it increases. The process is uneven, it is impossible to compensate for it using an expansion tank. The freezing sections are isolated, the static pressure on the pipe walls increases, and as a result, a rupture occurs. Aqueous solutions of polyhydric alcohols are devoid of these disadvantages. They freeze at much lower temperatures, without forming local foci. Their density during crystallization decreases much less than during the conversion of water into ice,

Very often, customers choose propylene glycol because it is not toxic. In fact, this is an approved food supplement E1520, which is used in baking and other food products as a water-retaining agent. Apply it in cosmetics and many more where else. If the system is filled with an aqueous solution of propylene glycol, special precautions are not needed: the customer will only need an additional tank to compensate for leaks. It is more difficult to work with ethylene glycol - this substance is classified as moderately toxic (third hazard class). Its maximum permissible concentration in the air is 5 mg / m ³ , but due to the low volatility at normal temperature, vapors of this polyhydric alcohol can be poisoned only if you breathe them for a long time.

The worst situation is with effluents: water and propylene glycol do not require disposal, but the concentration of ethylene glycol in the objects of general water use should not exceed 1 mg / l. Because of this, the owners of the data center will have to put in the estimate special drainage systems, insulated containers and / or a dilution system for the coolant to be drained with water: it is simply impossible to lower it into the sewer. The volumes of water for dilution are hundreds of times higher than the volumes of the coolant, and it is extremely undesirable to spill it on the ground or on the floor - toxic polyhydric alcohol must be washed off with plenty of water. Nevertheless, the use of ethylene glycol in modern air conditioning systems for data centers is also quite safe, subject to all necessary precautions.

Economy

Water can be considered almost free in comparison with the cost of heat carriers based on polyhydric alcohols. An aqueous solution of propylene glycol for the chiller-fan coil system is quite expensive - it costs about 80 rubles per liter. Given the need for periodic replacement of the coolant, this will result in impressive amounts. The price of an aqueous solution of ethylene glycol is almost half that, but with it you will have to include in the estimate the costs of disposal, too, however, relatively small. There are nuances related to viscosity and heat capacity: a propylene glycol-based heat carrier requires a higher pressure created by the circulation pump. In general, the cost of operating a system with ethylene glycol is significantly lower, so this option is often chosen, despite some toxicity of the coolant. Another option for reducing costs is the use of a dual-circuit system with a heat exchanger, when ordinary water circulates in internal rooms with a positive temperature, and non-freezing glycol solution transfers heat outside. The efficiency of such a system is slightly lower, but the volumes of the expensive coolant are significantly reduced.

Summary

In fact, all of the above options for cooling systems (except impossible in our latitudes purely water) have a right to exist. The choice depends on the total cost of ownership, which must be considered in each case at the design stage. The only thing that should not be done in any case: change the concept when the project is almost ready. Moreover, it is impossible to change the coolant when the engineering systems of the future data center are already being installed. Throwing and torment will result in serious expenses, so the choice should be determined once and for all.