Chiller cooling data center: which coolant to choose?

For conditioning in data centers, centralized multi-zone systems with water-cooled machines (chillers) are most often installed. They are more effective than freon air conditioners, because the coolant circulating between the external and internal units does not become gaseous, and the chiller compressor-condensing unit is only put into operation when the temperature rises to a certain level. One of the most fundamental questions when designing a chiller system: which heat carrier is better to use? This 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 safely poured into the ground or into the sewer. Unfortunately, in our latitudes water is used only indoors, because at 0 ° C it freezes. 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 areas of freezing are isolated, the static pressure on the pipe walls increases, and as a result, a rupture occurs. Aqueous solutions of polyhydric alcohols do not have these disadvantages. They freeze at much lower temperatures, without forming local foci. Their density in the process of crystallization decreases much less than during the conversion of water into ice, and therefore the volume does not increase so much - even frozen aqueous solutions of glycols do not destroy the pipe.

Very often, customers choose propylene glycol because it is non-toxic. In fact, it is an approved food additive E1520, which is used in baking and other foods as a moisture-retaining agent. Apply it in cosmetics and much 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 reservoir to compensate for leaks. It is more difficult to work with ethylene glycol - this substance belongs to moderately toxic (third hazard class). Its maximum permissible concentration in air is 5 mg / m ³ , but due to its low volatility at normal temperature, vapors of this polyatomic alcohol can be poisoned only if you breathe for a long time.

Worst of all things with drains: 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 drain systems, isolated tanks and / or a system for diluting the heat transfer fluid with water: you cannot simply drain it into the sewer system. 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 has to be washed off with a large amount 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 practically free compared to the cost of coolants based on polyhydric alcohols. An aqueous solution of propylene glycol for the chiller-fan-coil system is rather 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, it will have to be included in the estimate and the costs of utilization are also, however, relatively small. There are nuances associated with viscosity and heat capacity: a coolant based on propylene glycol requires a higher pressure generated by a circulating pump. In general, the cost of operating the system with ethylene glycol is significantly lower, so this option is often chosen, despite some toxicity of the coolant. Another way to reduce costs is to use a dual-circuit system with a heat exchanger, when ordinary water circulates in internal rooms with a positive temperature, and an antifreeze glycol solution transfers heat to the outside. The efficiency of such a system is somewhat lower, but the volume of the expensive coolant is significantly reduced.

Results

In fact, all of the listed cooling options (except the purely impossible ones in our latitudes) 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: to change the concept when the project is almost ready. Moreover, the coolant cannot be changed 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 made once and for all.