For air conditioning in data processing centers, centralized multi-zone systems with water-cooling machines (chillers) are most often installed. They are more efficient than freon air conditioners, because the coolant circulating between the external and internal units does not go into a gaseous state, and the compressor-condensing unit of the chiller is put into operation only when the temperature rises to a certain level. One of the most fundamental questions when designing a chiller system is: what is the best coolant 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 heat carrier. In addition, it can be safely poured onto the ground or into the sewer. Unfortunately, in our latitudes, water is used only indoors, because at 0 Β° C it freezes. 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 with the help of an expansion tank. Freezing areas 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 transformation of water into ice, which means that the volume does not increase so much - even frozen aqueous glycol solutions do not destroy pipes.
Very often, customers choose propylene glycol because it is non-toxic. In fact, this is an approved food additive E1520, which is used in baking and other food products as a moisture-retaining agent. It is used in cosmetics and much more. If the system is filled with an aqueous solution of propylene glycol, no special precautions are necessary: ββthe customer only needs an additional tank to compensate for leaks. It is more difficult to work with ethylene glycol - this substance is classified as moderately toxic (the third hazard class). Its maximum permissible concentration in the air is 5 mg / m3, but due to its low volatility at normal temperatures, vapors of this polyhydric alcohol can be poisoned only if you breathe them for a long time.
The worst situation is with wastewater: water and propylene glycol do not require disposal, but the concentration of ethylene glycol in public water facilities should not exceed 1 mg / l. Because of this, the owners of the data center will have to include special drain systems, insulated tanks and / or a system for diluting the drained coolant with water in the estimate: you canβt just drain 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 has to be washed off with a large amount of water. However, the use of ethylene glycol in modern air conditioning systems for data centers is also quite safe if all necessary precautions are taken.
Economy
Water can be considered practically free compared to the cost of heat transfer fluids based on polyhydric alcohols. An aqueous solution of propylene glycol for a 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 as much, but it will have to be included in the estimate and the costs of disposal, which are also relatively small, however. There are nuances associated with viscosity and heat capacity: propylene glycol-based coolant requires a higher pressure generated by the circulation pump. In general, the operating costs of a system with ethylene glycol are significantly lower, so this option is often chosen, despite some toxicity of the coolant. Another option to reduce costs is to use a two-circuit system with a heat exchanger, when ordinary water circulates in the interior with a positive temperature, and a non-freezing glycol solution transfers heat to the outside. The efficiency of such a system is somewhat lower, but the volume of expensive coolant is significantly reduced.
Results
In fact, all of the listed options for cooling systems (except for purely water ones that are impossible in our latitudes) have the right to exist. The choice depends on the total cost of ownership, which must be considered in each case already at the design stage. The only thing you should not do in any case: change the concept when the project is almost ready. Moreover, it is impossible to change the coolant when the installation of engineering systems of the future data center is already underway. Throwing and torment will result in serious expenses, so the choice should be made once and for all.
Source: habr.com