How to Make the Right Choice? Comparison between the Enclosed Cold Aisle and the Hot Aisle

As we all know, the isolated cold (hot) aisle originates from the requirement in TIA942 "Data Center Communication Infrastructure Standard" that computer equipment and racks in the computer room should adopt the installation mode of "hot and cold aisles". Its core guiding ideology is the "reasonable planning of the air distribution in the data center" that we elaborated in the previous section.

So, what should we do when encountering problems in the actual design and implementation process? What benchmarks should we use for analysis and handling? These are the two basic principles that I will share with you below.
00 Container Principle
a) Name Definition
Define the static pressure chamber below the electrostatic floor in the data center as the container volume (M);
Define the cold aisle area after the air distribution planning as the container volume (M1);
Define the cold air volume sent out by the precision air conditioner as (Q);
Define the cold air volume required by a single rack as (Q1);
Define the air supply speed of the static pressure chamber below the electrostatic floor as (S);
b) Principle Elaboration
After arranging the cold (hot) aisle in the computer room, the cold air volume (Q1) required by a single rack remains unchanged. Since the cold air sent out by the precision air conditioner passes through two different static pressure chambers, M and M1, at this time, we assume that the container M and its air supply speed S are constant. Then, whether the pressure Pa1 of the container M1 is sufficient depends on the size of the container M1, that is, Pa1 is inversely proportional to the size of M1; and when we assume that the air supply volume Q of the precision air conditioner is constant, if the container M1 increases, it will not get enough pressure and thus cannot dissipate heat for the rack quickly.
c) Conclusion
Through the above problem analysis, if we cannot obtain enough cold air volume Q1, it can be said that, under the condition of the same cabinet height, the cold aisle with a width of 1200mm will have a better effect than the cold aisle with a width of 1800mm.
Speaking of this, some people may ask why the height of our static pressure chamber is getting higher and higher. In fact, the increase in the clearance height of the electrostatic floor is to reduce the impact of the air supply pressure on IT equipment. Because the greater the pressure, the faster the air supply speed. After passing through the IT equipment, they just simply take away the heat without sufficiently cooling the IT equipment, and the satisfactory effect has not been achieved yet.
The most talked-about topic in the industry regarding the hot and cold aisle system is: "Should we arrange the enclosed cold aisle or the enclosed hot aisle?" So, in practical cases, how should we make the correct choice? For now, let's not discuss how to make the choice. First, let's take a look at the differences between these two solutions.
Air Distribution Diagram of the Isolated Cold Aisle
Air Distribution Diagram of the Isolated Hot Aisle
01 Prerequisites for Selecting Cold/Hot Aisle Enclosure
Not all data center computer rooms are suitable for arranging enclosed cold aisles and hot aisles. It mainly depends on the heat dissipation amount of the data center. If the heat dissipation amount is not very large (generally based on the heat dissipation of a single rack not exceeding 4KVA), at this time, either the cold aisle enclosure or the hot aisle enclosure can be selected. In other cases, the author believes that it is best to choose to arrange the enclosed cold aisle. Why do I say so?
First, after choosing to arrange the enclosed hot aisle, we regard the area in the computer room except the racks as a closed container (M1), and the heat dissipation amount (Q1) of a single rack is unchanged. Then, to dissipate these heats, doesn't the cold air volume sent out by the precision air conditioner need to be greater? That is to say, when the heat dissipation amount is constant, the size of the container is inversely proportional to the air supply volume. From this point of view, arranging the enclosed hot aisle will increase the power consumption of the air conditioner.
Second, after choosing to arrange the enclosed cold aisle, we also regard the cold aisle area as a closed container (M2), and the heat dissipation amount (Q2) of a single rack is unchanged. Then, to dissipate these heats, doesn't the cold air volume sent out by the precision air conditioner need to be smaller than that when arranging the enclosed hot aisle?
02 Computer Room Environment after Arranging Enclosed Cold/Hot Aisle
After arranging the enclosed hot aisle, the remaining space in the computer room is in a cold air area. People who often enter and exit the computer room will feel that the computer room is very cold, but the equipment may not get cold air with sufficient pressure. After arranging the enclosed cold aisle, the remaining space in the computer room is in a hot air area. People who often enter and exit the computer room may feel more adaptable to the temperature, and the equipment can also get cold air with sufficient pressure.

03 Prerequisites for Arranging Enclosed Cold/Hot Aisle
The prerequisite for arranging the enclosed cold aisle is that the precision air conditioner is required to be a system with bottom air supply and top air return. At the same time, to ensure the air supply volume, the clearance space of the electrostatic floor should be at least 450mm.
The prerequisite for arranging the enclosed hot aisle is that, in addition to the requirement that the precision air conditioner is a system with bottom air supply and top air return, it is also particularly required that the data center computer room must have a good return air and fresh air system. So, from this point of view, it is not suitable to arrange the enclosed hot aisle for the renovated computer room. Because the old computer room may not have a good return air system.
04 Comparison of the Construction Difficulty of the Two Enclosure Methods
From the perspective of the prerequisites for arranging the enclosed cold (hot) aisle, the construction difficulty of arranging the enclosed hot aisle is obviously much greater than that of arranging the enclosed cold aisle.

It is mainly reflected in the following aspects: 1) The installation and arrangement of the return air duct it involves require the arrangement of the ceiling (while most large data centers do not need to arrange the ceiling at present). 2) Arranging the return air duct requires adding thermal insulation materials and exhaust equipment so that the hot air can be effectively discharged. 3) For enclosing the hot aisle or the cold aisle, the materials and workload of the two are not much different.
05 Post-maintenance after Arranging Enclosed Cold/Hot Aisle
After arranging the enclosed cold aisle, there is basically no maintenance cost in the later stage. Even if it is necessary to replace the lighting in the computer room, the ceiling of the cold aisle can be easily moved away.
After arranging the enclosed hot aisle, the hot air may not be discharged quickly, so it is necessary to add exhaust fans, which also brings a lot of troubles to the later maintenance. The fan is always running, and it is impossible that there will be no failures. Then, the exhaust fan needs to be replaced, and the workload will also increase.
06 Energy-saving after Arranging Enclosed Cold/Hot Aisle
In the case where the arrangement of the enclosed hot aisle is allowed, there is still a difference in the energy-saving efficiency data after arranging the enclosed cold (hot) aisle. The cold aisle can save about 30% of energy, while the energy-saving efficiency of the hot aisle is obviously lower than 30% due to the need to install return air fans, and in some places, it may not even reach 10%. It is possible that a return air fan needs to be installed at the top of the back of each (row of) cabinet.
07 Linkage between Arranging Enclosed Cold/Hot Aisle and Fire Protection
Fire protection has always been the most concerned comprehensive safety prevention system in data centers. Here, we are not going to talk about how the fire protection is, but rather what the situation will be when a single computer room encounters a fire? Because the power supplies of all IT equipment and the cabinet power supplies are arranged at the back of the rack. Even if a part of the circuit board of the IT equipment is at the front, when the equipment catches fire, it will start from the back of the rack. In line with the principle of extinguishing the fire first when a fire occurs, the main fire sprinklers should be installed in the hot air area, and auxiliary sprinklers can also be installed in the cold air area.
The linkage between arranging the enclosed cold aisle and fire protection does not seem to have a great relationship. However, if we want to arrange the enclosed hot aisle, it has a great relationship with the joint defense linkage.

That is to say, when arranging the cold aisle, the practice of using a magnetic lock to close the top plate in order to achieve linkage with fire protection is inappropriate. This not only does not play a key role in fire protection but also increases the energy loss.
Generally speaking: From the construction perspective, the enclosed cold aisle must have a raised floor, and the enclosed hot aisle must have a return air device, such as an air duct, a return air fan, etc.; from the perspective of access control management, the cabinets in the enclosed cold aisle are face to face, and aisle-level access control and cabinet-level access control can be realized, while the cabinets in the enclosed hot aisle are back to back, and cabinet-level access control needs to be set; from the perspective of the comfort of operation and maintenance personnel, in the computer room with the enclosed cold aisle, the temperature outside the aisle is high, and the comfort of personnel is lower than that in the computer room with the enclosed hot aisle.