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The fact that in a cabinet with a performance loss of only 1 kW the limits of air cooling can be easily reached, lead to the initiative of the IEC Committee SC 48 D to develop a new standard for the cooling of cabinets (Picture 1). The aim of this standard is the provision of guidelines for the selection of air/water heat exchangers generated by different configurations. Users should receive a planning and configuration aide, proven in practice.
Picture 1: Water cooled VARISTAR with side assembly of air/water heat exchanger LHX 20
Cooling concepts - recommendations
The factor that is known in relation to the cooling of an installation is the expected performance loss and it is here, where the first decision has to be made for the cooling concept. If the expected performance loss is below 1 kW and the ambient temperature is below the expected cabinet temperature, in most cases conventional air cooling can be used. IEC 62 194 provides recommendations and guide lines for the thermal management. Together with measures for targeted air flow the generated performance loss is expelled from the cabinet.
If the expected performance loss lies above 1 kW, it may be necessary to deploy other methods for cooling. Water for instance has a heat transfer capacity which is higher than air by a factor of 4000 (related to its volume) and is therefore far more suited for the cooling of critical components. The higher density of the water allows to expel far more heat from the cabinet. Air/water heat exchangers work with an air and a water circuit and thereby use these properties of the water. Furthermore through the closed circuits of the air/water heat exchanger a cooling solution is created which is independent from the room. This means, that the room temperature can be above the expected internal cabinet temperature.
There are multiple air/water heat exchangers on the market; base or side assembly, horizontal or vertical air flow, different fixings, bolt-on versions to the top or bottom, etc. The selection process can be difficult, as the various methods can rarely be compared and each solution, depending on the existing performance loss, demands a particular cabinet dimension. It is here that users are looking for aids with which they can determine the required cooling concept precisely. Until now people worked with rough estimated values. Therefore it could easily happen, that expensive space was "given away" or if the calculation was too "tight", the generated performance loss could not be expelled optimally from the cabinet. Another factor, which should not be underestimated, is the generation of noise by the cooling solutions. In office areas the limits are lower as for instance in acoustically insulated rooms, where no personnel is working. This new standard, which was developed together with manufacturers and users, solves this problem and offers guidance, diagrams and calculation examples for a professional planning process.
The new standard
It is expected that the new IEC/TS 62454 Ed.1.0 will be issued at the beginning of 2007. It relates to cabinets for which the dimensions were defined in the well-known standards IEC 60297 (19") and IEC 60917 (metric). The standard divides the water cooling in cabinets into three levels.
Level I - Cabinet platform: Cabinet with air/water heat exchanger (base or side assembly). A global view point, whereby the entire control cabinet/electronic enclosure is cooled with an air/water heat exchanger.
Level II - Shelf (subrack) platform: Cabinet with a smaller air/heat exchanger, located at the side, to cool an individual or a group of subracks
Level III - Board platform: The most profound platform, where the water cooling has a direct impact on the boards or chips.
Level I
This level describes the framework conditions and supplies calculation examples for applications with air/water heat exchangers, which are either assembled at the base or the side of the cabinet. Through the different configuration of the air/water heat exchangers two basic air flow possibilities evolve: Vertical or horizontal air flow. With vertical air flow the air cirluates over the entire cabinet height. The heat exchanger is located at the bottom of the cabinet and directs the cold air from the front through the assembled components to the rear, then the air is returned to the base via fans of individual subracks or fans in the heat exchanger. With horizontal air flow the air circulates the cold air over the entire cabinet height from the front to the rear through the integrated components. The heat exchanger is located on the side.
Independently of which solution is selected (base or side assembly), the user can now establish the required footprint for his application with the aid of a diagram or through the calculation with formulas. The dimensions of the assembled components are defined with a depth of 400 mm and the cabinet widths is defined at 600 mm. For the noise generation the air speed vmax plays a substantial role. A diagram establishes the values at an air speed of vmax of 3 m/s, which corresponds to a noise level of ≤ 45 dB(A) in accordance with DIN EN ISO 11690-1. The second diagram is valid for an air speed vmax of 5 m/s, which corresponds to a noise level of ≤ 55 dB(A) in accordance with DIN EN ISO 11690-1. The starting values for the establishment of the required cabinet depth are the ΔT between the internal cabinet and ambient temperature in Kelvin as well as expected performance loss Q in kW.
Example 1 (Picture 2):
Air/water heat exchanger assembled at the base (vertical air flow), air speed vmax = 3 m/s. At a ΔT = 15 and an expected performance loss of Q = 10 kW it results in a required cabinet depth of 1100 mm. With the same values and an air speed vmax = 5 m/s a required cabinet depth of 900 mm is necessary.
The factor that is known in relation to the cooling of an installation is the expected performance loss and it is here, where the first decision has to be made for the cooling concept. If the expected performance loss is below 1 kW and the ambient temperature is below the expected cabinet temperature, in most cases conventional air cooling can be used. IEC 62 194 provides recommendations and guide lines for the thermal management. Together with measures for targeted air flow the generated performance loss is expelled from the cabinet.
If the expected performance loss lies above 1 kW, it may be necessary to deploy other methods for cooling. Water for instance has a heat transfer capacity which is higher than air by a factor of 4000 (related to its volume) and is therefore far more suited for the cooling of critical components. The higher density of the water allows to expel far more heat from the cabinet. Air/water heat exchangers work with an air and a water circuit and thereby use these properties of the water. Furthermore through the closed circuits of the air/water heat exchanger a cooling solution is created which is independent from the room. This means, that the room temperature can be above the expected internal cabinet temperature.
There are multiple air/water heat exchangers on the market; base or side assembly, horizontal or vertical air flow, different fixings, bolt-on versions to the top or bottom, etc. The selection process can be difficult, as the various methods can rarely be compared and each solution, depending on the existing performance loss, demands a particular cabinet dimension. It is here that users are looking for aids with which they can determine the required cooling concept precisely. Until now people worked with rough estimated values. Therefore it could easily happen, that expensive space was "given away" or if the calculation was too "tight", the generated performance loss could not be expelled optimally from the cabinet. Another factor, which should not be underestimated, is the generation of noise by the cooling solutions. In office areas the limits are lower as for instance in acoustically insulated rooms, where no personnel is working. This new standard, which was developed together with manufacturers and users, solves this problem and offers guidance, diagrams and calculation examples for a professional planning process.
The new standard
It is expected that the new IEC/TS 62454 Ed.1.0 will be issued at the beginning of 2007. It relates to cabinets for which the dimensions were defined in the well-known standards IEC 60297 (19") and IEC 60917 (metric). The standard divides the water cooling in cabinets into three levels.
Level I - Cabinet platform: Cabinet with air/water heat exchanger (base or side assembly). A global view point, whereby the entire control cabinet/electronic enclosure is cooled with an air/water heat exchanger.
Level II - Shelf (subrack) platform: Cabinet with a smaller air/heat exchanger, located at the side, to cool an individual or a group of subracks
Level III - Board platform: The most profound platform, where the water cooling has a direct impact on the boards or chips.
Level I
This level describes the framework conditions and supplies calculation examples for applications with air/water heat exchangers, which are either assembled at the base or the side of the cabinet. Through the different configuration of the air/water heat exchangers two basic air flow possibilities evolve: Vertical or horizontal air flow. With vertical air flow the air cirluates over the entire cabinet height. The heat exchanger is located at the bottom of the cabinet and directs the cold air from the front through the assembled components to the rear, then the air is returned to the base via fans of individual subracks or fans in the heat exchanger. With horizontal air flow the air circulates the cold air over the entire cabinet height from the front to the rear through the integrated components. The heat exchanger is located on the side.
Independently of which solution is selected (base or side assembly), the user can now establish the required footprint for his application with the aid of a diagram or through the calculation with formulas. The dimensions of the assembled components are defined with a depth of 400 mm and the cabinet widths is defined at 600 mm. For the noise generation the air speed vmax plays a substantial role. A diagram establishes the values at an air speed of vmax of 3 m/s, which corresponds to a noise level of ≤ 45 dB(A) in accordance with DIN EN ISO 11690-1. The second diagram is valid for an air speed vmax of 5 m/s, which corresponds to a noise level of ≤ 55 dB(A) in accordance with DIN EN ISO 11690-1. The starting values for the establishment of the required cabinet depth are the ΔT between the internal cabinet and ambient temperature in Kelvin as well as expected performance loss Q in kW.
Example 1 (Picture 2):
Air/water heat exchanger assembled at the base (vertical air flow), air speed vmax = 3 m/s. At a ΔT = 15 and an expected performance loss of Q = 10 kW it results in a required cabinet depth of 1100 mm. With the same values and an air speed vmax = 5 m/s a required cabinet depth of 900 mm is necessary.
Picture 2: Diagram to establish the cabinet depth with a air/water heat exchangers installed at the base (Example 1)
The same result can be obtained with the following calculation:
D = DR + DF + DE
(D = cabinet depth; DR = distance between the installed components and the rear panel; DF = distance between the front panels of components and the front door; DE = depths of the installed components)
DR = 1.25 x DF
D = 1.25 x DF + DF + DE
D = 2.25 x DF + DE
(Q = performance loss; ρair = air density; v = air speed; W = 600 mm cabinet width; Cpair = heat capacity of the air; ΔT = temperature difference between internal cabinet and ambient temperature)
If the same values as those used for the diagram example are applied, a cabinet depth of D + 1089.6 mm is obtained. Rounded up to the standard dimension of IEC 60297 this also results in a cabinet depth of 1100 mm. The advantage of calculating the cabinet depth with above listed formulas is the fact that for instance the depth of the installed components can be varied, and if the user so wishes, non-standard cabinet dimensions can be realised.
Example 2 (Picture 3):
Air/water heat exchanger assembled at the side (horizontal air flow), air speed vmax = 3 m/s and a ΔT = 15 and an expected performance loss Q = 10 kW. If these values are entered into the corresponding diagram, a necessary cabinet depth of 700 mm is obtained. With an air speed vmax = 5 m/s the necessary cabinet depth is even less, 600 mm.
D = DR + DF + DE
(D = cabinet depth; DR = distance between the installed components and the rear panel; DF = distance between the front panels of components and the front door; DE = depths of the installed components)
| DF = | Q |
| ρair x v x W x Cp;air x ΔT |
DR = 1.25 x DF
D = 1.25 x DF + DF + DE
D = 2.25 x DF + DE
(Q = performance loss; ρair = air density; v = air speed; W = 600 mm cabinet width; Cpair = heat capacity of the air; ΔT = temperature difference between internal cabinet and ambient temperature)
If the same values as those used for the diagram example are applied, a cabinet depth of D + 1089.6 mm is obtained. Rounded up to the standard dimension of IEC 60297 this also results in a cabinet depth of 1100 mm. The advantage of calculating the cabinet depth with above listed formulas is the fact that for instance the depth of the installed components can be varied, and if the user so wishes, non-standard cabinet dimensions can be realised.
Example 2 (Picture 3):
Air/water heat exchanger assembled at the side (horizontal air flow), air speed vmax = 3 m/s and a ΔT = 15 and an expected performance loss Q = 10 kW. If these values are entered into the corresponding diagram, a necessary cabinet depth of 700 mm is obtained. With an air speed vmax = 5 m/s the necessary cabinet depth is even less, 600 mm.
Picture 3: Diagram to establish the cabinet depth with a air/heat exchanger assembled at the side (Example 2)
Here, too, the cabinet depth can be calculted with formulas:
D = DR + DF + DE
(D = cabinet depth; DR = distance between installed components and rear panel; DF = distance between component front panels and front door; DE = depth of the installed components)
DR = 1.25 x DF D = 1.25 x DF + DF + DE D = 2.25 x DF + DE
(Q = performance loss; ρair = air density; v = air speed; Heff = 1600 mm cabinet height; Cpair = heat capacity of the air; ΔT = temperature difference between internal cabinet and ambient temperature)
In all diagrams and formulas a 25 % tolerance is included. This takes into account such things as the air flow being obstructed at the front and rear of the installed components through for instance cabling.
Level II
This level describes the framework conditions and supplies the calculation examples for applications with small air/water heat exchangers assembled at the side of the cabinet and only cool parts of the cabinet, such as individual subracks or groups of subracks (Picture 4). Solutions conforming to this standard are not currently available on the market, they offer, however, a substantial cost saving potential. Especially existing installations, which are upgraded retrospectively with additional electronics and where the cooling requirement subsequently increases, could benefit from this solution. The air/water heat exchangers planned for this solution by manufacturers and users and now defined by the standard, work with vertical air flow. This means, that above and below the subrack or subracks an area for air circulation and redirection has to be allowed. How many U the user has to calculate for this, can be worked out with diagrams (1 kW to 5 kW performance loss per subrack) or with formulas. The depth of the assembled subrack is defined with 400 mm and a possible deterioration of the air flow through geometric influences is taken into account with a 10 % tolerance.
D = DR + DF + DE
(D = cabinet depth; DR = distance between installed components and rear panel; DF = distance between component front panels and front door; DE = depth of the installed components)
| DF = | Q |
| ρair x v x Heff x Cpair x ΔT |
DR = 1.25 x DF D = 1.25 x DF + DF + DE D = 2.25 x DF + DE
(Q = performance loss; ρair = air density; v = air speed; Heff = 1600 mm cabinet height; Cpair = heat capacity of the air; ΔT = temperature difference between internal cabinet and ambient temperature)
In all diagrams and formulas a 25 % tolerance is included. This takes into account such things as the air flow being obstructed at the front and rear of the installed components through for instance cabling.
Level II
This level describes the framework conditions and supplies the calculation examples for applications with small air/water heat exchangers assembled at the side of the cabinet and only cool parts of the cabinet, such as individual subracks or groups of subracks (Picture 4). Solutions conforming to this standard are not currently available on the market, they offer, however, a substantial cost saving potential. Especially existing installations, which are upgraded retrospectively with additional electronics and where the cooling requirement subsequently increases, could benefit from this solution. The air/water heat exchangers planned for this solution by manufacturers and users and now defined by the standard, work with vertical air flow. This means, that above and below the subrack or subracks an area for air circulation and redirection has to be allowed. How many U the user has to calculate for this, can be worked out with diagrams (1 kW to 5 kW performance loss per subrack) or with formulas. The depth of the assembled subrack is defined with 400 mm and a possible deterioration of the air flow through geometric influences is taken into account with a 10 % tolerance.
Picture 4: Principle assembly of an air/water heat exchanger for the partial cooling of a subrack inside a cabinet
Example 3 (Picture 5):
Smaller air/water heat exchanger assembled at the side of the cabinet (vertical air flow), air speed vmax = 3 m/s, W = 600 mm, W1 = 800 mm, W2 = 200 mm, DE = 400 mm and D ≥ 600 mm. With ΔT = 2.5 K and a performance loss of the subrack of Q = 2.5 kW this results in 3.82 U (4 U are standardised) additional U for air circulation and redirection above and below the subrack. If the subrack has 14 U, the user has to plan for 18 U.
Smaller air/water heat exchanger assembled at the side of the cabinet (vertical air flow), air speed vmax = 3 m/s, W = 600 mm, W1 = 800 mm, W2 = 200 mm, DE = 400 mm and D ≥ 600 mm. With ΔT = 2.5 K and a performance loss of the subrack of Q = 2.5 kW this results in 3.82 U (4 U are standardised) additional U for air circulation and redirection above and below the subrack. If the subrack has 14 U, the user has to plan for 18 U.
Picture 5: Diagram to establish the required additional height units required for the cooling of a subrack
Here, too, a calculation with a formula is possible:
HB = 1.72 U
HB + HT = 3.44 U
(HB = height of the air channel below the subrack in [U]; HT = height of the air channel above the subrack in [U]; Q = performance loss; ρair = air density; v = air speed; 1 U = 44.45 mm; DE = 400 mm subrack depth; Cpair = heat capacity of the air; ΔT = temperature difference between the cabinet interior and the ambient temperature)
Level III
This level deals with water cooling of individual components/chips directly on the boards. The possible cooling solutions are so manifold due to the various dimensions and positions of the components that need to be cooled, that the members of the standards consortium decided not to restrict this by tight definitions. Therefore Level III only defines that water connections have to be present in the cabinet, leading to fast release connectors at the front or rear of the subracks. Here, only the principle of water cooling of individual components is outlined. The detailed configuration is left to the manufacturers/users (Picture 6).
| DF = | Q |
| ρair x v x 1U x DE x Cpair x ΔT |
HB = 1.72 U
HB + HT = 3.44 U
(HB = height of the air channel below the subrack in [U]; HT = height of the air channel above the subrack in [U]; Q = performance loss; ρair = air density; v = air speed; 1 U = 44.45 mm; DE = 400 mm subrack depth; Cpair = heat capacity of the air; ΔT = temperature difference between the cabinet interior and the ambient temperature)
Level III
This level deals with water cooling of individual components/chips directly on the boards. The possible cooling solutions are so manifold due to the various dimensions and positions of the components that need to be cooled, that the members of the standards consortium decided not to restrict this by tight definitions. Therefore Level III only defines that water connections have to be present in the cabinet, leading to fast release connectors at the front or rear of the subracks. Here, only the principle of water cooling of individual components is outlined. The detailed configuration is left to the manufacturers/users (Picture 6).
Picture 6: MicroTCA system with water cooled components on individual boards
Water inlet defined
In the presently last chapter of the IEC/TS 62454 it has been defined, where in a cabinet the water connections for the air/water heat exchanger and the connections for the direct cooling of the components on boards must not be assembled. Statements, of where these connections can be located, would again have restricted the users very much.
With this first standard for water cooling in cabinets, the user obtains a practice orientated tool, which provides rules and guidelines, concrete planning and calculation models, unrestricting where freedom is required.
In the presently last chapter of the IEC/TS 62454 it has been defined, where in a cabinet the water connections for the air/water heat exchanger and the connections for the direct cooling of the components on boards must not be assembled. Statements, of where these connections can be located, would again have restricted the users very much.
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1. The water intake should always be at the base
2. Areas are used for cabling (power and signals) must not be used
3. Areas where excess cable is stored must also be avoided
4. Corner areas in the cabinet uprights must not be used
With this first standard for water cooling in cabinets, the user obtains a practice orientated tool, which provides rules and guidelines, concrete planning and calculation models, unrestricting where freedom is required.
