1. Hot Aisle vs. Cold Aisle Containment
White Paper #135
This presentation, entitled Hot Aisle vs. Cold Aisle Containment is based on recently published APC White Paper #135 but it is extended with a case study, Hot Aisle vs Cold Aisle containment showing energy savings that can be achieved when installing and operating Hot Aisle Containment.

2. Learning objectives
Most of our competitors are promoting Cold aisle containment. The objective of the course is clearly to demonstrate the advantages of APC Infrastructure solution using Hot Aisle Containment System (HACS).
This presentation is based on recently released White Paper #135
The learning objectives are primarily to explain both concepts, hot aisle and cold aisle containments and clearly demonstrate advantages of the Hot Aisle Containment System (HACS).

3. Agenda Characteristics of traditional cooling methods
Hot Aisle Containment and Cold Aisle containment concepts
Common advantages of both Hot Aisle Containment and Cold Aisle containment concepts
Cold Aisle Containment system limitations
Hot Aisle Containment system advantages
Comparison of Hot Aisle and Cold Aisle Containment systems
Case Study – Hot Aisle vs Cold Aisle Containment System
Agenda of the presentation

4. Introduction Traditional Data Center Cooling Methods
Traditional cooling floods the entire space and mixes hot and cold air
Characteristics of Traditional Cooling
Perimeter placed cooling units
Raised Floor
Hot air mixes with cold air
Not consistent hot / cold aisle arrangement
Oversized power and cooling components
Low heat load densities
Energy cost used to be low concern
Traditional cooling approaches share the following characteristics (see Figure 1):
Perimeter cooling (CRAC units are placed on the outer perimeter of the rows of racks)
Raised floor (cold air is delivered to the rows of racks via a plenum under the raised floor)
The hot exhaust air from IT equipment mixes with cold air as the hot air makes its way back to the CRAC unit return vents.
Rows of racks are not set up in a consistent hot aisle / cold aisle arrangement.
Oversized power and cooling components reduce the data center efficiency.
The traditional environment developed and became accepted business practice because, for many years, rack densities were low (below 2 kW per rack), energy costs were negligible (in fact the fuel bill was invisible to most IT organizations), and a culture of “oversizing” hardware became a commonplace practice for reducing the risk of capacity shortage or downtime.
Now most data center professionals recognize that these past practices are inefficient, costly to the organization, and wasteful from a carbon footprint point of view. Two recent technology breakthroughs have helped to remedy this situation: row-based cooling and separation of hot and cold air streams. Row-based cooling brings the cooling source in very close proximity to the load (by being imbedded into the server rows) so that energy is not wasted forcing cold air across long distances, under a congested raised floor, to the load.
Present Trends
Past practices were inefficient, costly and carbon emissions low concerned
Row cooling and separation of hot and cold air streams are new breakthrough technologies to help to remedy the situation

5. Hot Aisle vs. Cold Aisle Containment System
Both Methodologies prevent Hot and Cold Air streams from mixing
This improves predictability and efficiency of the cooling solution compare to traditional cooling
Hot Aisle contains Hot Air
Room operates as Cold air plenum
Cold Aisle contains Cold Air
Room operates as Hot air plenum
In-Row Cooling Unit
Hot Aisle Containment System (HACS)
The Hot Aisle Containment System (HACS) encloses a hot aisle to collect IT equipments hot exhaust air and cools it to make it available for IT equipment air intakes. This creates a self-contained system capable of supporting high density IT loads.
Cold Aisle Containment System (CACS)
The Cold Aisle Containment System (CACS) is typically deployed in traditional perimeter-based cooling environments. Traditional cooling environments use the entire room as a hot air return plenum and use deliver cold air via the raised floor plenum to the cold aisles. The CACS encloses the cold aisle allowing the rest of the data center to become a large hot air return plenum. By containing the cold aisle, the hot / cold air streams within the data center are separated.
Note:
Although this is titled HACS vs. CACS the efficiency benefits are dominated by the superior InRow over the standard raised floor perimeter cooling.

6. Hot Aisle vs. Cold Aisle Containment System
Benefits of both
Hot Aisle Containment System (HACS) and Cold Aisle Containment System (CACS)
Prevent Hot and Cold air streams from mixing
Cooling systems can be set to a higher temperature
Reduction of humidification and dehumidification costs
Better overall physical infrastructure utilization that enables rightsizing
Containment, both hot aisle and cold aisle, offers the following benefits:
Cooling systems can be set to a higher temperature (thereby saving energy) and still supply the load with safe operating temperatures. The temperature of traditional cooling systems are set much lower than required by IT equipment (i.e. approx 55°F / 13°C). This is done to compensate for heat picked up by the cold air as it makes its way from the CRAC unit to the front of the racks.
Reduction of humidification / dehumidification costs - Typically when hot IT equipment exhaust air is captured and sent directly back to the CRAH unit, no humidity is removed from the air. If no humidity is removed, than adding humidity is not required which saves energy and water.
Better overall physical infrastructure utilization which enables rightsizing, which, in turn, results in equipment running at higher efficiencies - Larger oversized equipment experiences larger fixed losses than right-sized equipment. However, over-sizing is necessary for traditional cooling because the extra fan power is required to both overcome under the floor obstructions and to pressurize the raised floor plenum.
Those benefits lead to energy savings and better equipment utilization compare to traditional cooling methods

7. Cold Aisle Containment System (CACS)
Cold Aisle Containment Design
Eliminates hot and cold air mixing
Typically used with traditional perimeter cooling – cold air supplied via raised floor and perforated tiles. Alternatively CACS is designed with row based cooling system
The rest of the room becomes large, return hot air plenum
Cold Aisle Containment Limitations
Perimeter Cooling – Longer air paths and higher air pressure resistance - higher power consumption by fans
Lower return air temperature than available – lower cooling capacity and efficiency
Limited heat load density per rack - limited by the raised floor and perforated tiles
Limited predictability – variable room and raised floor dimensions
Limited ride through during cooling failure - confined to Cold Aisle air volume
Room acts as the Hot Aisle – contradicts with perception that data centers should be cold. Peoples expectation when entering the room need to be adjusted
Room acts as the Hot Aisle – difficulty in cooling free standing racks and equipment
Need for lower supply temperatures – higher power consumption and limiting free cooling period
Not modular & scalable due to perimeter Cooling
CACS efficiency limitations when deployed in a room-based approach
Although a CACS does offer efficiency improvements over traditional cooling, it does exhibit some drawbacks when deployed in a room-based, perimeter cooling environment:
Inefficiencies resulting from distances and pressures required for adequate air distribution - The single largest contributor to inefficiency in a room-based approach is the requirement to move cold air from a perimeter CRAC unit to a distant load. A row-based cooling approach brings the source of the cooling in close proximity to the load. As a result, much less energy is required to deliver the cold air to its destination. This is not the case if the data center owner chooses to deploy CACS with row-based cooling. See APC White Paper # 130, “The Advantages of Row and Rack-Oriented Cooling Architectures for Data Centers” for a detailed comparison of row and room based cooling approaches.
Density limitations of using cold air distribution through raised floor - The practical density limit when using a CACS approach is approximately 6 kW per rack. See APC White Paper # 46, “Cooling Strategies for Ultra High Density Racks and Blade Servers” for details regarding the reasons for this limitation. Higher densities can be achieved only if an investment is made in a customized design. To address some of the limitations of the raised floor plenum, some CACS solutions are offered with fan powered floor tiles. This improves airflow for higher density racks. The use of additional fan assisted devices further reduces the efficiency of the CACS. The extra fans contribute to the overall power consumption and add heat to the supplied cold air. The efficiency gains achieved by the CACS are therefore diminished by the added requirement for floor-based fans. This density limitation can be avoided if the data center owner chooses to deploy CACS with row-based cooling.
Predictability of the raised floor - Cold aisle containment helps improve predictability through the elimination of hot and cold air mixing. However, it does not eliminate the variable of the raised floor. Cabling, piping, and other obstructions are added below the raised floor as the data center evolves. These obstructions limit the delivery of sufficient cool air to the IT equipment. Figure 4 (reference to WP#135, page 6) provides an example of how air dams in the plenum under the raised floor can hinder the predictable delivery of cold air to the cold aisle. This is not the case if the data center owner chooses to deploy CACS with row-based cooling as the need for the raised floor might not exist.
CACS limitations when deployed in a row-based cooling approach
A row-base deployment of CACS is more advantageous, from an efficiency perspective than a traditional room-based approach. However some limitations still exist:
Availability of cold air during a loss of power / cooling - Containing the cold aisle minimizes the overall pool of cold air available to the servers, should a loss of power and / or cooling occur. The reduced volume of cold air results in more rapid temperature increases in the event of a failure. Figure 5 (reference to WP#135, page 7) depicts a sample data center and compares the volume of air in a contained cold aisle to the volume of air in an uncontained cold aisle. The uncontained cold aisle shows a volume of cold air that is 17 times greater than that of the cold air volume found in the contained cold aisle. This reduced air volume shortens the amount of time (seconds instead of minutes) it would take for the servers to overheat if a failure were to occur.
All of the cold aisles in the entire data center must be contained in order to realize benefits -Containing only some of the cold aisles in the data center will yield little benefit because any other cold air that is allowed to mix with hot air will diminish any expected savings. Mixing will cause the cooling system to operate in a less efficient manner (a smaller difference between return “hot” air and cooling coil temperatures). To minimize mixing and to maximize cooling system efficiency, all cold aisles must be contained. Only then will hot return air temperatures reach maximum potential thereby allowing the cooling equipment to operate at much higher efficiency levels.
Overall perception and operation of a hot data center – ASHRAE Standard TC9.9 recommends that server inlet temperatures range from 68-77° F (20-25° C). When cold aisles are contained, the air in the rest of the room becomes hotter (well above 80° F / 27° C and in some cases as high as 100° F / 38° C), and anyone entering the data center is exposed to unusually high temperatures. People are generally alarmed when entering such hot conditions, and tours become impractical. People’s expectations need to be adjusted so that they understand that the higher temperatures are “normal” and not a sign of impending system breakdown. This cultural change can be challenging for workers not accustomed to entering a data center operating at higher temperatures.
When operating a data center at elevated temperatures, special provisions need to be made for non-racked IT equipment. This is equipment that cannot be integrated into a CACS. Since, with a CACS system, the room is a reservoir for hot air, miscellaneous devices (such as tape libraries and standalone servers) will need to have unique ducting in order to enable them to pull cold air from the contained cold aisles. In addition, electric outlets, lighting, fire suppression, and other systems within the room will need to be evaluated for suitability of operations at elevated temperatures.

8. Hot Aisle Containment System (HACS)
Hot Aisle Containment Design
Hot Aisle Containment Concept is patented by APC (US ; Europe pending)
Eliminates hot and cold air mixing
Typically used with Row cooling – hot air is captured and neutralized via cooling units placed within the row of racks and supplied to cold aisle
The rest of the room becomes large, cold air plenum, no raised floor or ducting is used
Hot Aisle Containment Advantages
In-Row Cooling is Closed Couple Cooling – short air paths and low air pressure resistance - lower power consumption by fans (See White Paper #130)
Higher return air temperature – higher cooling capacity and efficiency of the cooling system
High heat load density per rack – all heat is neutralized and there is no limitation with the raised floor and perforated tiles
Predictable solution – independent of room and raised floor dimensions
Higher ride through during cooling failure – significantly larger cold aisle air volume
Room acts as the Cold Aisle – in agreement with the perception that data centers should be cold
Continues....
The Hot Aisle Containment System (HACS) encloses a hot aisle to collect IT equipments hot exhaust air and cools it to make it available for IT equipment air intakes. This creates a self-contained system capable of supporting high density IT loads.
Mixing of hot and cold air streams in the data center lowers availability of IT equipment. Returning the warmest possible air to the computer room air conditioners increases the efficiency and capacity of the system. The HACS ensures proper air distribution by completely separating supply and return air paths.
The design of HACS assimilates many of the advantages of the CACS and avoids many of the pitfalls. When upgrading a data center to be more efficient and less costly to operate, any move away from the traditional perimeter cooling approach is a step in the right direction. While CACS is a “better” scenario compared to traditional approach, the “best” scenario is embodied in a HACS system.
A HACS system consists of doors at either end of the row, a roof for containing hot air, in-row cooling with variable speed fans, and a temperature controlled air supply to the cold aisle (see Figure 6). (reference to WP#135, page 9)
Hot aisle containment system (HACS) efficiency benefits
Efficiency – The efficiency of the HACS will be higher because the hot aisle is capable of maintaining higher temperatures. In a typical high density server environment the temperature difference between the server exhaust air and the room temperature is typically around 30° F / 17° C). If the room is maintained within ASHRAE TC9.9 standards at 72° F / 22° C, a 30° F / 17° C temperature difference, would yield a server exhaust air temperature of 102° F / 39°F.
In a typical HACS environment the cooling units tend to move slightly more air than the servers and draw in a small amount of room air into the hot aisle. The effect of this can cause a slight reduction (about 2° F) in the return temperature, yielding a return temperature to cooling units of 100° F / 38°C. The net effect of this elevated return temperature (i.e., 100° F / 38° C) to the cooling unit enables better heat exchange across the cooling coil, better utilization of the cooling equipment, and overall higher efficiency. Figure 7 (see WP#135, page 10) gives an example of the effect of elevated return temperatures on sensible cooling capacities (the ability of an air conditioning system to remove heat from the air).
The effect of increasing return temperatures on cooling unit capacity holds true for virtually all air conditioning equipment. Some equipment may have limits as to the maximum return temperature they can handle, but, in general, all cooling systems will yield higher capacities with warmer return air.
In the case of the HACS, hot aisles operating at 100° F / 38° C with high density servers is typical. Contrast this with a cold aisle contained room where the entire room space would have to be maintained at 100° F / 38° C in order to achieve the same level of efficiency. While CACS would enable higher return temperatures, the typical data center operator will not operate the entire data center room at 100 F / 38° C in order to achieve the same efficiency as a HACS.

9. Hot Aisle Containment System (HACS)
Hot Aisle Containment Advantages
Room acts as the Cold Aisle – flexible solution that can be deployed with existing architectures
No need for too low supply temperatures – lower power consumption and higher potential for using free cooling and extended period of free cooling
Modular & scalable solution due to Row cooling
Effect of increased return air temperature on sensible cooling capacity
Improved Flexibility – Unlike CACS, a HACS does not impact the temperature of the surrounding room. A HACS is, in effect, a room neutral solution. For example, if the temperature of the data center is set for 75° F (24° C), and a CACS system is implemented, the room temperatures outside of the cold aisle will rise because hot air will mix in with the air outside of the cold aisle on its way to the intake of the cooling system. The hot air inside of the HACS is contained from the rest of the room. The HACS does not deliver any hot air to the outside room; therefore the existing cooling system is not rendered less efficient.
A HACS can be “dropped in” to the data center without requiring any changes to the existing data center cooling architecture. When utilizing a row-based cooling approach (as opposed to a room-based approach), no need exists for the installation of specialized duct work and no adjustments need to be made to the existing HVAC systems to handle elevated return temperatures.
Higher Availability – The “Cold Air Volume Sample Calculation” in Figure 5 demonstrates the differences in cold air volume when comparing CACS volume to room volume (uncontained cold air volume is 17 times greater than cold air in a contained cold aisle). This difference has a significant impact on the ability of the systems to support a cooling failure (i.e., runtime). A runtime that could be minutes in an uncontained room scenario might only be seconds if a CACS approach is deployed. With HACS only the hot air is contained, leaving the rest of the data center environment cool. Therefore, the servers will draw air from a larger pool of cool air outside the contained hot aisle, thereby extending available runtime.

10. Hot Aisle vs. Cold Aisle Containment System
Fire Suppression Considerations
Depending upon the location fire detection and/or fire suppression may be required inside the enclosed area of HACS or CACS
The National Fire Protection Association standard NFPA75 does not state an opinion whether suppression systems (sprinklers or gaseous agents) should be provided in HACS or CACS
NFPA 75 documents two requirements that could be applied to HACS and CACS (see White Paper #135, page 12 for details)
HACS have been successfully installed and approved with sprinklers and gaseous agent suppression in many sites
Depending upon the location of the data center, fire detection and / or fire suppression may be required inside the enclosed area of the HACS or CACS. The primary suppression mechanism is usually sprinklers, which are heat activated. Gaseous agents are usually a secondary system which can be initiated by smoke detectors. The National Fire Protection Association standard NFPA 75 does not state an opinion as to whether sprinklers or gaseous agents should be provided in a HACS or CACS. However, NFPA 75 documents the following two requirements that could be applied to HACS / CACS:
Automated information storage system (AISS) units containing combustible media with an aggregate storage capacity of more than 0.76m^3 shall be protected within each unit by an automatic sprinkler system or a gaseous agent extinguishing system with extended discharge. (Note: This information is significant because it sets a precedent for fire detection and suppression in an enclosed space within a data center).
Automatic sprinkler systems protecting ITE rooms or ITE areas shall be maintained in accordance with NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems
In practice, HACS have been successfully installed and approved with sprinklers and gaseous agent suppression in many sites. The AHJ should be contacted for specific requirements in a given location

11. Hot Aisle vs. Cold Aisle Containment System

12. Hot Aisle vs. Cold Aisle Containment System

13. Hot Aisle vs Cold Aisle Containment System - Summary
Hot Aisle and Cold Aisle Containment eliminate air mixing and it is superior solution compare to traditional cooling architecture
Hot Aisle Containment System (HACS) is more efficient approach than Cold Aisle Containment System (CACS) because the HACS methodology allows for the channeling the hottest air directly into coolers
HACS used with In-row cooling architecture provides closed couple cooling that allows higher cooling capacity utilization and efficiency
HACS is more flexible, it can be deployed anywhere within the room, it is more predictable and scalable solution, it better addresses the higher IT density requirements
HACS has higher potential for better heat utilization and using free cooling approaches
Hot Aisle Containment has better ride through capability
Conclusion
Prevention of hot and cold air mixing is a key to all efficient data center cooling strategies. Hot aisle containment (HACS) is a more efficient approach than cold aisle containment because the HACS methodology allows for the channeling of the hottest air directly into the coolers. Cooling set points can also be set higher while still maintaining a comfortable work environment.
Since it does not affect its outside environment, a HACS solution can also be deployed anywhere within the room. With a HACS solution, equipment inlets are uncontained and therefore can draw cooling airflow from the room in event of a cooling failure. This allows more time for a switch to generator or for a graceful shutdown of servers.
Both HACS and cold aisle containment (CACS) offer superior power density and efficiency when compared with traditional cooling approaches. CACS can offer some improvement in a traditional room-based perimeter cooling layout. However, HACS with a row-based cooling architecture is more efficient, more flexible, provides better ride through capability, and can more easily address the higher IT density requirements without increasing the temperature of the entire data center. For most users the additional heating of the uncontained operator space is an unacceptable condition which eliminates CACS as an option. For the above reasons, most high efficiency, high density data center projects, both for new designs and retrofits, incorporate some form of hot aisle containment.

14. Case Study: Hot Aisle vs Cold Aisle Containment System
Room size 120 m2
Raised floor height 1.2 m; plenum = 144 m3
44 racks with 11 kW/rack => 484 kW (design load)
Server equipment: 50% standard server; 50% blade
Rack Air Inlet temperature = 24C
Air turnover 108,900 m3/h (server only)
Add 25% air turnover for raised floor
Electrical cost 0.12 €/kWh
Chilled water cooling system is same for both (chiller and piping)
All calculations for N+1
Actual customer situation in Sweden. Telco / Internet provider in Stockholm.
All figures used are based on true data and existing building.
4 rows of racks respective 2 HACS
Raised floor calculation is based on 8 x Uniflair 4300A
HACS configuration includes 18 InRow RC (600mm) units
Chiller and other cooling equipment is identical; maximized use of free cooling is one of the goals
Air turnover calculation is based on 160 cfm = 75.5l/s = 271m3/h per kW for standard servers
And 105 cfm = 49.6 l/s = 178 m3/h per kW for blade servers

15. Saving HACS vs. CACS = 4,100 € per annum
CACS vs HACS
8 x CRAC (Uniflair_4300A)
Possible air flow 192,384 m3/h
Respective power draw 32kW
Needed air flow = 136,125 m3/h (71%)
Needed power draw =11.3 kW (35%)
365 x 24 x 11.3 x 0.12 = 11, €
18 x InRow RC
Possible air flow 210,888 m3/h
Respective power draw 54kW
Needed air flow = 108,900 m3/h (52%)
Needed power draw =7.4 kW (14%)
365 x 24 x 7.4 x 0.12 = 7, €
Power saving by reducing fan speed using the fan rules.
Power consumption lowers by the power of 3 related to the change of airflow.
Refer to S4 Energy efficiency presentation on sales tools portal.
Saving HACS vs. CACS = 4,100 € per annum

16. PLUS other benefits not calculated
Reduced usage of free cooling when implementing CACS efficiency ↓
Extra cost for extra height of raised floor (effect might be minimal, though)
PLUS some operational reasons
Predictability of raised floor plenum is critical
No standalone equipment can be placed in CACS room
For partial load the power ratio gets better for HACS solution
Warm air volume: CACS: 444 m3 HACS: 113m3
Cold air volume CACS: 180 m3 HACS: 367m3
Within the case study the extra cost for the raised floor was not considered but equalized with the estimated extra cost for the piping for the HAC solution.
Partial load benefit results from better adjustment of fan speed since there is no static pressure to be applied.
Ride through capability is better since the amount of cold air is double with HAC solution

17. Free Cooling – Hours/year vs Temperature
This graph shows how many hours per year (vertical axes) there is given ambient air temperature (horizontal axes) in a location in Germany. The graph was generated by office providing weather data.
For example, there is about 400 hours (16.7 days) per year when ambient temperature is 12C.
The green areas shows cumulative number of hours per year when ambient air temperature is below 12C.
400 hours are 16,666 days that means in the range between -2 through +12 °C
you can gain around 17 days of free cooling for each one °C you can raise your chilled water temperature. In other words if we can raise chilled water temperature we can use more days of free cooling and per every one deg C we can use higher ambient air temperature there is about 17 days more of free cooling.
Note:
Free cooling - this refers to a chiller equipped with free cooling option. In general, a chiller is a machine design to provide chilled water by removing heat (cools water) employing a refrigeration circuit with the compressor. A chiller with free cooling option is a machine that is design in such a way that when ambient air temperature is low (typically during winter/autumn/spring months) it keeps compressor off and rejects heat (=chills water) for free just by circulating water trough the coil and fans blows cold ambient air across the coil and cools water down.

18. CACS – Assumptions for Operation
Maximum allowed
ambient air temperature for (100%) FREE Cooling 8°C
17°C
11°C
30°C
31°C
CACS – assumptions for operation
This picture depicts assumptions for operation of CACS. Start with the cold aisle, rack air inlet temperature = 24C.
To achieve rack air inlet temperature of 24C you would need to supply cold air from the CRAC unit to be 20C. We assume that there is a need to have at least 4deg C (delta T) lower temperature of the supply air from CRAC because of distance the air travels and it will get warmer by the time it is supplied in front of the racks.
To have 20C supply air, you could use maximum chilled water return temperature of 17C. We assume that return chilled water temperature must be at least 3 deg C (delta T) colder than supply air temperature (20C). To maintain temperature difference between chilled water supply and return 6deg C (delta T) we would need to supply chilled water temperature at 11C.
To achieve 100% free cooling then ambient air temperature must be at least 3 deg C (delta T) lower than chilled supply temperature, it means 8C.
So in summary,
Chilled water supply = 11C
Chilled water return = 17C
CRAC supply air temperature = 20C
Cold aisle temperature = 24C
Maximum ambient air temperature for free cooling = 8C ∞
24°C
32°C
29°C
32°C
11°C
20°C
17°C

19. HACS – Assumptions for Operation
Maximum allowed
ambient air temperature for (100%) FREE Cooling 11°C
20°C
14°C
24°C
24°C
HACS – assumptions for operation
This picture depicts assumptions for operation of HACS. Start with the cold aisle, rack air inlet temperature = 24C.
To achieve rack air inlet temperature of 24C you would need to supply cold air from the InRow cooling units placed in HACS to be 23C. We assume that there is a need to have at least 1deg C (delta T) lower temperature of the supply air from InRow. Such a low temperature difference, only 1 deg C compare to 4deg C for CACS, can be assumed because there is very short distance air will travel, no resistance (no raised floor or any ducting), no additional heat load (or minimum) that would warm supply air by the time it is supplied in front of the racks.
To have 23C supply air, you could use maximum chilled water return temperature of 20C. We assume that return chilled water temperature must be at least 3 deg C (delta T) colder than supply air temperature (23C). To maintain temperature difference between chilled water supply and return 6deg C (delta T) we would need to supply chilled water temperature at 14C.
To achieve 100% free cooling then ambient air temperature must be at least 3 deg C (delta T) lower than chilled supply temperature, it means 11C.
So in summary,
Chilled water supply = 14C
Chilled water return = 20C
CRAC supply air temperature = 23C
Cold aisle temperature = 24C
Maximum ambient air temperature for free cooling = 11C
24°C
32°C
14°C
23°C
23°C
20°C

20. More free cooling available with HACS
Facts are:
CW return temperature has to be 3°C colder than supply air temperature
CW supply temperature has to be 3°C warmer than outside air temperature to enable 100% free cooling
Cold air supply of downflow units supporting raised floor cooling have to be 4°C colder than the expected server inlet temperature
Cold air supply of InRow units have to be only 1°C colder than expected server inlet temperature
So, we calculate back from server supply temperature to be 24°C
Server Inlet [°C]
Cold Air Supply [°C]
CW Return [°C]
CW Supply [°C]
Free Cooling Temp [°C]
Days Possible 100%
InRow 24 23 20 14 11
210
CRAC 17 8
160
The assumptions for operation of CACS and HACS have been presented in the previous two slides.
For the operation of the cooling compressor the average power consumption is about 1/3 of the cooling power.
For this sample this is 1/3 * 484 kW. [ 484/3 kW * 0.12 €/kWh * 24 h = € ]
For the case study the additional cost savings are 50 days * € = 23,232-€
The assumptions on the temperature difference might be subject to arguments but the relative distance will stay the same for both solutions.
For every day of free cooling we save another € on the electrical bill!
Savings HACS vs CACS (50 days of free cooling more) = 23,232 €/a

21. Case Study HACS vs CACS - Summary Cost Saving
Annual saving indoor units: 4,100.- €/a
Annual saving additional free cooling 23,232.- €/a
Total benefit for HACS over CACS 27,332.- €/a
Considering a life time of 10 years
Savings add up to 273,320.- €
CAPEX benefit for CACS over HACS 78,160.- €
Break even after years
Annual saving indoor units – this comes from difference in power consumption for airflow for HACS vs CACS, the HACS with InRow units does consume worth of 10,058 E/a in electricity bill less than CRACS in CACS – see slide #15 for details.
Annual saving additional free cooling – this comes from difference that can be achieved by operating HACS that allows more days of using free cooling – see slide 20 for details
Considering the savings of the indoor unit only would level the cost per kW over a life time of 10 years with the total cost for the downflow solution.
The real saving comes with the free cooling.
Pure product cost (purchase price) is 128,000 vs. 176,850 € at the day of purchase.
Difference of CAPEX is 204,800 (CRAC) vs. 282,960 € (HACS) including financing cost of 6% and a life time of 10 years.