FAC 8.5 Passive RDHx as a
Cost Effective Alternative
to CRAH Air Cooling
Jeremiah Stikeleather
Applications Engineer
FAC 8.5 Passive RDHx as a Cost Effective Alternative
to CRAH Air Cooling
While CRAH cooling has been a common data center
cooling solution, OPEX for RDHx cooling can be better
at minimizing today’s energy consumption and operating
costs. This will have increasing significance as we look
to the future and interest in sustainability grows.
• Benefits of the Passive Rear Door Heat Exchanger
• The Study: Comparing 3 Cooling Designs
o Traditional CRAH Units
o RDHx’s with a Primary Piping Manifold
o RDHx’s with CDU’s and a Secondary Water Loop
• Summary of the 3 Design Alternatives
Benefits of the Water-Cooled Passive Rear Door Heat
Exchanger (RDHx)
Replaces rear door on the server enclosure
Air-to-Water Heat Exchanger
Close-coupled cooling solution
Removes the heat at the source
Rear of
Front of
Benefits of the Water-Cooled Passive Rear Door Heat
Exchanger (RDHx)
Significant energy reduction versus a typical CRAH solution
Heat exchange process occurs at rear of the rack
Water thermal capacity is 3400 times greater than air
Significant reduction in maintenance costs
The Study: 3 Cooling Designs
Designs Compared:
• Design 1 – Traditional 30-Ton CRAH Units
• Design 2 – RDHx’s with a primary piping manifold system
• Design 3 – RDHx’s with Coolant Distribution Units
and secondary water loop
Study includes:
All aspects of deploying solutions in a 1 MW Data Center
Supply and installation of cooling system
Electrical connections, valves, piping
Building monitoring integration system
Leak detection, smoke/fire detection
Condensate removal
The Data Center Configuration:
1 MW of IT power in a raised floor environment
5,000 sq. ft. white space
Planned deployment of 177 IT enclosures
Infrastructure for a space loading of 200 watts /ft2
28 ft2 per IT enclosure (assume 5.7 kW / rack)
The Benchmark Air Cooling System:
Chilled water Computer Room Air Handlers (CRAHs).
(12) 30-Ton operating units around perimeter
CRAH unit air discharge temperature 68°F to 70°F
Two additional CRAH units installed for redundancy
Cold air discharged under an 18 inch raised floor
Hot aisle-cold aisle arrangement
The Benchmark Air Cooling System:
CRAH running at a reduced load of 80% (4.6 kW)
Chilled water for the CRAHs (100% water, no glycol)
Branch connected from a main chilled water loop running external to
the white space
Chiller, water supply, and related energy costs not included in any of
the cooling designs
Design 1 – Traditional CRAH Units
Fourteen 30-Ton CRAH units
12 active, 2 standby
Assume 25% CRAH performance reduction
Large area with unpredictable airflow
Obstructions (columns, cable runs, etc.) alter airflow
Wasted air (openings in tiles that do not provide direct access for rack
White space consumption and the required
service clearance is factored in
Footprint required by CRAH system complicates future expansion of IT
Design 1 Summary – Traditional CRAH Units
Cost includes:
Supply and installation of CRAH units
Space fit-out
Fire protection/suppression systems required for access and CRAH footprint
De-rating published sensible cooling by 25% considered conservative due to the
built-in inefficiencies of CRAH based air cooling systems
Power consumption much higher due to fans, humidification, and reheat functions
Increased rack power density will force a change in cooling infrastructure –
more CRAHs, supplemental cooling, hot aisle/cold aisle containment
Design 1 – Traditional CRAH Units
Cost Summary
Design 2 – RDHx with manifold system
Dedicated chiller
177 RDHx
Chilled water distribution from prefabricated manifold system
2 CRAH units for humidification control and room cooling backup
CAPEX reduced by not using additional pumps or plate-and-frame heat
Design 2 – RDHx with manifold system
Piping manifold alternatives:
Manifold and pump tapping into bypass/mixing line
(Manifold return water discharges back into the bypass)
Manifold and three-way valve tapping into supply and return lines
(Mixing building return water with supply water to achieve higher supply water
temperatures for the RDHx)
Similar to methods used in the radiant piping industry
Design 2 – RDHx with manifold system
Controls for the proposed system rely on supply air temperature sensors
for the racks and their corresponding RDHx that is connected to the
manifold, and a modulating control valve or circuit setter at the manifold
Design 2 Summary – RDHx with manifold
CAPEX comparable to CRAH design (Design 1)
OPEX for RDHx is minimal
(About 3% of the total power consumed by CRAH units)
RDHx units are passive
Small RDHx whitespace footprint
Reduce overall building footprint
Greenfield project savings
Construction savings for future expansion
RDHx ROI within first year of operation
Over 3x IT expansion, 5.7 kW to 18 kW / rack
(RDHx nominal cooling capacity 18 kW)
Design 2 – RDHx with manifold system Cost
Design 3 – RDHx with CDU and secondary water loop
• 4 Coolant Distribution Units (CDU’s)
• CDU is floor-mounted device – heat exchanger, pumps,
controls, distribution manifold
• CDU connects to water from chiller (or cooling tower)
• 2 CRAH units (Humidity control and room cooling backup)
• No condensate (Temperature/humidity
sensor regulates secondary loop
temperature 2 degrees above dew point)
• The RDHx water loop is isolated from
primary water loop
• CDU power consumption 3.7 kW each
Design 3 Summary – RDHx with CDU
• CDU increases CAPEX
• Low OPEX cost
CDU pumps use 15% of power for CRAH units
• Break-even point is in Year 3
• Design 2 and 3 are “future proof”
5.7 kW racks can grow up to 18 kW
Design 3 – RDHx with CDU
Cost Summary
At 5 kW per rack, CAPEX for RDHx and CRAH cooling approximately equal
(CAPEX could be further reduced by implementing alternating RDHx’s – CAPEX savings up to 25% compared to populating each rack
with an RDHx)
OPEX is significantly reduced with RDHx designs
Reduced energy consumption
Reduced demand charges
Reduced maintenance costs
RDHx allows for future growth without new construction costs
RDHx performs well with elevated water temperatures
Minimizing chiller energy usage
Reducing chiller OPEX
• OPEX savings increased using waterside economizers
(Free cooling window is increased using elevated water temperatures)
• Hybrid system including some CRAH units with RDHx adds
redundancy for greater system availability
• Increasing rack density to 18 kW can minimize infrastructure space
(CAPEX savings 30-40%)
Study Conclusions
• A common misconception that liquid cooling is too
expensive to deploy disproved
• CAPEX for liquid cooling and traditional air cooling is
approximately the same at 5 kW / rack
• Increasing energy costs encourage data center owners
and operators to consider liquid cooling
• Passive liquid cooling enables expansion and flexibility at a
lower, incremental, capital expenditure
Future Considerations
• A similar study done by a 3rd party consulting engineering
firm is comparing RDHx’s to IRC’s
• 4 MW Data Center, 5000 sq. ft.
• IRC CAPEX $4.5M, 6 MW cooling capacity
• RDHx CAPEX $2.5M, 7.5 MW cooling capacity
Thank You
Jeremiah Stikeleather
Applications Engineer
[email protected]
(603) 765-8305

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