Air Cooled Chillers In Mission Critical Facilities
New technology has improved energy efficiency and lowered operating costs
Mission critical facilities such as data centers demand equipment that is reliable, efficient, and easy to operate. When it comes to selecting an HVAC system for these facilities, it is important for facility managers and building owners to choose one that embodies all three of these traits, letting them focus on the data center’s operation instead.
Because of the importance of choosing the right chiller for data centers, building owners must consider all of the options when selecting one for their facility. One option that may be best for many buildings is an air cooled chiller. Let’s take a look at the advantages air cooled chillers present, such as reduced lifecycle costs, and how they may be best suited for your data center project.
PUSHING THE EFFICIENCY ENVELOPE
Water cooled centrifugal systems were once the undisputed champions of efficiency. Not anymore. Pushing the efficiency envelope, today’s air cooled chillers have part load values and energy consumption numbers that rival those of water cooled chillers. Current variable speed rotary screw chillers, for example, have part load values in the realm of 0.65 kW/ton or better, making their energy consumption arguably better than systems that include variable speed water cooled chillers, pumps, and cooling towers. In addition to lacking an energy efficiency advantage, water cooled chillers also have higher operating costs, as the number of gallons of water that must be used and treated yearly is measured in millions per year.
Improved energy efficiency and lower operating costs aren’t the only advantages air cooled chillers have. With a much wider range of leaving fluid temperatures — typically from 15°F and up to 70°F — positive displacement air cooled chillers are more versatile for ambient operating temperatures, which range from 0°F to 125°F. Building owners can maximize their chiller system’s inefficiencies by increasing the chilled water plant leaving fluid temperatures to match the operating temperature limits of data center equipment. And given the wide ambient temperature window of the equipment, designers are also afforded the flexibility to standardize on the design of chiller plants, regardless of a data center’s geographical location.
REDUNDANCY IS KEY
To properly plan for power losses in mission critical facilities, manufacturers have made it a best practice to design multiple redundancies into each air cooled chiller, including multiple circuits. This facet makes manufacturing for 2N redundancy simpler and more cost effective for air cooled equipment. In addition, individual compressors and a completely independent variable-frequency starter are available on each circuit of the chiller, giving each machine a built-in electrical and mechanical redundancy.
For mission critical facilities, seconds can make all the difference. Today’s air cooled chillers can return to operation in as few as 30 seconds after power restoration and reach full load in just five to six minutes, whereas previous iterations used to take 20 to 30 minutes to attain full load.
Another benefit of air cooled chillers is that, unlike water cooled systems, which have cooling towers and pumps, their entire condensing sections are built-in. This eases concerns about how they will respond to a quick start. Considerably smaller buffer capacity in the system design is also possible due to chiller rapid restore and fast loading options.
EASY INSTALLATION, GREATER FLEXIBILITY AND SCALABILITY
It’s crucial for facilities like data centers to equip themselves with chillers that are simple to install, as they frequently need to increase capacity. Since they are installed outdoors, and thus require minimal mechanical room space, air cooled chillers provide easy installation for building owners. They also offer the best scalability and flexibility in terms of fluctuating facility demands.
Freezing weather and related conditions must be taken into account with outdoor chillers, but they do not necessarily require freeze inhibiting additives such as glycol. In response to freezing conditions, many manufacturers offer the option of remote mounting evaporators indoors.
In addition to costing less and being more operator friendly than systems that use freeze inhibitors, systems equipped with a remote evaporator may provide greater performance. Remote evaporators also do not have cooling towers, simplifying operations and maintenance during the winter months in cold climates. Indoor evaporators also do not generate any discernable noise nor require external electrical connections.
SIMPLIFIED MAINTENANCE AND CONTROLS
In addition to what they do have, part of the appeal of air cooled chiller systems is what they don’t have. With no cooling tower, condenser pumps, or water treatment systems to service and maintain, and no required water treatment chemicals, air cooled chillers offer simplified maintenance compared to water cooled systems.
Facility managers also appreciate the simplistic controls of air cooled chillers. With most air cooled systems placing their frequently used components on the sides and ends of units for easy access, these chillers offer smooth use and operation. In addition, the convenient location of coils makes them easy to clean. These factors help reduce maintenance staff and budgets for air cooled chillers, making them a more cost-efficient alternative to water cooled systems.
Another way air cooled chillers provide superior efficiency at lower costs compared to water cooled systems is through water conservation. Consuming roughly 2,800 gallons of water per hour, a standard 1,000-ton cooling tower’s yearly water use can be measured in millions of gallons. With water rates in the United States ranging from $1.50 per 1,000 gallons at the low end, to more than $20.00 per gallon at the high end, this is not an insignificant cost. With considerably lower water usage, this makes air cooled chillers a more efficient way to cool data centers.
Water prices only account for part of the utility cost, as sewer related charges are typically tied directly to water consumption and can be just as, if not more, expensive than the water consumption rate. It is common for tower supply water to be separately metered for a reduced utility cost; however, this only decreases the sewer portion of the utility bill and does not eradicate its charges altogether.
MAXIMIZE REVENUE GENERATING SPACE
Using air cooled chillers is a great way to conserve valuable interior building space. By placing the large components of the mechanical equipment outdoors, interior space may be more efficiently used for revenue generating equipment instead of mechanical equipment. Placing the refrigerating equipment outdoors also eliminates the need for sophisticated ASHRAE 15 refrigerant safety compliance systems, such as refrigerant monitors, exhaust, and supply fans
OPERATIONAL COSTS: A COST COMPARISON
An energy simulation, provided using the Department of Energy EnergyPlus™ calculation engine and based on Daikin Applied Energy Analyzer II™ analysis program, further exemplifies the operational cost comparison between the available high efficiency, variable speed air cooled chillers and their centrifugal chiller counterparts. The simulation uses a fictitious 125,000-sq-ft data center located in Nashville as the locale, with the load estimate being approximately 2,500 tons of peak load (2000 tons of average operating load) and six 500-ton chillers sharing the load (N+1 equipment redundancy). The redundant chillers are anticipated to be operating and sharing the load. The variable speed centrifugal machines (HVAC A) have an NPLV of 0.319 kW/ton at the design condition of 50°F, leaving chilled water and variable primary flow. Air cooled variable speed machines (HVAC B) have an NPLV of 0.588 at the same design conditions. The assumed electric rate is $0.10/kWh; the water usage rate is assumed $2.14/kGal; and the sewer charge was estimated at one-third the total tower water usage (separate meter for blow-down) at a rate of $4.32/kGal.
The results of the analysis show an average annual operational cost savings of approximately 2.74% for the variable speed air cooled chiller.
The reduced water usage makes up the majority of the operational cost savings. The pump energy used in the water cooled system also greatly adds to the operating cost difference between the two systems. And while the water cooled system offers lower energy consumption in the peak summer months, the air cooled system offers lower energy consumption during the more moderate months.
These are just a handful of the benefits that installing air cooled chillers in mission critical facilities yield. In the long run, air cooled chillers offer a low total cost of ownership with competitive upfront costs, minimal operating and maintenance costs, and a reduced risk of expensive downtime. Air cooled chiller systems also offer many advantages when capacity needs to be expanded. When it comes to operating a mission critical facility like a data center, having a chiller that is reliable, easy to maintain, and affordable offers building owner and occupants the peace of mind they need.