Aaon Helps Project Achieve LEED Platinum

Signature Center, in Golden Colorado, recently received a LEED™ Platinum rating. This ambitious goal was realized with inspired design, and wise choice of mechanical systems.

The design featured:

• Underfloor Air Distribution
• Chilled Beams
• Evaporatively Cooled Chillers with Variable Speed Pumping
• Evaporatively Cooled Rooftop DX Air Handlers
• Non-CFC R-410a Refrigerant
  

Aaon made a natural choice because of their commitment to efficient packaged equipment. This project utilized a packaged LL Chiller Plant as well as a penthouse-type RL air handler. Both units utilized Aaon's unique, water-saving evaporative condensing system. Major project savings were achieved not just in energy, but in this other increasingly scare resource.

Not only did this project reach LEED™ Platinum--It also received the 2007 top award in the institutional building category from the Colorado Renewable Energy Society (CRES).

You can read more about this notable project in this Aaon case study.

Rethinking Air-Cooled Chillers

Air Cooled Advantages

Air cooled chillers offer many advantages to owners and designers. The first, and perhaps most compelling for many jobs is lower installed cost. Lower installed costs (compared to water cooled chillers) are driven by the following advantages:

• No Cooling Tower, Tower pumps, Tower and Pump Starters
• No equipment room required for the chillers
• Mounted starters

They also are easier to maintain, since the systems are significantly simpler than water-cooled systems:

• No on site Systems Engineer required
• No water treatment or make up water required
• No leaks on the roof
• No cooling tower, condenser pumps, associated starters

Generally, however, these advantages have come with significant trade-offs: Efficiency and Sound performance.

However, the introduction of Variable Speed oil-free air-cooled chillers by Smardt changes the balance.


First off, the Smardt Chiller is efficient. With IPLV's as low as 0.65 kw/ton, these chillers rival water-cooled system when the parasitic loads of the condenser pumps and cooling tower are considered. These chillers gain their efficiencies both from the inherent efficiency of the Turbocor compressor and the elimination of oil return issues that prevent other air-cooled chillers from capitalizing on the reduced head pressures available at low ambients.

This means these chillers use about 60-65% energy of other air-cooled chillers for the same load, and can nearly eliminate the energy benefit typically provided by moving to water-cooled systems. When you consider the cost of water (nearly $15/1000 gallons in Seattle, including sewer charges) this means the yearly cost of operation of these units is unrivaled. And energy conservation rebates are extremely attractive for these chillers.

The other major traditional trade off with using air-cooled equipment is sound. Screw chillers especially are known for their unfavorable sound characteristics. In most municipalities, sound ordinances are driven by occupancy and time of day. The most stringent criteria must be met during evening hours, typically when the units are not at their peak load. However, with constant-speed systems, the compressor is either on or off. This means it is either putting out its full sound or none at all. At full speed, such compressors can often exceed the evening sound criteria--even if they are on only momentarily. And the staging between on and off can be objectionable in its own right, regardless of sound level.

The Smardt chiller minimizes the problems with compressor sound in two ways. First, the variable speed drive allows the compressor to ramp slowly up and down to match the required output, eliminating the objectionable switching between compressors that constant-speed chillers exhibit. And secondly, they are just extremely quiet to begin with. Since no moving mechanical part is in contact with the chiller casing, very little mechanical noise is transmitted. Ninety-ton Turbocor compressors have been tested at 72 dBa at one meter, compared to screw compressors that can be as high as 80 dBa or higher in the same test. Five of these compressors operating together yield a sound level of 75 dBa at 10’.

More Benefits

But efficiency and sound are not the only benefits from using the Turbocor technology on air-cooled chillers. Other, less obvious ones exist.

Turbocor compressors have only one moving part, yielding un-matched reliability.

Reliability is enhanced by the elimination of oil in the refrigerant system. And the frictionless bearing requires almost no maintenance.

Since Turbocor compressors are variable speed driven, they provide an inherent soft-start on the compressor. Instead of kicking the motor up to full speed when power is applied to the system, the VSD slowly ramps the compressor up to the required speed for the load sensed by the system. This reduces stress on the already greatly simplified system to reduce wear and tear on the components.

But this soft start has another, very important advantage over standard air-cooled chiler systems--the use of the VSD eliminates inrush amperage. When an electrical motor is at rest, there is very little inductive resistance to current flow through the windings. As the motor starts to turn, this inductive resistance increases with the increase in RPM. What this means is when power is applied across the line (or even with a reduced voltage starter) to a stopped motor, there is a spike of electrical current far greater in amplitude than the design amp draw of the motor:

(example graph of inrush on a well pump motor)

This temporary increased amp draw heats the motor beyond where it is designed to operate for extended periods. This forces the chiller designer to provided anti-recycle timers to prevent rapid re-starts that could fatally overheat the motor. In practice, this usually means constant speed compressors cannot be started more often than every half-hour or so.

Additionally, this increased amp draw has effects that need to be addressed electrically. This becomes even more significant if the chillers are being served by emergency power. The emergency generators that serve the chiller must be sized to handle the inrush amperage. This can be a very costly addition, especially since the added amperage is only required for the first 30 second of operation or so.

Generators = $$$

Turbocor compressors on the Smardt air-cooled chillers eliminate inrush and provides a soft-start. This both heightens reliability and reduces electrical costs. For jobs where reliability is a primary concern, like data centers, this technology makes a lot of sense. First, it eliminates the need for increased generator sizing, it is an inherently more reliable compressor, and it frees the cooling system from reliance on a water utility service that could be disrupted.

Saving Water in Evaporatively Cooled Systems

Water is a limited resource, just like energy. Engineers are very aware of the need to save energy in their designs, and one of the best ways to do this is to take advantage of evaporative heat rejection for their cooling systems. The traditional cooling tower is an extremely effective way to reduce energy use at the compressors in a traditional cooling system. But introducing a cooling tower introduces a need for water to the system. It would be advantageous if this water use could be kept to an absolute minimum.

Especially since, in Seattle, water is expensive. As of this posting, the water utility rate per thousand gallons is $4.48 (summer) and the Sewer costs tack on an additional $9.96. When you consider that a cooling tower consumes a minimum of 1.8gph/ton (evaporation required to reject that heat), you can see that over a 1900 hour cooling season, these costs can really add up for a reasonably-sized cooling tower.



Earlier, I posted an article that highlighted ways to reduce water use in traditional cooling tower systems. For the most part, these recommendations address keeping the actual water use as close to the theoretical 1.8gph/ton evaporation figure as possible. Reducing the water use any further requires reducing the load on the tower, since evaporation is the only way a cooling tower can reject heat.

There are two ways to reduce load on a cooling tower--Reducing the total building load, or rejecting heat through some other method other than the cooling tower. Assuming the first option has already been exhausted through good engineering practices, the only other option is the second.

This is the approach taken by Aaon in their evaporative condenser systems. They essentially use a dry finned coil as the first stage of cooling before the refrigerant is cooled by evaporative methods. This essentially allows the system to reject as much heat as possible through a non-evaporative method before water is used. Every btuh that is rejected in this manner means less water used in the system.


This idea could be borrowed and applied to an open cooling tower by the use of a dry-cooler as a pre-cooler before a cooling tower. This way, the system rejects as much heat as possible in a dry fashion, and only uses water for what the dry-cooler can't do. This system gets to take advantage of the strengths of both methods of heat rejection--the water conserving function of a dry-cooler, and the lower water temperatures and more efficient heat rejection provided by a cooling tower.

Evapco has capitalized on this approach by creating a new, water-saving fluid cooler called the WDW:


This unit is a hybrid between a dry-cooler and an evaporative fluid cooler. It is provided with a control panel that controls both wet and dry sides of the unit, varying fan speeds with a VFD and determining when to run the evaporative pumps to optimize both water efficiency and fan energy.

Cutaway of an Evapco WDW unit

In practice, the evaporative system is only used for a small portion of the year, only when the design condenser water temperatures cannot be met by the dry-cooler side alone. What you see is a major reduction in water use compared to the same system served by a fully evaporative system:


Other advantages of this approach besides reduced water use are reduced chance of tower plume (since there are far fewer hours in which water is being evaporated, and when this does occur, it occurs in warmer temperatures) and the ability to provide some cooling even if city water is lost due to a service disruption.

But since a dry-cooler uses more fan energy per ton of cooling than a cooling tower, this system will inevitably use more energy to save water. Does this approach pay off?

An example from a real project might help demonstrate the economies involved. Below are the utility cost calculations from a project utilizing a 240 ton WDW installed in Seattle on a heat pump system with a portion of the load serving a 24/7 cooling application:



Note that even with the reduced water cost (to approximate the effective cost of using a deduct meter to avoid being charged wastewater charges for evaporated water) the hybrid system saves about 18% of the annual operational utility costs compared to a fully evaporative system. This affords a relatively quick payback for the added equipment costs associated with the hybrid system.

Cool Ways to Conserve Water

A few years back, I had an article published in the April 2005 issue of Plumbing Systems and Design Magazine that highlighted the many ways to optimize the water saving performance Cooling towers.



You can read that article right here.