Air Cooled Chiller Efficiency?

Last month I shared what ETC Group has learned with water cooled chillers. Unfortunately this month I am exposing how little we know about air cooled chillers. My goal is to push chiller manufacturers to support our industry in making air cooled chillers more efficient.

At least 20 years ago the refrigeration industry began focusing on reducing compressor energy. I am getting old but not old enough to know who took the lead; refrigeration vendors or large water-cooled chiller manufacturers. Since cooling compressor power decreases with lower lift (difference in pressure between condenser and evaporator), both industries focused on dropping the condenser pressure/temperature.

What about air cooled chillers? As more and more owners "simplify" their equipment, we see an expanded use of air cooled chillers vs water cooled chillers with cooling towers. We see air cooled chillers up to 500 tons, we see them serving year-round process cooling loads, and we have seen 5 to 10 of them in a single plant. So the original assumptions that went into their design (1 or 2 serving an office building or school) may no longer by valid and a deeper look is required.

Our curiosity into air cooled chillers first started when we not only noticed condenser fans cycling off when it was 60F outside but we also saw fans cycling off when it was 90F outside.

We have done literature searches and we have contacted the major manufacturers. This is the little bit that we have learned or heard:

• Air-cooled chillers are designed to keep a minimum lift (condensing pressure – evaporating pressure) assuming that the chiller will have its leaving set point reset up to 60F.
• The condensing pressure is set in the factory. This setting is either not field adjustable or only adjustable at the chiller.
• No chiller points are connected to the BAS to reset the condensing pressure/temperature
• A high enough lift has to be maintained to move refrigerant through the expansion valve
• Screw chillers require a high enough lift to prevent major problems.
• High enough lift is required for proper oil management.
• Free-cooling add-ons are the only option for reducing power during cold conditions. But SLC has 5,000 hours between 40F (when free-cooling would work) and 75F where better condensing control would deliver energy savings.

In general, we see chillers making 45F chilled water with condensing temperatures around 105F. If these chillers could operate in cold weather with 70F condensing temperature, you could potentially save 30% to 40% of the chiller power.

How can we get:

• Vendors to support operating changes to maximize chiller efficiency?
• Vendors to re-evaluate their chiller designs to maximize year-round efficiency for the ways air cooled chillers are actually operated?

We will keep searching for answers and will share whatever we learn. If anybody has relevant information, please share.