Central building cooling options include water chillers and direct-expansion (DX) A/C units. Chillers use a refrigeration cycle to cool water to 42º F to 55º F for pumping to chilled water cooling coils. Air is then blown over the chilled water cooling coils to provide cool air to the conditioned space. DX systems also use a refrigeration cycle, but distribute refrigerant directly to DX cooling coils – some are packaged in a single housing, others are split (evaporator coil remote to condenser and compressor). High-efficiency chillers can produce chilled water using less than 0.50 kW per ton of cooling capacity. A refrigeration system must also reject the heat that it removes using a cooling tower for water cooling the condenser (most efficient) or using fans for air cooling the hot refrigerant of the condenser coil.
Chiller Performance Facts
Chillers consume more than 50% of electrical energy during seasonal periods of building use. More than 120,000 chillers in the U.S. are expending more than 30% in additional energy through operational inefficiencies. (Estimated by DOE survey) Water cooled condenser water (cooling tower) temperature decrease of 1ºF can increase efficiency of the chiller compressor by 1% to 2 % in most situations; however, there is a limit and optimum lower condenser temperature for a given partial loading of the chiller compressor. A neglected or poorly maintained cooling tower can reduce chiller efficiency by 10% to 35% and a dirty coil condenser of an air cooled chiller as much as 5% to 15% Chemical cleaning of the inside of the condenser and evaporator heat transfer surfaces can result in a 5% to 10% energy savings – kw/ton Air-cooled chillers are less efficient than water-cooled chillers but cost less as well. to determine, with today's chiller machines this instantaneous input (kw)/(ton) output relationship. Most have a control panel that looks at all of the motor drives connected kw for comparison with the evaporator chilled water flow and temperature. However, if this panel is not available, the following formula represents the readings that must be taken to approximate the kw/ton values for comparison with the previous ARI certification data. This will determine operating cost and efficiency and also represents a logged report.
Chiller Performance Ratings
There are many chiller manufacturers of several compressor types and condenser cooling configurations.¹ These manufacturers have websites that are very educational to chiller operation in general and specific marketing uniqueness of their equipment. The important thing for chiller operation is to know how the unit is performing relative to optimum design efficiency after the initial installation, where efficiency is dependent on chiller service duty and maintained equipment performance worthiness. The Air-Conditioning and Refrigeration Institute (ARI) certiAirfies chiller performance both for capacity (tons) and power input according to very detailed procedure to ensure manufacturer’s performance statements are verified through actual running under partial load and full load conditions. As seen below there is a kw/ton range from a partial loaded condition to a full loaded condition that would be expected during installed operation. It is possible
Efficiency Recommendations ARI Standard 550/590 Air Cooled Chillers Part Load Optimized
Compressor Type & Capacity Scroll (30 – 60 tons) Reciprocating (30 – 150 tons) Screw (70 –200 tons) Recommended IPLV (kw/ton) Best Available IPLV (kw/ton)
Full Load Optimized
Recommended Full Load (kw/ton) Best Available Full load (kw/ton)
0.86 or less less 0.90 or less 0.98 or less
0.83 0.80 0.83
1.23 or less 1.23 or less 1.23 or less
1.10 1.00 0.94
Water Cooled Chillers
Part Load Optimized
Compressor Type & Capacity Centrifugal (150 –299 tons) 2,000 Centrifugal (300 – 2,000 tons) Rotary Screw greater 150 tons Recommended IPLV (kw/ton) Best...