Chiller

Most building chillers use the vapor-compression cycle. Warm water returning from the building passes through the evaporator, where refrigerant boils at low pressure and absorbs the heat. The compressor raises the pressure and temperature of the refrigerant vapor, and the condenser rejects that heat to outdoor air or to condenser water. An expansion device then drops the pressure so the cycle can repeat.

In a typical comfort cooling plant, the chiller supplies chilled water at around 42 to 45°F, and the water returns roughly 10 to 12°F warmer after passing through air handler and fan coil coils. Absorption chillers do the same job using a heat source such as steam, hot water or natural gas instead of a mechanical compressor.

Chillers are usually described by how they reject heat and by the type of compressor they use.

• Air-cooled chillers reject heat directly to outdoor air with condenser fans. They need no cooling tower and are common in small to mid-size buildings.
• Water-cooled chillers reject heat to condenser water that is cooled by a cooling tower. They are generally more efficient and are typical in large central plants.
• Scroll compressors are used in smaller chillers, often several in one unit.
• Screw compressors cover mid-size capacities.
• Centrifugal compressors are used in large chillers, including oil-free magnetic bearing designs.
• Absorption chillers use heat instead of an electric compressor to drive the cycle.

These three are often confused because they work together. The chiller makes cold water. Air handlers and fan coil units use that cold water to cool and dehumidify the air supplied to occupied spaces. On water-cooled systems, the cooling tower removes the heat the chiller pulled out of the building, using condenser water that typically returns from the tower at around 85°F on a design day.

Pumps move water through both the chilled water and condenser water loops, and the plant controls decide how many chillers, pumps and tower fans run to meet the load.

Chiller capacity is rated in tons of refrigeration, where one ton equals 12,000 Btu per hour of heat removal. Efficiency is usually expressed as kW per ton, where lower is better, or as COP or EER, where higher is better.

Part-load ratings such as IPLV, defined in AHRI Standard 550/590, matter because chillers spend most of their operating hours below full load. Minimum efficiencies for new chillers are set by energy codes based on ASHRAE 90.1.

Chillers are among the most expensive pieces of equipment in a building, so they get close attention. Typical maintenance includes logging temperatures, pressures, amps and approach temperatures, cleaning condenser tubes on water-cooled units (usually annually) with periodic eddy current testing of the tubes, oil and refrigerant analysis, leak checks, cleaning condenser coils on air-cooled units, and checking water treatment, flow switches and safety controls.

Common problems include fouled condenser tubes that raise head pressure and energy use, low refrigerant charge from leaks, low water flow trips, short cycling at low loads, and drifting sensors that cause poor staging. A rising approach temperature, the gap between the leaving water temperature and the refrigerant temperature, is an early sign of fouling or low charge.

In a digital twin such as Syyclops, the chiller's live BMS points, work orders and maintenance history are attached to the same asset, so current readings and past repairs can be reviewed together.