Chiller lift, chiller lift pressure or refrigerant temperature lift is a key component of the compressor’s efficiency within an HVAC unit used to maintain a building’s temperature. Lift represents the temperature difference between the hot water exiting the condenser and the cold water exiting the chiller to circulate through your building.
How do you calculate chiller lift?
To calculate the chiller lift, subtract the temperature of the leaving condenser water to be cooled by a cooling tower or fans (85-95 degrees F.) by the temperature of the chilled water exiting the building (42-46 degrees F.). Lift ranges from 5-50 degrees F. based on the equipment. For example, if the condenser water return temperature is 85 degrees F. and the evaporator leaving water temperature is 44 degrees F., then the chiller lift temperature is 41 degrees F.
What drives chiller efficiency?
The compressor converts energy into compression and represents most of the energy consumed by the chiller. The compressor is at the heart of the chiller and moves refrigerant throughout the system to create the pressure differential between the evaporator and condenser, also known as lift.
The warmed refrigerant evaporates into the compressor, which turns the refrigerant vapor back into a liquid before it flows into the condenser. The liquid refrigerant undergoes cooling via a heat-rejection process from outdoor fans and coils for air-cooled chillers, or through a cooling tower, or dry cooler for water-cooled units. The condenser’s expansion valve regulates the amount of cold refrigerant released into the evaporator to achieve your cooling needs.
Drive train efficiency, or the rotating components, in chiller systems, ensures the compressor operates at its most efficient level. The speed is a function of compressor speed, desired lift, capacity and type of refrigerant. Other key considerations include motor selection, drive train and bearing choice.
What is the chiller approach?
However, do not mistake the chiller lift for the chiller approach, the temperature difference between the water and the refrigerant. There is also an evaporator approach, measured by the temperature difference between the chilled supply water and the evaporating refrigerant as well as a condenser approach, measured by the temperature difference between the condenser water supply and the condensing refrigerant.
Chiller approach temperatures offer insights into how well the chiller is maintained. A lower approach temperature indicates a higher chiller efficiency. Inefficient heat transfer increases approach temperature and results in poor chiller efficiency and performance. Even with good maintenance, the chiller approach does gradually rise through years of use.
How does lift affect the chiller load?
Variable speed drives (VSDs) can reduce a chiller’s energy use by up to 30% yearly if the VSD chiller is specifically designed to function efficiently in off-design conditions. For example, in low-lift conditions, optimization of tower water temperatures or chilled-water-supply temperatures can result in up to four times more energy savings.
A unit’s cooling load affects chiller energy consumption. Efficiency can improve by 15% when a chiller at a constant entering condenser water temperature reduces load by 50%. The lessened load is achievable as the number of days the unit operates at full capacity is limited. Chillers usually operate in the 40-70% load range most of the operational time, facing conditions that deviate from standard conditions, according to the EPA.
