For comfort-cooling applications, water-cooled centrifugal chillers generally are designed for a set of standard conditions specified by the Air-Conditioning, Heating and Refrigeration Institute (AHRI). Typically, these include a leaving chilled-water temperature (LWT) of 44°F (6°C) and an entering condenser-water temperature (EWT) of 85°F (29°C).
But in a process-cooling application, AHRI standard conditions usually do not apply. While the condenser-water EWT may remain at 85°F, the chilled-water LWT may be 65°F (18°C) or higher.
For a centrifugal chiller to operate efficiently with higher chilled-water LWT, certain features are required that may not be standard on many chillers unless specified. To understand the importance of these features, it is necessary to understand the thermodynamics of “lift” and its relationship to chiller performance. This knowledge will facilitate proper chiller selection for process-cooling applications with low-lift conditions.
Understanding LiftLift (or head pressure) is the difference between condenser refrigerant pressure and evaporator refrigerant pressure. Using defined pressure/temperature relationships, lift can also be measured with the leaving chilled-water temperature and the leaving condenser-water temperature. Further, when the chilled-water LWT and condenser-water flow are constant, the condenser entering water temperature can be used as a metric for lift. Because most condenser water systems are designed for constant flow, the condenser EWT is the most common metric for lift, and I will use it in this discussion.
Lift in Comfort Applications. In comfort-cooling applications, lower condenser EWT indicates lower lift, which lowers the compressor work (figure 1). The relationship can be summarized as shown in figure 1.
In comfort-cooling applications, ambient weather conditions often allow facility owners to take advantage of condenser EWTs as low as 50°F (10°C), at AHRI conditions.
The ability to use lower condenser EWTs significantly improves chiller efficiency. In fact, greater chiller efficiency can be achieved by lowering lift than by lowering load. The efficiency improvements due to lower lift can be realized in both single-chiller and multiple-chiller installations.
Lift in Process Applications. In process-cooling applications, chilled-water LWT is the metric associated with lift: Higher chilled-water LWT means lower lift (figure 2). So for process-cooling applications with low-lift conditions, the formula changes slightly, as shown in figure 2.
But, the chiller must be designed to take advantage of the higher chilled-water LWT to see an effective reduction of compressor work. If it is, then process facilities also will see significant energy savings because efficiency is mostly impacted by lift and only slightly impacted by load. These efficiency improvements will be seen in both single-chiller and multiple-chiller installations.
Selecting a Chiller Designed for Handling Low LiftNot every chiller is designed to take advantage of conditions when high chilled-water LWT is specified. In fact, four design variables affect a centrifugal chiller’s ability to handle low-lift conditions encountered in process-cooling applications:
- Drive design.
- Orifice design.
- The oil-management system.
- Compressor aerodynamics.
A hermetic-drive motor is located inside a refrigerant-filled motor cavity. Unfortunately, this is a bad place to be under low-lift conditions. At all conditions, head pressure on a hermetic-drive motor must be high enough to ensure that refrigerant flows adequately through the motor cavity. Without sufficient flow, current draw can overheat the motor windings, and the chiller will shut down due to high motor temperature. For that reason, chillers with a hermetic-drive motor must maintain a greater pressure differential between the evaporator and the condenser to ensure adequate motor cooling. A common method for ensuring sufficient pressure differential for hermetic-drive chillers is to artificially limit the lift reduction. Limiting lift reduction will increase the compressor’s energy consumption.
By contrast, an open-drive motor is located outside the refrigerant circuit. Therefore, it can be air-cooled or, optionally, water-cooled. It does not depend on refrigerant flow for cooling and is, therefore, unaffected by changes in refrigerant flow during low-lift conditions.
Orifice Design. The orifice is the chiller component that creates a refrigerant pressure drop between the condenser and the evaporator. There are two orifice-design options: fixed or variable.
With a fixed orifice, it is difficult for a chiller to perform efficiently under low-lift conditions at full loads. That is because fixed orifices are sized for the high head pressure that exists at design-lift conditions. As a result, fixed orifices simply are not large enough to allow the required refrigerant flow at low-pressure conditions.
The variable-orifice design, however, is more accommodating. A variable-orifice valve automatically modulates to maintain proper refrigerant flow, taking into account the head pressure across the valve. At design-lift conditions, the variable orifice is partially closed, and at low-lift conditions, it opens to allow the proper refrigerant flow. This feature is especially important for multiple-chiller plants where additional chillers and associated auxiliaries (pumps, towers) have to operate to meet process/facility demand.
Without a variable orifice, the operator may resort to running more chillers and more associated auxiliaries than needed because the chillers are unable to load-up. This is an extremely inefficient way to operate a chiller plant.
To avoid the full-load problem of fixed-orifice chillers under low-lift conditions, some chiller manufacturers maintain a high minimum condenser EWT - up to 75°F (24°C). But the strategy to increase lift (head pressure) to maintain chiller stability sacrifices chiller efficiency in situations where low-lift conditions would be available to slash operating costs.
In terms of chiller design, the only way to achieve both full-load cooling capacity under low-lift conditions and off-design energy performance is to use a variable orifice as a refrigerant-metering device.
In conclusion, centrifugal chillers that can adapt to low-lift conditions, where head pressure is reduced because of high chilled-water leaving water temperature, are able to save energy in many process-cooling applications. To best take advantage of low-lift conditions, the chiller should incorporate an open-drive design to ensure proper motor cooling, a variable orifice to ensure proper refrigerant flow, an oil-eduction system to maintain oil in the sump, and a gear-drive compressor to optimize impeller tip speed. A chiller equipped with these design features can deliver good performance in low-lift conditions and provide energy savings in a range of process-cooling applications.