A common design goal for the small chilled-water system with one or two chillers (Figure below) is to minimize complexity while balancing energy consumption goals. Smaller chilled-water systems may have smaller budgets allotted for operation and maintenance and may run unattended more often than larger systems. Keeping it simple, while capitalizing on chilled water advantages, is the hallmark of a successful project.
The first cost of a small system is a common hurdle faced by a building owner. There are ways to minimize first costs without sacrificing operating costs. For example, a wider design ΔT reduces flow rates, which in turn reduces pipe and pump sizes. In addition to reducing pump and pricing costs, this may also allow the designer to avoid installing a storage tank to meet the required chiller “loop times.” On a system with multiple chillers, using a variable-primary-flow design can reduce the number of pumps, starters, electrical equipment, and space required.
Constant flow is simple and often applied to small systems up to 200 tons—as long as the system pressure drop is fairly low and a wider ΔT is applied to reduce the system flow rate. In constant flow systems, appropriate chilled-water reset reduces chiller energy. These two strategies for saving energy (reducing flow rates and/or chilled-water temperature reset) can be used successfully in the constant flow designs more common in small chilled-water systems.
Constant flow systems use either a balancing or pressure-reducing valve or, in a few cases, trim the pump impeller to set the system design flow. Pressure-reducing valves waste pump energy. Another option designers use to reduce pumping energy and increase system flexibility is to install a variable frequency drive on the pump motor and set it at a constant speed during system commissioning.
If, instead, system flow is balanced by trimming the pump impeller, flow adjustment is much more difficult. Using a variable frequency drive at a set speed allows the flow to be decreased or increased in the future if necessary. This approach is more cost effective because the cost of variable frequency drives has dropped. Any incremental cost will be offset by the elimination of the balancing valves and pump starter.
Although a variable-primary-flow system may cost more than a constant flow system, it is growing in popularity because it is less expensive than installing a decoupled system. Another reason for its increased popularity is that pump energy is reduced.
Some owners are concerned that the controls are more complex, but variable flow systems can work very simply in the small chilled-water system when there is only one chiller or when two chillers are piped in series.
Many small chilled-water systems use air-cooled chillers because of the lower maintenance requirements of the condensing circuit. Water-cooled systems are generally more energy efficient and have more options for features such as heat recovery, though some air-cooled chillers have partial heat recovery options.
To help the owner decide on the system selection, a comprehensive energy analysis is the best method of estimating the life cycle cost difference between air-cooled and water-cooled systems. Energy analysis is likely required for many facilities seeking LEED certification, so it may already be part of those jobs.
Number of chillers
The number of chillers to install is a function of redundancy requirements and first cost. In general, the more chillers installed, the higher the initial cost. Therefore, many small systems only use one chiller. Most chillers in the 20 through 200 ton range use multiple compressors with multiple refrigeration circuits and provide a reasonable level of cooling redundancy. The only system controls installed on a single chiller installation may be a clock and ambient lockout switch to enable and disable the chilled-water system. If only one chiller is used, a system that varies the flow rate through the chiller can be quite simple to operate. Minimum and maximum flows and maximum rate of change for the flow would still need to be addressed
As systems get larger, the owner may require more redundancy, leading them to install multiple chillers. Some designers use 200 tons as the maximum job size for a single chiller.
When there is more than one chiller, there are many more system control decisions to be made including:
- enabling the second chiller
- • turning the second chiller off, and
- failure recovery.
Two-chiller plants require higher system control intelligence than single chiller plants.
Parallel or series
Parallel configurations are more common than series configurations. In chiller systems with an even number of chillers, there are advantages to putting them into a series configuration, especially if low or variable water flow is desired. This offers the benefits of better system efficiency and higher capacity because the upstream chiller produces water at a warmer temperature. Series chillers should not be applied with low system ΔTs, because the maximum flow through the chillers may be reached. Efforts to eliminate the so-called “Low ΔT syndrome” must beaddressed for both configurations.
Part load system operation
For small chilled-water systems, especially those with only one chiller, part load system energy use may be dominated by ancillary equipment, especially in a constant flow system. At low loads, constant speed pumps and tower fans constitute a much larger portion of the chiller plant energy than at full load. Variable frequency drives for unloading tower fans and chilled-water pumps may provide benefits, depending on the costs, system operating hours, system type, and outdoor air conditions.