True Partners Consulting Engineers

The Importance of Low-Load Chillers

A low-load chiller is a type of cooling system designed to operate efficiently under conditions of low thermal load. They are especially useful in scenarios where the cooling demand is variable or doesn't always reach the capacity of larger, traditional chillers.

These chillers are engineered to provide effective cooling while consuming less energy when the cooling demand is low. This is achieved through various design elements such as variable-speed compressors, advanced control algorithms, and efficient heat exchangers.

In buildings or facilities where the cooling load frequently fluctuates or drops significantly during certain periods (like night time or winter), low-load chillers can result in considerable energy savings and improved overall system efficiency. This makes them an environmentally friendly and cost-effective choice for many applications.

In Australia, most office buildings will require low cooling in the central zones of the buildings in winter. The cooling load requirement is usually a fraction of the total cooling demand in peak summer conditions.

Exploring the Consequences: Absence of a Low-Load Chiller in a Cooling System

If a low-load chiller is not used in a scenario where the cooling demand frequently drops or fluctuates, several issues can arise:

Decreased Efficiency: Regular chillers are less efficient under low-load conditions as they are typically designed to operate at maximum capacity. Operating these chillers at low load can lead to energy wastage and decreased overall system efficiency.

Short Cycling: This refers to a situation where a chiller turns on and off more frequently than it should. It can happen if the chiller's capacity significantly exceeds the cooling demand, leading to rapid attainment of set temperatures and consequent system shut-offs. Short cycling can lead to increased wear and tear, resulting in reduced equipment lifespan.

Poor Temperature Control: Without a low-load chiller, it might be challenging to maintain consistent temperature levels, especially during periods of reduced cooling demand.

Increased Operational Costs: Due to decreased efficiency and potential short cycling, the operational costs could be significantly higher due to increased energy consumption and maintenance needs.

Therefore, not using a low-load chiller when it is appropriate can lead to less efficient operation, higher costs, and potential damage to equipment.

If a low-load chiller isn't a viable option, several alternative measures can be employed to manage low cooling load situations:

· Variable Speed Drives: Implementing variable speed drives (VSDs) on chillers can adjust the speed of the compressor in response to cooling demand. By modulating compressor speed, VSDs can maintain chiller efficiency at low loads.

· Chiller Sequencing: In systems with multiple chillers, chiller sequencing can be employed to optimize performance under varying load conditions. It involves adjusting which and how many chillers are operating based on cooling demand

· Thermal Energy Storage: By using thermal energy storage, excess cooling capacity during periods of low demand can be stored and then used when demand is higher. This can reduce the need for the chiller to operate at low efficiency during low-load periods.

· Buffer Tank: It is also referred to as inertia tank and acts as a shock absorber in a chilled water system, it will avoid chiller compressor to start and stop frequently. If the amount of water in the closed circuit is not large enough, the water temperature can reach the return water target temperature set in a very short period of time, at which point the compressor will stop working. Then in a very short time, the water temperature will rise again to the temperature that will trigger the compressor to start. This frequent starting and stopping will greatly reduce the life of the chiller compressor and waste a lot of electricity. Therefore, not only we need a buffer tank but also the capacity of the water tank should be adequately sized. All chiller manufactures have a limitation on the maximum number of compressor on/off cycle, if passed, chiller will stop and generate a fault alarm.

We will talk about the required chilled water buffer tank capacity in another post.

· Free Cooling Systems: Also known as economizer cycles, these systems use ambient air or water to cool spaces when the outdoor conditions are suitable. This can offset the need for mechanical cooling during periods of low cooling demand.

· Building Energy Management System (BEMS): BEMS can be used to manage and control the building's HVAC systems to ensure they operate at maximum efficiency under different load conditions.

Remember, the best solution depends on the specific circumstances, including the climate, the building's usage patterns, and the available resources.

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