Partial-Load Operation

Introduction 

In most industrial applications, a refrigeration system operates at its maximum cooling capacity only for a limited period. Once the target temperature has been reached, the thermal load decreases significantly, and the system's primary task becomes compensating for heat gains through insulation, cooling newly introduced products, and removing heat generated by internal sources. 

This operating profile is typical of climate chambers, low-temperature storage facilities, test chambers, pharmaceutical equipment, and biotechnology processes. It has become even more relevant since the COVID-19 pandemic, which led to a significant increase in temperature-controlled storage facilities and industrial processes with highly variable thermal loads. 

Under these conditions, the ability of a refrigeration system to operate efficiently at partial load has become one of the key factors influencing energy consumption, temperature stability, and the overall cost of designing and operating refrigeration equipment. 

Capacity Control in Conventional Refrigeration Systems

Most industrial refrigeration systems based on the vapor-compression cycle regulate cooling capacity using one of several conventional methods: 

• Compressor cycling (Start/Stop) 

• Capacity control through throttling or bypass valves 

While each of these methods enables the system to adjust its cooling capacity, they are all associated with specific technical limitations. 

The most common approach is the Start/Stop principle. Once the required temperature has been reached, the compressor shuts down and restarts when the temperature rises above the setpoint. Although simple to implement, this operating mode has several disadvantages. 

Each compressor start is accompanied by a high inrush current, placing additional stress on the facility's electrical network. In installations operating multiple refrigeration units, these repeated start-up events can significantly increase the demands on the power supply infrastructure. 

In addition, compressor start-up subjects the system to its highest mechanical and electrical loads. Frequent start/stop cycles accelerate wear of bearings, the electric motor, couplings, and other compressor components, ultimately reducing equipment service life and increasing maintenance costs. 

Another widely used capacity control method is throttling through regulating or bypass valves. However, this approach increases hydraulic resistance within the system and introduces additional pressure losses (pressure drop). As a result, part of the input energy is consumed overcoming these flow restrictions rather than producing useful cooling, reducing the overall energy efficiency of the refrigeration system. 

Capacity Control Principle in MIRAI Refrigeration Systems 

MIRAI refrigeration systems use a fundamentally different approach to handling varying thermal loads. 

Instead of repeatedly switching the compressor on and off, the system continuously adjusts its cooling capacity to match actual operating conditions. Capacity is modulated across a wide range—from 5% to 100% of full output—allowing the refrigeration system to precisely match the cooling demand of the application. 

As the thermal load decreases, the system automatically reduces its cooling capacity while maintaining a stable temperature without interrupting the refrigeration cycle. When the load increases, the cooling capacity is smoothly increased to the required level. 

This continuous modulation eliminates the need for constant transitions between full-capacity operation and complete system shutdown, ensuring stable and efficient performance across the entire operating range. 

Energy Efficiency at Partial Load 

The wide capacity modulation range enables the refrigeration system to produce only the amount of cooling required at any given moment. 

As a result, electrical power consumption is reduced, leading to lower operating costs. This advantage is particularly significant in long-duration industrial processes, where maximum cooling capacity is needed only during the initial pull-down phase, while the majority of operating time is spent compensating for relatively small and variable heat gains. 

Because capacity is regulated by directly adjusting the output of the refrigeration system itself, there are no additional energy losses associated with bypass control or flow throttling, resulting in higher overall energy efficiency. 

Reduced Load on the Electrical Network 

Another advantage of MIRAI refrigeration systems is the smooth start-up of the turbomachine. 

Unlike conventional refrigeration systems, where compressor start-up is accompanied by high inrush currents, the MIRAI system accelerates gradually to its operating speed. This significantly reduces stress on the electrical network, minimizes the risk of voltage drops, and enables more efficient use of the available electrical capacity of the facility (see the graph below for details). 

This advantage becomes even more significant in facilities operating multiple refrigeration units, as it reduces the combined peak electrical demand on the site's power distribution system.  

Improved Operational Reliability 

Eliminating frequent Start/Stop cycles not only improves energy efficiency but also extends equipment service life. 

Because the refrigeration system operates under steady-state conditions for most of its operating time, the mechanical and electrical stresses associated with compressor start-up in conventional refrigeration systems are significantly reduced. This minimizes wear on critical components and enhances long-term reliability. 

An additional level of reliability is provided by the robust design of MIRAI equipment, which is engineered with a threefold structural safety margin. This design philosophy ensures stable operation over many years of continuous service, even under variable thermal loads and demanding industrial conditions. 

Effect of Partial-Load Operation on the Cooling Water System 

Reducing refrigeration capacity under partial-load conditions also decreases the amount of heat that must be rejected through the cooling water circuit. As cooling capacity is reduced, the turbomachine consumes less electrical power, resulting in a lower heat load on the external water-cooling system. 

In systems equipped with variable water flow control, this allows the cooling water flow rate to be adjusted according to the current operating conditions. As a result, the load on circulation pumps is reduced, auxiliary power consumption decreases, and the required capacity of the heat rejection equipment can be optimized. 

It should be noted that partial-load operation does not alter the requirements for cooling water temperature, flow rate, or water quality. To ensure stable operation, the parameters of the water circuit must remain within the limits specified in the MIRAI technical documentation. 

No additional refrigeration system is required to produce the low-temperature air stream. The MIRAI unit generates the required cooling independently, while the external water circuit serves solely to remove heat from the refrigeration system. 

Maintaining the Full Temperature Range 

Capacity modulation does not reduce the available operating temperature range of the system. As the thermal load decreases, the refrigeration unit simply reduces its cooling capacity while continuing to maintain the required temperature within its specified operating range. 

The supply air temperature is generated directly within the MIRAI refrigeration cycle and is not determined by the ambient temperature. Provided that the specified operating conditions and cooling water requirements are met, the system maintains the target temperature regardless of changes in ambient conditions within the approved operating range. 

Conclusion 

Modern industrial refrigeration applications are characterized by highly variable thermal loads. As a result, refrigeration system performance should be evaluated not only by maximum cooling capacity, but also by its ability to operate efficiently under partial-load conditions. 

MIRAI refrigeration systems provide continuous capacity modulation from 5% to 100% without Start/Stop cycling and without the pressure losses associated with throttling or bypass control. 

This approach delivers several key advantages: 

• Reduced electrical power consumption 

• Lower operating costs 

• Elimination of high inrush currents 

• Reduced load on the electrical network 

• Lower mechanical wear 

• Stable temperature control across the entire operating range 

• High operational reliability, supported by a design engineered with a threefold structural safety margin 

Partial-load operation is therefore one of the key technical advantages of MIRAI refrigeration systems, combining high energy efficiency, stable process control, and long-term equipment reliability.