Which device transfers heat between two fluids without allowing them to mix?

The correct choice is a heat exchanger because it is specifically designed to transfer heat between two fluids, such as water or air, without those fluids mixing together. The fundamental principle behind a heat exchanger is its ability to allow thermal energy to flow from the hotter fluid to the cooler fluid while maintaining a physical barrier that separates them.

In data center cooling applications, heat exchangers play a crucial role in managing heat loads by efficiently transferring heat out of the data center environment or by recovering waste heat for reuse, thereby improving overall energy efficiency. This process is essential in minimizing the need for additional cooling resources and helps in maintaining optimal operating temperatures for critical equipment.

The other options, while related to energy usage or heat management techniques in some capacity, do not serve the specific function of transferring heat between fluids without mixing. For instance, a variable frequency drive adjusts the speed of motors, a chilled beam system uses convection to cool spaces with a mixture of water and air, and an economizer is a system that uses outside air for cooling purposes but does not involve direct heat transfer between non-mixing fluids. Thus, these devices do not fit the precise requirements of the question.