Examples of the disclosure relate to an apparatus comprising means for: receiving a plurality of input values comprising physical parameters of, at least part of, a cooling system; using the received input values to estimate dimensionless ratios providing a measure of at least one of; heat transfer within at least part of a cooling system, pressure loss within at least part of a cooling system; using the dimensionless ratios to determine at least one performance metric of at least part of a cooling system; and using at least one determined performance metric to control one or more physical parameters of at least part of a cooling system.