CRAC Heat Exchanger Response to Step Change in Chilled Water Flowrate

Abstract

This paper presents experimental data showing the response of a computer room air conditioning unit (CRAC) to chilled water (CHW) pump restart. The data are offered to improve and develop modeling of cooling equipment restart events following data center power failure. There are estimates that power failures will increase and limits on availability will affect data center operations at more than 90 percent of all companies over the next five years. Because providing backup power to cooling equipment increases data center first cost, it is important to have accurate models for cooling events and processes following power failure that help predict server inlet temperatures during the transient phase caused by a power failure. Since power density of computing equipment continues to rise, the temperature rise of air within the data center has been predicted to rise more quickly to an unacceptable level, increasing concern. Accurate models of CRAC response to pump restart can aid in data center cooling design, backup power infrastructure provisioning, and even compute equipment selection by predicting the air supply temperature after the generator provides power to the chilled water pump. Previous transient models include zonal models with large time scales and CFD/HT models with boundary conditions developed for steady state. These models can be improved by comparison with experimental data. The experiment consists of measuring the response of the CRAC heat exchanger to the step change in CHW flow rate upon pump restart. Inlet and outlet temperatures of both CHW and air were measured, as well CHW flow rate. A point measurement of air at the CRAC fan outlet was also taken to verify that airflow remained relatively constant. Outlet temperatures from the CRAC follow a first order response curve; it is found that the CRAC under consideration has fan outlet temperature time constant of 10 seconds. A delay of 20 seconds is observed between the fan outlet temperature response and the CHW return temperature response.