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Humidity Control for Data Centers

Data centers have more unique challenges to cooling and maintaining set points rather than cooling used for creature comfort. Data centers tend to have electrical equipment (servers, UPS, and storage units) and this equipment produces a high sensible heat load. To remove the high sensible heat load from the space there are specialty air conditioner units. These Computer Room Air Conditioner (CRAC) units are designed differently than comfort air conditioning. The biggest difference is the sensible heat ratio. For comfort air conditioning the heat ratio typically is 60% to 70%, that is, the coil/airflow design is to remove 30% to 40% latent heat load (moisture) and 60% to 70 % sensible heat load. The CRAC unit design is for 85% to 95% sensible heat load and 15% to 5% latent load. These units will remove the high sensible heat load produced by the electronic equipment in a data center.

The intent of the CRAC in the data center is to control the room temperature, humidity and continuously clean the air. This control is accomplished with the use of a microprocessor controller. The microprocessor will monitor the room conditions, control the temperature, control the humidity, alarm if there are any failures and, if connected to a building management system (BMS), letting the building owner know in real-time the condition of the data center.

The humidifier is activated when the relative humidity in the space drops to the humidity setpoint minus the dead band. The humidifier is deactivated as the relative humidity increases to the set point.

The humidity control logic is similar for most manufacturers of CRAC units. On a call for dehumidification, the cooling mode is activated. As the room temperature decreases the unit will activate the reheat. This reheat will temper the air leaving the cooling coil, allow the air to absorb more moisture. Reheat in general is a supplement to the existing heat load in the room. The reheat is not intended to overcome the cooling capacity of the CRAC unit. If there is a large difference in the heat load in the controlled space and the cooling capacity of the CRAC’s the space will be overcooled. This leads to more energy required to remove the humidity from the space.

It is important to control the temperature and humidity set points in critical spaces such as data centers. The safety and well being of the equipment is jeopardized if these factors are not measured and maintained properly.